However, the CPT test must be performed so that pore pressure dissipa- 

 tion curves can be determined, which means the cone must be stopped 

 periodically. As a result, the time required to perform a cone sounding 

 is very significantly increased. 



PILE DESIGN 



Pile design from CPT data is separated into two parts: end bearing 

 and side friction. Procedures for estimating end bearing resistance 

 were first developed in The Netherlands several decades ago and have 

 been under continuous development since then. Procedures for estimating 

 frictional resistance followed the development of the friction sleeve on 

 the CPT. The procedures that follow are for driven, straight-sided 

 displacement piles as summarized by Schmertmann (1978). 



End Bearing Resistance 



For all soils, the pile tip is assumed to be supported by a zone 

 soil from 0.7d to 4d (d = pile diameter) below the tip to 8d above the 

 tip. The lowest below-tip end bearing contribution is found using the 

 procedures shown in Figure 29. However, the cone record is searched to 

 lOd below the pile tip to check for a weaker layer of significance. If 

 such a weaker layer is found, this governs the weakest path rule in 

 Figure 29. For the above tip contribution, if there are a few abrupt 

 cone pressure reductions and recoveries, they can be ignored. Because 

 of uncertainties involved in developing cone tip pressures, a cutoff of 

 300 kg/cm 2 is usually applied to the cone pressure. Also, pile tip 

 pressures are limited to 150 kg/cm 2 in sands and 100 kg/cm 2 in very 

 silty sands. In clays, these procedures have been found applicable when 

 undrained shear strengths are less than 7 psi. For higher strength 

 clays, Schmertmann recommends reducing end bearing according to the 

 adhesion factors given in Figure 30. These factors will reduce tip 

 capacity by a larger percentage as soil strength increases. 



Side Friction 



Nottingham (1975) developed an empirical procedure for estimating 

 pile friction that can be applied to both sands and clay soils. This 

 procedure has the advantage that a direct measure of soil adhesion is 

 used in the design. The formula used is: 



Q = K 

 s s,c 







L 







s 



A' 

 s 



8d 



f 

 s 



A' 



s 



V i_ f 



£=0 8d 



where: Q = total ultimate pile friction 

 s 



K = correction factors for sands and clays to be applied 



S,C to f 

 s 



Si = depth to which f is being considered 



47 



