Mechanically Stabilized Earth Wall Structures (MSE)

1. Mechanically Stabilized Earth Wall Structures (MSE) NZGS Auckland Branch Mini Symposium 30th November 2018

2. 1. Introduction 1. Facing elements Mechanically Stabilized Earth Wall Structures (MSE) 2. Compacted granular fill Provides the compressive and shear strength 3. Horizontal reinforcement Provide tensile strength for the MSE system MSE consists of three main components;

3. - Moist unit weight of soil H - Total height of wall Mechanically Stabilized Earth Wall Structures (MSE) ka – coefficient of active earth pressure q – uniform surcharge Pq 2. Coulomb (1776) Earth Pressure Theory Coulomb earth pressure equation for earth Pa 2 Eq (2a) Pa = X H X ka 1/2x Surcharge pressure equation q Eq (2b) Pq = X ka X H ka – coefficient of active earth pressure is determined by Coulomb or Rankine (1857) Theory

4. = angle of failure plane from the horizontal = angle of internal friction of soil = angle of batter of wall from horizontal = slope angle above wall = batter of wall measured from vertical = angle of friction at back of wall By Rankine By Coulomb Mechanically Stabilized Earth Wall Structures (MSE) Pa R Pa R = 90 deg 3. Determination of ka – coefficient of active earth Force diagram Force diagram

5. 4. Bearing Capacity at base the of MSE 4.1.1 Determination by Meyerhof 4.1.2 Ultimate Bearing Capacity of soil can be calculated from Meyerhof equation ( ) Qult e Qult Mechanically Stabilized Earth Wall Structures (MSE) e Qult >2.5 Eq 4.4 Factor of Safety for bearing= B = Total length of base e Limit of eccentricity < B/6 W X L Mr = sum of resisting moments Mr = q X L + Mo = sum of overturning moments Mo = q X H/2 + Pah x H/3 = calculated applied bearing pressure

6. 5. External Stability Analysis Active Earth Pressure for static 5.1. External sliding Mechanically Stabilized Earth Wall Structures (MSE) Live load surcharge is included as Resisting force for the determination of eccentricity and bearing pressure 5.2 Overturning However live load surcharge q is not included as a resisting force in the external sliding and overtuning analysis

7. A = Peak Ground Acceleration 6. External Stability Analysis for seismic (dynamic) for A < 0.45, Am = A(1.45 – A) for A > 0.45, Am = A Am = Peak Structural acceleration kh (ext) = Ext. Hori. Seismic coefficient , for min displacement kh (ext) = Am for “10A” inch displacement kh (ext) = Am/2 kh (int) = Am, Internal horizontal Seismic coefficient kv = Vertical Seismic coefficient, usually assumed zore Surcharge varies from zero to 50% Mechanically Stabilized Earth Wall Structures (MSE) Total earth thrust = dynamic + static based on Mononobe-Okabe (Kae) Total surcharge thrust = dynamic + static Seismic Parameters Inertia Dynamic Forces Seismic thrust (dynamic) on external

8. 7. Internal Stability Analysis 7.1 The tensile elements do not exceed working stress 7.2 The tensile elements have adequate connection capacity to facing elements 7.3 The tensile elements have adequate anchorage beyond the potential failure plane to hold the wedge of the soil in place 7.4 There is no potential surface where the mass can shear internally 7.5 There is stability of facing elements against shear, bulging and overturning Earth pressure resisted by top reinforcement Mechanically Stabilized Earth Wall Structures (MSE) Failure plane - Rankine Earth pressure resisted by second reinforcement Earth pressure resisted by third reinforcement Earth pressure resisted by forth reinforcement Internal Stress Distribution Earth pressure resisted by fifth reinforcement

9. 8. Internal Stability Analysis 8.1 Tension Level Calculation For inextensible reinforcement the lateral earth pressure coefficient varies with depth from ko to ka for Coherent Gravity Method. Mechanically Stabilized Earth Wall Structures (MSE) kr ka kr ka Tn Whereas for the Simplified Method the lateral earth pressure coefficient kr varies with depth as well as the type of inextensible reinforcement kr ka Tn Tributary area Tension in each layer of reinforcement Tn ka q* ka extensible ka = coefficient of active earth pressure (Coulomb or Rankine) For extensible reinforcement - Geosynthetic reinforcement Inextensible kr q*kr For Simplified Method Tn

10. 9. Internal Stability Analysis 9.2 Simplified Method 9.1 Coherent Gravity Method Zone of maximum stress or potential failure surface 0.3H ko 1.7 2.5 Mechanically Stabilized Earth Wall Structures (MSE) Inextensible Inextensible Metal Strip H/2 Extensible Metal bar mats & welded wire grids Extensible - Rankine ka Inextensible 6.0m Steel reinforcement Geosynthetic H/2 Steel reinforcement 1.2 Lateral earth pressure using Coherent Gravity Method Lateral earth pressure using Simplified Method

