GEO102Geotechnical Applications for Transportation ProjectsChapter 6: MSE Walls Part 1

Retaining WallsChapter 6: MSE Walls Part 1Types of MSE facing and soil reinforcing materialsModes of Failure External and Internal Stability Estimation of Lateral Earth PressureSite EvaluationBackfill Selection Criteria

KEY Objective

Describe different types of MSE walls commonly used in transportation construction and discuss potential modes of failure

Learning ObjectivesUpon completion of this lecture, the learner will be able toIdentify the key components of a mechanically stabilized earth (MSE) wall systemDescribe the broad range of soil reinforcing materials and facing systems that are available for use in MSE wall construction Compare and contrast between internal and external stability requirements for MSE walls

Learning Objectives, contd.Identify site conditions that can have adverse impact on MSE wall performance Explain the significance of backfill gradation and corrosion potential in controlling the stability and longevity of MSE wall systems

ReferencesRetaining Wall Construction and Maintenance Recommendationshttp://www.dot.state.tx.us/services/bridge/retaining_walls.htm

Geotechnical Manual, Texas Department of Transportation, August 2006FHWA-NHI-00-043: MSE Walls and Reinforced Soil Slopes Design and Guidelines

Mechanically Stabilized Earth (MSE) Retaining Walls

MSE WallsLower cost of constructionLess design effortEasy to constructFast construction speedGood appearance

Mechanically Stabilized Earth (MSE) Retaining WallsFill Type Wall that is most widely used in transportation applications

Soil reinforcements

Mechanical Stabilization of Soil; The ConceptMechanical stabilization relies on reinforcing the soil backfill using inclusionsIn ancient times, straw, bamboo, and wire mesh have been used to reinforce soil for use in the construction of mud dwellings, dikes and erosion control structuresModern methods of soil reinforcing for retaining wall construction was pioneered by the French engineer, Henri Vidal in 1960s

MSE Retaining Walls; Historical DevelopmentHenri Vidals research led to the development of Reinforced Earth System that uses steel strips to reinforce soilReinforced Earth System; Precast concrete facing unit (21-24ft2); Galvanized ribbed steel stripsThe first retaining wall of this type in the US was built in 1972 on California State Highway 39The use of geosynthetic products in the US for soil reinforcement began around 1980; geogrid reinforced modular block walls now comprise a growing portion of the market

MSE Walls; Types of FacingSegmental precast concrete panels20-25ft2Min thickness, 5.5-inCruciform, square, hexagonal, diamondLarger precast concrete panels (up to 54ft2)

Precast Concrete Panelscruciformrectangular

Precast Concrete Panelshexagonal

MSE Walls; Architectural Finishes

MSE Walls; Types of FacingDry cast modular block wall units (MBW)Small, squat concrete units (1 SF used in highway applications)

Concrete (Modular) Block Walls

Gabion Faced WallsWire filled baskets

MSE Wire Faced WallsWire grid bent at the wall face

MSE Walls; Types of FacingMetallic FacingsOriginal ReCo, half cylindersLighter; may be used where access or handling is difficultGeosynthetic FacingGeotextile r/f looped around to form facingGeogrid used the same way; vegetation can grow through gridPostconstruction FacingShotcreting, cast-in-place concrete

Reinforcement MaterialTwo types of materialsMetallic reinforcements Typically mild steel (galvanized / epoxy coated)Strips; welded wire mesh Nonmetallic reinforcement Generally polymeric materials (polypropylene, polyethylene or polyester)

Mechanically Stabilized Earth (MSE) Retaining WallsGalvanized Steel Strips

Reinforcement MaterialGalvanized Wire Mesh

Reinforcement MaterialGalvanized Wire Mesh

Reinforcement MaterialGeogrids

Mechanically Stabilized Earth (MSE) Retaining Walls

MSE Retaining Walls; Potential Modes of Failure

External Stability RequirementsSlidingOverturningBearing Capacity FailureDeep Seated Shear FailureInternal Stability RequirementsReinforcement Pull-outReinforcement Rupture

External Stability; Failure due to Sliding

Original Ground SurfaceFinished GradeRetained FillReinforced Earth Mass

External Stability; Failure due to Overturning

Original Ground SurfaceFinished GradeRetained FillReinforced Earth Mass

External Stability; Failure due to Overturning

Original Ground SurfaceRetained FillReinforced Earth MassFinished Grade

External Stability; Bearing Capacity FailureOriginal Ground SurfaceRetained FillReinforced Earth MassFinished Grade

Original Ground SurfaceRetained FillReinforced Earth MassFinished GradeExternal Stability; Deep Seated Shear Failure

