Ground improvement by vacuum consolidation – a review

Abstract

This paper presents a review of literature related to the vacuum consolidation ground improvement technique, focusing on issues affecting efficiency of performance and the application of the technique in peaty ground.Although it is now widely considered to be effective for reducing post-construction settlement, studies on vacuum consolidation in peat have, to date, mostly been limited to field trials. This paper offers an overview of the development and theory behind the technique. Issues affecting the efficiency of vacuum systems and recent advances in predicting the ground response to vacuum preloading are described. The application of the technique in peaty soils is investigated; an overview is also provided of some geotechnical properties that distinguish peat from soft mineral soils, highlighting lessons learned from relevant case studies.

Case studies of Vacuum Consolidation Ground Improvement in Peat Deposits

“…The majority of vacuum consolidation projects to date have been performed on soft clayey deposits and hydraulic fills for land reclamation (see Griffin and O'Kelly, 2014a). This section summarizes the limited application of the technique in peat and other highly organic deposits (see Table 11.1).…”

“…Second, between July 29, 2010 and October 29, 2010 (end of the trial), a 2.2 kW liquid ring pump in combination with the jet pump was found to produce a more stable vacuum (Farrell et al, 2012;Griffin and O'Kelly, 2014a). To these applied vacuum pressures must be added the surcharge load due to the 0.45-m-deep granular bed and 0.5-m-deep peat covering layer; that is, $14 kPa initially reducing to $4.5 kPa when the underlying peat had Cross section (adapted from Osorio et al, 2010).…”

“…However, boundary effects were significant on account of the small treatment area and relatively deep peat layer, resulting in the formation of a concave depression, evident from Fig. The water inside of the vacuum lines also froze periodically over the course of an 11-week period (Farrell et al, 2012;Griffin and O'Kelly, 2014a), rendering the vacuum system inoperative. As expected, considerable tension cracking occurred around the test area perimeter near ground-surface level (Farrell et al, 2012) on account of the lateral inward displacement of the upper peat layer (Chai et al, 2005).…”

Sustainability of Combined Vacuum and Surcharge Preloading

“…Hence the amount of fill material required onsite can be reduced and a shorter period will also be required in achieving near/full consolidation. Discrepancies between predicted and actual performance of the vacuum consolidation technique may arise from difficulties in predicting the impact of smear effects on the PVDs and well resistance (Indraratna et al, 2012b, Griffin andO'Kelly, 2014), along with incorrect assumptions regarding soil behaviour and the vacuum pressure distribution achievable against increasing depth. Numerical models have achieved improved accuracy, although remaining discrepancies between predictions and field measurements of pore pressure and settlement for vacuum-assisted preloading require further attention.…”

Editorial

“…The vacuum consolidation technique aids stability through applying an isotropic component of pressure to the ground and can speed up construction. Griffin and O'Kelly (2014) perform a review of the method in general and of the few examples of its use in peaty soils. The authors find that peats can be successfully treated using vacuum consolidation but lateral deformations and secondary compression of peats under vacuum conditions needs further investigation.…”