Lecture 12- Earth Pressure and Sturucture Rankine Theory [1]
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Transcript of Lecture 12- Earth Pressure and Sturucture Rankine Theory [1]
Earth pressure and structure (Rankine theory)
Dr. Md Mizanur Rahman
School of Natural and Built EnvironmentsCIVE 3008-Lecture 12: Earth pressure and structure (Rankine theory)
Acknowledge: Dr. D. A. CameronPrevious course coordinator
Earth pressures on retaining structures
School of Natural and Built EnvironmentsCIVE 3008-Lecture 12: Earth pressure and structure (Rankine theory)
Earth pressures on retaining structures
School of Natural and Built EnvironmentsCIVE 3008-Lecture 12: Earth pressure and structure (Rankine theory)
Earth pressures on retaining structures
School of Natural and Built EnvironmentsCIVE 3008-Lecture 12: Earth pressure and structure (Rankine theory)
Earth pressures on retaining structures
School of Natural and Built EnvironmentsCIVE 3008-Lecture 12: Earth pressure and structure (Rankine theory)
Two Methods
School of Natural and Built EnvironmentsCIVE 3008-Lecture 12: Earth pressure and structure (Rankine theory)
- to estimate the earth pressures on structures
1. RankinePlausible stress states
2. CoulombPlausible failure mechanisms
Relative merits of approaches?
Rankine earth pressures
School of Natural and Built EnvironmentsCIVE 3008-Lecture 12: Earth pressure and structure (Rankine theory)
A lower bound estimate
Effective horizontal stress,
H = Kz
where, K = earth pressure coefficient
z = effective vertical stress
Earth pressure states (retaining walls)
School of Natural and Built EnvironmentsCIVE 3008-Lecture 12: Earth pressure and structure (Rankine theory)
“At rest” wall is not moving, so the soilan intermediate state
PassiveActive
Both are failure states
Earth pressure at rest
School of Natural and Built EnvironmentsCIVE 3008-Lecture 12: Earth pressure and structure (Rankine theory)
“AT REST” PRESSURE
The intermediate state
K = Ko = fn(soil type, density, OCR)
The soil is unable to move laterally - can’t expand, OR contract
e.g soil confined in a large body of soil
Active state (stress relaxation)
School of Natural and Built EnvironmentsCIVE 3008-Lecture 12: Earth pressure and structure (Rankine theory)
Normal stress
Shear stress
3f 3o 1
At rest stateActive state
failure envelope
Passive state (stress intensification)
School of Natural and Built EnvironmentsCIVE 3008-Lecture 12: Earth pressure and structure (Rankine theory)
Normal stress
Shear stress
1f3o 1
Passive stateAt rest state
failure envelope
3f
All three states
School of Natural and Built EnvironmentsCIVE 3008-Lecture 12: Earth pressure and structure (Rankine theory)
Normal stress
Shear stress
1o
Active state
Passive state
At Rest failure envelope
Note:Active state: stress relaxation
Passive state: stress intensification
The 3 States (consider a vertical retaining wall)
School of Natural and Built EnvironmentsCIVE 3008-Lecture 12: Earth pressure and structure (Rankine theory)
H/z
Wall movement
Kp
Ka
NB: Passive needs LARGE strains
KO
Equations for Rankine States
School of Natural and Built EnvironmentsCIVE 3008-Lecture 12: Earth pressure and structure (Rankine theory)
(Can be derived from Geometry of Mohr’s circles)
For ACTIVE STATE
Case 1A: c = 0
H = Kaz
and Ka =
[Ka max 0.333 for loose sand]
)sin(1
)sin(1
Active state
School of Natural and Built EnvironmentsCIVE 3008-Lecture 12: Earth pressure and structure (Rankine theory)
Normal stress
Shea
r str
ess
NB: Active state = a failure state
Failure, f , nf (
1 - 3 )/2
(1 + 3)/2
Active state
School of Natural and Built EnvironmentsCIVE 3008-Lecture 12: Earth pressure and structure (Rankine theory)
31
31sin
)sin1()sin1( 31
)sin1(
)sin1(
1
3
aK
From the geometry,
Active state (with cohesion)
School of Natural and Built EnvironmentsCIVE 3008-Lecture 12: Earth pressure and structure (Rankine theory)
Case 2A: c 0
H = Kaz - 2cKa
Notes:
• the 2nd term is a constant!
• z = (z) + z
i.e. stress due to self weight + extra due to surface load
Can soil undergo tension?
School of Natural and Built EnvironmentsCIVE 3008-Lecture 12: Earth pressure and structure (Rankine theory)
If z = 0, then H 0
Now if z = z,
At what depth will H = 0?
