# 6.4.1.3 Models based on the results of ground tests

6.4.1.3.1 General

6.4.1.3.1.1 The ultimate bearing resistance of a pile foundation may be calculated directly from the results of ground tests on soil and rock (i.e. without first converting those results to ground parameters).

6.4.1.3.1.2 The calculation of ultimate compressive resistance from the results of ground tests should conform to the main method given in BS EN 1997-1:2004+A1:2013, 7.6.2.3.

6.4.1.3.1.3 If this method is used, the characteristic ultimate compressive resistance of an individual pile (Rc,k) should be calculated as the smaller of the following two values:

(46)

where:

ξ3 and ξ4 are correlation coefficients that depend on the number of tests performed;

(Rc,calc)mean is the mean calculated ultimate compressive resistance of the pile; and

(Rc,calc)min is the minimum calculated ultimate compressive resistance of the pile.

6.4.1.3.1.4 The calculated ultimate compressive resistance of an individual pile (Rc,calc) should be determined from:

Rc,calc = Rs,calc + Rb,calc
(47)

where:

Rs,calc is the calculated ultimate shaft resistance; and

Rb,calc is the calculated ultimate base resistance.

6.4.1.3.1.5 The calculated ultimate shaft resistance (Rs,calc) may be determined from:

(48)

where:

As,i is the total circumferential area of the pile shaft (in layer i);

ps,i is the ultimate unit shaft resistance (in layer i) obtained from a field test; and

n is the total number of layers in contact with the pile shaft.

6.4.1.3.1.6 The calculated ultimate base resistance (Rb,calc) may be determined from:

Rb,calc = Ab × pb
(49)

where:

Ab is the total cross-sectional area of the pile base; and

pb is the ultimate unit base resistance obtained from a field test.

6.4.1.3.1.7 The values of the correlation coefficients ξ3 and ξ4 should be taken from the UK National Annex to BS EN 1997-1:2004+A1:2013.

6.4.1.3.2 Cone penetration tests (CPTs)

6.4.1.3.2.1 The ultimate unit shaft resistance in layer j (ps,j) may be calculated from:

ps,j = cs,j × qc,j
(50)

where:

cs,j is an empirical coefficient (for layer j) that depends on soil and pile type; and

qc,j is the measured cone resistance in layer j.

6.4.1.3.2.2 The ultimate unit base resistance, at a settlement equal to 10% of pile diameter, (pb,0.1) may be calculated from:

pb,0.1 = cb,0.1 × qc,b
(51)

where:

cb,0.1 is an empirical coefficient that depends on soil and pile type; and

qc,b is the average cone resistance measured over a distance ±1.5 pile diameters below the pile base.

6.4.1.3.2.3 In the absence of reliable test data, values of cs and cb,0.1 may be estimated from Table 11.

NOTE Further information about cb,0.1 can be found in Comparing CPT and pile base resistance in sand [57].

Table 11 Values of the empirical coefficients cs and cb,0.1 according to soil and pile type
 Soil type cs cb,0.1 Displacement piles Replacement piles Displacement piles Replacement piles High displacement Low displacement High displacement Low displacement Sand 0.0004–0.009 A), B), C) 0.0015–0.004 A), B), C) 0.003–0.006 D), E) 0.3–0.5 F), G) 0.15–0.25 F), G) 0.15–0.25 H) Silt 0.006–0.01 B), C) 0.003–0.006 D) Data not available Clay Medium to high strength/ over-consolidated 0.007–0.017 B), C) 0.008–0.012 D) 0.8–1.3 I) 0.4–0.65 I) 0.34–0.66 D) Low strength/normally consolidated to lightly over-consolidated Data not available 0.9–1.0 0.9–1.0 A) See Estimation of load capacity of pipe piles in sand based on CPT results [58]. B) See An approximate method to estimate the bearing capacity of piles [59]. C) See Fundações para o silo vertical de 100000 t no Porto de Paranaguá [60]. D) See On the prediction of the bearing capacity of bored piles from dynamic penetration tests [61]. E) See Pile capacity by direct CPT and CPTu methods applied to 102 case histories [62]. F) See Evaluation of a minimum base resistance for driven pipe piles in siliceous sand [63]. G) See Investigations into the behaviour of displacement piles for offshore structures [64]. H) See Determination of pile base resistance in sands [65]. I) See ICP design methods for driven piles in sands and clays [66].

6.4.1.3.3 Standard Penetration Tests (SPT)

6.4.1.3.3.1 The ultimate unit shaft resistance in layer j (ps,j) may be calculated from:

ps,j = ns,j × pref × Nj
(52)

where:

ns,j is an empirical coefficient (for layer j) that depends on soil and pile type;

pref is 100 kPa; and

Nj is the measured (uncorrected) SPT blow count in layer j.

