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Fig. 1. CPT profile from the example site. The two zones surrounded by the dashed lines are identified as consisting of sand and silty sand and are considered susceptible to liquefaction under the design earthquake. Sustained load from structure Sustained load plus drag load Fig. 2. Distribution of load and resistance along the pile before liquefaction 70 • Q4 • 2008
ensure that the magnitude of the soil settlement at the neutral plane is within acceptable limits or to ensure that the neutral plane lies in non-settling soil. It is noteworthy that the location of the neutral plane depends on the magnitude of the mobilized toe resistance and corresponding toe movement. It was determined that earthquake-induced liquefaction could occur in the sand layers at 3 m depth and between depths of about 7 m to 9 m. During a liquefaction event, the sand would experience compression and the overlying soil layers would move downward relative to the pile. The unfactored drag load due to accumulated negative skin friction above 9 m depth is about 700 kN. According to the AASHTO Specifications, the drag load, factored up by 1.25, is to be added to the factored sustained and transient structure loads, resulting in a total factored load of 3,060 kN after applying specified load factors on sustained and transient loads of 1.35 and 1.75, respectively (one could argue the actual factors and which AASHTO Specs edition to apply, and including the transient load along with the drag load is incorrect, the two cannot occur simultaneously, but that is beside the point here,). The sum of the factored shaft and toe resistances below 9 m depth is smaller than this load. Indeed, already the unfactored resistances are smaller. Therefore, the approach specified in the AASHTO Specifications implies that the pile is severely under-designed in the event of liquefaction. As a consequence, longer piles would be required (to increase capacity), or the number of piles would have to be increased (to reduce the sustained load per pile). However, the liquefying layer lies above the neutral plane, and the shaft shear down to 13 m depth is in negative direction before the liquefaction occurs. Therefore, as discussed below, the liquefaction will not change the forces in the pile and soil, nor cause the pile to settle. The factor of safety is only marginally affected by the small reduction of shear strength in the liquefying layer. Indeed, there is no need for increasing pile length or number of piles. It is interesting to note that some AASHTO-based designs allow the use of reduced (residual) strengths when computing the drag load in a liquefaction event. As a result, the design depends on the decrease in strength in layers above the liquefying layer in order to maintain an acceptable load‐to-resistance ratio. Because of the inherent uncertainty involved in the liquefaction prediction and soil behavior during earthquakes, this seems imprudent. Drag Load Evaluation According to the Unified <strong>Pile</strong> Design Method The authors propose to apply the unified design method to analyze the effect of liquefaction on the behavior of piles under axial load (Fellenius and Siegel, 2008). The load and resistance distributions in the pile when liquefaction occurs in soil above the static (or pre-liquefaction) neutral plane are shown in Fig. 3 for comparison to the static conditions. The effect of the liquefaction is limited to a loss of negative skin friction in the lique- Sustained load plus drag load for static condition Sustained load plus drag load for liquefied condition Fig. 3. Distribution of load and resistance during liquefaction above the neutral plane • Q4 • 2008 71
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