Preliminary Design of MSE and Steel Sheet Pile Retaining Walls

Mechanically Stabilized Earth Retaining Wall Design

A long, straight mechanically stabilized earth (MSE) retaining wall is to be constructed and you are asked to perform the preliminary design. The wall will have a height of H=12 m with a horizontal backfill and no surcharge loading will be allowed near the top of the wall.  The diagram provided below is a sample cross-section of the proposed wall.

The MSE wall unit is to be faced with precast concrete panels and will be reinforced with Tensar 80RE uniaxial geogrids that attach directly to the facing panels.  The geogrids have an ultimate tensile strength of Tult=34.8 kN/m of wall with reduction factor for installation damage RFID=1.07, reduction factor for creep RFCR=1.05, and reduction factor for durability (i.e. chem/bio degradation) RFD=RF_CBD=1.05.

 

The geogrids will be placed with 100% coverage (C_r=1.0) and experiments have determined the frictional interaction coefficient for the backfill and geogrids to be Ci=0.86.  Use a factor of safety for geogrid breakage, FSB=1.5 and factor of safety for geogrid pull-out, FSP=1.5. 

 

The backfill and wall material will be compacted well-graded sand and gravel with an effective drainage system keeping the water table well below the bottom of the wall.  The backfill soil has properties:  ?1=19 kN/m3,  ?’1=34°,  c’_1=0.

 

The in-situ foundation soil below the wall is a silty sand with properties:  ?2=17 kN/m3,  ?’2=29°,  c’_2=88 kPa.

 

a. You may copy the diagram provided below and modify it to include several more diagrams throughout your solution to illustrate the concepts and relationships.

b. Determine the maximum allowable vertical spacing of the geogrids, SV, and then choose an economical and practical spacing scheme.  Consider

 

(i) a single zone with all geogrids uniformly spaced, and

 

(ii) two zones with differing vertical spacing in upper and lower zones of the wall.

 

c. Determine the minimum required length of the geogrid reinforcing, L and then choose an economical and practical length scheme.  Consider

 

(i) a single zone with all geogrids having equal length, and

 

(ii) two zones with shorter geogrids in upper and lower zones of the wall.

 

Evaluate and correct for the external stability of the wall including

 

(i) overturning,

 

(ii) sliding, and

 

(iii) bearing capacity (use the improved bearing capacity factors).

 

Use this file as your worksheet to prepare you solution.  Add as many pages as necessary and use large, clear diagrams.  Include several sections, with headings, for the key steps ion the design process.

Determining Maximum Allowable Geogrid Spacing

 

When finished, convert to a PDF file to submit.

 

 

A long, straight anchored steel sheet pile retaining wall is to be constructed to form a water channel and you are asked to perform the preliminary design.  The wall will have a height of H=33 ft above the dredge line with a horizontal backfill and no surcharge loading will be allowed near the top of the wall.  The diagram provided below is a sample cross-section of the proposed retaining wall and water channel.
The water table will typically be maintained at the equilibrium level of H_1=6 ft shown in the diagram, with equal elevation on both sides of the wall.

 

The single row of anchor tie rods will be set at a depth of z_A=4 ft and horizontally spaced at s_H=5 ft on center along the length of the wall.
The sheet piles and tie rods will consist of mild steel with:  E=29,000 kip/in^2;??_allow=24 kip/in^2
The backfill and channel bottom is a loose sandy soil with properties:  

 

?^?=34°;?c^?=0;??=109 lb/ft^3 (above water table);??_sat=118 lb/ft^3

 

You may copy the diagram provided below and modify it to include several more diagrams throughout your design solution to illustrate the concepts and relationships.

 

a. Draw the earth pressure and water pressure diagrams for the sheet pile cross-section (i.e. FBD of the wall).  You may need several pressure diagrams to account for all forces acting on the wall.

 

b. Determine the minimum required theoretical and design depth of embedment required for stability (D_design=1.3D_t?eory) as well as the total length of sheet pile L=H+D

 

c. Determine the anchor force per unit length of wall, F_A and the minimum required diameter, d_min for each tie rod.

 

d. Determine the theoretical maximum bending moment in the sheet pile cross-section, M_max and choose an economical Z-section pile accordingly.

 

e. Perform at least one iteration of Rowe’s Moment Reduction Theory to determine if the theoretical M_max can be reduced and if a smaller section can be used.

f. Use this file as your worksheet to prepare you solution.  Add as many pages as necessary and use large, clear diagrams.  Include several sections, with headings, for the key steps ion the design process.

When finished, convert to a PDF file to submit.

 

Consider what would happen to the sheet-pile wall from Problem 2 if the water channel was rapidly drained.  In the case a such a rapid draw down, you may assume the water level behind the sheet-pile remains the same while the water level in the channel is lowered to the same elevation as the dredge line (i.e. ).

 

Repeat all the requirements from Problem 2 for this new situation.  Conclude by discussing the differences in your results for the two situations (anchor force, max bending moment etc.) and discuss, in general, the effect of rapid draw down of the water channel.

You may copy the diagram provided below and modify it to include several more diagrams throughout your design solution to illustrate the concepts and relationships.

 

Use this file as your worksheet to prepare you solution.  Add as many pages as necessary and use large, clear diagrams.  Include several sections, with headings, for the key steps ion the design process.

 

When finished, convert to a PDF file to submit.