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Geosynthetic reinforced soil walls as integral bridge abutments

April 1st, 2011 / By: / Feature, Geogrids, Reinforcement

From ER-2010 / Earth Retention Conference #3

Introduction

The first use of geosynthetic reinforced soil walls for integral bridge abutment construction in North America occurred on the Greenville Southern Connector (Interstate 185) toll road in 1999. A second bridge with a longer span and higher loads was constructed in 2000 for the same project.

Each of these four bridge abutment walls was constructed more than 20ft (6m) high using modular concrete block wall (MCBW) facing units and geosynthetic reinforcement, with a silty fine to medium sand backfill around vertically driven steel H-pile foundation elements. While the piles were designed to carry the vertical live and dead bridge loads, the lateral loads, due to momentum, braking, and thermal movement, would be transferred through the integrally cast-in-place concrete abutment to the piles to the wall facing elements through the piles, located 3ft (1m) behind the MCBW facing and resisted by the geosynthetic reinforcement within the abutment wall.

This article describes the engineering analysis, design procedures, and some of the installation details used for these geosynthetic reinforced mechanically stabilized earth walls (MSEWs) with MCBW facing for the traditional retaining wall loadings, plus the additional procedures to account for the pile induced lateral loads.

The design procedures were based on the prevailing guidelines at the time, the 1996 AASHTO “Standard Specifications for Highway Bridges” as revised in 1998. Lateral loads were apportioned to the wall facing and geosynthetic reinforcement using PY curves for laterally loaded piles developed by Reese & Matlock. Seismic loadings were addressed by pseudo-static procedures.

A review of the performance of these geosynthetic walls based on deformed shape measurement of the wall facing after 10 years of service is also presented to begin to assess their performance.

Project description

The Southern Connector toll road, I-185 in Greenville, S.C., opened to commercial traffic in February 2001 connecting the major east-west highway I-85 to the primary north-south route I-385.

The 16-mile toll road was built to FHWA standards in 1999–2000 by a private developer, but it is now owned and operated by the South Carolina Department of Transportation (SCDOT). MSEWs were used extensively on the project, with a total face area of more than 40,000ft2 in three roadway grade separation walls and six abutment walls.

There are 25 bridges on this route. Three have geosynthetic reinforced MSEW abutment walls. Two of those MSEW bridge structures have integral bridge abutments, where the bridge beams were rigidly fixed to the abutment bearing seat (i.e., cast into the concrete).

For straight short-span bridges, integral abutments reduce initial and maintenance costs by eliminating bridge bearings and battered piles to resist lateral loads. This rigid connection presented a unique MSEW design requirement not previously addressed in North America for MCBW facing units.

This article describes the engineering design for those four abutment walls and details the engineering performance to date.

Bridge 19, with a span of 175ft, carries Log Shoals Road traffic over I-185, using equal spans of 87.5-ft-long precast concrete beams to a center bent. Bridge 19 abutment walls are 52ft long to accommodate the two lanes of local traffic.

Bridge 24 supports I-185 traffic as it overpasses Laurens Road (state highway SC-417) in a single span of 137ft using steel beams. Bridge 24 abutment walls are 99ft long to support four lanes of interstate highway traffic. All four abutment walls are skew to the overpassed roadway, creating acute or obtuse angles at each abutment corner.

Selection of MSEW system

Southern Connector project specifications allowed the general contractor to select the MSEW installation contractor and MSEW system for use in these locations. This flexibility of choice required the contractor to provide detailed MSE design calculations and construction drawings for the system selected.

The specifications and engineering properties for the facing block and geogrid are in Tables 1 and 2.Table 1Table 2The geogrid specific MCBW facing connection strength (CS) for the system is in Table 3, with each represented as a bilinear strength envelope for both peak (ultimate), as well as deformation limited (0.75in.) “service state” connection strengths.Table 3

Definition of soil conditions

A foundation investigation for each bridge location, using standard penetration testing borings, provided subsurface information that was useful for the design of the MSEWs. The soil conditions for MSE designs were defined by the owner’s representative for both Bridges 19 and 24 (see Tables 5 and 6). Table 5Table 6

capacity for each abutment, prescribed as a function of the effective foundation width B ( B = L-2e ), which influenced both static and seismic design. Per design guidelines the cohesion strength component of the reinforced and retained soils was ignored throughout design, except for slope stability analyses.

