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Polytechnic University of Puerto Rico

Department of Civil Engineering

CE6110 – Assignment 8 GROUND ANCHORS AND EXCAVATION SUPPORT
1. Design a continuous sheet pile anchor wall to restrain the tie rod for Problem 1 in Assignment 7. For consistency, assume that the required tie rod force is 20 kips/ft. Provide both the required vertical dimension of the anchor wall (assuming the wall willextend down from the ground surface) and the required horizontal location of the anchor wall assuming there are no restrictions on where the wall can be placed. 2. Consider the excavation shown below. Compute the apparent earth pressure diagram for the excavation supports assuming the excavation will only be open for a short period of time. Assume that the strut spacing into the plane of the figureis 2 meters.

su  24 kPa

  20 kN
2.0 m 1.0 m

m3

6.0 m
2.0 m 1.0 m

Soft Silty Clay

su  30 kPa

  19 kN

4.0 m

m3

Clay

3. Compute the forces that each strut in Problem 2 must support based on the calculated apparent earth pressures. 4. As a part of value engineering efforts, the contractor is considering using tiebacks in lieu of struts to support the excavationin Problem 2. Compute the required anchorage length of tieback anchors that will support the excavation if placed at the same locations as the struts. Assume that the tieback anchors will be 8-inches in diameter and assume that the unit skin friction between the soil and the anchors is 0.5 times the appropriate shear strength for the anchor-soil interface. You should also assume that the anchorswill be inclined at 15 degrees from the horizontal (measured clockwise). 5. Compute the factor of safety against basal instability for the excavation considered in Problem 2.

Fall 2008

CE6110 – Design and Analysis of Earth Retaining Structures

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CE6110- Earth Structures

Assignment 8

CE6110-Fall 2008 Assignment 8, Problem 2: Braced Excavation - apparent earth pressurediagram General variables: H  6m susoft  24000 newton m spacing  2m B  4m gsoft  20000
2

suclay  30000

newton m
2

newton m
3

gclay  19000

newton m
3

all clay site so first check stability number to see if "soft" or "stiff" behavior: gsoft H  4.0 suclay

implies its between so use larger of "soft" and "stiff" apparent pressures

"Soft" clay apparentpressures: Ka  1  4 susoft gsoft H Ka  0.2

sapparentsoft  gsoft H Ka

sapparentsoft  24000.0 Pa

so apparent earth pressure diagram increases from zero at ground surface to 24kPa at 0.25H and then remains constant for remainder of depth "Stiff" clay apparent pressures: sapparentstiff  0.2 gsoft H sapparentstiff  24000.0 Pa

so apparent earth pressure diagram increases from zeroat ground surface to 24kPa at 0.25H, remains constant to depth of 0.75H, and then decreases back to zero at base of excavation

As a comparison, can look at Schabel's Apparent Earth Pressure diagram: convert height to feet: schnabel  25Hft H  19.7 ft Hft  19.7

schnabel  492.5 lb ft
2

sapparentschnabel  492.5

sapparentschnabel  23581Pa pretty close...as long as get unitscorrect!

I will use the "soft" clay diagram (won't make a lot of difference!): 0Pa      sapparentsoft  xsoft   1000   sapparentsoft    1000  
assign8-FA08soln.xmcd

 H    3 ysoft   H  4   0   
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0Pa    sapparentstiff  1000 xstiff    sapparentstiff 1000   0Pa 

        

 H     3H  4  ystiff     1H  4   0    CE6110- Earth Structures

Assignment 8

0Pa      sapparentschnabel    1000 xschnabel     sapparentschnabel  1000     0Pa  

 H     0.8H  yschnabel   0.2H     0 

6

Elevation above Excavation Base (m)

4

2

0

10

20

30

Apparent Pressure (kPa) soft clay stiff clay schnabel
Problem 3: Braced Excavation - strut forces apparent pressures are...