Preliminary design

7.7.1 Introduction

This section presents initial thoughts on the choice of pile type that would be appropriate at initial development of a foundation solution. The section stops short of design which is addressed in Section 7.8.

The comment in Section 7.2 suggesting that the reason for using piled foundations is primarily that spread footings are inappropriate is restated.

Hence if shallow ground strata (say within 2 to 3m of ground surface) are inadequate for support of the structure with reference to the allowable bearing pressures in Section 6.6 then consideration of piled foundations, or ground improvement (see Section 9.4 for introductory comments), would be appropriate.

7.7.2 Pile types and sizes

The choice of type of piled foundations relies on multiple considerations including the ground conditions, the groundwater conditions, the location of the site relative to sensitive receptors and structures (a sensitive receptor being human, ecological or animal which may be impacted on by the works, e.g. a residential area will be sensitive to excess traffic, noise, vibration and dust), the type and magnitude of loading, and site constraints. Within this Manual there is insufficient space to present a detailed discussion on the choice of piles, however general pile types are identified along with principal advantages and disadvantages in Table 7.4. The table can be used as a basis for discussion with a piling contractor.

Table 7.4 Summary of common pile types – advantages and disadvantages

Pile type Advantages Disadvantages Typical uses

Pre-cast concrete piles – Should have high quality assurance in – Can be installed raked

– Causes vibration – Noisy unless installed

with ‘silent’ hammer or piles jacked in – Limited section sizes (up

to 0.45m square)

– Working load up to 1.5MN

Driven steel H piles – High quality assurance in fabrication process – Can be installed raked

– Causes vibration when Driven steel pipe piles – High quality assurance

in construction process – Installed through water – Can be installed raked – Sizes up to 6m – Working load, can be

high for large diameter piles

Driven steel mini-piles – High quality assurance in construction process – Can be delivered in

short lengths for low

– A much used method – Sizes from mini to

large diameter (0.15 to 2.1m)

– Installed to large depths (70mþ) – Can be installed in all

materials with appropriate support of bore (bentonite, casing etc.)

– Can be base grouted in sands or

needs to be disposed of (important if ground is contaminated)

– All types of piles installed on land into soils and soft rocks – Working load up to

20MN

When using Table 7.4 the reader must consider the following in identifying appropriate pile types:

– Load carrying capacity – Efficient structural pile design usually involves optimising concrete usage whilst incorporating a construction methodology which is appropriate to the ground conditions being encountered. A pile section should be chosen that optimises use of concrete strength. As a first pass for compression, the pile section size (area of pile) can be based on the representative pile load divided by 30% of the concrete cylinder strength.

Design for bending moment etc. will need to be carried out using EC2 procedures⁵which include additional material factors on concrete strength for cast-in-place piles and reduced effective pile diameters. Tension pile design will be based on steel reinforcement alone. Consideration of downdrag is also necessary for assessing structural actions and pile displacements.

– Installation methodology – Support of the pile bore is critical in order to prevent ground movements outside the pile bore and to provide a pile which is capable of carrying load. In clays, open bores can be used below a length of temporary casing required to support surface layers and to cut-off Table 7.4 Continued

Pile type Advantages Disadvantages Typical uses

Continuous auger (includes continuous flight auger and segmental auger piles)

– Common product – Sizes from mini to

large diameter (0.3 to

needs to be disposed of (important if ground is contaminated) – Construction risk in very

soft soils

– All types of piles installed on land into soils and soft rocks – Working load up to

6MN

Percussive rotary piles – Can be installed in low head room and though range of soil types (albeit this is increasing all the time)

– Small diameter piles (e.g. up to 0.5m diameter)

Screw piles – Factory produced elements (screws) – Can be installed raked – No excavated soil – Can be recovered and

reused

shallow water thus preventing it from entering the pile bore (the casing is also necessary to provide a safe working area at piling platform level). In contrast piles in sand and gravel need full support as would be provided by continuous flight auger piles (also good for clays), piles constructed under bentonite or preformed piles.

– Effects on adjacent/nearby third parties – Noise and vibration preclude common types of driven piles being used at sensitive sites resulting in these types of piles only being suggested for industrial locations (jacked or pressed in piles may be permissible). Where it is considered that the use of driven piles is possible then it will be necessary to assess noise and vibration

‘pollution’ to ensure that it is within acceptable bounds; discussion with the local environmental health officer may be necessary. Consideration of adjacent properties and their uses as well as of protected animal species is necessary. (Also see comment in Section 5.4 above on noise and vibration.) – Consideration of aquifer protection – Needs to be provided such that the

choice of pile prevents contamination of aquifers both during construction and in the long term. Discussion with the Environmental Agency or other authority may be necessary.

Table 7.5 presents a preliminary indication of working loads that could be achieved with different pile types in different ground conditions; the abbreviations used are presented in the table notes. The table should be used for initial purposes and is in no way exhaustive. Refinement of pile types would usefully be carried out through discussions with a piling contractor or geotechnical engineer.

For detailed design (see Section 7.8) further consideration of pile type and pile dimension is necessary based on the following criteria:

– Geotechnical load carrying resistance, a ULS consideration.

– Settlement of pile and pile group, an SLS consideration.

– Structural performance in axial and bending modes.

– Construction limitations in terms of noise, vibration and risk of causing ground movements.

When each of these criteria is satisfied then a design may be considered to be complete.

Notes to Table 7.5

a It is assumed that sand and gravel are medium dense and clay stiff.

b Key to pile types:

AB Auger bored piles, with (in sand/gravel) or without (clays) support fluid. Lengths up to 40m.

BBG Bored base-grouted pile. Lengths up to 50m.

BUR Bored, cast-in-place pile with under-ream. Lengths up to 30m.

CFA Continuous flight auger. Lengths up to 27m.

DP Driven pile.

PCD Precast concrete driven pile. Lengths up to 25m.

PCD-R Precast concrete driven pile with rock shoe. Lengths up to 25m.

RS Drilled piled with rock socket. Lengths up to 20m.

SA Segmental auger. Lengths up to 20m.

Table 7.5 Example pile sizes and types

Location Working

load

Bearing stratum^a

Possible pile types, numbers and diameters F/square section

Residential area/

sensitive receptors

Small (,1MN)

Sand/gravel 1 to 3 SA, 0.45m

Sand/gravel 2 to 3 SA, 0.6m

Sand/gravel 2 to 3 CFA, 0.6m

Sand/gravel 3 to 5 CFA, 0.9m

1 BBG, 1.2m (dense sand) Stiff clay/soft

rock

4 to 5 CFA, 0.9m

1 BUR, 5.0m under-ream (in stiff/

very stiff clay) Hard rock 1 RS, 1.2m

Industrial/remote Small (,1MN)

Sand/gravel 1 to 3 SA, 0.45m

Sand/gravel 2 to 3 CFA, 0.6m

Sand/gravel 3 to 5 CFA, 0.9m

1 BUR, 5.0m under-ream (in stiff/

very stiff clay) Hard rock 3 to 4 RS,

0.6m

1 RS, 1.2m 4 to 5 PCD-R, 0.45m