Pre-Production Lateral Load Testing in Tamaopu Formation

Abstract

At a recent power plant project in Taiwan, large diameter (1,200 mm), cast-in-place concrete, drilled shafts were selected to support heavy equipment and structures. These were selected based on the available types of piling in Taiwan, the subsurface soil conditions, and the need for high lateral capacity under seismic conditions. The subsurface profile consisted of 3 to 5 m of a medium-to-stiff, highly plastic clay soil underlain by a dense gravel and cobble/boulder layer in a clayey matrix, which is the bearing layer for the drilled shafts. This latter soil layer, which extends to substantial depth and is locally known as the formation, is similar in appearance to a hardpan/glacial till material. The initial design required that the drilled shafts be able to withstand repeated lateral loading of up to 206 kN with less than 12 mm of movement. The lateral drilled shaft capacity calculation determined that to achieve this required capacity, the drilled shaft would require an embedment of 5 m into the Tamaopu beating material. During the final design phase of the project, as a result of the earthquake in Taiwan and a revision to the seismic design codes, the design lateral loading on the drilled shafts was increased by approximately 125 percent to 470 kN. To accommodate the higher lateral loading, the drilled shaft design was revised to extend the drilled shaft to a depth of 9 m into the Tamaopu material. At that time it was considered that the drilled shaft might not be able to achieve the 470 kN at less than 12 mm of movement. As a result of these concerns, a decision was made to perform pre-production testing of the drilled shafts of varying depths into the Tamaopu material to verify the design. Drilled shafts were constructed and tested with embedment depths of 5, 7, and 9 m into the Tamaopu material and then tested to a total load of 920 kN. The test data clearly showed that the actual properties of the Tamaopu material were considerably better than those assumed in preliminary design. As a result of these better-than-assumed properties, the load test results showed that even the shortest drilled shaft would provide the required lateral capacity. As a result, the drilled shaft design before installation of the production drilled shafts was reduced to a 5 m penetration into the Tamaopu material, yielding considerable cost and schedule savings to the project.