Soils II T2 Flashcards
Consolidation definition
Consolidation: is the densification of soils by expelling water from the voids and allowing the soil particles to become more tightly packed.
Recall:
Compaction: removal of air
Consolidation: removal of water
Settlement and compressive deformation result from
The major cause of settlement is compressive
deformation of soil below the structure.
Compressive deformation results from a reduction
in void volume.
Causes of settlement
Causes of settlement:
Deformation of soil particles
Relocation of soil particles
Expulsion of water or air from the void spaces
Reduction in void volume
Rearrangement of soil grains
Compression of material in the void
Consolidation of dry soil
Voids filled with air
Air is compressible
Rearrangement of soil grains occurs rapidly
Consolidation of saturated soils
Voids of saturated soil are filled with water
Water is incompressible
Before soil grains can be rearranged, water must be
extruded
High permeability soils
i.e. coarse grained soils (sand, gravel)
Short time interval
Most settlement occurs by completion of construction
High permeability soils
i.e. coarse grained soils (sand, gravel)
Short time interval
Most settlement occurs by completion of construction
Low permeability soils
i.e. fine grained soils (silt, clay)
Long time interval
Strain occurs very slowly
Settlement occurs slowly
Continues over long period of time
Long term uncertainty
Settlement phases
3 phases
1. Immediate Settlement
2. Primary Consolidation Settlement
3. Secondary Compression Settlement
Immediate Settlement of Loads
on Clay
Occurs rapidly after load application
Caused by soil volume distortion
Completed quickly
Constitutes small amount of total settlement
Caused by the elastic deformation of dry soil, of moist
and saturated soils
No changes in moisture content
Calculations are based on equations derived from
theory of Elasticity
Primary Consolidation
Occurs due to the extrusion of water from the voids
Volume change in saturated cohesive soils
Result of increased loading
Very slow
Continues over a long period of time
Results in primary consolidation settlement
Secondary Consolidation
Primary consolidation has ended
Soil compression and associated settlement continue at a very slow rate under a constant effective stress
Result of plastic readjustment of soil grains
New, changed stresses in soil
Progressive breaking of clayey particles and interparticle
bonds
Results in Secondary Consolidation Settlement
Consolidation Test
Test is performed over 24 hour period
A specific pressure is applied to the top of the
undisturbed soil sample
Dial readings = deformation
Recorded at various times
Prepare graph
X-axis (abscissa) → log scale, time (min)
Y-axis (ordinate) → Deformation (mm)
Pressure is doubled and test re-run
Results:
1. The e-log p curve (void ratio – log pressure)
2. The cv-log p curve (coefficient of consolidation – log
pressure)
3. The initial void ratio e0
CBR
The CBR Test measures the shearing resistance of a soil under controlled moisture and density conditions.
The test yields a bearing-ratio number. This number is not a constant for a given soil but applies only for the tested state of the soil.
The CBR number (or, simply CBR) is obtained as the ratio of the unit stress required to produce a certain depth of penetration of the penetration piston into a compacted specimen of soil at some water content and density to the standard unit stress required to obtain the same depth of penetration on a standard sample of crushed stone.
CBR Values
CBR = Test Unit Stress / Standard Unit Stress * 100%
- The CBR Value
- Moisture contents
- Dry unit weights
- Swell
- Curve of penetration
stress vs penetration
CBR on site
CBR values “on site” may not bear any relationship to the CBR values employed in the road design, due to softening from wet weather and trafficking from site vehicles.
“Capping layers” have been introduced to help solve the problem of sub-grades wetting up and losing strength during construction by protecting the subgrade from the worst of the damage caused by site traffic.
If CBR values are taken on site after the sub-grade has been exposed and dry weather has caused the moisture content of the soil to decrease, increasing soil stiffness, the CBR value will be higher than natural moisture content, this is an incorrect value for design purposes and will cause a serious under design of the road pavement.
Natural soil moisture content, after drainage, is the correct moisture content for determining CBR values for highway design purposes because in the course of time natural soil moisture conditions will be re-established.
Good drainage is an essential part of road construction to allow the optimum strength/CBR to be obtained, and maintained. Drainage must be kept operating efficiently during the life of the road to prevent the strength/CBR decreasing through weakening of the foundation by a rising water table.
