2 MSE Wall as a Gravity Wall An MSE wall is designed as a gravity wall. A gravity wall supports the retained soil using its self weight applied over a wide base at the bottom of the wall. Narrow, light weight walls do not function well as gravity walls.Reinforced soil slopes (RSS) are also gravity structures. However, the slope of the face makes some of the failure modes considered for walls highly unlikely.This is a Power point slide inserted in the notes view. This description was placed in the file outside the printable area of the motes pagesAn MSE wall is a gravity wall.Speaking PointsGravity walls work because of their weightAn RSS is also a gravity structureSlide ControlNone.Adult Learning TechniquesNoneBackground InformationNotesHeavy walls are good
3 External Stability Bearing resistance Settlement Sliding Overturning As design engineers, we calculate a factor of safety for each of these assumed failure modes. The factor of safety is the ratio of resisting forces to driving forces. If the resisting forces are just equal to the driving forces, then the factor of safety is equal to 1.0, which is not acceptable.As an Inspector, you should be aware of how certain changes during construction of a wall can either increase driving forces or reduce resisting forces, thus resulting in a decrease in the factor of safety of the wall (as compared to what the Design Engineer intended).Bearing resistanceSettlementSlidingOverturningOverall StabilityThis is a Power point slide inserted in the notes view. This description was placed in the file outside the printable area of the motes pagesSpeaking PointsFive potential failure mechanisms: bearing capacity, settlement, sliding, overturning, overall stability.Slide ControlNone.Adult Learning TechniquesThese will be discussed in subsequent slides.Background InformationOverall stability is often referred to as global stability or deep seated failure/stability. Overall stability is the term currently used in AASHTO LRFD specifications.
4 External Stability Bearing Speaking Points A bearing failure is the failure of the soil beneath the wall due to the applied weight of the wall. It is a function of the bearing pressure applied to the soil and the properties of the soil beneath the wall.Bearing resistance is typically not checked for RSS designs.This is a Power point slide inserted in the notes view. This description was placed in the file outside the printable area of the motes pagesSpeaking PointsThis is a bearing capacity failure.Note that the wall is a rigid body and remains intactBearing resistance is typically not checked for RSSSlide ControlWall moves down and generates failure through soil on click..Adult Learning TechniquesNoneBackground InformationNotesNone.Bearing
5 Bearing Failure Speaking Points MSE wall constructed on fill. This is a project in Texas for which the MSE wall was constructed on fill.Speaking PointsMSE wall constructed on fill.Slide ControlNone.Adult Learning TechniquesNoneBackground InformationNotesThis is a project in Texas for which the MSE wall was constructed on fill.
6 External Stability Settlement Speaking Points Settlement results from compression of the soil beneath the wall and the retained soil. Note that the wall itself may comprise only a small portion of the load applied to the soil.Settlement of the wall as a unit is generally not harmful as it results in little distress to the wall elements and no visible signs of a problem. However, if one portion of the wall settles more than another portion of the wall there is visible distress.MSE walls can accommodate significant settlements without damage. RSS can accommodate even more. In many cases the settlement may be more of a problem to structures or roadways supported by the MSE wall or RSS than it is to the supporting structure itself.This is a Power point slide inserted in the notes view. This description was placed in the file outside the printable area of the motes pagesSpeaking PointsThis is a settlement failure.Note that the wall and backfill settle as a unit.Note that the reinforced zone may be only a small portion of the load contributing to settlement.Note that settlement as a unit is generally not a serious problem but that if different portions of the wall settle different amounts it can be a problem.Slide ControlWall and retained soil moves down on click..Adult Learning TechniquesNoneBackground InformationNotesNone.Settlement
7 Flexibility of Wall Systems DefinitionTYPE OF FACINGLIMIT OFDIFFERENTIAL SETTLEMENTCIP Gravity Wall1/5002 In. in 100 Ft.Welded Wire Facing1/502 Ft. in 100 Ft.5’ x 5’ Panels with ¾” Joints1/1001 Ft. in 100 Ft.5’ x 10’ Panels with ¾” Joints1/2006 In. in 100 Ft.Modular Block WallsFull Height Facing PanelsDifferential settlement is the relative vertical movement of one point on the wall to another point on the wall. It is usually expressed as a ratio of that difference in vertical movement to the horizontal distance between the two points under consideration.Differential settlement causes distortion and damage to both traditional CIP walls and MSE walls. However, most wall systems are much more tolerant of differential settlement than are rigid concrete gravity or semi gravity walls. Thus CIP walls will typically require more competent foundations than MSE walls.Speaking PointsCompare the settlement tolerance of an MSE wall system to a CIP. This is why CIP walls typically require more competent foundation materials than MSE walls.Note that the tolerance for some MSE wall facing types is identical to that for a CIP wall.Note the extreme difference between CIP walls and welded wire facing walls. (a factor of 10)Slide ControlNone.Adult Learning TechniquesNotesMake sure to define (via a sketch at the flip chart) what is meant by differential settlement
