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Federal Highway Administration > Publications > Research > Infrastructure > Structures > Geosynthetic Reinforced Soil Integrated Bridge System Interim Implementation Guide
Publication Number: FHWA-HRT-11-026
Date: January 2011

Geosynthetic Reinforced Soil Integrated Bridge System Interim Implementation Guide

CHAPTER 7. CONSTRUCTION

7.1 INTRODUCTION

GRS construction uses basic earthwork methods, primarily for excavation and compaction, along with sound general construction practices. The materials are readily available, which is a benefit of the generic nature of the system. This chapter provides guidance on most field–related scenarios. All methods that are presented have been field–tested and applied during the construction of GRS-IBS. The techniques outlined can be applied to efficiently construct the layered system and have been proven to quickly construct the GRS–IBS. The contractor will ultimately choose the methods most efficient for the site, the crew, and the equipment on hand.

The guidance outlined here applies to GRS structures, specifically abutments built with CMU blocks. This guidance can also be adapted to other GRS structures built with different facing systems.

GRS construction has two principal components: (1) logistics and (2) aspects associated with actual construction. Logistics occur after the final design and before construction, outlining a plan for implementation and control of the construction process. Even though building a GRS abutment is as simple as a row of facing block, a layer of well–compacted granular fill, and a sheet of reinforcement, the process will be hampered without adequate planning to ensure optimum flow and placement of material during the course of the project.

As a result, the single–sheet plan was devised to provide information on the reinforcement schedule and the facing block schedule. The single–sheet plan also contains information on the limits of excavation and details about assembly of the GRS structure. A second sheet may be necessary to detail quantities and construction notes.

This chapter conveys the importance of the following details as a means to rapid GRS construction:

  • Careful attention to the first row of blocks. Since all other courses of block are built off the first row, it is essential to ensure that the bottom row is level and even for fast construction.

  • Optimization of crew size and equipment for enhanced productivity. Too many laborers or excess onsite equipment can cause confusion and slow down the construction process.

  • Allowing time for a labor crew to adjust to the construction of the GRS–IBS. Having each crew member do their part in one of three basic steps of GRS construction (laying a course of facing block, compacting a layer of granular backfill, and placing a layer of reinforcement) dramatically improves productivity.

  • Establishing the central position of the excavator. Typically, it is best to limit movement of the excavator by locating it toward the back of the abutment, where it can both reach and place material without moving.

7.2 LABOR AND EQUIPMENT

7.2.1 Labor Requirements

A typical labor crew on GRS–IBS projects has consisted of about five workers: four laborers and an equipment operator (see figure 40 ). The equipment operator is central to the project and provides support to the labor crew. The equipment operator is responsible for shaping the excavation to facilitate construction of the RSF and the GRS abutment in addition to placing fill material and moving facing units into the work area. Typically, one member of the labor crew has the role of foreman and is responsible for layout of excavation limits, grades, alignment of wall face, placement of facing blocks, compaction of fill, placement of geosynthetic reinforcement, and other activities to streamline production and the flow of material to the job site.

Photo showing a typical labor crew of 4 workers with a trackhoe centrally located on top of the cut slope. The trackhoe is dump spreading a layer of gravel on the geotextile. The workers are compacting the gravel. The abutment is being built in slightly flooded water condition with the water level to the top of the riprap.

Figure 40. Photo. Typical labor crew with centrally located track hoe.

7.2.2 Tool and Equipment Requirements

Specialized equipment is not required to construct GRS–IBS. Simple tools that are readily available and relatively inexpensive can be used. These include hand tools, measuring devices, and heavy equipment. The contractor may modify the included lists depending on the site, the crew, and the size of the IBS.

Typical hand tools include the following:

  • Gravel rake (concrete spreader).

  • Shovels (flat blade and spade).

  • Heavy rakes.

  • Broom to sweep top of blocks.

  • Wisk broom.

  • 2– to 3–lb sledgehammer and wood two–by–fours to align blocks.

  • Heavy rubber mallet.

  • Spade trowel.

  • Razor knives or utility knives to cut reinforcement.

  • Hand tamper with metal base plate.

  • Chainsaw to cut reinforcement roll.

  • Concrete saw.

  • 5–gallon bucket.

  • Block lifter.

  • Standard concrete mixing and finishing tools.

Typical measuring devices include the following:

  • Survey equipment.

  • Laser level.

  • String line to align blocks.

  • 4–ft carpenter's level.

  • Plum bob to check wall batter.

  • Measuring tapes.

  • Chalk line.

Typical heavy equipment includes the following:

  • Walk–behind vibratory plate tampers (200 lb and 18 inches wide or larger).

  • Track hoe excavator.

  • Riding smooth drum vibratory roller (compacting 3.28 ft from wall face).

  • Pallet forks for excavator (for moving CMU block in and out of work area).

  • Trash pump and hose for dewatering foundation excavation.

  • Backhoe (as needed for material staging).

