Soil Mechanics Lecture 8 Flashcards
What happens if the head at the bottom is higher than the head at the top (Decreasing hydraulic gradient)?
you get water uplift
What causes state liquefaction?
The uplift from water making the effective stresses so low that they start to become negative or 0
What is the value for the i that allows for liquefaction to happen?
i = 1
What is the condition of liquefaction?
sigma’_c = zgamma’ - iz*gamm_w = 0
sigma’_c = 0
What is darcy’s law?
based on the assumption that the relationship between velocity and hydraulic gradient is linear
v is proportional to k*i
k = hydraulic conductivity
or v = k*delta h/ l
v is the velocity of the water
How do we find permeability in the land?
- constant-head test (sands or gravel)
- falling-head test (fine-grained soil)
How does a falling-head test work?
- Soil sample is saturated
- standpipes filled with de-aried water
- allow water to flow through the sample untul the water in the standpipe drops to the lower limit
- measure the time taken for the water to flow from the upper to the lower limit
How does the constant head test work?
- water flows through a cylindrical soil sample under constnt pressure difference
- Test is carried out in the permeability cell
- the ratio of the cell diameter to the largest grain size should be higher than 12
- The soil sample the saturated
- the amount of water flowing through the soil column is measured at regular time intervals
- this is done in a similair way to how Q is measured in fluids, with the discharge from the lower resevoir being measured and then divided by time
What is the equation associated with the constant-head test?
k = Q/Aht
k: hydraulic conductivity Q: flow volume or outflow t: test duration h: constant head A: specimen area L: specimen length
What is the equation associated with the falling-head test?
k = 2.303(aL/at)log10(h1/h2)
h: function of time
k: hydraulic conductivity
t; test duration
A: specimen area
a: standpipe area
L: specimen length
What is differential settlement?
non-uniform movement of underlying soils
What can differenctial settlement cause?
Cracking in foundation, exterior and interior finishes.
In extreme cases it can also visibly warp buildings and result in ultimate failure
What is total settlement?
When all the soilsupporting a building settles, forcing the building to sink
What is the effect caused by the excessive expanion of soils called?
heave or swell
What structural damage happens as a result of foundation heave?
Broken pipes, cracked foundations…
What kind of failures can arise from soil liquefaction?
rotation, total limit
What is a foundation element?
An interfacing element that (properly) spreads the structural load to the ground
What are the basic principles of foundation design?
We need to know about the soils
and the magnitude, the nature and the direction of the applied loads
What parameters do we need to know about soils for foundation design?
Nature: soil type and composition
State: Fabric (density), stress (degree of saturation, hydrostatic vs. 1D flow), hydraulic conductivity, stiffness and strength (not in this course
What doe we need to know about the loads for designing foundations?
- Magnitude
- Geometrical feautres (shape, direction, point of application)
- type (static or dynamic, monotonic or cyclic)
- boundary conditions (rigid, flexible, 1D,2D,3D)
What is the limit state?
The point at which the design fails, it is to be avoided
How do we design for limit states?
Analysses of soil-structure interaction scenarios that may lead to excessive foundation settlement or collapse of the soil/structure
What doe we need to ensure when designing foundations (limit states in soils)?
- will be able to support the applied loads without moving excessively (servicibility state - SLS)
- will not collapse (also prevents structural collapse) (Ultimate limit state - ULS)
What is the objective when designing foundations?
keep settlement low.
There will be a settlement associated with a working load
the better we design our foundations, the more spread out the load will be, the less settlement will occur
What is the equation for allowable stress design?
Q_ult/ FS = Qa >=Qd Q_ult: ultimate load (leading to ULS) Qa: allowable load Qd design or working load FS - factor of safety
What are the usual values for factor of safety?
between 2 and 4
What is the equation for load and resistant factor design?
sum[(LF_i)Q_ni] ,= (RF)R_n
Q_n: nominal load
R_n: nominal resistance
the subscript i refers to different load types
What are the 2 typical types of foundation elements?
shallow foundation
deep foundation
What is the benefit of a shllow foundation?
It spreads the load out by distributing the forces under the ground, increases the bearing capacity
What is the benefit of a deep foundation?
The bearing pressure comes from friction and the bearing pressure from the soil at the bottom
What is required for a pile foundation?
A stiff/ strong layer of soil/ rock
What is the name of the shallow foundation element?
A strip footing
How are pile foundations put in thre ground?
Driven down by a large 'hammer' (concrete pile) Vibrated down (steel H pile - basically just an I beam in the ground)
How is a bored pile or drilled shaft installed?
- Soil is excavated with an auger to desired depth
- reinforcement is installed
- hole is filled with concrete
What is a mat foundation?
A mat is layed on top of the surface, with piles driven below, this is used to reduce the settlement of the mat
What are the effects of external loads on soil?
- stress changes may be followed by pore pressure changes, which can lead to changes in effective stress
- In practice, fabric changes are usually accounted for as changes in soil density and/ or water content
- Osil deformes as a result of stress changes, which leads to compression and consolidation
What assumptions do we tend to make when calculating the effect of external loads?
1) semi-infinite homogeneous, isotropic, linear-elastic soil mass
2) uniform contact stress distribution at the base of the rectangular area
What is the equation for the effect of vertical loads?
At the end of the lecture 8 notes
Key takeaways, principle of superposition can be applied
If C1
