Franki Guide

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T H I R D E D I T I O N

A Guide to Practical

GEOTECHNICAL

ENGINEERING

in Southern Africa

First Edition January 1976 written and compiled by IH Braatvedt Pr Eng, BSc (Eng), MICE, FSAICE Second Edition December 1986 revised and updated by

JP Everett Pr Eng, BSc (Eng), FSAICE Third Edition July 1995 re-written and updated by

G Byrne Pr Eng, BSc (Eng), MSAICE JP Everett Pr Eng, BSc (Eng), FSAICE K Schwartz Pr Eng, BSc (Eng), GDE, FSAICE

assisted by

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THE PURPOSE OF THIS BOOK

When Frankipile South Africa first published "The Guide" in 1976 the main purpose was to create a practical reference on all aspects of soil investigation and piling as carried out by the company in Southern Africa at that time. Judging from the popularity of the first edition this objective was achieved and most design engineers in Southern Africa have a copy on their bookshelves.

The second edition was published in 1986 as an update of the first and it was equally popular. This, the third edition, is in fact a re-write of the book as Frankipile has expanded its activities into soil improvement and lateral support as well as environmental engineering. The name of the Guide has thus changed to include all aspects of Geotechnical Engineering as carried out by the Company in Southern Africa.

The purpose of the book, however, remains the same; it is a reference with a wealth of practical information on geotechnical topics which we are confident all those who receive a copy will find extremely useful.

The contents of this book are presented in good faith. As in all geotechnical design the methods and data presented in the book must be interpreted and used with a degree of knowledge, experience and judgement. Frankipile South Africa (Pty) Ltd does not hold itself in any way responsible for any inaccuracies or errors in the book or for any interpretation thereof by persons other than its own employees.

The company acknowledges, with appreciation, the contribution by Messrs. OVE ARUP & PARTNERS to the section on pilecap design.

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FOREWORD

by PROFESSOR KEN KNIGHT

Over the twenty years since Ian Braatvedt wrote the original Guide to Piling and Foundation Systems the book has become a standard text for all those in the industry in Southern Africa. It is also widely used by many outside the industry and don't be surprised i/you come across a copy in any country of the world.

What has made the Guide such a valuable text is the wealth of practical information it contains on piling as well as a number of other geotechnical topics.

Franki's product diversification has been dramatic since the publishing of the second edition in

1986. Through its subsidiary GeoFranki the company has entered the lateral support market

and has considerably increased its market share in soil improvement. There have been other product improvements which have been developed and the company is also involved in environmental engineering. These developments are all part of Franki's ongoing drive for improvement which is backed up by some of the most experienced geotechnical engineers in the country.

With all the changes it is not surprising that John Everett and his editorial team decided that the third edition of the Guide had to involve a change in name which in turn signifies that the book now covers a much wider cross section of geotechnical engineering. As such this edition will no doubt prove an even more valuable reference.

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1.0 FRANKIPILE SOUTH AFRICA (PTY) LIMITED

The South African company in the worldwide Franki group was started by Mr. Wally Rowland in 1946. The initial seeds had already been sown early in 1939 but the second World War broke out in September and Wally joined up with the South African forces. At the end of the war The Franki Piling Company of South Africa, as it was initially named, was registered and the first contract was secured. This involved the installation of eight piles for a building in Paarden Eiland and a steam driven piling machine was used to install the piles which were standard Franki driven cast-in-situ piles.

By 1952 Franki had branch offices in Johannesburg, Cape Town and Durban. In 1955 Wally Rowland returned to the UK to take up the position of Assistant Works Director with the British Franki company.

Ian Braatvedt took over as Managing Director in 1961 and under his guidance the company grew steadily. Large contracts such as the Alusaf Bayside Smelter in Richards Bay, the Mondi Paper Mill in Durban and Iscor Steel Works in Newcastle were secured in the late sixties and early seventies and these really helped Franki to establish itself as the leading piling company in the country .

In 1968 Franki started a soil investigation subsidiary which is known as Soiltech. Today it has a full complement of soil investigation and field testing equipment as well as a fully equipped soils laboratory .It has recently entered the environmental investigation field. The need to diversify into other geotechnical fields led to the formation of GeoFranki in 1987. GeoFranki's main areas of activity are lateral support, ground improvement, micropiling, grouting and cut-off walls.

The South African company today has over forty major production rigs and an employee complement in excess of 600. It has offices in Johannesburg, Cape Town, Durban and Harare. It operates in Africa and the Indian Ocean Islands.

FRANKI INTERNATIONAL

Frankipile South Africa is a wholly owned subsidiary of SA Franki BV which is a Belgian company. SA Franki BV has a number of subsidiaries around the world and interests in many other international companies and this group is commonly referred to as Franki International. There is continual commercial and technical communication within the group as well as a common product development interest. Frankipile South Africa can thus draw on this international experience as well as obtain additional plant resources and personnel from the

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The following is a summary of the products and services which Frankipile South Africa and its subsidiaries can presently offer its clients and which are described in greater detail in this guide.

FIELD INVESTIGATION PILING

• Auger trial holes • Franki cast-in-situ pile • Test pits • Driven tube pile • Bulk and undisturbed sampling • Precast pile • Dynamic cone penetration test • Steel H-pile • Cone penetration test • Timber pile • Rotary core drilling • Auger pile • Standard penetration test • Underslurry pile

• Vane shear test • Continuous Flight Auger (CFA) pile • Pressuremeter test • Forum Bored pile

• Lugeon test • Oscillator pile • Piezometer installation • Caisson pile • Shelby and piston tube sampling

• Core orientation UNDERPINNING

• Rotary percussion drilling

• Plate load test SOIL IMPROVEMENT

• In-situ density test

• Geophysical techniques • Vibratory compaction • Ground water monitoring • Dynamic compaction • Well installation • Compaction grouting • Environmental investigation • Vibratory replacement

• Dynamic replacement

LABORATORY TESTING • Driven stone columns

• Accelerated consolidation • Triaxial compression • Jet grouting

• Unconfined compression • Lime columns • Shear box

• Permeability LATERAL SUPPORT

• Odeometer

• Grading/Sieve analysis • Steel sheet piles • Hydrometer • Steel soldiers

• Atterberg limits • Concrete soldier piles

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Frankipile South Africa has always adopted a policy of combining innovative design with many years of practical experience to provide the most economical solution to a geotechnical problem. The company thus maintains a strong design capability as well as its professionally run contracting activities. Whilst not directly marketing this design capability, Franki makes every use of it in negotiations with clients and in tenders where alternate designs are permitted. The fact that Franki can offer the wide selection of products and services indicated above also results in the economic optimisation of design and so it is not surprising that the company secures a large percentage of its work through negotiation and through innovative design.

With this considerable expertise Franki can offer a complete package deal including investigating the site, the complete design of the foundation system and any lateral support requirements, pricing and drawing up the contract, execution of the work and final handover. It also has strategic partners it can draw on to form joint ventures where it considers the combined skills and resources of the partners will provide the client with a more comprehensive service and at a more competitive price.

