Pore Pressures in Partially Saturated Soil: A Case History of the Diamond Valley Lake Project

Abstract

The three earth-core rockfill dams of the Diamond Valley Lake Project were instrumented with vibrating wire piezometers during their construction. The vibrating wire piezometers were buried within the central earth core of the dams and as additional embankment was constructed the pore pressures within the core material were recorded. The primary purpose of the piezometers is to monitor pore water pressures in the core during first filling and long term operation. During construction the piezometers provide baseline readings with which to compare readings taken during first reservoir filling and long term operation. The construction data also provided information on the development and dissipation of pore pressures within partially saturated soil due to fill placement. The earth-core rockfill dams range in height from 58 to 114 meters (190 to 375 feet).The central earth-core material generally consists of clayey sand and sandy lean clay excavated from the upstream alluvial valley. Immediately after compaction, the material has a degree of saturation of approximately 85 percent. Vibrating wire piezometers with coarse porous tips are installed at regular intervals within the core material by placing them in shallow trenches and compacting core material directly around the piezometer. Measurements were taken immediately. For this paper the term pore pressure refers to the measurements of the coarse porous tipped vibrating wire piezometers. This type of piezometer measures approximate pore air pressure in partially saturated soil, and pore water pressure in saturated soils. Initially after compaction the pore air pressure is about atmospheric pressure and the pore water pressure is below atmospheric pressure due to capillary suction. As additional fill is placed and the soil compresses the difference between the pore air and pore water pressure decreases. Once the reservoir is filled the core will become saturated and the piezometers will measure pore water pressures. This paper presents a general description of the earth-core rockfill dams with details provided on the core material properties and construction methods. A general discussion of the instrumentation program with details on the vibrating wire piezometers and their installation are also provided. Finally, the pore pressure measurements are presented.