Earth pressures on twin circular culverts backfilled with controlled low strength material

Abstract

In many instances, twin rigid culverts need to be constructed at a very close spacing. The narrow spacing between culverts does not permit placement of granular fill in lifts and achieve the required degree of compaction. In these situations, controlled low strength material (CLSM) can be used instead of soil fill. CLSM is a self-compacting, selfleveling, cementitious material that goes by many names: flowable fill, lean-mix concrete, and controlled density fill to name a few. Even though CLSM has been used in rigid pipe construction, very limited research is reported on earth pressure distribution on twin culverts backfilled with CLSM. To develop better understanding of earth pressure distribution, a twin circular culvert installation was instrumented for this research project. The specified pipes were 140D (2400 mm ID) with a wall thickness of 200 mm, bringing the outside diameter (Bc) to 2800 mm. A spacing of only 280 mm was specified at the inside springlines of the two culverts. CLSM was used as fill in between culverts up to the height of the springlines instead of soil fill. The final embankment height was 7.4 m up to the sub-grade elevation. Sixteen sensors in total were installed at the site. Fourteen earth pressure sensors and two NATM (New Austrian Tunneling Method) type sensors were selected for this study. The earth pressure sensors were installed in the surrounding soil fill around two culvert sections of each pipe, while NATM cells were installed in CLSM. This thesis presents the results of field monitoring and compares the data with the numerical analysis predictions for the constructed installation. Practical significance of the results obtained are discussed. The use of CLSM as opposed to uncompacted soil fill did not significantly affect earth pressures on culverts, and the main benefits were found to be from a constructability perspective.