Evaluation of Differential Settlement along Bridge Approach Structure on Soft Bangkok Clay

Abstract

Excessive differential settlement along bridge approach is commonly found in the lower central region of Thailand. To overcome the differential settlement problem, engineers have developed different types of bridge approach structure and improved design procedures. Two types of bridge approach structure are currently used in Thailand, including the approach slab on ground and the approach slab on pile. This research conducts field investigation, subsoil exploration and laboratory testing, and long-term geotechnical instrumentation and monitoring program of two bridge approaches which were constructed in Bangkok and vicinity. After construction for 3 to 7 years, the study results show that the main cause of differential settlement along bridge approach is due to the foundation of bridge approach structure is rest on different soil layers having much different compressibility properties and thickness. The differential settlement problem is found at locations where the bridge approach foundation changes from end bearing pile to friction pile, and to shallow foundation. The long-term observations show that the differential settlement could cause a sudden change in the longitudinal slope and discomfort driver. Moreover, it could create large voids underneath the approach slab, failure of concrete slab, and damage of pavement and structure members along bridge approach. Back analysis results show that the settlement prediction using the Terzaghi consolidation theory and elastic theory with equivalent foundation approach for vertical stress distribution gives 0.44 to 1 times the value observed in the field.