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Before the strip starts to sink, the soil load acting on the strip equals the earth pressures acting at the depth of the horizontal support.
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Arching can be achieved by gradually lowering a strip-shaped section A-B (see Figure 1) of the horizontal support. Terzaghi described arching in ideal sand due to the local yield of a horizontal support. The mechanism of arching has been described by Terzaghi (1943). As a result, arching also reduces the total amount of soil load acting on the installed pipe. However, arching in the surrounding soil contributes to the stability of the borehole. A programme of four tests has been performed to investigate the phenomena described below:Īrching During drilling a borehole is protected from collapsing by filling it with bentonite. Tests have been carried out in the geotechnical centrifuge of GeoDelft (the GeoCentrifuge) in order to study the above processes and increase our understanding of the mechanisms that play a role in the stability of a borehole filled with bentonite. This is of importance because this pressure determines the maximum length over which a mud flow can be established and thereby controls the maximum length of the boring. The third issue addressed is the maximum allowable mud pressure during drilling. Better knowledge of the soil load acting on the pipeline can result in a reduction of the wall thickness or a reduction of the required ring stiffness of the pipeline. Secondly, it is considered that the magnitude of the load from the soil that acts on the pipeline is not well understood. However, the influence on the process of arching of different overburden cover-height/borehole diameter (H/D) ratios and the presence of cohesive soil layers is not well understood. Firstly, it is known that arching in the surrounding soil contributes to the stability of the borehole to some extent. One of the issues being examined is the stability of boreholes during and after drilling and installing the pipeline this is the subject of this present paper. In order to improve the cost-effectiveness of the technique, a group of Dutch contractors, engineering consultants and knowledge institutes are working together to enhance the understanding of the geotechnical and hydraulic mechanisms that play a role in horizontal directional drilling. These crossings are often installed using the horizontal-directional-drilling-method (HDD-method). Introduction In the densely populated areas in the Netherlands pipeline-crossings beneath important waterways, railways, roads and residential areas are often unavoidable.