This is very much the case, for example, on large marine construction projects where Franki has the piling and other geotechnical skills which are often a major feature of marine work. A joint venture partner with general marine civil experience thus forms a strong combination with which to tackle the design and construction of any marine contract. This type of arrangement, however, is not limited to marine construction but can be arranged for any civil or building project with a significant geotechnical content.

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2.0 GEOTECHNICAL INVESTIGATION

Soiltech, the division of Frankipile responsible for Geotechnical Investigations was established in 1968 and offers a complete Geotechnical Service to consulting engineers and client bodies as well as to the Company. The importance of obtaining adequate and reliable knowledge of sub-surface conditions at a sufficiently early stage cannot be over emphasised when considering:

• The choice and design of an economical and technically sound foundation. • Possible delays and additional expense due to inadequate soils information. • Expensive foundation failures or overdesign.

• Potential contractor's claims based on inaccurate and/or inadequate soils information. Soiltech is able to offer a complete geotechnical investigation service comprising:

• Planning of the investigation.

• Execution of the field work and laboratory testing. • Interpretation and reporting.

The range of field work and laboratory testing that Soiltech can offer is outlined below in Table 2.0.l.

Table 2.0.1- Range of Soiltech Services

FIELD INVESTIGATION AND IN-SITU TESTING

LABORATORY TESTING

• Auger trial holes. • Triaxial compression tests • Test pits. • Unconfined compression tests • Bulk and undisturbed soil sampling. • Shear box

• Dynamic cone penetration tests. • Permeability • Cone penetration tests. • Oedometer test • Rotary core drilling • Grading/sieve analysis • Standard Penetration Tests. • Hydrometer

• Vane shear tests • Atterberg limits

• Pressuremeter tests. • Moisture density relationship • Lugeon tests • California bearing ratio (CBR) • Piezometer installations • Specific gravity

• Shelby and piston tube sampling • Bulk density • Core orientation • pH

• Rotary percussion drilling • Conductivity • Vertical and horizontal plate load tests

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A further important aspect of Soiltech's activities lies in the environmental engineering field. This service provides for the collection of data with respect to potentially contaminated soils, surface and ground waters. For further details refer to SECTION 19.0 ENVIRONMENTAL ENGINEERING.

2.1 GUIDE FOR PLANNING A GEOTECHNICAL INVESTIGATION

The objectives of a geotechnical investigation may embrace any combination of the following. (British Standard Code of Practice B35930: 1981):

• To assess the general suitability of the site for the proposed engineering works. • To enable an adequate and economical design to be

prepared-• To foresee and provide against difficulties that may arise during construction owing to ground and other local conditions.

• To determine the causes of defects or failure in existing works and the remedial measures required.

• To advise on the availability and suitability of local materials for construction purposes. Taking the above objectives into consideration, the planning of a geotechnical investigation will be influenced by the following main factors:

• The nature of the proposed engineering development. "If you do not know what you are looking for in a site investigation you are not likely to find much of value" (Glossop, 1968).

• The Geology and Geomorphology of the site. • Access to and the remoteness of the site. • The site topography, vegetation and drainage. • The nature of adjacent developments.

• Knowledge of previous geotechnical investigations or foundation installations carried out in the area. In particular the opinions of persons such as local engineers, farmers and contractors.

• Evidence of problem soil conditions (expansive or collapsible soils, dolomites, dispersive soils, soft clays).

The cost of an adequate investigation is very low in comparison to the total cost of the project. The consequences of not providing sufficient, accurate and reliable geotechnical information, however, can have a significant effect on a project and can lead to delays and extras during construction with associated costly claims. Experienced engineers have come to realise that a thorough geotechnical investigation is invariably paid for by the client, whether

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define the problem. This is followed by phase two which is a far more extensive investigation in which the site geology is studied in greater detail and all the design parameters are determined. The phased approach will generally commence with a desk study and site reconnaissance, followed by the fieldwork and laboratory testing.

Conditions vary from site to site. Consequently, a variety of techniques has been developed to enable both the geotechnical engineer and specialist contractor to select the appropriate investigation procedures.

An accurate description of the soil profile forms the basis of the geotechnical investigation. In some cases this maybe all that is required. In the majority of investigations, however, it will be necessary to supplement an accurate description of the soil profile with appropriate in-situ testing and sampling, and possibly associated laboratory testing.

Under appropriate conditions, particularly where the water table is at depth which is applicable to large areas within the hinterland of Africa, the drilling of large diameter trial holes and/or the forming of test pits for visual inspection by a geotechnical engineer or engineering geologist, can be carried out. The advantages of this procedure are as follows: • It allows for the soil profile to be examined in-situ in its natural

state-• Good quality undisturbed block samples can be cut from the auger hole or test pit sidewalls. Disturbed samples can also be taken from specific horizons identified during profiling.

• In-situ testing such as hand shear vane tests and horizontal plate bearing tests can be executed within the trial holes or test pits.

• The procedures adopted are fast and economical and provide for accurate and comprehensive evaluation of site geotechnical conditions.

Safety procedures when profiling and sampling in trial holes and test pits are extremely important. All investigation work with trial holes and test pits must be carried out in accordance with the SAICE Code of Practice for the safety of persons working in small diameter shafts and test pits for civil engineering purposes (1990).

For certain projects it may be necessary to supplement the auger trial holes/test pits with additional investigation procedures. A variety of techniques are available. These could include dynamic cone penetration tests, rotary core drilling with associated sampling and in-situ testing techniques (standard penetration tests, vane shear tests, lugeon tests etc.).

In the coastal regions and on sites with a high water table the use of trial holes and test pits is often not feasible due to collapse of the sidewalls. In these areas the two standard methods used in a geotechnical investigation are boreholes with standard penetration tests and cone penetration tests. These are supplemented where necessary with, amongst others, rotary core drilling, vane shear tests, dynamic cone penetrometer tests and the recovery of undisturbed samples using the Shelby tube method or piston sampling.

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that could be carried out in stable soil profiles above the water table (Table 2.1.1) and in saturated variable soils (Table 2.1.2).