H = Kaz - 2cKa
This depth is called the depth of
cracking, zc, & defines the potential
tension zone
z
Depth of Cracking
School of Natural and Built EnvironmentsCIVE 3008-Lecture 12: Earth pressure and structure (Rankine theory)
By definition:
At zc, H = 0
Therefore,
H = 0 = Ka zc - 2cKa
Therefore,
zc = [2cKa][ Ka ]
Or a
cKγ'
c2z
z
The tension zone
School of Natural and Built EnvironmentsCIVE 3008-Lecture 12: Earth pressure and structure (Rankine theory)
The pulling power of cohesive soil is ignored in calculations of pressures behind retaining walls over the depth zc because:
- tension is unsustainable
i.e. short term only!
However, no compressive pressures exist in this zone = a dead zone
Evidence of a tension zone
School of Natural and Built EnvironmentsCIVE 3008-Lecture 12: Earth pressure and structure (Rankine theory)
How can unsupported, vertical-sided trenches be cut to metres depth in clay soils?
What depth is possible?
What happens if it rains?
Warning: people laying pipes have died in collapsed trenches!
OH&S???
Summary of active state
School of Natural and Built EnvironmentsCIVE 3008-Lecture 12: Earth pressure and structure (Rankine theory)
• Stresses relaxedcommon retaining wall situation
• Ka = (1 – sin)/(1+ sin )clean sand, Ka 0.33 usually
• Theoretical tension or crack zone from cohesive strength (c)
may be applied to slope stability
Passive state
School of Natural and Built EnvironmentsCIVE 3008-Lecture 12: Earth pressure and structure (Rankine theory)
Again, from Geometry of Mohr’s circles
Case 1P: c = 0
H = Kpz
and Kp =
[Kp min 3 for loose sand]
aK
1
)sin(1
)sin(1
Passive state (with cohesion)
School of Natural and Built EnvironmentsCIVE 3008-Lecture 12: Earth pressure and structure (Rankine theory)
Case 2P: c 0
H = Kpz + 2cKp
Note:
1. the 2nd term provides greater constant passive pressure component
Orientation of Failure Planes
School of Natural and Built EnvironmentsCIVE 3008-Lecture 12: Earth pressure and structure (Rankine theory)
From Mohr’s circlesActive state:
(45 + /2) to horizontalPassive state:
(45 - /2) to horizontal
Orientation of Failure Planes
School of Natural and Built EnvironmentsCIVE 3008-Lecture 12: Earth pressure and structure (Rankine theory)
Sliding surfaces?
ACTIVE
PASSIVE
School of Natural and Built EnvironmentsCIVE 3008-Lecture 12: Earth pressure and structure (Rankine theory)
Tension is ignored!
ACTIVE-2cKa
zc
+2cKp PASSIVE
Typical Lateral stresses, c 0
The Influence of Pore Water
School of Natural and Built EnvironmentsCIVE 3008-Lecture 12: Earth pressure and structure (Rankine theory)
Steady state (and seepage) pressures add to lateral stresses on walls
Should Ka be applied to the pore water pressure?
NO WAY!
Hydrostatic means K = 1
The Influence of Pore Water
School of Natural and Built EnvironmentsCIVE 3008-Lecture 12: Earth pressure and structure (Rankine theory)
No Water
c = 0
H = H = Ka z
Water
c = 0
H = Ka z u = wz
+
TOTAL LATERAL STRESS
The Influence of Pore Water
School of Natural and Built EnvironmentsCIVE 3008-Lecture 12: Earth pressure and structure (Rankine theory)
In the previous example
uniform soil, no surface load and with or without a Water Table at
ground level,
Almost twice the total lateral pressure is
experienced with the high Water Table
Effective lateral stresses are halved, BUT full pwp
is exerted!