External Stability; Deep Seated Shear Failure

Internal Stability; Breakage of Reinforcement

Original Ground SurfaceRetained FillReinforced Earth MassFinished Grade

Internal Stability; Breakage of Reinforcement

Internal Stability; Breakage of Reinforcement

Internal Stability; Pullout of Reinforcement

Original Ground SurfaceRetained FillReinforced Earth MassFinished Grade

MSE Retaining Walls; Other Performance Requirements

Wall DeformationLateral displacement, H H/250 (inextensible reinforcement)Vertical displacement (settlement), V ... MSE walls with precast concrete panels can typically accommodate differential settlements in the order of 1/100

MSE Retaining Walls; Other Performance Requirements

Corrosion of ReinforcementCorrosivity of backfill soilUse of de-icing salts, marine structures

MSE Retaining Walls; Other Performance Requirements

Backfill WashoutAesthetics

Satisfactory Performance must be Accomplished through ..Careful Site EvaluationProper Design and Material SelectionGood ConstructionProper Maintenance

Site EvaluationTopographyCut, Cut/Fill or FillVolume of excavation needed in side-hill situations; temporary shoring neededSlopes in front of the wall?Geologic FeaturesRock outcrops

Site EvaluationSubsurface Soil ConditionsWeak soil conditions (low TCP blow count)?Concerns: Bearing capacity failure/Deep seated shear failureWall height 20 blows, then detailed investigation not warrantedTall walls (>20ft), weak soils, slopes in front wall require more detailed analysis!Compressible soils; large differential settlement!

Site EvaluationSite accessibility & Available ROWFor work force and construction equipmentExisting and Proposed Utility Lines and Drainage StructuresExisting utility lines: realignment necessary?Surface/Subsurface DrainageSurface drainage patternsSubsurface seepageLikelihood of inundation

Subsurface Exploration ProgramBorings spaced on 200 foot centers for wall taller than 10 feetBoring spacing increased up to 300 feet where walls are less than 10 feet tallDallas District Type C borings for fill walls are 25 feet below base of wall or 5 feet into rock

Example 6.1Site Evaluation GIVEN:Cross-sectional profile and TCP data pertaining to site where an MSE wall is to be built

REQD:Evaluate the site based on data provided and recommend appropriate course of action

Example 6.1Site Evaluation

1333 psfTCP DataTCP Shear Strength667 psf467 psf467 psf1067 psf467 psf667 psf1067 psf

Global Stability Analysis based on TCP Undrained Shear StrengthC = 1200 psf, f = 0oC = 667 psf, f = 0oC = 467 psf, f = 0oC = 750 psf, f = 0oC = 2000 psf, f = 34oFS = 0.97Based on the analysis the wall would not be stable.

Rotate the vane slowly until failure occurs.Record the applied torque to cause failure.

PEAK UNDRAINED SHEAR STRENGTH

Vane Shear Test

4(6)3(6)3(6)4(6)5(6)5(6)8(6)10(6)10(6)10(6)TCP DataVane Shear Test DepthClay, tan (CL)Clay, brown (CL)Clay, brown (CL)Clay, tan (CL)151013Vane Shear Test ResultsSu(actual) = m * Su (raw)m = 1 0.5 Log (PI(%)/20))

Global Stability Analysis based on Vane Shear StrengthC = 1300 psf, f = 0oC = 1141 psf, f = 0oC = 934 psf, f = 0oC = 750 psf, f = 0oC = 2000 psf, f = 34oFS = 1.49Based on the analysis the wall would be stable.

Material SelectionSelect Backfill

Select BackfillThe select backfill is the most important component of an MSE wall The required type of select backfill will be indicated in the contract plansThe backfill must be sampled, tested and approved prior to beginning wall constructionSampling and testing should continue throughout wall construction

Select BackfillSelect backfill is checked for several propertiesGradation controls strength, drainage, constructabilitySoundness makes certain that the gradation will remain through compaction and service lifeChemical testing controls corrosion of galvanized steel reinforcements

Select Backfill GradationsType A is the premium, coarser material for permanent MSE walls Type B is the default for permanent MSEType C is the default for temporary MSE walls, and is not used in permanent wallsType D is a clean rock, for use in walls that will be flooded

2004 Backfill SpecificationsType D required for areas of wall subject to inundation or below the 100-yr flood elevationDefaultTemporary WallsFor enhanced performance

MSE Select BackfillTesting for soundness is at the option of TxDOTIf the backfill appears to contain shale, caliche, or other soft particles, request the test

Backfill Material; Corrosion PotentialFor permanent MSE walls with galvanized metallic earth reinforcementsor

MSE Select BackfillShould be sampled and approved prior to the beginning of constructionShould be sampled and tested regularly, in accordance with Construction Contract Administration ManualIf you see a change in the backfill, dont hesitate to sample and test!Remember, the select backfill is the most important component of the MSE wall!

Example 6.2Backfill Selection GIVEN:Gradation curves for 3 candidate backfill material sources, Source 1, Source 2 and Source 3

REQD:Determine whether they meet requirements for Type A, B, C or D MSE Wall select backfillWhat other information would you require about each material before you can make a final determination about their suitability

Example 6.2Backfill Selection

Chapter 6 Feedback

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