Table 2.1.1 -Guide to planning a soils investigation in stable soil profiles above the water table (usually residual soils or cohesive transported soils)

Parameter Field Test Laboratory Test

Description of the soil profile Auger trial holes Test pits

Boreholes with SPT Consistency of the soil

profile Dynamic cone penetrometer(DPSH) In-situ profiling of trial holes/test pits

Undrained shear strength Recover undisturbed samples from auger trial hole, test pit or borehole Vane shear test in borehole

Undrained triaxial

Unconfined compression test Effective angle of friction –ǿ

Effective cohesion-ć Recover undisturbed samplesfrom auger trial hole, test pit or borehole

Drained triaxial Drained shear box test Undrained triaxial with measurement of pore water pressure

Modulus of compressibility Cross-hole jacking test or Plate load test or

Pressuremeter test

Oedometer test Index property tests Recover disturbed samples

from auger trial hole, test pit or borehole

Grading analysis Atterberg limits Moisture content Permeability Recover undisturbed samples

from auger trial hole test -pit or borehole

Falling or constant head permeability

Collapse Recover undisturbed samples from auger trial hole, test -pit or borehole

Double oedometer Collapse potential test

Heave Recover undisturbed and/or disturbed samples from auger trial hole, test pit or borehole

Double oedometer Swell under load test Index property test(disturbed

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Table 2.1.2- Guide to planning a soils investigation in saturated, variable soils usually encountered in coastal areas or adjacent to water courses

Parameter Field Test Laboratory Test

Description of the soil profile Boreholes with SPT Consistency of the soil

profile

Dynamic cone penetrometer (DPSH) Cone penetrometer test(CPT) Boreholes with SPT

Undrained shear strength Recover undisturbed samples from borehole Vane shear test in borehole Correlate with in-situ penetrometer tests

Undrained triaxial

Unconfined compression test

Effective angle of friction –ǿ Effective cohesion-ć

Recover undisturbed samples from borehole

Correlate with in-situ penetrometer tests (sandy soils only)

Drained triaxial Drained shear box test

Undrained triaxial with measurement of pore water pressure

Modulus of compressibility Pressuremeter test Correlate with in-situ penetrometer tests

Oedometer test Index property tests Recover disturbed samples

from borehole

Grading analysis Atterberg limits Moisture content Permeability Recover undisturbed samples

from borehole

Falling or constant head permeability

Collapse Recover undisturbed samples

from borehole Double oedometer Collapsepotential test Heave Recover undisturbed and/or

disturbed samples from borehole

Double oedometer Swell under load test Index property test (disturbed sample)

Level of Water Table Drill a borehole and leave for a period of time and measure

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2.2 FIELD INVESTIGATION TECHNIQUES

The field investigation techniques that Soiltech offers and which are summarised in Table 2.0.1 are discussed in more detail below.

2.2.1 AUGER TRIAL HOLES

The auger trial hole involves the drilling of a large diameter auger hole using a powerful auger machine. A qualified person is then lowered in stages down the hole by means of a small winch and is able to profile the hole by visually inspecting the sidewalls and the base. Furthermore, it is possible to cut large undisturbed samples from the sidewalls or base of the hole for later testing in the laboratory, as well as carry out cross-hole jacking tests and plate load tests as described in SECTION 2.2.7 in the trial hole excavation. Bulk sampling for the purposes of evaluating the mineral content of materials on old dumps or within soil and weathered profiles, can also readily be accomplished using this technique.

Auger trial holes provide a very quick and economical method for obtaining reliable geotechnical information for a variety of engineering solutions and it is favoured by most engineers and geologists. For the successful application of this technique, however, it is essential that the sIde walls of the trial holes remain stable during drilling and profiling. It is thus not suited to areas with a high water table where collapse of the sidewalls is most likel'l. It is possible to drill a large number of trial holes in a relatively short space of time which makes this an economical form of investigation. A minimum hole diameter of 750 mm is required for in-situ profiling purposes but trial holes of up to 2000 mm in diameter are possible. Depths of up to 36 metres can be drilled in suitable materials. The technique is ideally suited to sites with deeply weathered profiles. The auger trial hole can also penetrate into soft rock and even harder fractured rock.

Under suitable site conditions approximately eight 750 mm diameter auger holes to a depth of 10 metres can be drilled within a normal working day. It is also possible to profile this number of holes within the same working day. The auger rig with its crew and ancillary equipment is normally hired on a daily basis. Soiltech can arrange for the profiling of the holes by experienced qualified personnel from an independent geotechnical engineering firm should this be required.

To facilitate the profiling of the trial holes, a tripod frame fitted with a winch is positioned over the trial hole, the winch being connected via a steel wire rope to a specially designed bosun's chair. All operations are carried out in accordance with the S.A.I.C.E. Code of Practice for the safety of persons working in small diameter shafts and test pits for civil engineering purposes (1990). Plastic sample bags, cling wrap, labels, tape measures and

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AUGER RIGS FOR DRILLING TRIAL HOLES

Soiltech has a variety of truck mounted auger rigs available for drilling trial holes. Details of these rigs are given in Table 2.2.1.1. Plate 2.2.1.1. shows the Williams LDH50 rig used for drilling auger trial holes. The overall dimensions of the auger rigs are given in SECTION 23.2 PILING RIG DIMENSIONS

Table 2.2.1.1- Range of Auger Rigs Available

Type of Auger Rig Max. Drilling Torque (Kgm) Gross Vehicle Mass (Kg) Max Depth (m) Max. Hole Diameter (mm) Hotline 16MI20 3000 24400 16 1000 Soilmech RTAH 11000 30000 32 1500 Williams Digger LDH50 6818 28580 15 1000 Williams Digger LDH80 6818 30000 24 2000 WilliamsDiggerLLDHI20 13636 39310 36 2000 2.2.2 TEST PITS

The use of test pits as an investigation technique offers the same advantages in terms of profiling and sampling as described for auger trial holes. Test pits are easily formed with a mechanical excavator or by hand, and therefore have the advantage of being relatively inexpensive. The main disadvantages are that they are limited to depths of two to three metres and cannot be used in areas of shallow water table. Test pits are therefore most appropriate in areas with a relatively deep water table where competent soils or rock are anticipated at a relatively shallow depth. They are often used to investigate areas where there is poor access for other types of equipment.

In view of cost advantages, test pits are often used as a preliminary or first phase of the investigation. Where a competent soil or rock stratum occurs close to the ground surface, the profiling and sampling of test pits may provide sufficient information for design purposes and no other form of testing is required. If, on the other hand, the excavation of test pits discloses a much deeper soil profile, then it is essential to follow up the first phase with additional investigation work. This is normally carried out using techniques which can reach to greater depths, such as auger trial holes and boreholes.

It is extremely important to follow the correct safety procedures when profiling and sampling in test pits, and the SAICE Code of Practice for the safety of persons working in small diameter shafts and test pits for civil engineering purposes (1990) should be strictly adhered to at all times. Experience has shown that test pits are far more prone to collapse than auger

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2.2.3 DYNAMIC CONE PENETRATION TESTING Dynamic Probe Light (DPL)

This local standard of the Dynamic Probe Light test (ISSMFE Technical Committee on Penetration Testing, 1988) is used in many applications in South Africa. A 20 mm diameter, 60° cone is driven into the soil by an 8 kg weight dropped through 575 mm. The results are expressed as millimetres per blow. The original test (Van Vuuren, 1969) was designed for the rapid determination of the California Bearing Ratio (CBR) to depths of about one metre for investigation into road pavement performance and design. Besides the original application in the field of pavement evaluation and design, the test has also been used as a rough guide in compaction control and for estimating soil conditions for the design of shallow footings. The main advantage of this type of equipment is that it is light, portable, inexpensive to operate and provides a continuous rough record of soil consistency over the depth tested. The disadvantages are that no sample is recovered, the nature of the equipment limits its depth capability to three metres below surface and the equipment is not able to penetrate hard lenses or other obstructions (large gravel, boulders etc.). The ease and low cost with which results can be obtained, is therefore somewhat offset by the limitations of the test and the indirect approximation to soil conditions that it provides. A guide to the interpretation of the results of this test can be found in SECTION 3.0 SOIL AND ROCK CLASSIFICATION AND DESIGN PARAMETERS.

