Experimental Investigation Examining Influence of Burial Depth on Stability of Horizontal Boreholes in Sand

Horizontal directional drilling (HDD) is now widely used for pipe installation by the trenchless industry. Horizontal borehole stability under excessive mud pressure can be a concern during the drilling processes. The Delft equation has been applied to calculate the maximum allowable mud pressure, but the results are usually not safe. Meanwhile, little experimental evidence is available in the literature to examine the relationship between the maximum mud pressure and different burial depths, or to explain the mud flow paths through the soil up to the ground surface. Therefore, a series of mud loss experiments using different burial depths in sand have been undertaken. An example of mud pressure history along with the development of cracks and a shear plane reaching the ground surface were observed. The manner in which maximum measured mud pressures $P_{\max }$ rose with increasing burial depth was quantified ($P_{\max }$ rises from 17 kPa with $H/D=7.5$, a ratio of the distance from the ground surface to the springline $H$ and borehole diameter $D$, to 181 kPa with $H/D=31.5$). In addition, two test sites were excavated to explore the mud infiltration zones. These show that a clear shear failure plane developed extending from near the borehole to the ground surface. Contractors and consultants often estimate $P_{\max }$ using the Delft equation, but this equation overestimated pressures compared to the experimental results by 90%–170%. These discrepancies are explained. In addition, earth pressure sensors (null gauges) were buried at three times the borehole radius below the boreholes in the experiments to measure axial, radial, and hoop stresses. These data were compared to the closed form theoretical solutions and confirmed the influence of the initial geostatic ($K_0$) stress conditions on stress histories.