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    Term ProjectSpring 2015


    Shayan AminJohn ODonald

  • Purpose

    The purpose of this presentation is to demonstrate our knowledge of slope stability and stabilization methods as a product of this course

    Term project considers a hypothetical project to replace a failed culvert

    The focus will be on the components of the proposed design that will require stability evaluation

  • Project Background

    Hypothetical Large culvert washed out due to historic storm event, collapsing a

    two lane roadway closure and detours until replacement Replacements considered must support roadway, facilitate a

    major stream flow, and retain fill

    Image 1: Culvert washout on I-88, Delaware County, NY

  • Site Parameters

    Collapsed culvert was 20-7 x 13-2 SPPA 40-0 roadway typical section (2) 12-0 lanes and

    (2) 8-0 shoulders Limited ROW outside of roadway boundaries Stream invert 20-0 below proposed top of roadway

    elevation H&H analysis performed to determine required

    hydraulics Geotechnical investigation performed to determine soil


  • Soil Parameters

    Site soil parameters per geotechnical investigation

    , 120

    , 30

    , 0

    , 9


  • Replacement Options

    A. Precast Dual Cell Concrete Box Culvert with Cast-In-Place (CIP) Cantilever Wing Walls

    B. Geosynthetic Reinforced Soil Integrated Bridge System (GRS-IBS)

  • Option A

    Precast Dual Cell Concrete Box Culvert with Cast-In-Place (CIP) Cantilever Wing Walls (2) 12-0 x 12-0 box culverts, wall thickness 1-0 all sides 20-0 from top of roadway to box invert 7-0 earth fill over culvert CIP concrete cantilever retaining walls to retain backfill for

    roadway support Wall at each corner of culvert to be 30-0 long parallel to

    roadway Stepped footings will be used to save costs and materials,

    resulting in two different wall sections to be designed Tall retaining wall section Short retaining wall section

  • Option A - Plan


  • Option A - Elevation


  • Option A Wall Sections



  • Retaining Wall Design

    Assumption: The dual box culverts have been adequately designed to resist all load effects and have met all design criteria. The following will focus on the design of the retaining walls in regards to external stability. Internal stability will not be covered, i.e. reinforced concrete design

    Design Criteria: LRFD per AASHTO LRFD Bridge

    Design Specifications, 7th

    Coulomb Earth Pressure Theory

    Design Steps:

  • Proportioning the Wall

    Certain assumptions on the wall dimensions are made for preliminary design as a guide

    Only the tall wall design will be detailed here

    , 12"min 1 6

    , 2 0"min 3 0

    0.5 0.5 22 11 ; 0.7 0.7 22 15.4 14 6

    , 0.1 0.1 22 2.2 3 0

    , 0.1 0.1 22 2.2 1 6"

  • Check External Stability

    In order to be considered suitable, the wall must pass all external stability requirements for: SlidingOverturning Bearing CapacityGlobal Stability

    Preliminary Wall Dimensions

  • Design Criteria - LRFD

    Limit States evaluated: Strength I-a Strength I-b Service I

    General design equation:

    ; 1.0

  • Design Criteria - LRFD


    Load Factors:

    Resistance Factors:

    DC EV LS EHStrengthIa 0.90 1.00 1.75 1.50StrengthIb 1.25 1.35 1.75 1.50Service I 1.00 1.00 1.00 1.00




    Category Strength1a Strength1b ServiceISl iding(Friction) 0.8 0.8 0.8Sl iding(Pass ive Pressure) 0.5 0.5 0.65Bearing 0.45 0.45 1.00

  • Design Criteria Coulomb Theory

    Coulomb Theory accounts for friction at wall-soil interface

  • Load Calculations

  • Sliding Stability

    Passive pressure above toe conservatively neglected

  • Overturning/Eccentricity

  • Bearing Capacity

  • Global Stability

    Performed using GeoStudio Slope/W software Entry and Exit Range settings: 10 increments over range and 30 radius increments Spencers Method

    Rigorously satisfies static equilibrium by assuming that the resultant interslice force has a constant but unknown inclination (Abramson).

