Engineering and Construction

The analysis for the load carrying capacity and settlement behavior of piles and piers in cohesive soil is more complex than shallow foundations. Some of the factors that may need to be considered in the analysis are as follows:

1. Immediate settlement. Provided there is an adequate factor of safety for the bearing capacity of the piles and pile groups, the immediate settlement si is often ignored. This is because the load is usually transferred to a great depth and the overburden pressure limits the ability of the saturated clay to squeeze out from beneath the bottom of the piles.

2. Pile load tests. Prior or during the construction of the foundation, a pile or pile group could be load tested in the field to determine its carrying capacity. Because of the uncertainties in the design of piles based on engineering analyses, pile load tests are common. However, the load test only determines the short-term load-deformation behavior of the pile. In addition, the zone of influence of a single pile will be much less than the zone of influence of the entire building.

Thus long-term settlement of the entire structure due to consolidation of the cohesive soil must still be evaluated. For example, the pile load test may demonstrate a high capacity, but once constructed, the structure experiences excessive settlement due to consolidation of clay layers located well below the pile tips.

3. Downdrag loads. Previously presented a discussion and the equations needed to determine downdrag loads for single piles and pile groups. Piles that penetrate soft clay or organic soil will often be subjected to downdrag loads as the soil consolidates.

4. Bearing capacity. The bearing capacity of clay layers underlying the piles must be evaluated. The danger is that the piles will punch through the bearing layer and into underlying softer clay strata.

Similar to solutions for shallow foundations, charts have been developed for estimating the distribution of stress beneath deep foundations. For example, Fig. 4.17 shows the pressure distributions for pile foundations for four different soil conditions. For easy reference, Fig. 4.17 has been reproduced as Fig. 8.17, with an asterisk included that shows the zone of clay that would need to be evaluated for primary consolidation settlement and secondary compression. These charts are valid for relatively rigid pile caps, where the rigidity of the foundation system tends to restrict differential settlement across the pile cap. In this case, the 2:1 approximation is ideal because with a relatively rigid foundation system, it is desirable to calculate the average settlement of the pile cap. The four conditions in Fig. 8.17 are individually discussed below:

Friction Piles in Clay. The upper left figure shows the pressure distribution for a pile group embedded in clay. The load P that the pile cap supports is first turned into an applied stress q by using Eq. 4.32.

In essence, the 2:1 approximation starts at a depth of 2/3 L below ground surface. In Fig. 8.17, it is assumed that there is a hard layer located below the clay. In this case, the primary consolidation and secondary compression would be calculated for the zone of clay defined as H in the upper left diagram of Fig. 8.17.

The clay of thickness H may have to be divided into several layers in order to calculate the primary consolidation and secondary compression.

Friction Piles in Sand Underlain by Clay. The upper right diagram in Fig. 8.17 shows the pile group embedded in sand with two underlying clay layers. For this situation, two conditions would have to be evaluated. The first is a bearing capacity failure of the pile group where it punches through the sand and into the upper soft clay layer. The second condition is the primary consolidation and secondary compression of the two clay layers located below the pile group. Similar to the case outlined above, the 2:1 approximation is used to calculate the increase in vertical stress σz due to the pile cap loading for the two clay layers.


Point Bearing Piles in Sand Underlain by Clay. The lower left diagram in Fig. 8.17 shows the condition of a soft clay layer underlain by a sand stratum. The sand stratum provides most of the vertical resistance and hence the pile group is considered to be end bearing. For this case, the upper sand layer is assumed to be thick enough that a punching failure is not a concern. As shown in Fig. 8.17, the 2:1 approximation is assumed to start at the top of the sand layer and is used to calculate the increase in vertical stress (σz = Δσy) and hence the primary consolidation settlement and secondary compression of the soft clay layer having a thickness defined as H.

Friction Piles in Clay with Recent Fill. The lower right diagram in Fig. 8.17 shows the condition of piles embedded in clay with the recent placement of a fill layer at ground surface. In this case, the piles will be subjected to a downdrag load due to placement of the fill layer. Previously presents a discussion and the equations needed to determine downdrag loads for single piles and pile groups.

As shown in Fig. 8.17, the 2:1 approximation is assumed to start at a distance of L3 below the top of the clay layer, where L3 = 2/3 L2. The value of q applied at this depth includes two additional terms, the first is the total unit weight of fill γt times the thickness of the fill L1 and the second is the downdrag load converted to a stress. As discussed in Sec. 6.3.4, two cases should be evaluated, the first where the downdrag load is determined for each pile and summed, or (nQD)/(BA), and the second is the downdrag load along the entire perimeter of the pile group. The higher downdrag load would be used in the analysis. Once q is known, the primary consolidation and secondary compression can be calculated for the zone of clay located below a depth defined as L3 in Fig. 8.17.

If the pile caps are spaced close together, there could be additional primary consolidation settlement as the pressure distribution from one pile cap overlaps with the pressure distribution from a second near-by pile Posts.


This answer is close to the calculated values previously because for this example problem where the piles terminate above the clay layer, they have little effect on the stress increase experienced by the clay layer. The settlement of the foundation due to primary consolidation could be eliminated if the piles were long enough so that they penetrated through the clay layer.