Investigation of Induced Trench Method Using a Full Scale Test Embankment

The construction of highway embankments in urban areas often interferes with existing underground facilities such as sewer lines and other buried conduits. In many instances, the extra loads imposed by embankment construction on buried conduits would be unacceptably high. Severe consequences of overstressing an underground utility conduit include damage and interruption of services for both the utility and highway. Typical alternatives considered to address this problem include: relocation of utility lines; embankment load reduction using lightweight fill; at-grade pile cap bridging; at-grade geogrids-reinforced bridging; and induced (imperfect) trenches. The relocation, lightweight fill and pile cap alternatives are usually cost prohibitive. A full-scale instrumented test embankment is constructed by Ontario Ministry of Transportation to study the effects of embankment construction on the underground utilities. The test embankment comprised four sections, which facilitated the evaluation of four configurations including the conventional backfill, induced trenching and two at-grade geogrid-reinforcing bridging with different spans. Each configuration consisted of a 3 m deep trench overlain by a 10 m wide, 10 m long and 6 m high embankment section. Earth load cells were installed to monitor stresses at the fill/ground interface and at depths of 1.5 and 3 m. The numerical models of the test embankment are generated using the finite difference program FLAC. This paper presents the results of stress measurements in the 3 m wide trench under embankment loads and the results of numerical varification analyses. The results showed that the use of induced trench method (ITM) resulted in substantial reduction in vertical stresses relative to positive projection method case. It was also observed that horizontal stresses increased when ITM is employed.