  • Option A External Stability Summary

    Sliding OK Overturning/Eccentricity OK Bearing Capacity OK Global Stability OK All stability criteria have been met and the wall

    design is satisfactory

  • GRS-IBS Design

    Geosynthetic Reinforced Soil Integrated Bridge System (GRS-IBS)

    The Federal Highway Administration (FHWA) specifies design considerations for GRS-IBS.

    A GRS-IBS supported structure consists of three main components: the reinforced soil foundation (RSF)

    the GRS abutments and wing walls

    and the integrated approach.

    FHWA Figure 2-1d


  • Important Design Considerations

    Seismic design considerations were not included within this analysis. According to FHWA, seismic is not

    required for any single span bridge regardless of seismic region.

    However, superstructure connections and minimum support lengths are required.

    For this design we are taking the concept of a GRS-IBS to support the bridge structure and treating it as a MSE wall for external analysis.

    Overall concept of GRS-IBS and MSE are the same, and the main focus for the design will be external stability of the MSE Mass.

  • Assumptions Made

    The spacing of the reinforcement (12 in or less) is a principal factor in the performance of GRS-IBS.

    A GRS mass is a composite material that is stabilized internally.

    Both the compacted soil and the reinforcement layers strain laterally together in response to vertical stress until they reach a failure condition.

    A GRS mass is not supported externally, and the facing system is not considered a structural element in design.

  • Assumptions Made (Cont.)

    The element of a GRS mass are frictionally connected to the reinforcement.

    Reinforcement creep is not a concern for the sustained loads. Therefore, individual reduction factors for reinforcement creep are not necessary.

    The external stability is assumed to have a wall friction angle of 0, and Rankine Theory is used for the design.

    MSE Wall compound stability is a concern and additional measures need to be considered for flood events.

  • MSE Wall External Stability

    Reinforcement Length, L initially equal to 0.7H = 14ft

  • External Stability Sliding Check

    Factor of Safety equivalent to the MSE Mass Resisting forces divided by the driving forces.

  • External Stability Overturning Check

    Because a GRS mass is relatively ductile and free of tensile strength, overturning about the toe is not a possible due to earth pressures at the back of the mass or loading on its top (FHWA 2011).

    The integrated superstructure functions as a strut to resist overturning, and each GRS mass has a reinforced integration zone above its heel, also resisting the overturning mode of failure (FHWA 2011).

  • External Stability Bearing Pressure Check

    Additional weight of the bridge superstructure was not considered to cause bearing capacity failure check

  • External Stability Global Stability Check

    SlopeW Analysis performed to check global stability, FOS = 1.677

    Spencer Method used for design with slip surface entry and exit and 10 increments over the surface range.

  • GRS-IBS Plan View

  • GRS-IBS Wall Profile

  • MSE Wall Facing

    A Modular Block Wall was selected to utilize locally available CMU blocks. Allows for consist geotextile spacing every course of block equivalent to 8, less than 12.

    The upper 2.0 feet of facing block elements are susceptible to movement due to the reduced weight. To prevent displacement, the hollow cores of these upper block courses shall be filled with concrete fill and pinned together with No. 4 epoxy coated rebar embedded with a 2-inch minimum cover.

  • Drainage System

    Proper drainage is essential in MSE wall design.

    Proper flow of water through the reinforced material will reduce hydrostatic pressures acting within the wall and reduce tension forces

    An internal drainage system will be installed at the bottom face of the wall to capture and outlet trapped water

    #57 Coarse Aggregate will be used below the 100 year flood line

  • Internal Stability

    Not specified in this project design Modular Block Walls typically use

    geotextile reinforcement because they are easier compatibility with connection requirements

    Most common GRS-IBS MBW reinforcement is a biaxial, woven, polypropylene (PP) geotextile in the abutment.

    Any biaxial geosynthetic meeting the load requirements and soil conditions can be used in the abutment and the wing walls.

  • List of Advantages of GRS-IBS

    A faster bridge completion and traffic reopening (as little as five days)

    Jointless bridge system with no bump transition Reduced cost. The FHWA and U.S. states report that the

    GRS-IBS reduces project costs from 25 to 60%, depending on the application

    No special skills required for construction, minimum equipment needs

    Can be used in poor soil sites with standing water Smaller crew and increased safety Use of local