Importance of Drainage for Retaining Walls (Drains, Filters & Weep holes)
School of Natural and Built EnvironmentsCIVE 3008-Lecture 12: Earth pressure and structure (Rankine theory)
Weep holes
Granular zone or geofabric drain
Examples
School of Natural and Built EnvironmentsCIVE 3008-Lecture 12: Earth pressure and structure (Rankine theory)
From Whitlow - modified
Find the total resultant thrust and its point of action behind vertical-backed retaining walls of height, 12 m, resulting from earth and water pressures given the following situations
1. Surface horizontal; no surcharge; single soil layer, c = 0, = 30, = 18 kN/m3
2. Surface horizontal; uniform surcharge of 10 kPa; single soil layer: c = 0, = 30, = 18 kN/m3
3. Surface horizontal; no surcharge; two soil layer:
0-5 m depth, c = 0, = 30, = 18 kN/m3
> 5 m depth, c = 0, = 36, = 20 kN/m3
Examples
School of Natural and Built EnvironmentsCIVE 3008-Lecture 12: Earth pressure and structure (Rankine theory)
Thrust = lateral pressure x area
= average pressure x height over which it acts, per m length of wall
9 m
40 kPa
360 kN
20 kPa
360 kN
60 kPa3 m
Examples
School of Natural and Built EnvironmentsCIVE 3008-Lecture 12: Earth pressure and structure (Rankine theory)
Resultant Thrust = Resultant, Pa = (all thrusts)
Point of action found by summing moments about a point and dividing by Pa
10 kPa12
m
360 kN
60 kPa4 m
120 kN
X mLocation of
resultant force
Examples
School of Natural and Built EnvironmentsCIVE 3008-Lecture 12: Earth pressure and structure (Rankine theory)
AnswerResultant, Pa = (120 + 360)
Pa = 480 kN per m length of wall
Point of action found by summing moments about the base and dividing by Pa
120 x 6 + 360 x 4 = Pa x X
X = (720 +1440)/480 = 4.5 m
Examples
School of Natural and Built EnvironmentsCIVE 3008-Lecture 12: Earth pressure and structure (Rankine theory)
Q1. c = 0, = 30, = 18 kN/m3
Ka = 0.333
72 kPa
12 m
432 kN
8 m
Examples
School of Natural and Built EnvironmentsCIVE 3008-Lecture 12: Earth pressure and structure (Rankine theory)
Q2. As for 1 but 10 kPa surcharge
Ka = 0.3333.33 + 72 kPa
432 kN
8 m
40 kN
10 kPa
ANSWER 472 kN/m, 4.17 m
Examples
School of Natural and Built EnvironmentsCIVE 3008-Lecture 12: Earth pressure and structure (Rankine theory)
Q3. Two granular soils
Ka1 = 0.333
12 m
5 m
Ka2 = 0.26
At z = 5 m, z = 90 kPaAt z = 12 m, z = 140 kPa
ANSWER
366 kN/m, 4.15 m
Example
School of Natural and Built EnvironmentsCIVE 3008-Lecture 12: Earth pressure and structure (Rankine theory)
From Whitlow, cont’d6. Surface horizontal; no surcharge; single soil layer, cu = 45 kPa, u = 0,
= 18 kN/m3
7. Surface horizontal; no surcharge; single soil layer, c = 15 kPa, = 20, = 18 kN/m3
11. Surface horizontal; no surcharge; two soil layer,
0-4 m depth, c = 0, = 30, = 19.6 kN/m3
> 4 m depth, c = 25 kPa, = 15, = 18.2 kN/m3
ANSWERSQ6 441 kN/m, 2.33 mQ7 408 kN/m, 3.21 mQ11 458 kN/m, 3.61 m
Example
School of Natural and Built EnvironmentsCIVE 3008-Lecture 12: Earth pressure and structure (Rankine theory)
Q7. c = 15 kPa, = 20, = 18 kN/m3
(0.49x216 - 300.49 kPa
OR 84.8 kPa
12 m
408 kN
zc
Ka = 0.49
zc = 2.38 m 21 = 0.49x18xzcANSWER: 408 kN/m, 3.21 m
Limitations of Rankine
School of Natural and Built EnvironmentsCIVE 3008-Lecture 12: Earth pressure and structure (Rankine theory)
1. Vertical backs of walls only
2. Backfill surface must be regular
– a solution exists for a sloping backfill, provided slope angle, <
– BUT pressures act parallel to the slope - theoretically wrong!
3. Backfill loads / surcharge effects approximated
4. Wall friction ignored!
– friction is beneficial
Summary
School of Natural and Built EnvironmentsCIVE 3008-Lecture 12: Earth pressure and structure (Rankine theory)
1) Earth pressures are needed for design of retaining walls & excavations
2) Three major states: at rest, active and passive
─ Last 2 are failure states
3) Earth pressure coefficients are based on effective stresses
4) Water pressures are important
− total lateral stresses
5) Cohesion leads to potential cracked zone for Active state
Excavation Bracing
School of Natural and Built EnvironmentsCIVE 3008-Lecture 12: Earth pressure and structure (Rankine theory)
Trench
Support systems:soldier beams (vertical)& shuttering between them or steel sheeting
Strut
Possiblefailure shape
Steelsheeting
Wale
PLAN
Example
School of Natural and Built EnvironmentsCIVE 3008-Lecture 12: Earth pressure and structure (Rankine theory)
Design of Bracing
School of Natural and Built EnvironmentsCIVE 3008-Lecture 12: Earth pressure and structure (Rankine theory)
Earth pressures are not simple
- propping forces from struts
- progressive construction
Empirical design earth pressures
- struts designed for thrust
Refer to Notes for guidance
Information Only