# geotech project

Post on 14-Apr-2018

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Introduction

The purpose of this project was to design a Mechanically Stabilized Earth Retaining Wall (MSE

wall) with square facial precast concrete blocks with one steel strip of circular cross-section

attached to each block. The reinforcement was assumed to be 60 ksi steel. The square panel sizes

used for comparison in the analysis: 1x1ft, 1.5x1.5, 2.5x2.5, 3x3, and 5x5.

The design of the wall was controlled by the internal stability requirements that need to be

satisfied for MSE walls. The factor of safety against yielding was checked in the bottom

reinforcement layer where the lateral stress is the highest and the resulting tensile force in the

reinforcement is the greatest because no point loads were used in the design and the lateral

pressure distribution simply increased linearly with depth. The factor of safety against pullout

was computed assuming a total length of reinforcement into the backfill. Various bar sizes were

used in the analysis. Initially, a constant bar diameter of 0.2 in was assumed in order to

determine the factor of safety for pullout and yielding. For the larger sized panels, the factor of

safety against pullout and yielding was lower than 1.5 because each panel contained only one barand the tributary area (svxsh) used to calculate the tensile force in the reinforcement was much

greater than the smaller panels. The bar size was later revised using the solver in Excel to find

the bar size at which the factors of safety against pullout and yielding were greater than or equal

to 1.5. The bar size corresponding to the 1x1ft panel was used in the final design because it gave

the lowest cost per square foot of face of the MSE wall.

Data

The following soil properties were assigned and used in the design:

Table 1. Soil Properties

H (ft) 15

25

i 16.66667

(kN/m^3) 15.6

(lb/ft^3) 99.2316

Ka 0.405859

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The factor of safety was calculated keeping the bar size constant resulting in the two black

curves. However, for some of the panels the bar size was not great enough to have an adequate

factor of safety, because the design required one strip per panel. The bar size needed to have a

high enough factor of safety was solved for in excel. The factor of safety for yielding controlled

for the larger panel size because the tributary area that each strip occupied was so large it

required a thicker bar in order to meet the internal stability requirement.

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For a square panel:

S = b^3/12

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The volume of steel need to have a factor of safety greater than or equal to 1.5 was calculated foreach size of panel. The minimum size is for B = 1 ft.

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MSE walls range from (\$19 to \$37 per ft2) of face, generally as function of height, size of

project and cost of select fill. (FHWA) Estimated total cost = \$28/sf

Percent of Total Estimate

Contractor 20 to 30%

Reinforcement 20 to 30% ------------------

Facing System 25 to 20% .25*28 = \$7.00/sf

Backfill w/ Placement 35 to 40% .35*28 = \$9.80/sf

Finish \$1/sf

The cost of reinforcement was changed in order to make comparisons for the design.

The weight of reinforcement required was calculated for each design and this was multiplied the

cost of steel at \$.9/lb.

The optimum design was chosen to be B = 1 ft since it resulted in the lowest cost per ft^2 of

face.

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Method of construction

Preparation of subgrade- Removal of unsuitable materials- If necessary use ground improvementPlacement of a leveling pad- Generally unreinforced concrete- Serves as a guide for facing panelsErection of first row of panels- First row must be shored up to maintain stability and allignmentPlacement and compaction of backfill to the first layer- Fill should be compacted to 95 to 100% of the AASHTO specified maximum density- Water should be added to reach the max density-

Moisture content less than optimum is recommended- Compaction should be performed a minimum distance of 3 ft from the edge of the wall- Closer to the wall a smaller hand compactor could be usedPlacement of first layer of reinforcement- Place and connect reinforcement perpendicular to the facing panelsPlacement of backfill over the reinforcement- Repeat the steps above for each layer

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Design Conclusion

The design consists of a 15 ft high retaining wall with 15 layers of reinforcement. The bar

size for each layer is constant and is equal to .136 in. The factor of safety against pullout and

yielding of the reinforcement were checked and were greater than or equal to 1.5. The length

of each layer of reinforcement was kept constant at 15 ft. The bottom layers do not requirethe same embedment length but they were all kept the same to make construction easier. The

proper measures need to be taken in order to have adequate drainage for the wall. This

ensures there is not a lot of hydrostatic pressure against the wall and that the water drains

away from the reinforcement. The methods of construction must be followed by compacting

each layer of reinforcement and connect them to the face panels. The costs computed may be

different depending on the commercial availability of materials and the area where the

project is to be completed.