6.4.1.3.3.2 The ultimate unit base resistance, at a settlement equal to 10% of pile diameter, (pb,0.1) may be calculated from:

pb,0.1 = nb,0.1 × pref × Nb
(53)

where:

nb,0.1 is an empirical coefficient that depends on soil and pile type;

pref is 100 kPa; and

Nb is the measured (uncorrected) SPT blow count at the level of the pile base.

6.4.1.3.3.3 In the absence of reliable test data, values of ns and nb,0.1 may be estimated from Table 12.

Table 12 Values of the empirical coefficients ns and nb,0.1 according to soil and pile type
 Soil type ns nb,0.01 Displacement Replacement Displacement Replacement Sand 0.033–0.043A)B) 0.014–0.026A)B)C) 2.9–4.8A) 0.72–0.82C) Silt 0.018–0.03A)B) 0.016–.023C) 1.1–2.6A)B) 0.41–0.66C) Clay 0.020–0.029A)B) 0.024–0.031C) 0.95–1.6A)B) 0.34–0.66C) A) See An approximate method to estimate the bearing capacity of piles [59]. B) See Fundações para o silo vertical de 100000 t no Porto de Paranaguá [60]. C) See On the prediction of the bearing capacity of bored piles from dynamic penetration tests [61].

6.4.1.3.3.4 Correlations with SPT blow count should be treated with caution, since they are inevitably approximate and not universally applicable.

6.4.1.4 Models based on static pile load tests

COMMENTARY ON 6.4.1.4

In the UK, static pile load tests are mainly used to verify resistance calculated using estimated soil parameters.

Static pile load tests are ill suited for designing piles in variable ground conditions, where it is impossible to determine the resistance provided by different strata.

Pile designs based on the results of dynamic impact tests alone can be unreliable when downdrag occurs.

6.4.1.4.1 The ultimate bearing resistance of a pile foundation may be calculated directly from the results of static pile load tests.

6.4.1.4.2 The calculation of ultimate compressive resistance from static pile load tests should conform to BS EN 1997-1:2004+A1:2013, 7.6.2.2.

6.4.1.4.3 If this method is used, the characteristic ultimate compressive resistance of an individual pile (Rc,k) should be calculated as the smaller of the following two values:

(54)

where:

ξ1 and ξ2 are correlation factors that depend on the number of tests performed;

(Rc,m)mean is the mean measured ultimate compressive resistance of the pile; and

(Rc,m)min is the minimum measured ultimate compressive resistance of the pile.

6.4.1.4.4 The values of the correlation factors ξ1 and ξ2 should be taken from the UK National Annex to BS EN 1997-1:2004+A1:2013, Table A.NA.9.

6.4.1.5 Models based on dynamic impact tests

COMMENTARY ON 6.4.1.5

According to BS EN 1997-1:2004+A1:2013, 7.6.2.4(1)P, the ultimate bearing resistance of a pile foundation may be calculated from dynamic impact tests provided the validity of the results «have been demonstrated by previous evidence of acceptable performance in static load tests on the same type of pile of similar length and cross-section and in similar ground conditions».

In the UK, dynamic impact tests are mainly used to verify resistance calculated using estimated soil parameters.

Dynamic impact tests are ill suited for designing piles in variable ground conditions, where it is impossible to determine the resistance provided by different strata.

Pile designs based on the results of dynamic impact tests alone can be unreliable when downdrag occurs.

6.4.1.5.1 The calculation of ultimate compressive resistance from dynamic impact tests should conform to BS EN 1997-1:2004+A1:2013, 7.6.2.4.

6.4.1.5.2 If this method is used, the characteristic ultimate compressive resistance of an individual pile (Rc,k) should be calculated as the smaller of the following two values:

(55)

where:

ξ5 and ξ6 are the correlation factors that depend on the number of tests performed;

(Rc,m)mean is the mean measured ultimate compressive resistance of the pile; and

(Rc,m)min is the minimum measured ultimate compressive resistance of the pile.

6.4.1.5.3 The values of the correlation factors ξ5 and ξ6 should be taken from the UK National Annex to BS EN 1997-1:2004+A1:2013, Table A.NA.11.

6.4.1.6 Models based on pile driving formulae

COMMENTARY ON 6.4.1.6

According to BS EN 1997-1:2004+A1:2013, 7.6.2.5(2)P, the ultimate bearing resistance of a pile foundation may be calculated from dynamic pile driving formulae provided the validity of the formulae has been demonstrated «by previous experimental evidence of acceptable performance in static load tests on the same type of pile, of similar length and cross-section, and in similar ground conditions».

6.4.1.6.1 The ultimate bearing resistance of a pile foundation may be calculated from pile driving formulae. However, preference should be given to another method of calculating the resistance.

6.4.1.6.2 The calculation of ultimate compressive resistance from pile driving formulae should conform to BS EN 1997-1:2004+A1:2013, 7.6.2.5.

