6 Spread foundations
7.18 UK practice for design using ground test results
Conventional UK design adopts the following approach for assessment of pile resistance with modifications in nomenclature to account for EC7
modifications:
R ¼ RsþRb
Rs;k¼pD L qs;k¼(p D L) a cu;k clay Rs;k¼pD L qs;k¼(p D L) Kss0vtan(w0cv;k) sand/gravel Rs;k¼pD L qs;k¼(p D L) a(sci;k)b rock
Rb;k ¼pD2/4 9 cu;k_base clay
Rb;k ¼pD2/4 Nqs0v_base sand/gravel
Rb;k¼pD2/4 fn(sci;k) rock where:
a is the factor linking undrained shear strength to shaft friction Ks is the factor linking horizontal to vertical effective stresses Nq is the bearing capacity factor based on characteristic value of w0 sci;k is the characteristic unconfined compression strength of rock a and b represent constants
General pile design equations in the following ground conditions are given in Tables 7.23 to 7.26:
– Pile design in clay soils.
– Pile design in sands and gravels.
– Pile design in chalk.
– Pile design in rock.
Tables 7.23, 7.24, 7.25 and 7.26 are provided as an outline guide to pile design; the reader is advised to read the references cited and other appropriate references to understand the limitations of a specific design method being used. Where ground strata are highly variable over the length of a pile (e.g. inter-bedded clay and sand layers) or where ground conditions
deteriorate below the base level of a pile (risk of block failure) then specialist advice should be obtained.
Figure 7.2 (Fleming87) shows how the shaft adhesion factor varies with the ratio of undrained shear strength to vertical effective stress. The undrained shear strength to vertical effective stress ratio (cu/s0v) is a measure of the soil over-consolidation. For values of cu/s0vless than 0.2 to 0.3 (depending on soil plasticity) the clay can be considered to be unconsolidated which implies that it is settling and as such no beneficial effect can be obtained from it, while for values of cu/s0vin excess of 0.8 the ground can be considered to be Table 7.23 Pile design equations: clay soils
Pile design in clay
qs;k a cu:k a is shaft adhesion factor
a ¼ 0.5 for firm and stiff clays, limit of qs;k_ave¼110kN/m2(see Figure 7.2) a ¼ 1.0 to 0.5 for very soft to soft clays depending on strength (see Figure 7.2) References:
– Piling engineering87
– Shaft capacity of driven piles in clay93
qb;k Nccu:k Ncis the bearing capacity factor ¼ 9
Table 7.24 Pile design equations: sand and gravel soils Pile design in sands and gravels (silica soils only)
qs;k s0vKstan dk Ksis the earth factor to convert vertical to horizontal effective stresses. This is usually taken as 0.7 for bored piles and between 0.5 and 0.9 for CFA piles (lower value for silty sands). For driven piles a value of 1.0 may be assumed.
dkis the characteristic angle of friction on the shaft of the pile. This is usually taken to be between win-situ;kand wcv;kfor conventionally bored and CFA piles and to be wcv;kfor preformed driven piles.
s0vis the vertical effective stress.
Reference:
– Piling Engineering87
qb;k Nqs0v Nq is the bearing capacity based on w0p;kas per Figure 7.387.
Base resistance is usually limited to a maximum of 11MN/m2in very dense sands, less in looser deposits. It should be noted that that the bearing capacity factor is also seen to reduce with depth (others have looked at reducing bearing capacity factor with increasing effective stresses).
References:
– Load bearing capacity and deformation of piled foundations94 – Piling engineering87
– Pile design and construction practice88
Table 7.25 Pile design equations: chalk Pile design in chalk
qs;k a þ b s0v a and b are constants (values below)
s0vis the vertical effective stress (the increment below top of chalk is approximately equal to 10z (kN/m2) where z is distance below top of chalk) Driven cast-in-place and (non-CFA) bored piles:
a ¼ 0, b ¼ 0.8; qs;k300kN/m2
CFA piles: a ¼ 0, b ¼ 0.45; qs;k100kN/m2 Low displacement driven piles (steel):
a ¼ 20kN/m2, b ¼ 0: Low and medium density chalk Low displacement driven piles (steel):
a ¼ 120kN/m2, b ¼ 0: High density chalk Large displacement driven piles:
a ¼ 20kN/m2, b ¼ 0 qb;k c Nk(kN/m2) c is a constant
N is the SPT blow count per 300mm penetration.
All bored piles: c ¼ 200
Driven cast-in-place piles: c ¼ 250 Driven piles: c ¼ 400
Reference:
– Engineering in Chalk73
0.2; 0 0.2; 1 0.35; 1
0.8; 0.5
0 0.2 0.4 0.6 0.8 1 1.2
0.0 0.5 1.0 1.5 2.0
Peak adhesion α
Undrained shear strength cu/Vertical effective stress σv
Under-consolidated zone
Fig 7.2 Shaft adhesion factor with undrained shear strength cu
moderately to highly over-consolidated. Further comments on how to design for under-consolidated soil are provided in Section 7.11.1 for downdrag effect on piles.
