Using TDA as an Engineered Stress-Reduction Fill over Preexisting Buried Pipes

Routing utility conduits underneath buildings is not always an avoidable situation, especially in densely populated and congested urban centers. Placing the pipe at a shallow depth attracts substantial additional earth pressures and loads causing overstressing and/or unacceptable deformations of the buried pipe. The backfill material characteristics control the pipe-soil interaction mechanism and sequentially the amount of exerted pressures. Tire-derived aggregate (TDA) is an engineered construction, lightweight backfilling material produced from recycled scrap tires. In the literature, TDA is often referred to as tire chips or tire shreds. TDA has excellent geotechnical properties, maintains its structural integrity, and weighs 50–60% less than conventional earth fill. In this paper, two full-scale field tests were conducted to evaluate the usefulness of using a layer of TDA above existing metal pipes to enhance the stress arching mechanism (i.e., stress bridging). In addition, the complicated pipe-soil interaction was investigated by monitoring the surface settlement, change of the pipe’s wall strains, and the pressure distribution over and around the pipe. The field results showed that using a layer of TDA over the pipe is significantly effective in reducing the pipe’s stresses and the magnitude of transferred pressures compared to using conventional backfill. Furthermore, three-dimensional finite element models (FEM) of the tests were developed to study the interaction mechanism of the considered problem, in which the developed models were validated against the field tests results. An excellent agreement was observed between the measured pipe’s strain values and surface settlements and the numerically obtained results. Additionally, an extensive parametric study was conducted to examine the effect of changing some key parameters on the performance of the investigated system (i.e., the thickness of the TDA layer, shape and configuration of the TDA cross section, and the pipe’s stiffness). This study illustrated that using a layer of TDA backfill above preexisting buried pipes is an excellent construction alternative to enhance the stress bridging mechanism under static loading conditions. The proposed system may lead to avoiding the costly pipeline rerouting option when there is a need for building over preexisting pipes.