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Soils Improvement in Construction

Improving Performance in Expansive Soil

removal by excavation & replacement with non expansive fill

Removal of surface expansive soil to depths of from 1.2m - 2.5m and replacement with compacted non expansive fill usually eliminates most potential soil heave because the depth of moisture change is often limited to about 2.5m

placement of vertical moisture barriers

vertical moisture barriers placed adjacent to pavements or around the perimeter of foundations down to the maximum depth of moisture, within the barrier. Differential movements are minimized. Long term soil wetting with uniform heave beneath impervious foundations may occur from lack of natural evapotranspiration

lime stabilization

lime injected or mixed into expansive soil can reduce potentialfor heave by reducing the mass permeability thereby reducing amount of water seeping into the soil, by cementation, and by exchange of sodium for calcium ions. Fissures should exist in situ to promote penetration of lime injected slurry. Lime may be detrimental in soils containing sulfates

potassium injection

potassium injected into expansive soil can cause a base exchange, increase the soil permeability and effectively reduce the potential for swell


free water is added by ponding to bring soil to the estimated final water content prior to construction. Vertical sand drains may promote wetting of subsurface soil


placing 0.3 - 0.6m or more of permanent compacted fill on the surface of a level site prior to construction increases the overburden pressure on the underlying soil reducing the negative (suction) pore water pressure, therefore, the potential for swell is less and tends to be more uniform. This fill also increases elevation of the site providing positive drainage of water away from the structure.

Soil Improvement Methods



most suitable soils & types

maximum effective treatment depth m

advantages and limitations



shock waves cause liquefaction, displacement, remolding

saturated, clean sands partly saturated sands & silts after floodings


rapid, low cost, treat small areas, no improvement near surface, dangerous


densify by vertical vibration, liquefaction, induced settlement under overburden

saturated or dry clean sand (less effective in finer sand)

20m (ineffective above 4m depth)

rapid, simple, good under water soft under layers may damp vibrations, hard to penetrate over-layers

vibratory rollers

densify by vibration, liquefaction, induced settlement under roller weight

cohesionless soils

2 - 3m

best method for thin leyers or lifts

dynamic compaction (consolidation) or heavy tamping

repeated high intensity impacts at the surface gives immediate settlement

cohesionless soils best, other soils can be improved

15 - 20m

simple,rapid,must protect from personal injury &property damage from flying debris; groundwater must be > 2m below surface faster than preloading but less uniform


densify by horizontal vibration &compaction of backfill materia

cohesionless soil with less than 20% fines


economical &effective in saturated & partly saturated granular soil

hydro- compaction

densify by vibration or repeated impact on surface of prewetted soil

collapsible soil

< 3m

most effective method to densify silty loose collapsible sands

Compaction Piles

compaction piles

densify by displacement of pile volume &vibration during driving

loose sandy soils, partly saturated clayey soils loess

20m limited improvement above 1 to 2

useful in soil with fines, uniform compaction easy to check results,slow

sand compaction piles

sand placed in driven pipe; pipe partially withdrawn & redriven using vibratory hammer



compressed air may be used to keep hole open as casing partially withdrawn



load applied sufficiently in advance of construction to precompress soil

normally consolidated soft clays, silts,organic deposits, landfills


easy, uniform, long time required (use sand drains or strip drains to reduce time)

surcharge fills

fill exceeding that required to achieve a given settlement; shorter time; excess fill removed

same as for preloading


faster than pre-loading without surcharge (use or strip drains to reduce time)

electro- ormosis

DC current causes water flow from anode towards cathode where it is removed

normally consolidated silts &clays

10 - 20m

no fill loading required; use inconfined areas; fast; nonuniform properties between electrodes; useless in higly pervious soil


mix-in-place piles & walls

lime, cement or asphalt placed by rotating auger or in place mixer

all soft or loose inorganic soils

> 20m

uses native soil, reduced lateral support required during excavation; difficult quality control

strips & membranes

horizontal tensile strips or membranes buried in soil under footings


< 3m

increased allowable bearing capacity;reduced deformations

vibro- replacment stone

hole jetted in soft fine-grain soil & back-filled with densely compacted gravel

very soft to firm soils (undrained strength 0.2 to 0.5 tsf


faster than recompressions, avoids dewatering required for remove and replace; limited bearing capacity

vibro- displacement stone

probe displace soil laterally, backfill discharge through probe or placed in layers after probe removed

soft to firm soils

undrained strength 0.3 - 0.6 tsf


best in low sensitivity soils with low ground-water

Grouting and injection

particulate grouting

penetration grout fills soil voids

medium to coarse sand & gravel


low cost; grout high strength

chemical grouting

solutions of 2 or more chemicals react in soil pores to form gel or soil precipitate

medium silts and coarser


low viscosity, controllable gel tim, good water shutoff; high cost; hard to evaluate

pressure injected lime, & lime-flyash

lime slurry & lime-flyash slurry injected to shallow depths under pressure

expansive clays, silts and loose sands

unlimited (usually 2 - 3m)

rapid &economical for foundations under light structures; flyash with lime may increase cementation and strength &reduce permeability

displacement or compaction grout

highly viscous grout acts as radial hydraulic jack when pumped under high pressure

soft, fine grained soils; soils with large voids or cavities


corrects differential settlement; fills large voids; requires careful control

jet grouting

cement grouts injected to replace & mix with soils eroded by high pressure water jet (soilcrete column)

alluvial cohesive, sandy gravelly soils miscellaneous fill & others


increases soil strength & decreases permeability; wide application

electro- kinetic injection

stabilizing chemicals moved into soil by electro-osmosis

saturated silts, silty clays


soil & structure not subject to high pressures, useless in pervious soil


remove and replace

soil excavated with competent material or improved by drying or admixture and recompacted

inorganic soil

< 10m

uniform, controlled with replaced; may require large area dewatering

moisture barriers

water access to foundation soil is minimized and more uniform

expansive soil


best for small structure & pavements; may not be 100% effective


soil is brought to estimated final water

expansive soil


low cost, best for small, light structure; soil may still shrink and swell

structural fills

distributes loads to underlying soft soils

soft clays or organic soils marsh deposits


high strength; good load distribution to underlying soft soils

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