Earth Pressure Behind a Retaining Wall

Principal Investigator:

Joseph Labuz, Professor & Department Head, Civil, Environmental and Geo-Engineering

Co-Investigator

Project Summary:

The objectives of the proposed research are (1) to determine strength parameters of a typical backfill material; (2) to instrument and monitor a retaining wall; (3) to estimate earth pressures and wall displacements (translation and rotation); (4) to compare the response of the wall to the assumed design behavior; and (5) to calculate load and resistance factors for the retaining wall. It is well known that the magnitude of earth pressure on a retaining structure depends on the physical properties of the soil and the character of the displacement. In relation to wall movement, soil pressure is defined as at-rest, active, or passive. Pressure at-rest is assumed where no displacement of the retaining structure occurs. Active or passive soil pressures exist where the relative displacement between the soil and structure may cause the soil either to expand (active state) or to contract (passive state). Reliable estimates of earth pressures involve the use of earth pressure cells. A special calibration device has been designed and fabricated for determining the sensitivity of earth pressure cells. This device provides a more reasonable simulation of sensor response, which translates to an accurate estimate of earth pressure. Earth pressures used for design would increase significantly with the addition of AASHTO specifications that require at-rest pressures for the design of abutments and retaining walls. Traditionally, a linear horizontal pressure distribution equivalent to 33 psi (1.6 kPa) per foot has been used by MnDOT with satisfactory performance. Consideration of at-rest pressure is expected to significantly increase costs without apparent benefit. This work will provide a fundamental basis for earth pressure calculations, by measuring the strength of common backfill materials and monitoring movements of a retaining wall. Readings from earth pressure cells will be compared to calculated values. For the instrumented wall, load and resistance factors will be computed.

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