48 terms

Soil Mechanics

Chapters 7-12
soils permeable due to existence of interconnected voids through which water can flow from high energy points to low energy points
hydraulic gradient
i = ∆h/L
Darcy's Law
discharge velocity of water through saturated soils
v = ki k is hydraulic conductivity (coefficient of permeability)
seepage velocity
velocity of water through void space
constant-head test
laboratory determination of hydraulic conductivity where the difference of head between inlet and outlet remains constant
falling-head test
laboratory determination of hydraulic conductivity where water from a standpipe flows through a soil. head difference and time are recorded at t1 and t2
hydraulic conductivity and void ratio for granular soils
increases linearly
3 methods to estimate in situ hydraulic conductivity of compacted clay soils
boutwell permeameter, constant-head borehole permeameter, porous probes
boutwell permeameter
hole is drilled and casing placed in it. casing filled with water and falling-head permeability test is conducted, hole is deepened, repeat
constant-head borehole permeameter
constant head is maintained by supplying water, rate of flow q is measured
porous probes
driven into soil, constant or falling-head permeability test is performed
accuracy of k determined in lab depends on 6 variables
temperature of fluid, viscosity of fluid, trapped air bubbles present in soil specimen, degree of saturation of the soil specimen, migration of fines during testing, and duplication of field conditions in the lab
laboratory consolidation tests
hydraulic conductivity of saturated cohesive soils can be determined
flow line
line along which a water particle will travel from upstream to downstream side in the permeable soil medium
equipotential line
line along which the potential head at all points is equal
flow nets
can be used to determine the uplift pressure at the base of a hydraulic structure
stress in saturated soil without seepage
stress = Hgamma w + (HA - H)gamma sat
stress = effective stress + pore pressure
sigma prime
effective stress (intergranular stress) sum of vertical components of all intergranular contact forces over a unit gross cross-sectional area
neutral stress or pore water pressure
FS against heaving
submerged weight of soil in heave zone per unit length of sheet pile / uplifting force caused by seepage on the same volume of soil
effective stress in partially saturated soil
consists of intergranular, pore air, and pore water pressures
capillary rise in soils
continuous void spaces in soil can behave as bundles of capillary tubes of variable cross section
effective stress in soil size
present in granular soils, in fine grained soils the intergranular contact may not be there physically because the clay particles are surrounded by tightly held water film
elastic settlement (immediate settlement)
caused by elastic deformation of dry soil and of moist and saturated soils without any change in the moisture content
primary consolidation settlement
result of a volume change in saturated cohesive soils because of expulsion of the water that occupies the void space
secondary consolidation settlement
observed in saturated cohesive soils and is the result of the plastic adjustment of soil fabrics. additional form of compression that occurs at constant effective stress
one-dimensional consolidation test, where soils sample is placed under water between two porous stones loading it for 24 hours with different load amounts
overconsolidation ratio: preconsolidation pressure/present effective vertical pressure
normally consolidated clay - present effective overburden pressure is the maximum pressure that the soil was subjected to in the past
overconsolidated clay- present effective overburden pressure is less than that which the soil experienced in the past.
preconsolidation pressure
maximum effective past pressure experienced by overconsolidated clay
6 assumptions to find total settlement caused by primary consolidation
clay water system homogeneous, saturation complete, compressibility of water negligible, compressibility of soil grains negligible (but soil rearranges), flow of water is in one direction only, Darcy's Law is valid
methods for accelerating consolidation settlement
sand drains, prefabricated vertical drains in soft NC layers to achieve precompression before the consrutction of a desired foundation
highly compressible, NC clayey soil layers lie at a limited depth and large consolidation settlements are expected as a result, precompression minimizes postconsruction settlement
shear strength
internal resistance per unit area that the soil mass can offer to resist failure and sliding along an plane inside it
phi '
friction angle
laboratory tests to determine shear strength
direct shear test, triaxial test, direct simple shear test, plane strain triaxial test, torsional ring shear test
direct shear test
most commonly used test, soil forced to fail along horizontal plane, shear stress distribution over soil is not uniform
drained direct shear on sat sand and clay
keep the rate of loading slow enough so excess pore pressure is dissipated completely by drainage
triaxial shear test types
consolidated drained CD, consolidated undrained CU, unconsolidated undrained UU
CD test
saturated soil subjected to confining pressure (sigma 3), then pore pressure is drained and consolidation occurs, drainage line open and deviator stress applied to cause failure
CU test
most common type, consolidated by confining pressure (sigma 3) resulting in drainage, drainage line closed, deviator stress applied to cause failure
UU test
no drainage allowed during confining pressure, deviator stress applied to cause failure
unconfined compression test
UU for saturated clay specimens. confining pressure = 0. axial load applied rapidly to cause failure
vane shear test
undrained shear strength for very soft to medium cohesive soils (use correction factor)
sensitive clays
sensitive clay deposits must be properly identified so as not to cause failure due to vibratory loads