Vibratory ground improvement techniques

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4.5.5Suitability of ground conditions

Vibratory ground improvement techniques shall only be conducted on suitable ground and be appropriate for the site conditions. Issues to be taken into account include:

  1. unsuitable ground conditions
  2. detrimental factors
  3. ground water conditions.

The engineer should assess the ground and be satisfied that it is suitable for treatment. Conditions acceptable for treatment are only those within zones A and B of Chart 1.

Zone A – range of materials suitable for deep compaction (vibro-compaction) techniques.
Zone B – range of materials suitable for stone column (vibro-replacement) techniques.

Unsuitable ground conditions

Table 2: Ground conditions not generally acceptable for treatment

Soil composition
ClaysGround with a Plasticity Index greater than 40%.
Soft claysGround with soft clays with an undrained shear strength less than 30kN/m2.
For clay strength less than 30kN/m2 additional consideration must be given to group effects, ground heave and settlement due to installation. Any proposals will be subject to NHBC agreement.
Ground with peat layers Ground with peat layers close to foundation level or the base of the stone column, or where intermediate layers of peat are thicker than 200mm either as a single layer, or the sum of the thicknesses of individual layers, throughout the length of the stone column.
Highly sensitive soils Ground liable to collapse or remoulding.

Detrimental factors

When specifying vibratory ground improvement techniques, the following factors should be considered:

  • Partial depth treatment of filled ground. The engineer should be satisfied with the anticipated performance of both the treated and untreated zones.
  • The specialist contractor should take responsibility for the treated zone and the depth of treatment.
  • Alterations to the oversite level before or after treatment, or the disturbance of ground by excavations after treatment.
  • Soils with a Modified Plasticity Index of 10% or greater; foundations should be designed to accommodate volume changes.
  • Obstructions and variations in the density of fill and natural ground (hard spots) and the location of changes in the profile of the natural underlying ground, e.g. edges of pits or quarries, slopes, or manmade obstructions such as soakaways or drainage runs.
  • The minimum depth of soil treated, which should allow for the interaction of adjacent foundations.
  • Stone columns that may form vertical drains, allowing the passage of water to a moisture-susceptible strata, or provide seepage paths for gases.

Ground water conditions

When specifying vibratory ground improvement techniques, the following factors should be considered:

  • Long-term lowering of the water table causing settlement of existing adjacent buildings.
  • Short-term rise in local water table due to large volumes of water used in wet process during construction causing settlement or heave of existing adjacent buildings.

Surface water sewers should be used for rainwater disposal where possible, but where soakaways are necessary, these should be positioned so that their construction and operation is not detrimental to the treated ground.

The effect of any new or existing sustainable drainage systems (SuDS) should be taken into account when vibro improvement techniques are proposed.