MODULE 1 Flashcards
Saturated soils
voids are filled completely with water
Dry soil
voids are filled completely with air
Fines
Clay and Silt
Sand
0.06 - 2 mm
Gravel
2 - 60 mm
Cobbles
60 - 200 mm
Boulders
> 200mm
Coefficient of Angularity
Fu = 4πA/l^2
where A = cross-sectional area
and l = perimeter
Roundness Coefficient
R = r/x
Plastic Limit
wPL: water content below which fine soil behaves brittle and crumbly
To determine wPL
- roll out threads of clay (3mm diameter)
- wPL is defined as the moisture content w when threads become brittle
Liquid Limit
wLL: water content above which fine soil behaves as a liquid
To determine wLL
BS: standard fall-cone tests
- usually carry out several tests at different w and draw a straight line between w and fall distance
US: use a Casagrande apparatus, cut V-shape in soil - count number of standard taps to cause the cut to disappear
- test requires more “judgement” than the fall - cone
Clays
- have a plate-like structure
- the higher the wLL, wPL and PL, the more plastic the clay (i.e. the more it will shrink, swell and change volume)
Non-plastic silts
- are like extremely fine sand
-
Soil Classification
35% fines = “fine”
Static Compaction
- sheepsfoot roller: shallow compaction
- smooth-wheel roller: shallow compaction
- pneumatic (multi-tired) roller: shallow compaction
Dynamic Compaction
- vibratory roller: shallow compaction
- High Energy Impact Compaction (HEIC): deep compaction
- Impact Load (weight dropping): very deep compaction
Vibro-compaction
very deep compaction
Proctor Compaction Tests
laboratory method of experimentally determining the optimal moisture content at which a given soil type will become most dense and achieve its maximum dry density
Standard Proctor Test Method
- cylindrical 1-litre mould (H=12.7cm and D=10cm)
- standard hammer (M = 2.5kg; dropping height 300cm)
- build 3 equal-volume layers and apply 27 blows per layer: compaction energy = 600kJ/m^3
Modified Proctor Test Method
- cylindrical 1-litre mould (H=12.7cm and D = 10cm)
- standard hammer (M = 2.5kg; dropping height 450cm)
- build 5 equal-volume layers and apply 25 blows per layer: compaction energy = 2700 kJ/m^3
Compaction
densification of soil by expulsion of air and the rearrangement of particles
Benefits of compaction
- increases strength
- reduces permeability
- decreases compressibility (settlement)
Effects of compactive effort
- increase the dry unit weight (density)
- decrease the optimum water content
Effects of gradation
(uniformity coefficient): poorly graded and gap graded granular soils are easy to compact
effective stress, unsaturated soils
σ’ = (σ - Ua) + X(Ua - Uw)
where X depends on the degree of saturation
Ua = pore air pressure
Uw = pore water pressure
C(u)
Coefficient of Uniformity
- C(u) less than or equal to 10 “uniformly graded”
- C(u) greater than 10 “well graded
C(z)
Coefficient of Curvature
- C(z) between1 and 3 = “well graded”
- C(z) less than 1 or greater than 3 = “gap graded”
Clay size
less than 0.002 mm
Silt size
0.002 to 0.06 mm
amount of fines for “coarse” soil
less than 35%
amount of fines for “fine” soil
more than 35%
very dense
D(r) more than 85%
dense
D(r) 65 - 85%
med. dense
D(r) 35-65%
loose
D(r) 15 - 35%
very loose
D(r) less than 15%
Dense soils have superior engineering properties
- higher stiffness (less deformability
- higher strength (improved stability)