8 External Stability Sliding Speaking Points This is a sliding failure. A sliding failure involves horizontal movement of the wall along the foundation soil. It is a function of the horizontal forces acting on the wall and the friction developed between the wall and the foundation soils.For RSS, the sliding stability nay not be specifically evaluated. However, it is included as part of the overall stability evaluation.This is a Power point slide inserted in the notes view. This description was placed in the file outside the printable area of the motes pagesSpeaking PointsThis is a sliding failure.Note that wall remains as a rigid body.Slide ControlWall slides on click.Adult Learning TechniquesNoneBackground InformationNotesNone.Sliding
9 Sliding Failure Speaking Points This project is from Mississippi, A geomembrane was placed at the bottom of the wall presumably to limit water migration to the underlying swelling clays. The geomembrane is a well-defined planar surface whose interface shear strength (with the adjacent soil) is relatively low.Speaking PointsGeomembrane at wall bottom was to limit water migration to swelling clays.Slide ControlNone.Adult Learning TechniquesNoneBackground InformationNotesThis project is from Mississippi, A geomembrane was placed at the bottom of the wall presumably to limit water migration to the underlying swelling clays. The geomembrane is a well-defined planar surface whose interface shear strength (with the adjacent soil) is relatively low.
10 External Stability Overturning Speaking Points An overturning failure involves rotation of the wall about a point near the toe of the wall. It is a function of the weight of the wall and the magnitude of the overturning forces.Do to the slope of a RSS, an overturning failure will never be critical to the design.This is a Power point slide inserted in the notes view. This description was placed in the file outside the printable area of the motes pagesSpeaking PointsThis is an overturning failure.Note the wall is assumed to acted as a rigid body though the flexibility of the wall system would not permit this in practice.Overturning is not critical for RSS due to the slope of an RSS.Slide ControlWall rotates on click.Adult Learning TechniquesNoneBackground InformationNotesInform students that overturning is not a viable concept for an MSE/RSS structure because it would require the whole mass to act as a rigid block and rotate about its toe. We use this notion to evaluate the design of the wall.Overturning
11 External Stability Overall Stability Speaking Points Overall stability failure involves failure of the entire soil mass behind and beneath the wall. It is controlled by the strength of the soil along the failure plane and the geometry of the wall (height, width, etc.).Overall stability is sometimes called global stability, slope stability, or deep seated failure.Overall stability is the primary external failure mechanism evaluated for RSS.Overall stability is often not evaluated in designs performed by suppliers. This is because evaluation of overall stability requires more extensive knowledge of the subsurface conditions than is provided in typical contracts. Overall stability evaluation may be performed by the owner or the owner may provide additional information to the supplier to permit them to evaluate the overall stability. Regardless of who performs the overall stability evaluation, it is an essential part of the design for an MSE wall or RSS.This is a Power point slide inserted in the notes view. This description was placed in the file outside the printable area of the motes pagesSpeaking PointsThis is a overall stability failureAlso known as global stability, slope stability or deep seated failure.Note that wall remains intact and acts as one rigid bodyThis is the primary failure mode evaluated for RSS.This failure mode is often not evaluated in designs performed by suppliers.Slide ControlWall and soil mass rotates on clickAdult Learning TechniquesNoneBackground InformationNotesNone.Overall Stability
12 Forces Surcharge Earth Pressure Water Pressure Earthquake DefinitionEarth pressure: the horizontal force applied to the wall due to the weight of the retained soil and the fact that the soil can not support itself. This force will always be present.Water pressure: The horizontal force applied to the back of the wall due to the weight of the water since water has no strength and can not support itself.Surcharge: horizontal extra load applied to the wall due to the weight of objects or soil above the top of the wall. This force is transmitted through the retained soil to the back of the wall.Earthquake loading: The force applied to the wall due to the inertia of the wall during an earthquakeEarth PressureSpeaking PointsThese are common types of loads that are considered in MSE wall design.Slide ControlWith each click, a different loading is shown.Adult Learning TechniquesNone.Background InformationNoneNotesRemind students that earth pressures from retained fill are always there. These forces are simply the result of “gravity”.Water PressureEarthquake
13 Forces Back Slope Surcharge Speaking Points Surcharge on a wall. An example of A RSS being placed above an MSE wall. The additional weight of the RSS acts as a surcharge to the MSE wall below.Back Slope SurchargeSpeaking PointsSurcharge on a wall.Slide ControlNone.Adult Learning TechniquesBackground InformationNone
14 Forces Surcharge Speaking Points Surcharges can be temporary as in vehicle loads or permanent as in loads from supported buildings.SurchargeSpeaking PointsSurcharges on a wall can be temporary (live loads) or permanent (dead loads).Slide ControlNone.Adult Learning TechniquesBackground InformationNone
