Ground Improvement Techniques
Table of Contents
What is Ground Improvement
Ground improvement techniques are used to prepare construction sites before any foundation, or superstructure is built. The ground improvement will change the properties of the soil (i.e. increase the density, stiffness, shear strength and permeability) and this will determine the type of foundation to be used on site (i.e. deep or shallow foundations). This can be used to build a more cost effective foundation design.
Ground improvement techniques developed greatly over the last 30 years. It is becoming more popular due to the increased brownfield site development and sustainability requirements. There is a lot of pressure from the government and sustainability objectives to retain and reuse marginal fills due to landfill and aggregate tax.
This article looks into the improvements of ground properties rather than ‘remediation’ of contamination.
Initial Site Investigation
Before any ground improvement techniques are undertaken, a site investigation and desk study is required.
These studies will determine the engineering properties of the ground and what kind of foundation is required (i.e. 450mm diameter, 18m contiguous flight auger pile) and whether ground improvement is required. The desk study will show whether the ground improvement is feasible.
General Ground Improvement Techniques Available
- Improvement by Compaction/Vibration methods
- Addition of load or increase in effective stress
- Improvement by structural reinforcement
- Use of structural fill
- Use of admixtures
- Improvement by grouting
- Improvement by thermal stabilization
- Improvement by Vegetation
Soil Compaction
Conventional compaction plant are used commonly under highway specification (plant rollers and hand held plant), and involves applying a static stress onto the soil. The applied stress increases the soil density, decreasing air volume, and inhibits root penetration and movement of air and water in the soil.
Impact Compaction involves lifting and dropping a non-circular rotating mass. This compacts layers between a thickness of 600-1500mm but improvement can occur at 4m.
Rapid Impact Compaction was developed by the military for runway repairs, which can involve dropping a 7 tonne weight from a height (1.2m height) at a rate of 40-60 drops per minute.
Vibro-compaction is often used in granular soil – ideally saturated sand sized particles. The first stage of the vibro-compaction involves using a vibrator penetrates the soil at a design depth and is surged up and down to remove fines using full water pressure. Once at depth, the water pressure is reduced and with the vibrator remaining in the sand, sand infill is added from ground level and compacted at the base of the vibrator. When the required compaction resistance is achieved, the vibrator is raised and more sand infill added and compacted as before. This procedure is repeated until compaction point is built to ground level.
Vibro Stone Columns
Another ground improvement techniques vibro stone columns, which involves vibrator to penetrate and consolidate the soil. This stabilizes granular soil and strengthens the soil. Stone is introduced into the hole and is compacted and tightly interlocked with the surrounding ground. This compacts the ground soil and increases the bearing capacity of the soil.
Vibro Stone columns are primarily used to reduce potential for settlement and is typically 0.6 to 0.8m diameter, 1m (under foundations) -3m (floor slabs) centres. This can be used in cohesive materials but the shear strength needs to be greater than 15 kPa. For cu 15 kPa consider concrete columns. Also, it can’t be used in sites where peat > 0.6m thick.
Improvement by adding load
Soil compaction by adding load to induce settlements in advance of the proposed construction. Larger pre-loads will reduce time required for the settlement to occur. The shear strength, stiffness, bearing capacity and stability of the soil.