Dynamic Probe Super Heavy (DPSH)

In Southern Africa, considerable use is made of a local standard of the Dynamic Probe Super Heavy test (ISSMFE Technical Committee on Penetration Testing, 1988). A 60° disposable cone, 50 mm in diameter, is fitted onto the bottom of an "E" size rod and driven into the ground by a 63.5 kg hammer falling through 762 mm. The number of blows required to drive the cone through each successive 300 mm of penetration is recorded. This provides an empirical indication of consistency. Once refusal depth is reached (more than 100 blows per 300 mm), the driving rods are withdrawn by 600 mm. The disposable cone remains at the base of the hole. The rods are then re-driven with the number of blows per 300 mm being recorded. These re-drive blow counts provide an indication of the skin friction acting on the drive rods. Data collected from the DPSH test (including the re-drive figures) are presented on a report sheet.

A feature of the test is that it is very economical and can be rapidly and easily performed. A major disadvantage of the test is that no soil sample is obtained. In many instances this disadvantage can be overcome by adopting a variation to the test procedure by fitting a Raymond split-spoon sampler to the "E" rods, instead of the solid cone. This technique provides a continuous disturbed representative sample of the soil profile. Any blow counts

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The DPSH rig is designed so that tests can be undertaken in areas that are not readily accessible, such as inside existing buildings and in narrow passage ways between buildings. Plate 2.2.3.1 shows a typical DPSH test rig.

The DPSH is used under the following conditions:

• As economical supplementary data between boreholes on larger sites.

• On sites with erratic profiles (alluvial, colluvial or lacustrine deposits), it will locate softer areas.

• Probing for rock or hard

strata-• In conjunction with a soil profile it will provide rough consistency readings which can be plotted graphically.

• As the test closely approximates a driven pile, it is extensively employed for determining an estimate of skin friction and installation depths of driven cast-in-situ piles. In non-cohesive materials it is very reliable, but must be used with caution in non-cohesive soils. The test will also indicate pile driving conditions.

Limitations of the test are:

• Driving refusal is frequently experienced on hard layers (such as very dense ferricretes or calcretes, boulder horizons) which may be underlain by soft soil horizons.

• Differences in remoulding caused by the small diameter cone on the one hand and the considerably larger piling tube on the other,can lead to erroneous prediction of pile installation depth.

• Similar differences may occur when excessive pore pressures are set up during the driving of a pile whereas this does not occur with the DPSH test.

A graphical presentation of this data is presented in Figure 2.2.3.1. The interpretation of the test results is generally associated with local experience. As a preliminary evaluation the blow counts can be taken as being roughly equivalent to the SPT N value (See SECTION 2.2.5). In the interpretation, however, it is essential to take into account the influence of the rod friction.

2.2.4 CONE PENETRATION TESTING (CPT)

This method was initially developed in the Netherlands in the 1930's where it was first used as a means of determining the ultimate bearing capacity of driven piles founded in sand. Over the years the test has been called the Dutch sounding test, the Dutch probe and the static cone penetration test. In terms of acceptable international standards (ISSMFE Technical Committee on Penetration Testing, 1988) it is now referred to as the cone penetration test

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penetration resistance (point resistance), total penetration resistance and the side friction resistance of the friction sleeve are made continuously throughout the test.

The main advantages of the CPT are that the testing procedure is relatively simple and repeatable, and the test results are more amenable to a rational analysis rather than relying entirely on empirical correlation. The CPT also gives a virtually continuous record of soil resistance values throughout the depth of penetration.

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The main limitations of the CPT test are as follows:

• Penetration depth limitations due to machine capacity.

• The technique is rarely effective in gravels and boulder horizons and is also not suited to weathered rock profiles.

• No samples are recovered.

The data obtained from the cone penetration test may be employed to: • Assist in the evaluation of the type and stratigraphy of the soils present. • Interpolate ground conditions between control boreholes.

• Evaluate engineering parameters of soils (relative density, shear strength, compressibility characteristics, liquefaction potential).

• Assess driveability, bearing capacity and settlement of piled foundations.

Mechanical cone penetrometers (Begeman, 1965) have a telescopic action which requires an outer probe sleeve and an inner rod. These mechanical cone penetrometers offer the advantage of low equipment cost and simplicity of operation. They do, however, have the disadvantages of a slow incremental procedure, limited accuracy in very soft soils and labour intensive data handling and presentation.

With the electrical cone penetrometer the friction sleeve and cone point advance together as a single system. The point resistance and local side shear are recorded continuously with the use of built-in load-cells. An electrical cable located inside the rods connects the load cells to recording equipment at ground surface. Electrical cones carry a high initial equipment cost and require skilled operators as well as adequate back-up for calibration and maintenance. They do, however, offer advantages over the mechanical penetrometer such as a more rapid procedure, higher accuracy and repeatability , automatic data logging, reduction and plotting. One of the important applications of the CPT test is to evaluate variations of soil type within the soil profile. With mechanical and electrical cones extensive use is made of what is known as the friction ratio as a means of soil classification (Jones, 1974, Schmertmann, 1975). The friction ratio is the ratio between sleeve friction and the point resistance and is expressed as a percentage.

The most significant recent development in electric cone penetration testing is the development of the piezo-cone penetrometer test (CPTU) which incorporates a pore pressure sensor in the cone. This allows for the measurement of the pore water pressure present in the soil during penetration. Pore pressure measurements during cone penetration testing provides more details on the stratification and has added a new dimension to the interpretation of

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Further advantages of the CPTU test over the conventional CPT are as follows (Campanella and Robertson, 1988):

• The ability to distinguish between drained, partially drained and undrained penetration. The ability to evaluate flow and consolidation characteristics.

• The ability to assess equilibrium groundwater conditions.

A guide to the interpretation of the results of CPT and CPTU tests can be found in SECTION 3.0 SOIL AND ROCK CLASSIFICATION AND DESIGN PARAMETERS.

2.2.5 ROTARY DRILLING, IN-SITU TESTING AND SAMPLING

The rotary drilling technique is used to drill a borehole which is normally cased through the upper soil profile. Various methods for testing and sampling the soil during the drilling of the borehole are available and described later in this section. The most common of these is the standard penetration test or SPT. Once the borehole reaches strata of rock consistency, rotary core drilling is used to recover samples.

