Soil Improvement

An Introduction

Soil Improvement

• If poor soil conditions are encountered:– Bypass poor soil, use deep foundations– Remove poor soil, replace with engineered fill– Improve soil properties in place

Factors to be considered in Selecting Soil Improvement Method

• Soil type; fines content (silt and clay size)• Area and depth of treatment• Soil properties – strength compressibility• Proposed structure and settlement criteria• Availablity of skills, equipment and materials • Adjacent improvements• Economics

Primary objectives of soil improvement

• Clay– Increase bearing capacity or slobe stability– Reduce foundation settleement

• Sand– Reduce liquefaction potential– Increase bearing capacity– Reduce foundation settlement

Mechanism of Soil improvement for Clay

• Consolidation– Preloading

• Reinforcemnt– Soil-Cement columns– Vibro-Replacement stone columns– Geopiers® and Vibro Piers™

Mechanism of Soil improvement for Sand

• By vibration– Impacts at surface : dynamic Compaction– Depth vibrator: Vibro-Compaction

• By vibration and Displacement of backfill– Vibro-replacement stone column– Vibro Piers™

• By displacement of backfill material– Compaction grouting

Soil Improvement MethodsMethod Clay Sand

Preloading X

Dynamic Compaction X

Soil-Cement Columns X X

Vibro-Compaction X

Stone Columns X X

Geopiers and Vibro Piers X X

Compaction grouting X

Preloading• Performed by placing fill over soft clay• A proportion of the final settlement can be achieved prior

to construction by pre-loading the soil• The larger the pre-load, the less time it will take to achieve

the final settlement• Pre-consolidating the ground in this way tends to be an

expensive solution compared with the use of piles to support localized loads such as columns

• Pre-consolidation may be a cost-effective way of reducing the settlement due to lightly distributed loads from roads or warehouse or supermarket floors provided that material is readily available to provide the pre-loading.

Drainage

• Pumping water out of the ground will cause a local lowering of the ground water level and a decrease in water pressure

• Low permeability implies slow • Decreasing the water pressure increases the

effective stress, which increases the shear strength and causes settlement.

Dynamic Compaction (Densification)Dynamic Compaction is the dropping of heavy weights on the ground surface to densify soils at depth

• Dynamic Compaction Applications – Reduce foundation settlements– Reduce seismic subsidence– Permit construction on fills– Densify garbage dumps– Improve mine spoils– Induce settlements in collapsible soils

• Important Dynamic Compaction Construction Conditions– Minimum 100-150 ft clearance from any structure– Review site for vibration sensitivity

Vibro-Compaction• Vibro-Compaction is used to densify clean, cohesionless soils. The action of

the vibrator, usually accompanied by water jetting, reduces the inter-granular forces between the soil particles, allowing them to move into a denser configuration

Vibro-Replacement Stone Columns• Vibro-Replacement Stone Columns extends the range of

soils that can be improved by vibratory techniques to include cohesive soils.

References

• www.haywardbaker.com• www.treadwellrollo.com• http://environment.uwe.ac.uk