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Geotechnics > Soil Mechanics Lecture 8 > Flashcards

Soil Mechanics Lecture 8 Flashcards

1
Q

What happens if the head at the bottom is higher than the head at the top (Decreasing hydraulic gradient)?

A

you get water uplift

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2
Q

What causes state liquefaction?

A

The uplift from water making the effective stresses so low that they start to become negative or 0

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3
Q

What is the value for the i that allows for liquefaction to happen?

A

i = 1

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4
Q

What is the condition of liquefaction?

A

sigma’_c = zgamma’ - iz*gamm_w = 0

sigma’_c = 0

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5
Q

What is darcy’s law?

A

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

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6
Q

How do we find permeability in the land?

A
  • constant-head test (sands or gravel)

- falling-head test (fine-grained soil)

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7
Q

How does a falling-head test work?

A
  • 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
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8
Q

How does the constant head test work?

A
  • 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
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9
Q

What is the equation associated with the constant-head test?

A

k = Q/Aht

k: hydraulic conductivity
Q: flow volume or outflow
t: test duration
h: constant head
A: specimen area
L: specimen length
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10
Q

What is the equation associated with the falling-head test?

A

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

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11
Q

What is differential settlement?

A

non-uniform movement of underlying soils

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12
Q

What can differenctial settlement cause?

A

Cracking in foundation, exterior and interior finishes.

In extreme cases it can also visibly warp buildings and result in ultimate failure

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13
Q

What is total settlement?

A

When all the soilsupporting a building settles, forcing the building to sink

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14
Q

What is the effect caused by the excessive expanion of soils called?

A

heave or swell

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15
Q

What structural damage happens as a result of foundation heave?

A

Broken pipes, cracked foundations…

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16
Q

What kind of failures can arise from soil liquefaction?

A

rotation, total limit

17
Q

What is a foundation element?

A

An interfacing element that (properly) spreads the structural load to the ground

18
Q

What are the basic principles of foundation design?

A

We need to know about the soils

and the magnitude, the nature and the direction of the applied loads

19
Q

What parameters do we need to know about soils for foundation design?

A

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

20
Q

What doe we need to know about the loads for designing foundations?

A
  • Magnitude
  • Geometrical feautres (shape, direction, point of application)
  • type (static or dynamic, monotonic or cyclic)
  • boundary conditions (rigid, flexible, 1D,2D,3D)
21
Q

What is the limit state?

A

The point at which the design fails, it is to be avoided

22
Q

How do we design for limit states?

A

Analysses of soil-structure interaction scenarios that may lead to excessive foundation settlement or collapse of the soil/structure

23
Q

What doe we need to ensure when designing foundations (limit states in soils)?

A
  1. will be able to support the applied loads without moving excessively (servicibility state - SLS)
  2. will not collapse (also prevents structural collapse) (Ultimate limit state - ULS)
24
Q

What is the objective when designing foundations?

A

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

25
Q

What is the equation for allowable stress design?

A 
Q_ult/ FS = Qa >=Qd
Q_ult: ultimate load (leading to ULS)
Qa: allowable load
Qd design or working load
FS - factor of safety
26
Q

What are the usual values for factor of safety?

A

between 2 and 4

27
Q

What is the equation for load and resistant factor design?

A

sum[(LF_i)Q_ni] ,= (RF)R_n
Q_n: nominal load
R_n: nominal resistance
the subscript i refers to different load types

28
Q

What are the 2 typical types of foundation elements?

A

shallow foundation

deep foundation

29
Q

What is the benefit of a shllow foundation?

A

It spreads the load out by distributing the forces under the ground, increases the bearing capacity

30
Q

What is the benefit of a deep foundation?

A

The bearing pressure comes from friction and the bearing pressure from the soil at the bottom

31
Q

What is required for a pile foundation?

A

A stiff/ strong layer of soil/ rock

32
Q

What is the name of the shallow foundation element?

A

A strip footing

33
Q

How are pile foundations put in thre ground?

A 
Driven down by a large 'hammer' (concrete pile)
Vibrated down (steel H pile - basically just an I beam in the ground)
34
Q

How is a bored pile or drilled shaft installed?

A
  1. Soil is excavated with an auger to desired depth
  2. reinforcement is installed
  3. hole is filled with concrete
35
Q

What is a mat foundation?

A

A mat is layed on top of the surface, with piles driven below, this is used to reduce the settlement of the mat

36
Q

What are the effects of external loads on soil?

A
  • 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
37
Q

What assumptions do we tend to make when calculating the effect of external loads?

A

1) semi-infinite homogeneous, isotropic, linear-elastic soil mass
2) uniform contact stress distribution at the base of the rectangular area

38
Q

What is the equation for the effect of vertical loads?

A

At the end of the lecture 8 notes

Key takeaways, principle of superposition can be applied
If C1

Geotechnics (12 decks)

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