ROTARY DRILLING

The borehole is typically drilled through the upper soil layers using a casing fitted with a diamond/tungsten tipped casing shoe. A drilling fluid is used to remove the cuttings and flush them to the surface where they can be sampled. This technique for advancing the borehole is called wash boring and the samples are known as wash samples. The borehole is advanced in stages with samples taken at the various depths required. Plates 2.2.5.1 and 2.2.5.2 show two types of rotary drilling rigs.

When materials of rock consistency are encountered and wash boring is no longer effective, rotary core drilling is used to advance the borehole and recover core samples. The cores are drilled using a core barrel which is fitted with a diamond tipped or impregnated drill crown. The core barrel with drill crown is rotated by the drilling rig which also has the means to hydraulically crowd the drill stem. A drilling fluid is pumped through the core barrel to cool the drill bit and flush the cuttings to the surface.

The conventional core barrel can recover a 1.5 metre length of core at a time. Once the core barrel is full, the drill stem with core barrel is withdrawn from the hole and the core sample is recovered and stored in a core box. Core boxes are marked with the depths drilled so that a visual inspection of the core box shows what percentage of core was recovered relative to the depth drilled. Cores are sometimes waxed to retain their natural moisture content. UCS and point load tests are often carried out on rock cores so as to determine the strength of the rock. This is an important factor when carrying out a geotechnical investigation for a contract on which piles will be required to penetrate the rock, as the piling contractors need to know the hardness of the rock to be able to assess penetration rates at the time of tender.

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A variety of core barrels and appropriate crowns is available, allowing the driller to select the most suitable type for the particular materials being cored. Core barrel designs, such as double or triple tube, help to maximise the core recovery especially in the very soft and weathered rock strata. Heinz ( 1989) gives a detailed description of rotary core drilling techniques and equipment. Soiltech complies with the Standard Specifications for Subsurface Investigations (CSRA, 1993) in carrying out rotary drilling operations.

IN-SITU TESTING

Standard Penetration Test (SPT)

This process was standardised in the 1920's and 1930's into what we know now as the

Standard Penetration Test. In the execution of this test a standard 51 mm diameter split spoon sampler known as a Raymond Spoon is driven into the soil at the bottom of a borehole. A free-fall hammer of 63.5 kg operating off a trip mechanism and falling through a height of 762 mm provides the driving force. The number of blows required to drive the sampler each 150 mm increment of a total of 450 mm penetration is recorded. The blow count for the first 150 mm increment is discarded and the sum of the blow counts for the second and third 150 mm increments is known as the SPT "N" value.

The standard penetration test has become accepted world wide as a useful test in geotechnical investigation and foundation design. SPT results in boreholes give an empirical qualitative guide to the in-situ engineering properties of cohesive and cohesionless soils and provide a sample of the soil for classification purposes.

The results of the SPT can be affected by incorrect drilling and sampling procedures some of which are given below (refer also to the Canadian Foundation Engineering Manual, 1985): • Inadequate cleaning of the bottom of the borehole.

• Driving the spoon above the bottom of the casing.

• Failure to maintain sufficient hydrostatic head in the borehole. • Not using the standard hammer drop or correct mass.

• Free fall of the hammer is not obtained. • The tip of the spoon is damaged.

• Not recording blow counts and penetration accurately.

It is thus extremely important that the drilling crew carrying out the tests is experienced in this type of work. Even then it is advisable to carry out some CPT tests close to the borehole positions to check the correlation between the two. This will give an indication as to whether the SPT values are reliable. The relationship between the SPT N value and engineering

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Plate 2.2.3.1 -DPSH Test Rig Plate 2.2.5.1- Skid Mounted Rotary Core Drilling Rig

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Vane Shear Tests

The vane shear test is routinely used to obtain undisturbed peak and remoulded undrained shear strength. The test consists of placing a four bladed vane in the undisturbed soil and rotating it from the surface to determine the torsional force required to cause a cylindrical surface to be sheared by the vane. This force is then converted to a unit shearing resistance of the cylindrical surface as shown in Figure 2.2.5.1.

A typical example of the equipment employed to apply torque to the steel rods from surface is also shown in Figure 2.2.5.1. The steel rods are housed in a sleeve in order to prevent flexing and to protect the rods. The vane which is connected to the base of the steel rods is housed within a "torpedo" attached to the base of the sleeve. The vane consists of a four bladed cruciform. For standard tests the height of the vane should be twice the diameter. The selection of the vane size is directly related to the consistency of the soil being tested, with larger vane sizes being used in the softer soils.

The test procedure is to advance the vane from the bottom of the torpedo in a single thrust to the depth at which the test is to be conducted. Once the vane is in position, torque is applied in a rotational sense at a slow rate using the gear driven surface equipment. Torsional force is measured and converted to unit shearing resistance in accordance with the following assumptions:

• Penetration of the vane causes negligible disturbance, both in terms of changes in effective stress and shear distortion.

• No drainage occurs before or during shear: • The soil is isotropic and homogeneous. • The soil fails on a cylindrical shear

surface-• The diameter of the shear surface is equal to the width of blades.

• At peak and remoulded strength, there is a uniform shear stress distribution across the shear surface.

• There is no progressive failure, so that at maximum torque shear stress at all points on the shear surface is equal to the undrained shear strength.

The results of a vane shear test may be influenced by many factors: • Type of soil, especially when permeable fabric exists.

• Strength and anisotropy.

• Disturbance due to insertion of the vane. • Rate of rotation or strain rate.

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Pressuremeter Tests

The pressuremeter test was originally developed by Menard (1956) and comprises a horizontal in-situ loading test carried in a borehole by means of a cylindrical expandable probe. A major difference between categories of pressuremeter tests lies in the method of installation of the device in the ground. In accordance with Mair and Wood (1987), the following two broad categories of tests can be distinguished in terms of installation method: • Menard type pressuremeter (MPM) test in which the device is installed in a borehole. • Self-boring pressuremeter (SBP) test in which the device bores its own way into the

ground usually from the bottom of a borehole.

The following parameters can be deduced from the results of the pressuremeter test. • Deformation modulus (i.e. compressibility).

• Undrained shear strength for clays or weak rocks. • Effective angle of friction for

sands-• In-situ total horizontal stress.

The degree of success in obtaining any of these parameters is essentially dependent upon the type of test and the interpretation of the data. Consideration must also be given to possible differences in the properties of soil horizons measured in a horizontal direction by the pressuremeter, and those required for many design problems which are more concerned with vertical properties.

For more details with regard to pressuremeter testing and its interpretation reference should be made to Baquelin et al (1978), Windle and Wroth (1977) and Mair and Wood (1987).

