Nude vibrating wire piezometer installations — no filter response zone and no engineered grout

Authors: Mazur, AJ; Schouten, LM; Lam, WY; Setiawan, PH Download Paper

DOI https://doi.org/10.36487/ACG_rep/1508_33_Mazur

Cite As:
Mazur, AJ, Schouten, LM, Lam, WY & Setiawan, PH 2015, 'Nude vibrating wire piezometer installations — no filter response zone and no engineered grout', in PM Dight (ed.), FMGM 2015: Proceedings of the Ninth Symposium on Field Measurements in Geomechanics, Australian Centre for Geomechanics, Perth, pp. 487-500, https://doi.org/10.36487/ACG_rep/1508_33_Mazur



Abstract:
This paper presents and compares the field performance of nude vibrating wire piezometer (VWP) installations with conventionally installed VWPs (with bentonite seal and filter response zone). The term ‘nude’ is used here to describe VWP instruments installed inside drill holes, without any filter response zone, bentonite seals or engineered grout materials. The success of this technique is dependent on the natural soil materials collapsing and enveloping the VWP sensor, forming a satisfactorily impermeable seal. Once the nude VWP instrument is lowered to the desired depth in a drill hole, the drill hole support casing is quickly removed and the surrounding saturated, soft, fine grained soils are allowed to collapse around and remould above the instrument to provide a relatively impermeable seal. Any remaining open area at the collar of the drill hole is backfilled with drill cuttings. In 2003 at the Sixth International Symposium on Field Measurements in Geomechanics, VWPs in fullygrouted boreholes were proposed as an acceptable and even superior alternative to the conventional filter sand/bentonite seal response zones commonly used with VWP instruments. Several authors have contributed further research over the ensuing twelve years, and the fully-grouted borehole technique has been adopted by a growing number of practitioners. We now put forward that under certain geotechnical conditions, the natural ground profile in soft soils can form an adequate low-permeability seal around the piezometer, and the engineered grout used with the fully-grouted borehole method may also be eliminated. In this case study, nude VWP performance was found to be characteristic of readings observed from conventionally (filter zone and bentonite seal) installed VWP instruments. Our findings are the result of full-scale field trials on a major Hong Kong reclamation project covering 150 hectares that were subject to staged loading using a sand surcharge. These piezometers were placed within low permeability, soft marine sediments, with data retrieved automatically during several stages of surcharge loading and unloading. The following arrangements were tested: Absolute readings, sensitivity, response times and dissipation signatures were compared in all instruments, in response to a rapidly applied and staged sand surcharge of 10 metres maximum height.

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