15 Drainage If the wall backfill “fills” with water …. Improper consideration of drainage and improper construction of drainage elements is a significant cause of many wall and slope failures. Water effects the wall in many ways.Saturated soil weighs more and can have lower strength than dry soil which can increase the loads on the wall.Water moving through the soil tends to cause settlement of the soil and can erode fine soil grains from the wall backfill.A static water table adds horizontal water pressure to the wall and adds water pressure to the bottom of the wall (buoyancy). Both combine to decrease the factor of safety for sliding, overturning, and overall stability.Drainage systems incorporated into wall and slope designs are intended to prevent the accumulation of water and safely discharge it around the wall or slope.If the wall backfill “fills” with water ….Weight of soil increasesShear strength of backfill decreasesPressures on the wall increaseGreater tendency for fill to settlePotential for movement of soils as water exits wall or slope.Buoyancy needs to be consideredAppropriate drainage systems are critical !!Speaking PointsDrainage is perhaps the most important design element of a MSE wall and RSS.Slide ControlBullets enter one at a time.Adult Learning TechniquesNone.Background InformationNoneNotesThe Instructor should use the flip chart to sketch an MSE wall and show a sketch of a backslope drain to show how subsurface drainage is intercepted and conveyed away from wall backfill. Also note the illustrations on following pages of drains for MSE and RSS.
16 Drainage System Installation Installation of a drainage system using perforated pipe, gravel and prefabricated drainsSpeaking PointsInstallation of a drainage system using perforated pipe, gravel, and prefabricated drains.Slide ControlNone.Adult Learning TechniquesBackground InformationNone
20 Instructor Guide Design Concepts Slide controlKey MessageIndicate where we are within the courseBackground InformationInteractivityNotesThis is a Power point slide inserted in the notes view. This description was placed in the file outside the printable area of the motes pagesTopic 2Internal Stability Design of MSE WallsLesson Duration45 MinutesInstructor MaterialsLaptop Computer with RC RemoteLCD ProjectorGeogrid samplesParticipant MaterialsParticipant WorkbookPencilMaterials for demonstration 2-2
21 What is a Failure Surface ?? Critical failure surface is at locus of maximum tensile forces (Tmax) in each reinforcement layer. For design, we assume the location of this failure surface. The assumed failure surface is a function of the type of reinforcement (e.g., geosynthetic or metal) and the geometry (height) of the wall or slope.The zone behind the failure surface is termed the resistant zone. Horizontal forces from the backfill which push against the wall are carried in friction by the reinforcement in the resisting zone.This is a Power point slide inserted in the notes view. This description was placed in the file outside the printable area of the motes pagesGeosynthetic Reinforced WallMetal Reinforced WallPotentialFailureSurfaceActiveZoneResistantZoneHSpeaking PointsWith this and next slide, will show students what is meant by a failure surface and it how it used for MSE designPoint out resistance zone and active zone.Slide ControlNone.Adult Learning TechniquesNoneBackground InformationSoil ReinforcementReinforcedSoil MassL
22 How do Reinforcements Hold Up the Wall ? Reinforcements are used to support the wall through the generation of resistant forces in the “resistant zone”. The zone in front of this, the “active zone” is assumed to behave as a wedge of soil which wants to fall away from the reinforcements.If the tensile forces developed in the reinforcements become too large, the reinforcements can break in tension or pull out.“Active” ForceResistant Length, LeActive Length, LaSpeaking PointsReinforcements hold up the wall by developing friction and tension resistance in the resisting zone.Slide ControlNone.Adult Learning TechniquesNoneBackground InformationSoilReinforcementL
23 Internal Stability Reinforcement Failure Modes Internal failure of an MSE wall can occur in three different ways:1-The tensile forces in the reinforcement become larger than the pullout resistance (the force required to pull the reinforcement out of the soil mass). Can lead to large movements and possible collapse.2-The tensile forces in the reinforcements become so large that the reinforcements elongate or break, leading to large movements and possibly collapse of the structure3-The forces imparted at the connection of the reinforcement to the facing are so large that the connection fails or breaksInternal stability is used to size the reinforcements to preclude internal failure. The process includes determining the maximum developed tensile forces, their location along a slip surface, and the resistance provided by the reinforcements both in pullout capacity and tensile strength.Bullet slide indicating reinforcement failure modes.Pull outReinforcement tensile strengthConnectionSpeaking PointsDemonstrate the various failure modes for soil reinforcement.Internal stability checks are used to size the reinforcement so that the wall behaves a a gravity wallSlide ControlNone.Adult Learning TechniquesInvite the class to participate in the demonstration. Pass out the 2“ wide strips of paper towel.Background InformationExperiment with different paper types, width, and length to make sure the outcome is as planned before performing in class.NotesDo not use writing paper as it is too strong and smooth. 2” wide strips allow you to grab the entire width of the strip with your thumb and forefinger yet will tear out a discrete hole with the paper clip or pencil.
