Strengthening soft soils
What are Stone Columns?
Soils with appreciable silt or clay content do not respond to deep vibratory compaction. To improve these cohesive soil types to allow building and other heavy construction, it is necessary to create stiff reinforcing elements in the soil mass. The stone column technique, also known as vibro-replacement or vibro-displacement, is a ground improvement process where vertical columns of compacted aggregate are formed through the soils to be improved. These columns result in considerable vertical load carrying capacity and improved shear resistance in the soil mass.
Stone columns are installed with specialized vibratory probes, generally having a horizontal mode of vibration. Column diameters of 2 to 5 feet can be achieved, depending upon soil conditions and design requirements.
The vibrator first penetrates to the required depth by vibration and air or water jetting or by vibration alone. Gravel is then added at the tip of the vibrator and progressive raising and repenetration of the vibrator results in the gravel being pushed into the surrounding soil. The soil-column matrix results in an overall mass having a high shear strength and a low compressibilty.
What Soils are Suitable?
Any soil type that does not respond to vibration alone is a candidate for stone columns. These soils include silty and clayey sands, silts, clays, and some layered soils where damping of vibrations occurs. Often very fine sands will not respond well to vibration, because of their low permeability, but can be improved with stone columns.
The largest worldwide application of stone columns, however, is for improvement of old fill soils, including rubble fills.
What Sites are Suitable?
Stone columns have been used in nearly every type of civil construction. Projects in the U.S. have included residential, commercial, and industrial buildings, dams, storage tanks, power plants, highways, landslide corrections, liquefaction mitigation, stabilization of cofferdams, and other applications. Stone columns can be used adjacent to existing buildings without causing damage from vibrations.
How Much Improvement is Possible?
The degree of improvement is dependent on the soil characteristics and the spacing of the columns. The stone columns can be arranged to fit varying soil conditions and design requirements, but generally vary from 5 to 10 feet on center. Although the soil-column system must be considered as a unit in terms of both compressibility and shear strength, the individual columns often have axial load-carrying capacities of 40 tons or more. Generally, for building construction, stone columns are used only beneath the building foundations.