the friction angle also seemed independent of soil density. No tests 

 were done on the crushed silica sand. This result is in agreement with 

 the work done by Beringen, et al. (1982). 



The contribution of skin resistance developed on the surface of a 

 pile to total pile capacity was given in Equation (1) as f A , where f 

 is the unit skin friction and A is the side surface area of the pile in 

 contact with the soil. As shown in Figure 37, f can be expressed as: 



f = a' tan 6 - K a' tan 6 



where: a' = effective lateral stress acting on the pile surface 

 n 



a' = effective vertical stress (overburden pressure) 

 v 



K = coefficient of lateral earth pressure 

 6 = soil-pile friction angle 



The results of these tests demonstrated that calcareous soils have 

 very high friction angles. They also have soil-steel friction angles 

 that are higher and are comparable to silica sands. Furthermore, grain 

 crushing caused by pile driving does not reduce the soil-steel friction 

 angle. It seems, therefore, that the reason for the low capacities of 

 piles in calcareous soils might be the low effective lateral stress, a', 

 acting perpendicular to the pile surface. 



It is hypothesized that when a pile is driven into a noncalcareous 

 sand, pile vibration helps density the sand around the pile which in- 

 creases lateral pressure on the pile. In calcareous soils, the soil 

 matrix does not densify around the pile under vibration, and the earth 

 pressure coefficient, K, is small. Consequently, the developed skin 

 resistance is much lower than expected. Soil cementation may further 

 influence the soil-pile interaction (Agarwal et al., 1977). Laboratory 

 model pile tests and full-scale field tests can help clarify the in- 

 fluence of lateral pressure and soil cementation on the behavior of piles 

 in calcareous sands. 



Crushability During Pile Driving 



A study was conducted to investigate the relationship of grain 

 crushing to pile-soil driving friction as a function of cementation, 

 density, and carbonate content of a soil. The results clearly indicate 

 that the behavior of piles in calcareous sand is complex. The results 

 establish certain definitive behavior patterns with respect to crush- 

 ability, degree of cementation, density, driving resistance, and pullout 

 resistance. It is clear that the frictional characteristics of a pile 

 in calcareous sand depend on the interrelated effects of these items and 

 none of these items alone can adequately explain the behavior. 



The frictional characteristics of a pile in calcareous sands are 

 functions of the friction parameter between the pile and soil, and the 

 lateral soil stress on the pile. As discussed earlier, grain crushing 

 might reduce the coefficient of friction between the soil and pile 



27 



