Transition treatment design for bridge approach embankments – from a practitioner’s perspective

Transition treatments in soft soil are implemented for the purpose of achieving differential settlement criteria between a rigid piled abutment and an approach embankment founded in soft soil. With carefully designed transition treatments, bridge abutments and their supporting piles can be constructed early in the construction program without the need to wait for embankment settlement to conclude – thus, facilitating an efficient construction process.

Methods to reduce or control settlement, include the use of piles/columns in the embankment foundation, and other ground improvement techniques such as preloading/surcharging. Piles/columns are used in varying frequency and quantity within the approach embankment to achieve the desired transitional effect and “protection” for the abutment piles from the lateral movement of the foundation soils. Overburden loads are often transferred to the piles/columns through a gravel mattress embedded with geotextiles, commonly referred to as a load transfer platform (LTP). The design of LTP’s depends heavily on the degree of support provided by the soil matrix surrounding the pile/column treatment.

The success of transition treatments can be measured by the magnitudes of settlement within the transition zone and achievement of differential settlement criteria. Methods to reduce long term creep settlement include the use of surcharging pressures to over-consolidate clay soils. In this paper, the measurement of differential settlement in the form of change-in-grade (CIG), i.e. change of slope between two straight lines, are discussed and clarified – removing some ambiguities and confusion which currently exist in practice.