Civil PE - Geotech
Ph on a retaining wall
(1/2) x gamma x H^2
Two main design considerations when designing shallow foundations
1. ensuring against bearing capacity failures 2. and excessive settlements
Minimum factor of safety for permanent slopes
1.5
Unit weight of concrete
150 pcf
Shallow Foundation
Depth (measured to bottom of footing) is shallower than width
a continuous footing is
a long footing supporting a continuous wall
a spread footing is used to support
a single column
another name for a continuous footing
a wall footing or a strip footing
another name for a spread footing
an individual column footing or an isolated footing
why does sand behave poorly in excavations?
because it lacks cohesion - when sand is loose and saturated a major loss in supporting material occurs
cohesive soils are
clay-type soils with angles of internal friction of close to zero
SG x density of water =
density of material
what are the most damaging sort of settlement?
differential settlements - those that are not uniform across the supported area
another name for buoyant unit weight
effective unit weight
clay is good in ___ but poor in ___
good in excavations, poor in foundations
two criteria for USCS and AASHTO soil classification systems
gradation and Atterberg limits
where do local shear failures occur?
in looser, more compressible soils at high bearing pressure
a cantilever footing
is a combined footing that supports a column and an exterior wall or column
a combined footing
is a footing carrying more than one column
what happens to soil usually before it fails in shear?
it deforms significantly
Atterburg has to do with..
liquid limit, plastic limit, placisticy index
qnet =
qult - gamma x depth of foundation
which has higher strength: clay or sand?
sand
which makes a better foundation material - sand or clay?
sand
granular soils are
sand and gravel-type soils with value of cohesion, c, of close to zero - also known as noncohesive soils
effective unit weight =
saturated unit weight - unit weight of water
clay is subject to long-term ___
settlement
Examples of a Shallow Foundation
spread footing, continous (or wall) foorings, mats
settlement
tendency of the soils to deform (density) under applied loads
bearing capacity
the ability of the soil to support the foundation loads without shear failure
what soil pressure is used to design the foundation?
the allowable bearing capacity
the area of the cone of influence
the area of the horizontal plane enclosed by the influence boundaries
earth pressure is
the force per unity area exerted by soil on the retaining wall
in consolidation, the depth of drainage is divided by two if..
the layer of soil is sandwiched between two porous layers like sand
the total mass =
the mass of the solids + the mass of the voids
allowable bearing capacity
the net pressure in excess of the overburden stress that will not cause shear failure or excess settlements aka: the net allowable bearing pressure or safe bearing pressure
the standard penetration test (SPT) N-value provides and indication of ...
the relative density of cohesion less soils
what happens in a general shear failure?
the soil resists an increased load until a sudden failure occurs
consolidation is
the squeezing out of water from the pores as the soil comes to equilibrium with the applied loads, volume change occurs
the wall translation (or strain) of passive state vs active state . . .
the strain required to achieve the passive state is at least twice that required to reach the active state
the effective pressure or effective stress is the difference between
the total pressure and the pore pressure
the volume of voids Vv=
the volume of air Va + the volume of water Vw
effective stress =
total stress - pore water pressure
unit weight =, density =
weight/vol, mass/vol -> the thing that makes it unit weight is gravity
what is the cohesion of ideal sand?
zero