6.4.1.6.3 If this method is used, the characteristic ultimate compressive resistance of an individual pile (Rc,k) should be calculated from equation (55).

6.4.1.6.4 If a pile exhibits reduced resistance when redriven, and the resistance does not increase again significantly, then care should then be exercised in applying pile driving formulae and preference given to designing on the basis of 6.4.2.1 or 6.4.2.2.

6.4.1.6.5 Pile driving formulae should be used in combination with other verification methods, such as those given in 6.4.2.1, 6.4.2.2, and 6.4.2.3.

6.4.1.7 Models based on wave equation analysis

COMMENTARY ON 6.4.1.7

According to BS EN 1997-1:2004+A1:2013, 7.6.2.6(2)P, the ultimate bearing resistance of a pile foundation may be calculated from wave equation analysis provided the validity of the analysis has been demonstrated «by previous evidence of acceptable performance in static load tests on the same pile type, of similar length and cross-section, and in similar ground conditions».

6.4.1.7.1 The ultimate bearing resistance of a pile foundation may be calculated from wave equation analysis. However, preference should be given to another method of calculating the resistance.

6.4.1.7.2 The calculation of ultimate total resistance from wave equation analysis should conform to BS EN 1997-1:2004+A1:2013, 7.6.2.6.

6.4.1.7.3 If this method is used, the characteristic ultimate compressive resistance of an individual pile (Rc,k) should be calculated from equation (55).

6.4.1.7.4 Wave equation analysis should be used in combination with other verification methods, such as those given in 6.4.2.1, 6.4.2.2, and 6.4.2.3.

6.4.1.8 Modifications for downdrag (also known as «negative skin friction»)

COMMENTARY ON 6.4.1.8

The term «downdrag» is used in this standard to refer to the phenomenon whereby the ground surrounding a pile settles a significant amount relative to the pile head. Downdrag is particularly relevant to piles installed in low strength clay or in coarse soils subject to upfilling or a lowering of the groundwater table.

The additional axial force in the pile due to downdrag is termed the «drag force» and the additional settlement of the pile is called the «drag settlement» (after A practical design approach for piles with negative skin friction [67]).

It is a common misconception that downdrag reduces the ultimate bearing resistance of a pile. In many situations, the settlement of the pile at the ultimate limit state is sufficient to cancel the effects of downdrag, resulting in no reduction in ultimate bearing resistance.

Although downdrag might have little effect on a pile's ultimate bearing resistance, the drag force does influence the structural design of the pile and the drag settlement influences its serviceability.

Downdrag causes shaft friction over the upper part of a pile to act as an additional force applied to the pile, instead of as a resistance. Because the direction (or «sign») of this shaft friction is reversed from the norm, it is commonly termed «negative skin friction».

The depth at which there is no relative movement between the pile and the surrounding ground is known as the «neutral plane». The neutral plane occurs where the ground settlement (sg) at a particular depth equals the pile settlement (sp) at the same depth, as shown in Figure 4.

The depth of the neutral plane can be over-predicted by linear elastic settlement analyses for friction piles in strata whose stiffness increases gradually with depth.

6.4.1.8.1 The characteristic compressive force (Fc,k) acting on a pile that is subject to downdrag should be calculated from:

Fc,k = Pc,k + Wk + Pdd,k
(56)

where:

Pc,k is the characteristic compressive force applied to the pile by the structure;

Wk is the characteristic self-weight of the pile; and

Pdd,k is the additional characteristic compressive force owing to downdrag, given by:

where:

(57)

Ldd is the length of pile subject to downdrag (defined in Figure 4);

Cs is the circumference of the pile shaft at depth z; and

qs,k,sup is the «superior» characteristic (defined below) unit shaft friction at depth z.

6.4.1.8.2 The «superior» characteristic unit shaft friction (qs,k,sup) should be selected as a cautious upper estimate of the mean shaft friction acting over the length of pile that is subject to downdrag.

NOTE A cautious upper estimate of the mean is one that has a 5% probability of being exceeded during the design working life.

6.4.1.8.3 The characteristic shaft resistance (Rs,dd,k) of a pile subject to downdrag should be calculated from:

(58)

where:

L is the total length of the pile;

Ldd is the length of pile subject to downdrag (equal to the depth of the neutral plane defined in Figure 6);

Cs is the circumference of the pile shaft at depth z;

qs,k,inf is the «inferior» characteristic (defined below) unit shaft friction at depth z; and γRd is a model factor.

6.4.1.8.4 The "inferior" characteristic unit shaft friction (qs,k,inf) should be selected as a cautious lower estimate of the mean shaft friction acting over the length of pile that is not subject to downdrag.

NOTE A cautious lower estimate of the mean is one that has a 5% probability of not being achieved during the design working life.

Figure 4 Calculation model for downdrag

BS 8004:2015 Code of practice for foundations