manual 02 en cantilever-wall
TRANSCRIPT
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Engineering manual No. 2
Updated: 02/2016
1
Design of Cantilever wall
Program: Cantilever wall
File: Demo_manual_02.guz
In this chapter, the design of cantilever wall and its overall analysis is described.
Assignment:
Design a cantilever wall with a height of 4,0 m and analyze it by EN 1997-1 (EC 7-1, Design
approach 1). The terrain behind the structure is horizontal. The ground water table is 2,0 meters
deep. Behind the wall acts a strip surcharge with a length of 5,0 meters and with a magnitude of 10
kN/m2. The foundation soil consists of MS –Sandy silt, stiff consistency, 8,0r S , allowable bearing
capacity is 175 kPa. The soil behind the wall will consist of S-F – Sand with trace of fines, medium
dense soil. The cantilever wall will be made of reinforced concrete of class C 20/25.
Scheme of the cantilever wall - Assignment
Solution:
For solving this problem, we will use the GEO5 program, Cantilever wall. In this text, we will
explain solving this example step by step.
In the frame “Settings” click on “Select” and then choose analysis setting No. 3 – “Standard –
EN 1997 – DA1”.
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Dialog window “Settings list”
In the frame “Geometry” choose the wall shape and enter its dimensions.
Frame “Geometry”
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In the frame “Material” enter the material of the wall.
Frame “Material” – Input of material characteristics of the structure
Then, define the parameters of soil by clicking “Add” in the frame “Soils”. Wall stem
is normally analyzed for pressure at rest. For pressure at rest analysis, select “Cohesionless”.
Dialog window “Add new soils”
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Note: The magnitude of active pressure depends also on the friction between the structure and
soil. The friction angle depends on the material of construction and the angle of internal soil friction –
normally entered in the interval ef 3231
Table with the soil parameters
Soil
(Soil classification)
Profile
m
Unit weight
3mkN
Angle of
internal
friction
ef
Cohesion
of soil
kPacef
Angle of friction
structure – soil
S-F – Sand with trace of
fines, medium dense soil0,0 – 4,0 17,5 28,0 0,0 18,5
MS – Sandy silt, stiff
consistency, 8,0r S from 4,0 18,0 26,5 30,0 17,5
In the frame “Terrain” choose the horizontal terrain shape.
Frame “Terrain”
The ground water table is at a depth of 2,0 meters. In the frame “Water” select the type of
water close to the structure and its parameters.
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Frame “Water”
In the next frame define “Surcharge”. Here, select permanent and strip surcharge
on the terrain acting as a dead load.
Dialog window “New surcharge”
In the frame “FF resistance” select the terrain shape in front of the wall and then define
other parameters of resistance on the front face.
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Frame “FF resistance”
Note: In this case, we do not consider the resistance on the front face, so the results will be
conservative. The FF resistance depends on the quality of soil and allowable displacement of the
structure. We can consider pressure at rest for the original soil, or well compacted soil. It is possible to
consider the passive pressure if displacement of structure is allowed. (for more information, see HELP
– F1)
Then, in the frame “Stage settings” choose the type of design situation. In this case,
it will be permanent. Also choose the pressure acting on the wall. In our case, we will choose active
pressure, as the wall can move.
Frame “Stage settings”
Note: Wall stem is dimensioned always on earth pressure at rest, i.e., the wall can´t be moved.
The possibility of evaluating the stem and the wall of the active pressure is considered only in
exceptional cases - such as the effects of the earthquake (seismic design situation with partial
coefficient equals 1.0).
Now, open up the frame “Verification”, where you analyze the results of overturning and slip
of the cantilever wall.
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Frame “Verification”
Note: The button “In detail” in the right section of the screen opens a dialog window with
detailed information about the analysis results.
Analysis results:
The verification of slip is not satisfactory, utilization of structure is
Overturning: 52,7 % 75,10917,208 ovr res M M [kNm/m] SATISFACTORY
Slip: 124,5 % 83,8174,65
act res H H [kN/m] NOT OK
Now we have several possibilities how to improve the design. For example, we can:
Use better soil behind the wall
Anchor the base
Increase the friction by bowing the footing bottom
Anchor the stem
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These changes would be economically and technologically complicated, so choose the easiest
alternative. The most efficient way is to change the shape of the wall and introduce a wall jump.
Change of the design: change of the geometry of the wall
Return to the frame “Geometry” and change the shape of the cant ilever wall. For increasing
the resistance against slip we introduce a base jump.
Frame “Geometry” (Changing dimensions of cantilever wall)
Note: A base jump is usually analyzed as an inclined footing bottom. If the influence of the base
jump is considered as front face resistance, then the program analyses it with a straight footing
bottom, but FF resistance of the construction is analyzed to the depth of the down part of the base
jump (More info in HELP – F1)
Then analyze the newly designed construction for overturning and slip.
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Frame “Verification”
Now, the overturning and slip of the wall are both satisfactory (Utilization: 49.4 % and 64.9%)
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Then, in the frame “Bearing capacity”, perform an analysis for design bearing capacity
of the foundation soil 175 kPa.
Frame “Bearing capacity”
Note: In this case, we analyze the bearing capacity of the foundation soil as an input value,
which we can get from geological survey, resp. from some standards. These values are normally
highly conservative, so it is generally better to analyze the bearing capacity of the foundation soil in
the program Spread footing that takes into account other influences like inclination of load, depth of
foundation etc.
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Next, in the frame “Dimensioning” chose wall stem check. Design the main reinforcement
into the stem – 10 pcs. Ø 12 mm, which satisfies in point of bearing capacity and all design principles.
Frame “Dimensioning”
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Then, open up the frame “Stability” and analyze the overall stability of the wall. In our case,
we will use the method “Bishop” , which result in conservative results. Perform the analysis
with optimization of circular slip surface and then leave the program by clicking “OK”.
Results or pictures will be shown in the report of analysis in the program Cantilever wall.
“Slope stability” program – frame “Analysis”
Conclusion Result of analysis bearing capacity:
Overturning: 49,4 % 94,10735,218 ovr res M M [kNm/m] SATISFACTORY
Slip: 64,9 % 38,6426,99 act res H H [kN/m] SATISFACTORY
Bearing capacity: 80,2 % 31,140175 d R [kPa] SATISFACTORY
Wall stem check: 80,4 % 25,14592,169 Ed Rd M M [kN·m] SATISFACTORY
Overall stability: 39,2 % Method – Bishop (optimization) SATISFACTORY
This cantilever wall is SATISFACTORY.