AbstractThe use of controlled low-strength material (CLSM) to backfill the cut-and-cover pipelines trench under busy streets is getting popular nowadays. Due to the slow hydration rate of cement in CLSM, extra waiting hours are usually needed before it can develop sufficient strength to sustain traffic. To shorten the waiting time, this paper presents a method that can provide a good immediate bearing capacity for the CLSM backfilled trench right after completing narrow trench work. This modified method adds extra gravel to the top of the fresh CLSM, which is already in the trench. The gravel is then pushed down and mixed with CLSM and forms a strong upper layer for the CLSM trench. It can provide a good immediate bearing capacity to endure traffic weight. To verify the suitability of this modified CLSM trench filling method, a numerical analysis method [discrete-element method (DEM)] was adopted in this study, along with the laboratory model and field tests. The DEM results showed that high-friction material (gravel) on the upper portion of the trench could form a strong layer that sustains a large share of the surcharge load. So, it could result in a significant pressure reduction for the lower portion of the trench and around the buried pipes, even though the CLSM had not hardened yet. Such a result was confirmed by the laboratory model test and the full-scale field test, in which it showed an increase in the initial load-bearing capacity and a much smaller settlement on the surface of the CLSM backfilled trench.

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