SITE INVESTIGATIONS
INTRODUCTION
A thorough and comprehensive site investigation is an essential preliminary to the design and construction of a civil engineering project. The investigation of a potential contruction site is necessary to evaluate its general suitability for the proposed works. Comprehensive accumulation of information on the ground and its characteristics will be used in an appropriate foundation design, and enables a practical, safe and economic construction process to be planned.
PRIMARY OBJECTIVES
The requirement of a ground investigation is to provide geotechnical data which are representative of the ground conditions and relevant to the development considered. It will also provide data which relate to ground conditions beyong the immediate vicinity of the site where the soil, groundwater, and exisiting or future structures may effect or be effected by the works.
The British Standard Code of Practice BS 5930:1991 gives a list of the primary objectives of site investigation and is given below :
a) To assess the general suitability of the site and neighbourhood for the proposed works, from a geological and geotechnical point of view.
b) To provide suitable getechnical data for all aspects of an economic, safe and reliable design of foundations, earthworks and temporary works, including assessment of the effects of any previous uses of the site.
c) To assess the problems and constraints associated with the construction of the works arising from the soil or groundwater conditions and to plan the best method of construction.
d) To assess the quantity, quality and ease of extraction of construction materials suitable for the works (where relevant).
e) To determine the changes in the stability, drainage and other geotechnical aspects of the site and the surrounding ground and buildings, which might be initiated by the construction works.
f) To make comparison on the construction works by alternative methods or at alternative sites.
SEQUENCE OF STAGES IN SITE INVESTIGATION
Figure 1 : Generalised considerations and sequences for a ground investigation
CLIENT/CONSULTANT LIAISON
DESK STUDY
SITE RECONNAISSANCE SURVEY
PRELIMINARY REPORT
PRELIMINARY DISCUSSIONS WITH SELECTED SI CONTRACTORS
REQUEST FOR QOTATION FOR GROUND INVESTIGATION
TENDER FOR GROUND INVESTIGATION (for large or complex investigations (for small or straightforward investigations
PRELIMINARY INVESTIGATION
MAIN
GROUND INVESTIGATION
DESIGN OF LABORATORY SCHEDULE INTERMEDIATE
PROGRESS REPORT
SUBCONTRACTED SPECIAL ACTIVITIES
LIAISON
ENGINEERING
DESIGN REPORT FACTUAL REPORT
FINAL REPORT
ISSUE OF TENDER DOCUMENTS FOR CONSTRUCTION with factual
REVIEW DURING CONSTRUCTION
COMPLETE DEVELOPMENT MONITORING OF
A DESK STUDY
Desk Studies provide an opportunity to gather valuable information for negligible cost. They are carried out at the start of the Site Investigation, and involve reading existing information about the site. This existing information could include;
• topographical maps
• geological maps
• aerial photographs
• existing SI records
• geotechnical journals
Information obtained during the Desk Study will be taken into account when planning the SI.
Desk Study Limitations :
Whilst Desk Studies provide invaluable information, it must be noted that the usefulness of this data can be limited. It will be of no great use if the data obtained is not up-to-date.
Topographical maps are not updated frequently and what is shown on the map may not be the case at the time of the study. Also, features which may be relevant to the project may not have been added to the map, due to considerations of time, cost and 'readibility' of the map. It is dangerous to assume that a lack of indication of a feature means that the feature does not exist.
Aerial photographs can also be similarly out of date, if expenses allow, aerial photographs of a rapidly changing site should be taken often.
B SITE RECONNAISSANCE
Following the Desk Study, further information can be gained through site reconnaissance, this involves a visit to the site for inspection. Information can also be obtained by talking to local inhabitants during the visit.
Site reconnaissance can provide information on the following; slopes and mounds
site geology
surface water (ponds and streams) and erosion features groundwater
site access
condition of existing structures
existence of grave sites or archaeological features
C DETAILED SITE EXPLORATION AND SAMPLING
Site Investigation fieldwork is undertaken to gain information on groundwater levels, geological strata, bedrock depth etc. Samples, which are later sent to laboratories for testing, can also be obtained. Investigation of detailed geology and sub-surface soil conditions using surface surveys (land surveys), trial pits, boreholes, soundings (CPT, etc), geophysical methods, etc.
Trial pits
Generally, the pit is dug using the back-shovel of a mechnical excavator so trial pits can be excavated in all soils types, even those in which obstructions such as cobbles and litter are encountered.
Trial pits are limited to 4-5m depth, sides must be sloped at a safe angle, stepped or shored. If the excavation is beneath the groundwater level in permeable soils, dewatering will be required.
BOREHOLES
The most common method used to obtain soil and rock details directly is to drill a borehole. To estimate the engineering properties, plasticity etc. mazier and U76 samples can be taken from the borehole and sent to a laboratory for testing.
D. LABORATORY TESTING OF SAMPLES
Tests are conducted in the laboratory on disturbed and undistrubed samples submitted from the site. These tests include classification tests, quality, permeability, shear strength, compressibility of soil tests, etc, tests on rock cores and samples for strength and durability. Tests on constructional geo-materials such as compaction tests. Tests on groundwater; chemical and petrographic analysis.
E. IN SITU TESTING
Tests carried out on the site either prior to or during the construction process. Examples : Standard Penetration test (SPT), Cone Penetration test (CPT), Vane shear test, plate bearing test, etc.
F. REPORTING OF RESULTS
The factual Site Investigation report should include but not limited to the following details which are important to geotechnical design :
a) intoduction - statefor whom the SI works was done, the nature and scope of SI, purpose of SI, etc.
b) Site description - describe access, terrain, vegatation, land use, geoligical features and informations about the site.
c) Field works – SI methods, testing, procedures results of borings (borelogs)
d) Comments and recommendations relating to the design and construction of the proposed works
e) Recommendations relating to further investigating or testing, and to on going or post completion monitoring.
