4.5.6 Suitability 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:
- unsuitable ground conditions
- detrimental factors
- groundwater 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.
4.5.6.1 Unsuitable ground conditions
Table 2: Ground conditions not generally acceptable for treatment
4.5.6.2 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 (Figure 3)
- 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, eg 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 (Figure 4)
- stone columns that may form vertical drains, allowing the passage of water to a moisture-susceptible strata, or provide seepage paths for gases (Figure 5).
Figure 3: Vibro foundation depth in volume change potential soils
Figure 4: Interaction of foundations in close proximity
Figure 5: Stone column acting as vertical drains
4.5.6.3 Groundwater 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.
Figure 6: Effect of depressed water level on adjacent building
Figure 7: Effect of raised water level on adjacent building
Surface water sewers should be used for rainwater disposal where possible, but where soakaways are necessary, they 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 compaction improvement techniques are proposed.
Last updated: 2nd January 2025