Ground improvement for sites underlain by soft or loose cohesive and mixed soils.

Sites underlain by soft cohesive or loose mixed soils generally require some form of ground remediation, primarily to reduce total and differential settlements, to be viable for development. A simple and effective solution is the use of pre-loading or surcharge loading.

Pre-loading involves placement of load on the ground surface prior to construction such that the imposed load is equivalent to the final project loading conditions.  Sufficient time is then allowed for the ground to settle and strengthen as consolidation occurs under the imposed load.  Once sufficient consolidation has been achieved the pre-load can be removed and the project construction can commence.

Surcharging consists of applying load on the ground surface in excess of that associated with the long term development conditions to accelerate consolidation.  This can take the form of temporary fill embankments, constructed to a height that exceeds the design finished surface level, which are cut back to the design level following an appropriate period of consolidation settlement. 

The consolidation process requires sufficient time to reach the required conditions.  For this reason, it is imperative that, if this simple and relatively low cost solution is to be adopted, planning, design, site access and installation of monitoring is all carried out in advance of the construction commencement date.  For deep deposits of clay strata the required consolidation period can be many months and so early consideration and adoption is mandatory. 

Successful use of the pre-load or surcharge loading process requires provision, processing and detailed interpretation of reliable monitoring data.  In particular pore water pressures and surface settlements must be accurately and reliable measured such that consolidation progress can be computed and the required duration of treatment confirmed. 

The purpose of pre-loading and surcharging is to ensure that:

  • Long term primary and secondary consolidation settlements are acceptable.
  • The treated ground is sufficiently uniform that anticipated differential settlements are acceptable.
  • The treated ground has improved strength and stiffness properties that are consistent with deign assumptions and project specifications.
  • The treated ground has improved strength and density properties such that liquefaction potential is reduced to an acceptable level.
  • Surcharge embankment design must consider both primary consolidation settlement and secondary compression (creep) settlement of the soft soils.

To accelerate the rate of primary consolidation in low permeability strata, prefabricated vertical drains (PVD) or sand drains can be installed to significantly shorten the drainage path length and take advantage of the possible higher horizontal soil permeability. The PVD drains typically comprise a corrugated core through which water can flow, with a permeable geotextile outer cover. The drains are usually installed using a rig mounter push in mandrel.  Sand drains are commonly formed by drilling a small diameter borehole and filling it with a clean coarse sand.

CMW has significant experience with these methods and can undertake pre-load and surcharge design, incorporating embankment height and PVD/sand drain spacing design and specification, to optimise the surcharge solution.

Our design solutions incorporate details of the anticipated construction timeframe, the post construction settlement tolerances and detailed assessments of the surcharge fill and in-situ material characteristics.  We also provide monitoring specification, installation, measurement, processing and interpretation services to ensure correct implementation of the design. 

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