The reinforced fill soil source was identified and tested by the contractor. A quarry manufacturing byproduct, “course screenings,” was selected based on cost, available quantity, location, and consistency.

The engineering properties of the reinforced fill used throughout this project are shown in Table 4, which complies with AASHTO’s standard MSEW backfill specification, as required by the project.Table 4

The groundwater table was measured 5-8ft. beneath the bearing elevations of Bridge 19’s MSEWs. This was close enough to the base of the Bridge 19 MSEWs to install a protective gravel blanket drain to intercept potentially rising groundwater levels and analytically model the groundwater level at the base of those MSEWs. Bridge 24 groundwater levels were at sufficient depth to eliminate the blanket drain.

Design of MSEW system

The 1996 AASHTO Bridge Manual, as amended by the 1998 interim specifications, was used as the design guideline for these MSEWs. The lone exception was connection strength requirements, where a single lumped overall safety factor of 1.5 was applied to the peak strength defined by testing (Table 3).

The 20in.-deep MCBW facing unit provided a residual friction connection, even after the geogrid would rupture, so a single lumped safety factor applied to friction connections subject to pullout failure was implemented for these MSEWs. The wall height and foundation soil conditions at each of the four abutments varied enough to dictate a slightly different geogrid reinforcement layout.

Each abutment MSEW design utilized a 300psf uniform surcharge to account for vehicle traffic loading and 0.12g design seismic loading. The seismic loading conditions controlled both the number of reinforcement layers and the length of the reinforcement layers (i.e., internal and external stability, respectively), as affected by wall height and soil conditions.

Figures 1 and 2 show typical sections for geogrid reinforcement layout at Bridges 19 and 24, respectively.Figure 1 | Bridge 19 typical abutment sectionsFigure 2 | Bridge 24 typical abutment sectionsLateral sliding and settlement limited allowable bearing capacity during seismic loading, controlling length of the reinforcement at each MSEW. Seismic tensile capacity and static connection strength of the geogrid reinforcement controlled the type and number of reinforcement layers required at each MSEW.

Overall global stability analyses for the seismic controlled geogrid lengths and strengths under both seismic and static loading conditions found safety factors exceeding the minimum required by the AASHTO design method.

The vertical loads of the bridge superstructure were carried by H-pile foundations driven to end bearing on bedrock, about 50-70ft deep. With the H-piles rigidly connected to the concrete abutment seat, lateral movements of the bridge beams due to thermal expansion/contraction and braking loads placed additional lateral loads on the pile.

The applied lateral pile load was calculated (Table 7) based on the design deflection provided by the bridge designer, which varied due to bridge beam material.Table 7 | Lateral loadP-Y curves, as presented by Reese & Matlock (1956, 1961, 1962) and later modified by Davisson (1963, 1970) were used to determine transfer of lateral load into the surrounding soil and directly apportioned into each geogrid layer, (Figures 3 and 4).Figure 3 | B19 – Geogrids loadsFigure 4 | B24 – Geogrids loads

Matlock, H. and Reese, L.C. (1962) “General Solutions for Laterally Loaded Piles” Trans. ASCE, Vol 127, Part I, pp. 1200-1247

Reese, L.C. and Matlock, H. (1956) “Non-Dimensional Solutions for Laterally Loaded Piles with Soil Modulus Assumed Proportional to Depth,” Proceedings Eighth Texas Conference on Soil Mechanics & Foundation Engineering, Austin, Texas, ASCE

This approach is conservative, ignoring pile spacing and stress distribution through the soil. These static lateral loads were used in calculating the total static tension, and included as a component of applied seismic tension, but were not directly increased pseudo-statically, for the seismic analysis.