Lugeon Testing

Lugeon testing (also known as water pressure or packer testing) is carried out to measure the permeability of the soil or rock at specific depths in a borehole. The equipment consists of two packers comprising steel tubes surrounded by inflatable rubber sleeves separated by a perforated length of steel tube. The spacing of the packers can be adjusted to the specific length of soil or rock to be tested. The minimum length of packer sleeve is 700 mm to ensure a watertight seal. The packer arrangement is connected via high pressure tubing to a suitable pump on the surface. Data collected from the system is obtained by flow metres and pressure gauges. The above arrangement is known as a double-packer system. The system can be adapted, however, for so-called single packer tests, where testing is carried out between the packer and the bottom of the hole.

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The test consists of pumping water into the isolated zone of the borehole at three different pressures, in the following typical sequence:

1 st 10 min. at low pressure a 2 nd 10 min. at medium pressure b 3 rd 10 min. at high pressure c

4 th 10 min. at medium pressure b -repeated 5 th 10 min. at low pressure a- repeated

The actual duration of each pressure stage is accurately timed. The pressures selected are dependant on the depth at which each test is carried out. The required pressures are maintained to an accuracy of 5% during each pressure stage.

Piezometer Installations

Piezometers are installed in boreholes in order to provide information regarding the at rest levels of the ground water table. In addition, ground water pressure can be measured via more specialised piezometers i.e. hydraulic, electrical and pneumatic.

In general piezometers are installed into pre-cleaned holes by lowering a selected porous tip to approximately 500 mm above the bottom. The tip is surrounded by a filter of graded, washed silica sand and sealed off with a bentonite plug. The remainder of the borehole is sealed by introducing cement/bentonite grout.

SAMPLING

Shelby and Piston Tube Sampling

This sampling technique is employed to obtain undisturbed material from soft and very soft cohesive soils. The Shelby tube used to recover the samples, consists of a thin walled stainless steel tube with an internal diameter of approximately 75 mm. The leading edge of the tube is beveled and crimped such that the entry diameter is fractional smaller than the body diameter. The tube is usually a half metre in length with the top end designed to fit into an adapter. The adapter has a one-way valve built into it to allow water to escape so as to prevent compression of the sample.

The Shelby tube sampler is attached to the drill string in place of a core barrel and is lowered to the base of the borehole and pressed into the soft material using the drill rig hydraulics. The sample and tube are then raised and the sample extruded on site. The sample should be sealed and packed so as to maintain the in-situ moisture content and to resist damage during normal handling and transport.

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rests on the top of the sample as it is pushed into the tube. The piston creates a vacuum which allows for retention of the sample within the tube.

Core Orientation

Such surveys are carried out where information is required regarding the spatial orientation of planar features, palaeontological studies, etc. The techniques employed include the following:

• Impression Core Orientation: this technique employs a hollow tube fixed to the base of the drill string filled with a suitable Plasticine material. The tube is lowered to the base of the pre-washed borehole and the orientator is pushed to seat onto the proud core break. The tube is withdrawn and the impression in the Plasticine matched with the bottom of the previous core run. Correct orientation is maintained during the raising and lowering of the drill string.

• .Integral Core Orientation: this technique involves the drilling of a pilot hole (E size or similar) to 1.5 metres below the base of the main borehole using centering bushes to centre the pilot hole in the main borehole. An orientated bar or pipe is placed into the pilot hole and cemented into position. The orientated bar is overdrilled once the cement has set. The technique can be employed in vertical or inclined holes, and is specifically used where highly fractured formations have been intersected or the impression technique cannot be employed.

2.2.6 ROTARY PERCUSSION DRILLING

There are two types of rotary drilling equipment. The one is known as a top drive rig and this consists of a drive head which remains above the surface and is connected via drill rods to a drill bit. The drive head rotates the drill string as well as imparts an impact force into the rods. The drill bit impacting on the rock chips the rock and the chips are air flushed to the surface.

The other type of drilling equipment is very similar to the above but the impact force is generated by a down-the-hole hammer. This is a percussion hammer which is driven by air and which imparts a rapid series of impacts to the drill bit which is part of the hammer. The rotation drive to the drill stem is provided by a top drive head. The down-the-hole hammer is favoured for geotechnical investigation purposes because of greater versatility and sensitivity particularly when recording penetration times.

The standard procedure in terms of geotechnical investigation is for percussion chips, which are flushed to the surface by compressed air, to be collected at one metre intervals. During

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A borehole log is compiled from the inspection of the chip samples, an evaluation of penetration times and the other relevant information supplied by the driller. The nature of the technique is such that the compilation of the borehole log can be influenced by a number of factors that can lead to inaccurate interpretation of the soil/rock conditions. Some of the more important of these factors are as follows:

• The highly disturbed nature of the chip samples recovered and the possibility of contamination of these samples.

• Total loss of sample in loose or soft layers.

• Incorrect interpretation of the penetration rate in relation to the hardness of the material being penetrated.

From the discussion presented above it is apparent that, in terms of geotechnical investigation, rotary percussion drilling can only be used to obtain a rough indication of the soil/rock profile and is subject to a large number of inaccuracies which include to a large extent the experience of the driller and the logger.

The advantages of the rotary percussion technique are that it is relatively inexpensive when compared with rotary coring, being about one tenth of the cost of rotary coring. Drilling production is also fast when compared to rotary coring with production rates of 80 to l00 metres per day possible. It is also one of the few techniques that can be used to economically penetrate boulder horizons or layers of chert which are often encountered in dolomitic terrain.

In South Africa the technique has been used successfully used as part of the overall geotechnical investigation procedures used in dolomitic terrain (Wagener, 1984). The technique is also used for the following applications:

• As probe holes to determine rock head depths.

• As probe holes to determine the depth and extent of old mine workings.

• To form boreholes for the conducting of in-situ tests (pressuremeter, lugeon tests). • To form boreholes for the installation of geotechnical instrumentation (piezometers,

extensometers, inclinometers, etc.).

2.2.7 PLATE LOAD TESTS

Plate load tests are usually carried out to determine the compressibility and occasionally the bearing capacity of soils and rocks. The test is a convenient and direct method of obtaining these parameters and is often used in soils or rocks which cannot be sampled or where the structure (joints etc.) may control the engineering behaviour of the soil/rock mass.

In its simplest form, the plate load test comprises a rigid plate placed on the surface to be tested. The load is provided by an hydraulic jack, using kentledge or an anchored beam as

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The following procedures are adopted for the test:

• The test site is carefully levelled and the plate bedded into the layer being tested using Plaster of Paris and/or bedding

sand-• Load is applied to the plate using a hydraulic jack in a series of predetermined steps. This application of load and the maximum load applied must be designed to conform with the type and purpose of the testing being carried out.

• Plate settlement is usually measured by means of dial gauges. In order to measure any tilt of the plate it is advisable to have four measuring points. The dial gauges are usually fixed to a beam supported by posts bearing on the soil some distance from the loaded area to avoid the readings being influenced by the settlement of the plate.

A variation to the standard test procedures can be implemented to allow the soil below the plate to be saturated at a specific load. The objective of this procedure is to allow the determination of any collapse properties associated with the material being tested.

