Ground improvement

Engineering properties
Drainage
Pre-consolidation
Compaction
Grouting

Geo-textiles

Where poor ground conditions make traditional forms of construction expensive, it may be economically viable to attempt to improve the engineering properties of the ground before building on it. This can be done by reducing the pore water pressure, by reducing the volume of voids in the soil, or by adding stronger materials.

Engineering properties

Compression
Consolidation
Shear strength
Permeability

The properties of soil which most affect the cost of construction are strengthand compressibility. Both can be improved by reducing the volume of the voids in the soil mass. Water must be displaced from saturated soils in order to reduce the volume of the voids. This may take months if the permeability of the soil is low.

Compression

Soil which is highly compressible is prone to volume change when a load is applied. This leads to settlement. Fine-grained soils which have been compressed and then allowed to swell, experience a smaller volume change when re-compressed. Loosely-compacted coarse-grained soils may exhibit little change in volume under static loads, but become unstable and exhibit large volume changes when either vibrated or flooded and then drained.

Consolidation

The sudden application of a load to a saturated soil produces an immediate increase in porewater pressure. Over time, the excess porewater pressure will dissipate, the effective stress in the soil will increase and settlement will increase. Since shear strength is related to effective stress, it may be necessary to control the rate of construction to avoid a shear failure. This was the case, for example, when approach embankments were constructed on soft alluvium, for the bridge which carried the M180 motorway over the River Trent near Scunthorpe. The rate at which the excess water pressure dissipates, and settlement occurs, depends on the permeability of the soil, the amount of water to be expelled and the distance the water must travel.

Shear strength

Collapse will occur if the shear stress along a potential failure surface exceeds the shear strength of the soil. Shear strength depends on the effective normal stress, which depends on the porewater pressure. Undrained loading causes an increase in porewater pressure equal to the change in the total normal stress so that there is no increase in strength to match the change in the shear stress. The shear strength can be increased either by decreasing the water pressure or reducing the void ratio of the soil to produce a peak strength which exceeds the critical shear stress.

Permeability

Fine-grained soils have a lower permeability than coarse-grained soils, thus excess porewater pressures take longer to dissipate. Consolidation reduces the void ratio of the soil and further decreases the permeability. Real soils are not hydraulically isotropic: the natural orientation of particles in soils which have been consolidated vertically tends to produce a horizontal permeability which is greater than the vertical permeability. Thin horizontal layers of coarse-grained soil in a mass of fine-grained soil may dramatically increase the horizontal permeability while having little effect on the vertical permeability. It is possible to increase the drainage rate without changing the permeability of the bulk of the soil by introducing layer drains (sandwicks) or fracturing the soil. The most effective way to reduce seepage into an excavation, through or under a dam, or away from contaminated ground is to create a low permeability zone perpendicular to the direction of flow.