Experience with a new high pressure self boring pressuremeter in weak rock. Part II : Ground EngngV22, N6, Sept 1989, P45–52

Abstract

This article describes the development of a new high pressure self-boring pressuremeter (rsbp), which allows the in situ direct measurement of strength and deformation characteristics of keuper marl for foundation design beneath a new United Kingdom power station site. A specific weathering scheme for the keuper marl at this site grades the material a to d according to the proportion of clay and condition of the joints; this is shown in table 1. The authors first establish the need for a self-boring pressuremeter which could be used in weak weathered rock, overcoming the problems of predrilling, allowing horizontal stress to be assessed directly and giving more reproducible values of modulus, less affected by membrane compression. The new instrument is designed to operate through h casing or in nx boreholes. It is 73.6 mm in diameter and 450 mm long; it can be expanded up to 12% cavity strain or 20 mpa. It uses a 2 rod drilling system but can be operated with a rotary rig. Rock cuttings are flushed to the surface using water or liquid polymer mud. Oil pressure is used to expand the pressuremeter; the thin composite membrane is made of nylon and neoprene. Transformer oil and power for electronics and transducer signal readings is supplied via 2 hydraulic hoses. Full details are given of the calibration tests used. The second part of this article is published in the September 1989 issue of ground engineering.