The widespread use of auger trial holes and test pits in Southern Africa has led to the development of light and portable horizontal plate load equipment suitable for use in trial holes and test pits. By carrying out the tests in a horizontal direction, the necessary reaction is provided by the opposing faces of the trial hole or test pit. The bearing plates on either side are of equal size and the test procedure is essentially the same as that used for vertical plate load tests. The distance between the plates is measured and the movement of each plate is taken as half the total on the assumption that the two plates have moved equally.

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2.2.8 IN-SITU DENSITY TESTS

In-situ density tests are mainly used for compaction control in roads and earthworks construction. In certain instances the determination of in-situ density may, however, form part of an overall geotechnical investigation fieldwork programme. Both the sand replacement method and nuclear methods are used for the determination of in-situ density. In the sand replacement method, the in-place dry density is determined by forming a hole in a layer and dividing the mass of the material removed from the hole by the volume of the hole, the latter being determined by filling the hole with a fine sand of known density .The disadvantage of this test is that the material removed from the hole needs to be dried to a constant mass, usually overnight in a suitable oven. This means that a period of at least 12 to 18 hours is required before results become available. The advantage of the test is that it gives an accurate value of in-situ dry density and in-situ moisture content.

Nuclear systems for the determination of wet density and moisture content have become popular in recent years. One of the main advantages of this test procedure is that results are immediately available. The disadvantage is that there are some potential inaccuracies associated with the results produced from this test. The inaccuracies are generally associated with the measurement of moisture content and can easily be overcome by taking a sample at each test position for the laboratory determination of moisture content. To a large extent this negates the advantages of having results available immediately. On most roads and earthworks contracts the results of nuclear gauge tests are generally only accepted as a control procedure after suitable calibration with sand replacement tests has been carried out. Soiltech is able to offer both sand replacement and nuclear gauge density tests. These tests are carried out in accordance with the procedures recommended in TMH 1 ( 1986).

2.2.9 GEOPHYSICAL TECHNIQUES

Geophysical exploration is a form of field investigation in which a set of physical measurements relating to the underlying soil or rock strata is made at ground surface or in boreholes. The measurements indicate variations in space or time of certain physical properties of the soil/rock materials. Geophysics is therefore a blend of physics and geology since the physical measurements are interpreted in terms of subsurface geological conditions. The properties of soils/rock which are of significance in geophysical exploration are density, magnetic susceptibility, electrical conductivity , elasticity and thermal conductivity. Since these physical properties vary widely in soils/rocks at least one of these properties usually shows marked changes from place to place which can be measured by sufficiently sensitive instrumentation.

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The main advantages of geophysical techniques are as follows:

• It is possible to carry out investigations of large areas rapidly and economically. • The techniques can be used to locate critical areas for further field investigation.

The disadvantage of the technique is that the results are dependent on the interpretation of physical measurements. These measurements are not in themselves geological or geotechnical parameters relative to the site subsurface conditions. It is therefore essential that geophysics is carried out and interpreted in conjunction with a carefully planned drilling programme. The main application of geophysics in geotechnical investigations is the interpolation of subsurface geological strata between carefully controlled drilling positions. The more common geophysical techniques used in geotechnical investigations are magnetics, gravity and resistivity. For more detailed information reference should be made to Darracott (1976), Bullock (1978), Griffiths and King (1965), Kleywegt and Enslin (1973) and West and Dumbleton (1975).

2.3 GEOTECHNICAL ENGINEERING LABORATORY SERVICES

Standardised and consistent soil mechanics and materials testing, often forms the basis for design and site quality control in geotechnical and materials engineering. Soiltech has a fully equipped soil mechanics and materials laboratory facility which provides a testing services to clients, consulting engineers and the Frankipile Group. A guide to testing procedures and requirements for the commonly specified soil mechanics and materials tests is presented in Table 2.3.1. All relevant road type materials testing is carried out in accordance with TMH 1 (1986). Soil mechanics testing is carried out in accordance with accepted published or International standards.

In certain instances non-standard testing may be required. Through in-house expertise Soiltech can assist clients to define the testing programme and ensure that the testing is carried out to specified requirements.

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Table 2.3.1 -Guide to Laboratory Procedures and Requirements

Laboratory Test Parameter Determined Duration of Test in Days

Sample Requirements Triaxial compression test

Unconsolidated Undrained (UU)

Undrained shear strength of cohesive soils (Cu)

3 Consolidated Undrained

with pore water pressure. measurements (CU)

Effective shear strength parameters c' or «I>'

5 to 7 Consolidated drained test

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Effective shear strength parameters c' or «I>'

7 to 10

Undisturbed: Good quality sealed block sample 300 mrn x 200 mrn x150 mm thick. Shelby tube or piston sample

Disturbed or remoulded: 2 kg of representative sample

Shear Box Test

Drained Shear Box Effective shear strength

parameters c' or «I>' Residual shear strength parameter «I>'

4 5 to 7

Undisturbed Good quality sealed block: sample 300 mm x 200 mm x150 mm thick. Shelby tube or piston sample

Consolidation Tests

Consolidation test soaked at11 kPa loaded to 1600 kPa and rebounded

Compressibility characteristics

7 Double oedometer test for

collapse

Compressibility and collapse characteristics over full loading spectrum

7 Collapse potential test.

Sample loaded to 200 kPa, saturated and rebounded

Compressibility and collapse characteristics Collapse potential index

3 Double oedometer test for

heave

Swell characteristics over full loading spectrum

7

Swell under load test Swell characteristics at

specified load

3

Undisturbed Good quality sealed block sample 300 mm x 200 mm x: 150 mm thick. Shelby tube or piston sample Disturbed or remoulded: 2 kg of representative sample

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Table 2.3.1 (Cont.) -Guide to Laboratory Procedures and Requirements

Laboratory Test Parameter Determined Duration of Test in Days Sample Requirements PERMEABILITY TESTS Falling head or constant head Coefficient of permeability 3 for sandy soils 7 to 10 for clayey soils Undisturbed:

Good quality sealed block sample 300 mm x 200 mm x150 mm thick. Shelby tube or piston sample.

BULK DENSITY Bulk density Dry density Moisture content

3 Good quality sealed block sample 300 mm x 200 mm x150 mm thick Index properties Grading/sieve analysis Particle size distribution to0.075 mm 3 2 kg sample of undisturbed or disturbed soil

Hydrometer Particle size

distribution from0.075 mm to 0.002 mm Atterberg limits Liquid limit, plastic

limit, plasticity index Moisture content Moisture content Moisture density

relationship Mod AASHTO Proctor

Max. dry density and optimum moisture content under specified compactive effort 2 40 kg of representative sample CALIFORNIA

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3.0 SOIL AND ROCK CLASSIFICATION AND

DESIGN PARAMETERS

3.1 NOTES ON SOIL PROFILING

As indicated in SECTION 2.0 GEOTECHNICAL INVESTIGATION, an accurate description of the soil profile forms the basis of the geotechnical investigation for any engineering development. It is important that each layer is described in a consistent way to ensure accurate interpretation of the soil profile by those involved in the geotechnical design and construction process.

