Vibratory techniques are often used to improve soil conditions prior to installing foundations.
Although this method can be a cost effective foundation solution there are potential issues if you don’t know what you’re letting yourself in for.
Here are the key factors you need to consider to ensure you comply with building regulations and avoid any problems.
1. A suitable site investigation should be commissioned, and all ground hazards identified.
2. The Engineer should assess the ground and be satisfied that it is suitable for treatment based on the following criteria:
- The type of soil, its strength, particle size, plasticity etc.
- The variability of the soil across the site and with depth.
- The presence of made ground or fill, (voided, degradable, unconsolidated).
- The presence of any soluble soils or contaminative substances or gases.
- Presence of a water table, and whether it fluctuates (tidal?).
- The influence of any vegetation.
- The existence of previous buildings, with associated underground obstructions, services or drains.
- Partial treatment of made ground.
- Stone columns acting as vertical drains or seepage paths for ground gases.
- Changes in the profile of the natural ground, i.e., edge of a quarry.
- Changes in the water table affecting adjacent buildings.
- Re-grading and/or disturbance of the ground by excavations after treatment.
- Use of soakaways or surface water sewers.
- Limitations on the building configuration, i.e. vulnerability of long blocks.
- Proximity of new drainage and service trenches.
- Building design to take account of predicted ground movements (spacing of movement joints, bed joint reinforcement and type of masonry mortar).
3. The Specialist Vibro Contractor shall confirm in writing that the site is suitable for the proposed ground improvement system.
4. The stone fill used for forming columns must be suitable for vibratory ground improvement and compatible with the ground conditions.
- Stone fill should be clean, hard and inert.
- Natural gravel or crushed rock gravel within a range 20 -75mm.
5. The Specialist Vibro Contractor should visit the site and provide competent supervision throughout the ground treatment process, giving consideration to:
- Location, depth and alignment of the stone columns.
- Unforeseen circumstances.
6. The Specialist Vibro Contractor needs to verify the ground treatment is satisfactory. This should take into account suitable insitu testing. The testing needs to be compatible with:
- the nature of the ground,
- quality of the site investigation,
- depth of treatment and foundation design.
7. On completion of the treatment the Engineer should satisfy himself that the treated ground has achieved the anticipated improvement in bearing capacity and settlement required by the design.
8. The Developer/Builder and Engineer should ensure that the treated ground is not disturbed by subsequent excavations, i.e. sewer trenches.
9. On completion of works the improved areas should be subjected to insitu testing.
To learn more about the differences between using vibro compaction compared to piling check out our blog article “To shake or not to shake”.
By Anna Symington