Standpipe Piezometer Installations – Lessons Learnt

Standpipe piezometer installations are frequently commissioned as part of site investigations to monitor groundwater levels and chemistry. On a recent site investigation for a large tunnel infrastructure project in Sydney, Australia, 15 standpipe piezometers were installed and developed. Some of the well screen depths exceeded 100 m below ground surface. Several of these wells, especially the deeper ones, returned unexpectedly high pH values after development and sampling. The values were potentially misleading for the assessment of infrastructure durability and environmental impacts.

A hypothesis for these high pH readings was the potential ingress of the cement-bentonite grout in the annulus to the standpipe piezometer via the bentonite seal and/or the casing threads. The latter was confirmed by borehole imaging. Following this observation, a literature review and trials were carried out to investigate the impacts of typical well construction methodology and materials on the pH of the groundwater sampled. In particular, threads from several PN18 nominal pressure rated casing were tested, with their elastomeric joints, at different confining pressures. The effectiveness of the bentonite seal above the screened section was also tested by varying curing time and seal thickness for different overburden pressures. This paper discusses the results of these trials and describes measures to reduce the risk of groundwater contamination induced by cement-bentonite grout leakage.