The description of the soil in profile, based on the work of Jennings, Brink and Williams (1973), is related to the following:

Designation Heading Example

M Moisture Moist

C Colour Reddish Brown C Consistency Stiff

S Structure Intact S Soil Tvpe Clay

O Origin Residual shale

Moisture

The moisture content is assessed as: DRY, SLIGHTLY MOIST, MOIST, VERY MOIST and WET. The assessment at the moisture content is dependant on the soil type. With a moisture content of say 20%, sand will probably be described as wet, whilst clay will probably be described as slightly moist.

Colour

Colour is important for description and for correlation. Colour is described from the soil in profile and also from a small sample of soil made into a creamy paste with water. A profile is MOTTLED when small exposures of different colours occur. A profile is BLOTCHED when larger exposures (say 75 mm and larger) of different colour occur. Colour charts obtainable from the South African Institution of Civil Engineers illustrate the main colours as well as variations in hue and lightness of each colour. These charts illustrate the following colours.

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Blue: Dusky Red: Dusky, Dark Pale Pale, Light. Green: Dusky. Grey: Dark

Pale Light

Olive: Dark Orange: Dark reddish Light Light reddish Brown: Dark Dark yellowish

Light Light yellowish Dark reddish Yellow: Dark

Light reddish Light

Consistency

Consistency is a measure of the strength or density of the soil. Observations are based on the effort required to dig into the soil or to mould it with the fingers. The consistency of cohesive soils is based on the undrained shear strength and described as VERY SOFT, SOFT, FIRM, STIFF AND VERY STIFF. Consistency vs. Undrained shear strength guidelines are set out in SECTION 3.3. Non-cohesive soil consistency is based on the angle of shearing resistance of the soil and described as VERY LOOSE, LOOSE, MEDIUM DENSE, DENSE AND VERY DENSE. Consistency vs angle of shearing resistance guidelines are given in SECTION 3.3.

Structure

The presence and type of discontinuities in the soil mass define the structure. Structural characteristics are generally related to cohesive soils in the following terms:

INTACT Absence of fissures and joints, though tension cracks may occur in firm samples when broken with a pick.

FISSURED Presence of closed joints.

SLICKENSIDED Highly polished fissures, usually indicative of expansive soils.

SHATTERED Indicates fissures which have opened up and allowed entry of air, often associated with expansive soils.

MICRO-SHATTERED Shattering on a small scale with shattered fragments the size of sandgrains. If well developed, the soil appears granular when cut, but the grains break down into clay and/or silt when wetted and rubbed. Indicates the presence of a highly expansive soil.

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Soil Type

The soil type is described on the basis of the grain size of the individual particles. The basic grain size classes are given below. Most natural soils occur as a combination of these classes e.g. Silty clay or gravelly sand.

BOULDERS Fragments of rock > 200 mm GRAVEL COBBLES 60mm -200mm

COARSE 20mm -60mm MEDIUM 6mm -20mm FINE 2.0mm -6mm

The range of size of boulders and gravel, the shape, the proportion by volume of the matrix and the description of the matrix are important.

SAND COARSE 0.6mm -2.0mm MEDIUM 0.2mm -0.6mm FINE 0.06mm -O.2mm

Sand particles are visible to the naked eye. SILTS 0.002mm -0.060mm

Silts are barely gritty between fingers and thumb when wet, but are gritty on tongue against teeth. Silts are not easily rolled into threads when moist. Silts exhibit dilatancy when moulded with water into a pat, (i.e. it increases its volume when shearing occurs which is illustrated by the film of water on the surface being absorbed if the pat is distorted.) Silts dry moderately quickly and can be dusted off the fingers. Dry lumps possess cohesion but powder easily in the fingers.

CLAY Particles less than 0.002 mm

Clay particles are flaky (not powdery) when broken and will soften with the addition of water. They have a soapy or greasy feel when wetted and rubbed on the palm of the hand. Clay sticks to fingers and dries slowly. There is no dilatancy or grittiness on tongue against teeth.

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Origin

In any soil profile there are four basic categories of origin: • Rock

• Residual soil • Pedogenic material • Transported soil

In the South African context, the demarcation between residual soils and overlying transported soils is often defined by the "pebble marker". This horizon is generally characterised by a gravel layer overlying the residual soil.

• Rock

Materials described as rock comprise igneous, metamorphic or sedimentary (not pedogenic) horizons with unconfined compressive strengths of the intact or unjointed material in excess of 1000 KPa.

• Residual Soil

A residual soil is formed from in-situ decomposition of rock. Decomposition can be caused by chemical weathering or mechanical disintegration which is a function of potential evaporation (temperature, humidity, wind) and average annual precipitation.

• Pedogenic Material

Pedogenic material is residual or transported soil that has become strongly cemented or partially replaced by one of the cementing agencies.

Description Cementing Agency

Ferricrete Iron oxide Calcrete Calcium carbonate

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• Transported Soil

This is soil which has been transported by a natural agency (water, wind, gravity) during relatively recent geological times (Pleistocene or Tertiary) and which has not undergone lithification into a sedimentary rock or cementation into a pedogenic material.

Type Agency Source Resulting Soil Talus (scree and

coarse colluvium) Gravity Rock outcrops Unsorted gravel and bouldersangular Hillwash (fine

colluvium) Run-off Acid crystallineBasic crystalline Arinaceous sediment Argillaceous sediment Clayey sand Clay Sand Clay or silt Alluvium (gully wash)

Rivers, streams and gullies

Various rocks and soils Boulders Gravels Sands Silts Clays Lacustrine Deposits Streams terminating

in lake, pan or pool

Various rocks and soils

Sand Silt Clay Estuarine Deposits Tidal rivers and

waters

Mixed Sand Silt Clay Littoral Deposits Waves Mixed Beach sand

Aeolian Deposits Wind Mixed Sand and clayey sand

Subsurface Water Condition

The water table is that level or those levels in the soil where the water in the pores of the soil occurs at atmospheric pressure, i.e. the level to which the water finds its own way in a borehole. The perched water table is a table which is only present in the soil temporarily. It will disappear and sometimes re-appear depending upon seasons or drainage conditions of the site. The permanent water table is the water table which persists throughout the seasons of the year with only minor seasonal fluctuations of level.

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A typical soil profile and a tabulation of the various soil symbols are given in Figures 3.1.1 and 3.1.2 respectively.

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Franki Guide

T H I R D E D I T I O N

A Guide to Practical

GEOTECHNICAL

ENGINEERING

in Southern Africa

FOREWORD

by PROFESSOR KEN KNIGHT

CONTENTS

1.0

FRANKIPILE SOUTH AFRICA (PTY) LIMITED

2.0

GEOTECHNICAL INVESTIGATION

3.0

SOIL AND ROCK CLASSIFICATION AND

DESIGN PARAMETERS