Improving Performance in Expansive Soil |
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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 |
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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 |
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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 |
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potassium injection |
potassium injected into expansive soil can cause a base exchange, increase the soil permeability and effectively reduce the potential for swell |
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prewetting |
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 |
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surcharge |
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. |
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Soil Improvement Methods |
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method |
principle |
most suitable soils & types |
maximum effective treatment depth m |
advantages and limitations |
Vibrocompaction |
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blasting |
shock waves cause liquefaction, displacement, remolding |
saturated, clean sands partly saturated sands & silts after floodings |
20m |
rapid, low cost, treat small areas, no improvement near surface, dangerous |
terra-probe |
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 |
vibroflotation |
densify by horizontal vibration &compaction of backfill materia |
cohesionless soil with less than 20% fines |
30m |
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 |
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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 |
all |
- |
compressed air may be used to keep hole open as casing partially withdrawn |
Precompression |
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pre-loading |
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 |
Reinforcement |
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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 |
all |
< 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 |
20m |
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 |
15m |
best in low sensitivity soils with low ground-water |
Grouting and injection |
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particulate grouting |
penetration grout fills soil voids |
medium to coarse sand & gravel |
unlimited |
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 |
unlimited |
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 |
13m |
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 |
unlimited |
increases soil strength & decreases permeability; wide application |
electro- kinetic injection |
stabilizing chemicals moved into soil by electro-osmosis |
saturated silts, silty clays |
unknown |
soil & structure not subject to high pressures, useless in pervious soil |
miscellanoeous |
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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 |
5 |
best for small structure & pavements; may not be 100% effective |
prewetting |
soil is brought to estimated final water |
expansive soil |
2m |
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 |