11. 10. Internal Stability Analysis 10.1. Strip loads Pv reinforcement Failure surface reinforcement Potential Failure surface for inextensible reinforcement Mechanically Stabilized Earth Wall Structures (MSE) Tn reinforcement Active Zone Tributary area Resistant Zone 10.2 Abutment – Simply supported Distribution of vertical stress 2V to 1H profile with depth q *kr Tn kr + Kr * Strip loads Internal Design for inextensible reinforcement of MSE

12. 11. Internal Stability analysis 11.1 True Bridge Abutment – Simply Supported = calculated applied bearing pressure at base of abutment Ph taken as minimum 5% of live load of Pv Limit eccentricity e< bf/6 and < 200kpa for Static Limit eccentricity e< bf/4 and < 335kpa for seismic F2 Mechanically Stabilized Earth Wall Structures (MSE) Sum of horizontal forces F = Ph + F1 + F2 (to determine the horizontal stress) M reinforcement Tn reinforcement F1 reinforcement Similarly there are limit of calculated bearing pressure specified by FHWA for foundation soil of MSE, as follows; Potential Failure surface for inextensible reinforcement The horizontal force is distributed into the MSE structure as an inverted triangle Limit eccentricity e< bf/6 and < 335kpa for Static Limit eccentricity e< bf/4 and < 718kpa for Static Foundation

13. 12.2 Semi Integral Abutment 12. True Bridge Abutment 12.1 Simply Supported As such the abutment is exerted by seismic thrust Mechanically Stabilized Earth Wall Structures (MSE) Approach slab – 6.0m Introduce a MSE wall on the bridge approach Simply supported true abutment need a large approach slab at least 6.0m in length, to prevent sliding of abutment due to seismic thrust Detail a gap between the MSE wall and the abutment to relieve the abutment from static and dynamic lateral forces. Simply supported where abutment retain approach fill & live load This reduces the bearing pressure exerted on the approach side of abutment and shield the abutment from seismic thrust Eccentricity and bearing pressure unlikely an issue Limit eccentricity e< B/6 and < 200kpa for Static

14. 13. Pullout Capacity of reinforcement 13.1 For Extensible reinforcement Pullout capacity = 2 X Le X X X F* Hov Mechanically Stabilized Earth Wall Structures (MSE) Potential failure surface for extensible based Rankine theory F* = interaction coefficient of the reinforcing 1 side 2 side AASHTO 2012 for Extensible & Inextensible Pull out diagram for Extensible 2 Moment = wl /8 2 Moment = w (Z2-Z1) /8 As the spacing of the reinforcement decreases the moment decreases as well This is one of the main requirement of GRS-IBS, limit the spacing of the reinforcement <300mm Stability of Facing

15. 14. Geosynthetic Reinforced Soil - Integrated Bridge System ( GRS-IBS ) Integrated Approach Requirement of GRS-IBS Beam Seat Mechanically Stabilized Earth Wall Structures (MSE) - Spacing of reinforcement Sv< 300mm - Minimum Ultimate strength of reinforcement 70kN/m (4800 lb/ft) i. For FS 3.5, Allowable strength of reinforcement 20kN/m (1371 lb/ft) ii. Strain of reinforcement < 2% (T@e<2%) < 300mm - Specify (dmax) maximum grain size of backfill - B=Minimum 1.8m or 0.3H Scour Protection If required Required reinforcement strength - Treq B=Minimum 1.8m or 0.3H Treq D=0.25B W = B + 0.25B

16. 15. Conclusion 1. The salient feature of MSE structures is the ability to accommodate settlement 2. Though the AASHTO code has specified an allowable of 1% maximum differential settlement, MSE tolerates much higher differential magnitude. As such, MSE structures are excellence option for retaining function. 3. The application of MSE to directly support the bridge, true bridge abutment, the strict adherence of the maximum allowable of 1% differential settlement is desirable to ensure that the serviceability requirement is not compromise. Mechanically Stabilized Earth Wall Structures (MSE) 5. As a guide, to exploit the self-weight of MSE for the dissipation of pore water pressure which consequently induced consolidation and settlement, the coefficient of consolidation of the soil shall not be less than /s. 6. The emergence of high strength low extensible geosynthetic has allowed accelerated bridge construction, GeosyntheticReinforced Soil - Integrated Bridge System ( GRS-IBS) to be realised. 4. Since a high percentage of total settlement is due to the self-weight of MSE, this phenomena is commonly exploited in the true bridge abutment structures as elevation losses can be compensated prior the construction of the abutment. 7. As MSE structures are economical, simple and environmentally friendly in construction, these structures provide a good solution for retaining as well as load supporting options when site conditions suit their application.

17. Acknowledgement and References Mechanically Stabilized Earth Wall Structures (MSE) Questions?