Unit 4: Consolidation Flashcards
Define Settlement
The downward soil movement due to applied loads.
What is Effective Stress
Stress carried by soil skeleton, excluding pore water.
Name the causes of settlement (6)
- Low bearing capacity of soils
- Addition to an existing house
- Improperly backfilled soils/poor compaction
- Irregular rainfall or drought
- Poor drainage around the house
- Changes in the Groundwater Table
What is consolidation?
Soil compression from water loss under load.
What is compaction?
Reduction of air voids in unsaturated soils.
Define the following and represent in a graph:
a. Initial Compression
b. Primary Consolidation
c. Secondary Consolidation
Stage I: Initial Compression - Primarily caused by preloading.
Stage II: Primary Consolidation - Excess pore water pressure dissipation/distribution which result in a change of soil volume.
Stage III: Secondary Consolidation - After dissipation of excess pore water pressure (plastic deformation of soil particles – cohesive soils).
(See notes for diagram)
What is the Coefficient of Consolidation?
Rate of soil consolidation under pressure.
Differentiate the following:
a. aO
b. as
c. a50
d. a100
e. af
a. a0: Initial Dial Gauge Reading – The first recorded reading before consolidation begins.
b. as: Start of Primary Consolidation – Determined by selecting two points on the curve (A & B) where time values (t) are in a 1:4 ratio. The vertical distance ζ between these points is measured. An equal distance ζ above point A determines as, corresponding to U=0 (no consolidation).
c. a50: 50% Consolidation – Found midway between the as and a100. The corresponding time at this point is t50.
d. a100: 100% Consolidation – Identified as the intersection of the two linear portions of the curve.
e. af: Final Dial Gauge Reading – The last reading recorded before applying a new load increment.
What is the void ratio?
Ratio of volume of voids to volume of solids.