24 Internal Stability Reinforcement Failure Modes – CORROSION/DEGRADATION STEEL REINFORCEMENTArea of steel is increased to account for the anticipated corrosion loss over the design lifeGEOSYNTHETIC REINFORCEMENTUltimate tensile strength increased to account for strength loss due to installation damage, creep, and durability
25 Design Checks Class Exercise Compare the internal stability of the Design MSE wall and the As-Built MSE WallSpeaking PointsBy comparing a baseline design to a poorly implemented as-built design, the students should recognize how specific changes can affect internal stability.Slide ControlNone.Adult Learning TechniquesProvide the students with the cross section of an MSE wall (as designed) and the cross section of an as-built MSE wall. For this exercise, let the students know that absolutely no inspection was performed on this job. By comparing the as-built to the design, the students will identify differences which increase internal stability and which decrease internal stabilityBackground InformationNoneThe internal stability of this wall seems OK!
26 Design H = 20 ft 4 ft L = 20 ft Section Continuous Geogrid ReinforcementH = 20 ftL = 20 ft21SectionBackfillMedium sand with 15% (max) non-plastic fines at95% compactionSpacing18”Chimney Drain4 ft3 ft3Turf reinforcement mat, topsoil, and seedFull size drawing appears on the following page.Speaking PointsThis is the Design.Note that reinforcement length is measured from the back of the wall face per FHWA design guidelines.Slide ControlNone.Adult Learning TechniquesBackground InformationNone
27 As-Built H = 25 ft L = 20 ft Plan Section L = 15 ft 2 ft 1 1.5 BackfillMedium sand with 35% plastic fines at 95% compactionSpacing 36”GeogridReinforcementNote: Geogrid Coverage = 50%2 ft2Full size drawing appears on the following page.Speaking PointsThis is the As-BuiltFull size drawings are in the participants workbook.Let participants know they have 5 minutes to come up with differencesSlide ControlNone.Adult Learning TechniquesAsk for differences and write them on the flip chart (See differences noted below).Go through each difference and indicate if it increases or decreases internal stabilityNotesDifferences in the As-Built and Design include:1 - Length of reinforcement2 - Horizontal coverage of reinforcement3 - Water table elevation4 - Height of wall5 - Slope of backfill over wall6 - Type of backfill material7 - Type of drainage
28 Durability MSE Walls Corrosion of steel reinforcement All walls are subject to deterioration over time. The difference between CIP walls and MSE walls are the specific durability issue that are most important.For CIP walls, deterioration mechanisms consist of corrosion of the steel reinforcement which results in spalling of the concrete from the exposed surfaces of the wall. This mechanism is well understood and methods for designing to prevent such deterioration have long been established.For MSE walls, the primary durability issue of concern is loss of reinforcement strength over time. The rate and causes of strength loss in MSE wall reinforcements is not as well understood as the deterioration of reinforced concrete.MSE WallsCorrosion of steel reinforcementDegradation of geosynthetic reinforcementDurability of the facingSpeaking PointsAll wall types must deal with durability.For CIP the concern is reinforced concrete durability.For MSE the concern is primarily the durability of the reinforcement and secondarily the durability of the facingSlide ControlNoneAdult Learning TechniquesNone.Background Information
29 MSE Facing Durability Speaking Points The relatively weak facing on MSE walls is more susceptible to damage from unintended loads. However, an MSE wall may be better able to sustain that damage without collapse.If a CIP wall sustained the kind of damage shown in the slide, it would likely have failed completely. In this case the reinforced soil mass supported the beam load without the facing.Speaking Points…MSE walls may be better able to sustain damage to the facing without collapse.Slide ControlNone.Adult Learning TechniquesBackground Information