Soil stresses Barnes Chapter 4

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Soil stresses Barnes Chapter 4

• D. A Cameron
• Intro to Soils 2006

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Content

• Dead weight stresses
• Pore water pressures
• steady state
• no flow
• water table
• Effective stress
• Horizontal earth pressure at rest, Ko

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VERTICAL STRESSES ? ?z in a soil mass, are due
to the dead weight of the soil
?z
z
?x
z
x
y
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VERTICAL STRESSES ? ?z force from weight of
prism above soil (area of soil in x-y plane)
?z
z
?x
z
?z
x
y
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The dead weight stresses are termed
TOTAL soil stresses
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PORE WATER PRESSURES, ? u in a soil mass with
a water table, are due to the dead weight of
water u ?wzw
GL
Saturated zone
z
u
u
z
x
y
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How do pore water pressures and total stresses
interact to affect soil behaviour?

• Concept of EFFECTIVE stress
• Terzaghi 1923
• PWP reduces the stress felt by the soil in a
  saturated soil system (with no air voids)

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EFFECTIVE STRESS

• Air is very compressible
• Both H2O and soil (the solids) are incompressible
• A SATURATED SOIL
• Stress on a soil element is taken by
  inter-particle contacts the pore water
• Volume change can only occur if water drains out
  of the soil, i.e.

?V ?Vw
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Box of Soil
P
N
P
After Craig Soil Mechanics
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Inter-particle contact
Interparticle force
OR ? ?? u
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B UNSATURATED SOIL

• The line of inter-particle contacts intercepts
  water and air in the voids

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? proportion of water in the voids along
the inter-particle line
?A ??Auw?A ua(1-?)A ? ?? ua - ?(ua -
uw)
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Soil Suction (ua - uw)
The difference between the pore air and the pore
water pressure affinity for water due to

• Clay mineral charges
• electro-chemical forces
• Capillarity
• pore size
• Osmosis
• salts

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Diameter of tube, d
Height of rise fn(d)
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The meniscus
ua 0?
T liquid tension
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Dead weight soil stress- total vertical stress
80 kPa
152 kPa
?v
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Dead weight soil stress- effective vertical
stress
0 m
? 16 kN/m3
2 m
? 18 kN/m3
5 m
? 20 kN/m3
9 m
166 kPa
?v
u
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Effective Stress Distribution
0 m
? 16 kN/m3
2 m
? 18 kN/m3
5 m
? 20 kN/m3
9 m
?v? ?v - u
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Alternative approach effective unit weight, ??
? - ?w
0 m
?? 16 kN/m3
2 m
?? 8.2 kN/m3
5 m
?? 10.2 kN/m3
9 m
?v? ?v - u
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• HORIZONTAL STRESSES ?
• ??H in a soil mass,
• is not the same as the
• vertical stress, ???z

?z
Z

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An Earth Pressure State

• AT REST PRESSURE
• K Ko
• The soil is unable to move laterally
• - it cannot expand OR contract
• e.g. soil confined in a large body of soil
• no buildings, no cuttings

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NOTE

• The earth pressure coefficient is a ratio of
  EFFECTIVE soil stresses
• horizontal vertical stress
• SO,
• must take into account the pore water pressures
• ? ?? ? - u

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AT REST PRESSURE

• Ko fn(soil type, density, OCR)
• OCR overconsolidation ratio (explained in later
  course)

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Values of Ko?
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Example Barnes 4.1

• A river 5 m deep flows over a sand deposit.
• ?sat 18 kN/m3
• At a depth of 5 m below the river bed, determine
• Total vertical stress
• Pore water pressure
• Effective vertical stress
• Ans 139, 98 and 41 kPa
• Barnes 4.2
• What happens to these stresses if the water level
  falls 5 m?

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0 m
No soil, no stress no pwp
? 9.8 kN/m3
5 m
? 18 kN/m3
10 m
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Effective stress
0 m
? 9.8 kN/m3
5 m
0 kPa
? 18 kN/m3
10 m
41 kPa
??z
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Barnes 4.2 river is dry
0 m
??z is unchanged!
5 m
? 18 kN/m3
10 m
49 kPa