Alternative designs based on in situ testing exist. These are most often used in mainland Europe and may become of value in the UK as site investigation techniques change. Such design rules are presented in the appendices to EC7 as follows:
– Annex D of EC7 Part 22: Pile design using the CPT method.
– Annex E of EC7 Part 22: Pile design using the Me´nard Pressuremeter method.
Table 7.26 Pile design equations: rock (generic) Pile design in ‘rock’
qs;k a sci;kb
a and b are constants (values below).
a varies between 0.45 and 0.6 for small and medium diameter piles depending on the roughness of the rock socket (all sockets assumed clean rock to concrete interface) with 0.6 being for clean rough sockets. Where large rock sockets are constructed, greater than 0.6m in diameter, then it is necessary to investigate the effect of socket diameter on shaft resistance. The a value can reduce to less than 50% of the values for smaller diameter piles.
b is conventionally taken as 0.5.
It should be noted that shaft friction is usually limited to 5% of the concrete
characteristic cube strength (6.25% of the cylinder strength) based on unfactored loads (working stress).
It should be noted that sciis always measured in MN/m2in this equation with the resulting shaft friction also in MN/m2.
qb;k a sci;kb a is typically 4.8 albeit the lower bound value of 3.0 is recommended unless there is ample case history data available to fully justify a higher value.
b is found to be 0.5.
It should be noted that sciis always measured in MN/m2in this equation with the resulting shaft friction also in MN/m2.
Notes:
– Due to the potential brittle nature of rock sockets it is recommended that design be based on either shaft or base alone in order to control settlement and prevent the risk of brittle behaviour.
– For combined shaft and base behaviour attention to A design method for drilled piers in soft rock95is recommended. Propriety software based on A new rock socket roughness factor for prediction of rock socket shaft resistance96also exists which models full pile performance.
– Further information particularly in relation to ‘softer’ rocks can be found in Piled foundations in weak rock97.
References:
– A design method for drilled piers in soft rock95
– A new rock socket roughness factor for prediction of rock socket shaft resistance96 – End bearing capacity of drilled shafts in rock98.
7.19 Illustration of design process
Refer to Appendix D.
7.20 Summary
– Prior to adoption of a particular pile type for support of building loads it is necessary to confirm the following:
– Use of piles are appropriate when compared to spread foundations with or without ground improvement
– The process of pile installation is compatible with the ground conditions (installation method, impact on stability of pile bores for bored piles, contamination of aquifers below contaminated ground etc.) at the site and to the locality of the site (noise and vibration, site access for piling plant etc.)
0 25 50 75 100 125 150 175 200
25 30 35 40 Bearing capacity factor Nq
ϕ′ (°)
D /B = 5
D /B = 20
D /B = 70
Note
D = pile depth, B = pile diameter Fig 7.3 Variation of Nqwith w0(Fleming)
– Pile design (ULS) is typically carried out using geotechnical parameters obtained from ground investigations and equations for shaft and base resistance. Pile design using continuous profiles of ground data (e.g. a CPT profile) to calculate a ‘location specific’ pile capacity is also possible but not common in UK practice. Pile design methods are based on the results of pile load tests (project specific or archive).
– The use of preliminary and construction pile load tests is relatively common place, allows validation of the design assumptions, provides confidence in the construction method, and allows for reduction in design partial and model factors.
– Design of piles using the results of pile load tests directly (justified by experience or calculation) is permitted but again not usually carried out in the UK.
– Design of piles differs from most other geotechnical designs in that partial factors are applied to calculated resistances (shaft or base – these resistance factors are applied after the geotechnical calculation) rather than to the geotechnical parameter (e.g. to cuor w0– these partial factors are applied prior to the geotechnical calculation).
– ULS design of piles using geotechnical parameters (the typical approach in the UK) has the sets of factors listed in Table 7.27.
Table 7.27 Partial factors used in ULS pile design Partial factors used Comments
Model factors:
– Table 7.15
Used to account for the presence, or not, of relevant pile load test data to calculated failure load. The model factor reduces the calculated resistance to the characteristic resistance
Resistance factors (R):
– Table 7.7 for EQU – Table 7.9 for STR/GEO
(DA1 C1)
– Table 7.11 for STR/GEO (DA1 C2)
– Table 7.13 for UPL
Used to reduce the characteristic resistance to the appropriate design resistance. Different resistance factors are provided for different pile types (bored, driven, CFA), for shaft and base components, for compression or tension loading, to account for the use of contract pile load tests, to verify performance in the working load range and finally as a function of the importance of reliable settlement at working load
Factor on actions (A):
– Table 7.6 for EQU – Table 7.8 for STR/GEO
(DA1 C1)
– Table 7.10 for STR/GEO (DA1 C2)
– Table 7.12 for UPL
Load factors are applied appropriate to the calculation being carried out (EQU, STR or GEO and UPL)
Factor on materials:
– Table 7.14
Factors are applied as appropriate to EQU, STR, GEO and UPL Negative skin friction:
– Table 7.18
Factors used in assessing the design value of negative skin friction for STR/GEO load cases
– Pile design must consider both pile settlement due to loading, which includes both actions applied by the structure and actions which are generated by ground movements. In particular negative skin friction (downdrag) should be included in pile design calculations for both ULS and SLS considerations.