Vertical Pressures Produced by Controlled Low Strength Material (CLSM) Poured into Pipe Trenches

Abstract

This paper presents data and analysis of the measurements from earth pressure cells installed to measure vertical pressures in two trenches during the installation of a vitrified clay pipe (VCP) embedded in Class 5 material (concrete). The trench walls above the Class 5 embedment were temporarily supported with steel plates and hydraulic struts. The installed earth pressure cells measured the vertical pressures induced during the placement of Controlled Low-Strength Material (CLSM) backfill and under subsequent traffic loads. Vertical pressures induced on the top of the concrete-encased pipe by the CLSM backfill were continuously recorded during various stages of trench completion. The measured vertical pressures demonstrate that arching developed between the CLSM and the steel trench plates as the trench was being backfilled. Upon removal of the trench plates, substantial increases in vertical pressures, approximately corresponding to the full unit weight of the slurry, were measured indicating that a complete loss of arching occurred because of the trench plate removal. The combination of loss of arching, high stiffness, and extending the CLSM up to the base of the pavement system causes a significant increase in the loads imparted onto the pipeline from traffic loads. When slurry backfill is used, care must be exercised in estimating pipe loads and developing the construction sequence, particularly the timing of trench plate removal. Additionally, provisions such as placing compacted granular backfill in the upper few feet of the trench, below the pavement system, would likely be helpful in mitigating the magnitude of traffic loads imparted to the top of the pipe.