Load-settlement behaviour of geosynthetic-reinforced granular soil cover over the conduit

Author Identifiers

ORCID: 0000-0002-7192-9218

Date of Award


Degree Type


Degree Name

Doctor of Philosophy


School of Engineering

First Advisor

Associate Professor Sanjay Kumar Shukla

Second Advisor

Dr Alireza Mohyeddin


The buried conduits/pipes have been used for a comprehensive transport purpose, such as transporting water, gas, oil, sewage and other services. In recent years, many researchers have investigated the behaviour of the buried conduits under soil cover. Some researchers have attempted to use geosynthetic reinforcement within the soil cover to protect the buried conduit to extend their service life. Some studies have been conducted to investigate the behaviour of the reinforced-conduit covered with sandy soil, sandy gravel, silica or fly ash, etc. The researchers proposed some different methods, such as experimental, numerical and analytical methods which can improve the performance of the conduit. In recent years, the researchers have used different types of geosynthetics to protect the conduit, such as geotextile, geofoam or geogrid. However, an optimal design and the effectiveness of employing geosynthetic material for protecting the conduit require an extensive knowledge of the pressure distribution around conduit covered with reinforced soil cover as well as the deflection of conduit. In this research, an attempt has been made to investigate the behaviour of geotextile-reinforced sandy soil over the conduit to achieve the benefits of reinforcement. For this purpose, extensive laboratory measurements, numerical analysis and analytical investigation have been carried out.

In the laboratory experimental phase, the model footing tests were conducted in a rigid tank filled with sandy soil compacted at a relative density of 70% which is most common relative density in real life. The PVC pipe of 150 mm diameter was placed at a depth of 320 mm below the footing base. The geotextile reinforcement (single layer as well as two layers) of varying widths was installed at several depths, but always above the conduit. The experimental investigation aimed at: (i) observing the effect of the width of a single-layer of geosynthetic reinforcement on the footing settlement, and pressure around the conduit and its deflection, and (ii) studying the effect of the depth of geosynthetic reinforcement on footing settlement, and pressure around the conduit and its deflection. It was observed that the optimum width of geosynthetic for pressure reduction around the conduit, footing settlement reduction and deflection reduction around the conduit is equal to four times conduit diameter, and optimum depth is equal to 1.5 times conduit diameter below the surface. When the depth of geosynthetic layer is 1.5 times the conduit diameter depth, pressure reduction is 26.62% on the crown of the conduit, compared to unreinforced case when applied pressure is 200 kPa. It was also noted that the two layers of geosyntheitc reinforcement bring more benefits for pressure reduction. For example, when the depth of first layer of geotextile is 1.5 times the conduit diameter, the reduction in pressure on the crown of the conduit is about 34% for the applied pressure of 200 kPa.

A numerical model was developed and validated with the experimental findings to investigate the effect of depth of the geosynthetic reinforcement on footing settlement and conduit deflection. The numerical model also helped analyse the behaviour of reinforced soil cover over the large diameter conduit. The parametric study was carried out to investigate the effect of governing parameters (such as cohesion of the soil, friction angle of the soil and the interface reduction factor between the soil and geosynthetic layer) on the footing settlement and pressure on the crown of the conduit. In the large-scale conduit case, it is found that the optimum reduction takes place when the spacing between pipe and geosynthetic layer is equal to the conduit diameter. The pressure on the crown of the small diameter HDPE and PVC pipes reduced by 31% and 27%, respectively.

In the present work, analytical expression has also been developed for evaluating the pressure on the crown of the two-layer geosynthetic-reinforced sandy soil cover over the conduit. Using the analytical expression, a parametric study has been carried out. An illustrative example is also presented to explain how the analytical expression can be used.

This thesis has several graphical presentations, which can be quite useful for the practising engineers for designing the conduits covered with geosynthetic-reinforced soil cover without depending on expensive commercial software.

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