Combining these lateral loads with soil loads controlled the design, requiring stronger geogrids be used higher in the section. Figure 5 shows installation detail for geogrid reinforcement around the piles.Figure 5 | Geogrid installation at piles

Matlock, H. and Reese, L.C. (1962) “General Solutions for Laterally Loaded Piles” Trans. ASCE, Vol 127, Part I, pp. 1200-1247

Reese, L.C. and Matlock, H. (1956) “Non-Dimensional Solutions for Laterally Loaded Piles with Soil Modulus Assumed Proportional to Depth,” Proceedings Eighth Texas Conference on Soil Mechanics & Foundation Engineering, Austin, Texas, ASCE

Monitoring of the MSEW system

The authors received permission to begin performance monitoring these MSEW abutment structures in October 2009. Unfortunately, a baseline survey immediately after construction (1999–2000) was not performed, eliminating the possibility of separately evaluating all post-construction movements from original construction in 1999-2000 to the present.

Consequently, the authors will evaluate performance to date, using the current position of the wall facing relative to its original stacked batter and the anticipated position after construction. A plot of the October 2009 wall face position for four monitoring sections on Bridge 19 are shown in Figures 6 and 7, and eight sections on Bridge 24 (Figures 8 and 9).Figure 6 | B19 – Bent 1Figure 7 | B19 – Bent 3Figure 8 | B24 – BentFigure 9 | B24 – Bent 2

Each section is located at the center of the travel lane supported above it. The measured existing wall facing position is about where expected after completion of construction except for Bridge 24–bent 1, which shows an increase in wall batter of almost 1 degree from 3.57–4.44 degrees.

Increase in wall batter, while unusual, is generally caused by settlement of fill immediately (< 3ft) behind the MCBW units, dragging the reinforcement down and pulling the MCBW facing units backward. This is also probably affecting section “K” of Bridge 24–bent 2. The October 2009 wall facing position for every section measured was within industry accepted performance tolerances of +2 degrees of the stacked batter of 3.57 degrees.

Collectively, these wall facing position measurements indicate that, overall, these MSEW abutments are performing well, in addition to looking good aesthetically. Additionally, the lack of any measured bulging near the top of wall indicates the geogrid reinforcement is adequately restraining the additional lateral loads being applied to the MCBW facing units by the H-pile foundations during the first decade (10%) of service life.

Summary and conclusions

The design requirements of the geogrid reinforcement for four MSEW integral abutment walls subjected to lateral loading from H-pile foundations located immediately (1.8–3.0 ft.) behind the MCBW facing is presented.

While seismic loading (0.12g) controlled the geogrid reinforcement length, lateral loading from pile foundations dictated geogrid reinforcement strengths necessary to restrain the MCBW facing from excessive horizontal displacement. The initial 10 years of structure performance has been excellent, with no deleterious movements associated with the laterally loaded piles and or the typical imposed soil loading.

More research into long-term structural performance is needed. The authors intend to monitor these MSEW abutments during the next decade to quantify the amount and rate of horizontal movement to assess current performance prediction models for deformation, connection strength, and distribution of load into geogrid reinforcement.

Mike Simac, P.E., M.ASCE, is the principal engineer at Earth Improvement Technologies Inc., Fort Mill, S.C., mike@earthimprovement.com.
Dave Elton, P.E., M.ASCE, is a professor of civil engineering at Auburn University in Auburn, Ala., and is the immediate past president of the North American Geosynthetics Society, eltondj@eng.auburn.edu.

References

AASHTO (1996), “Standard Specifications for Highway Bridges” and 1998 interims.

Davisson, M.T. and Gill, H.L. (1963), “Laterally Loaded Piles in a Layered Soil System,” Journal Soil Mechanics & Foundation Engineering. ASCE Vol. SM3.

Davisson, M.T. (1970), “Laterally Load Capacity of Piles,” Highway Research Record, No. 333, pp. 104-112.

Elias, V.E., Christopher, B.R., and Perkins, S. (1997), “Mechanically Stabilized Earth Walls and Reinforced Soil Slopes, Design and Construction Guidelines,” prepared for FHWA, Demo Project 82, Contract No.: DTFH61-93-C-000145, 371 p.

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