In situ pore pressures: What is their significance and how can they be reliably measured?

The intergranular fluid pressure in a soil matrix (pore pressure) has fundamental influence on the engineering behaviour of soils. Often assumptions are made regarding pore pressures, however, in critical analyses direct measurement of pore pressures should be made. This paper discusses the pore pressure measurements made at the Ormen Lange field. Pore pressure measurements using piezometers indicate pore pressures in excess of hydrostatic at the back wall of the slide and the Stornese feature. Hydrostatic pressures have been identified on the North Flank and in the basin of the slide. Dissipation tests performed during the field investigation did not yield reliable in situ pressure estimates. The pore pressure measurements from the piezometers are consistent with the assumptions required in the theoretical slope stability analyses explaining the Storegga slide mechanism. As no standard commercially available technology for autonomous pore pressure monitoring exists, the paper describes also the technical solutions designed and implemented for pore pressure monitoring at the Ormen Lange field.