 

7.3 SITE PREPARATION

GRS is built from the bottom up and generally from within the footprint of the structure. Staging and delivery of materials to the site should allow for continuous GRS construction and effective use of the space. Delivered material should be easily accessible to the excavator, which is the central piece of equipment. As shown in figure 41, the excavator is positioned inside the wall area for easy placement of fill, block, and other materials. Labor should be organized to assemble construction materials as needed on the work platform.

Photo showing the excavator positioned inside the wall area for easy placement of fill, block, and other materials from the staging area to the geosynthetic reinforced soil (GRS) abutment. The GRS abutment is being built behind an existing concrete abutment wall. The top of the existing concrete was removed to allow for the added width of the new bridge.

Figure 41. Photo. Cut slope of retained soil.

7.3.1 Site Layout

Site preparation begins with a survey of the bridge site to stake limits for the excavation. Reference stakes should be located in an area where they will remain undisturbed during construction of the base of the wall, usually about 5 ft from the excavation.

The base of the GRS abutment and wing walls should be constructed to within 1 inch of the staked elevations. The external GRS abutment and wing walls should be constructed to within ±0.5 inches of the surveyed staked dimensions.

7.3.2 Excavation

All excavations should comply with Occupational Safety and Health Administration requirements.( 21 )Excavation of the site involves shaping the slope for temporary slope stability, safety, and constructability. The temporary cut in the retained soil should be designed to accommodate movement of labor. The design of a temporary excavation needs to consider the loading imposed by heavy equipment and the reach limits of the excavator. figure 41 shows a typical cut slope in stiff clay. The excavation should include provisions for drainage with a sloped cut to facilitate the movement of water. Any open excavations that form a pit should be backfilled with crushed aggregate and compacted. Excavation also includes the clearing and grubbing of vegetation. In situations where the retained fill is stable, the volume of excavation can be limited to reduce the size of the GRS mass. In the case of an abutment application, this would form a horseshoe shaped excavation, as shown in figure 40 and figure 42.

Photo showing the excavator positioned on top of the stable cut slope above the abutment. The excavation is in the form a horseshoe to reduce the volume of excavation and the amount of geosynthetic reinforced soil (GRS) backfill. The wing walls are being built around the stable core of the excavation back into the slope. The abutment is being built in slightly flooded water condition with the water level to the top of the riprap.

Figure 42. Photo. Horseshoe–shaped excavation with native soil still intact in middle.

7.3.3 Placement of Abutment Behind Existing Substructure

In some situations, it may be beneficial to build GRS–IBS behind an existing substructure. Project feasibility, environmental considerations, and other factors need to be assessed before selecting this type of project layout. Building the bridge behind an existing substructure often requires the removal of the top part of the abutment walls to provide additional space for the width of the new GRS–IBS. figure 43 through figure 45 illustrate this technique. Note that the design of the GRS-IBS will be the same whether it is built behind an existing abutment or not.

Photo showing a face view of a geosynthetic reinforced soil (GRS) abutment with a superelevation that is built behind an existing concrete abutment wall. The top of the concrete wall was removed and capped with concrete. Formwork is shown for this concrete cap. A crane is onsite behind the abutment to place the beams.
Courtesy of St. Lawrence County , NY

Figure 43. Photo. GRS–IBS built behind an existing concrete abutment.

 

Sketch showing a cross section of a geosynthetic reinforced soil integrated bridge system (GRS-IBS) built behind an existing concrete abutment. A wrapped-face reinforced soil foundation (RSF) is shown directly behind the existing abutment with the GRS abutment built on top of the RSF. The concrete beams bear directly on the GRS mass with the integrated approach behind the concrete beams.
Courtesy of St. Lawrence County , NY

Figure 44. Illustration. Cross section of GRS–IBS built behind an existing concrete abutment.

 

Photo showing the construction of the reinforced soil foundation (RSF) behind the existing concrete abutment. There are three laborers compacting, holding, and raking out the fill. The concrete wall has spray painted lines at 8-inch increments as a guide for the compacted layers of fill.

Figure 45. Photo. Building the RSF behind an existing abutment.

 

7.4 RSF

The depth and footprint of the excavation for the RSF should be based on external stability, as described in chapter 4. The base of the RSF should be cut smooth. It should be excavated to uniform depth, and all loose, unstable material should be removed from the site (see figure 46 ). If the base of the excavation is left open, it should be graded to one end to facilitate the removal of any intrusion of water with a pump. If flooded, all water should be removed along with soft, saturated soils. The excavation should be backfilled as soon as possible to provide a suitable foundation and avoid adverse weather delays. The construction of the RSF can typically be completed in less than one day but is dependent on the size and depth of excavation, type of materials, equipment, and experience.

Photo showing the construction of a reinforced soil foundation (RSF). The excavator is positioned on top of the stable cut slope above the abutment. The RSF excavation is below the river level. The workers are in the pit, building the RSF. One worker is measuring elevation with a survey level rod. The abutment is being built in slightly flooded water conditions with the water level to the top of the riprap.

Figure 46. Photo. RSF excavation below stream level.

The base of the excavation should be compacted before construction of the RSF. This may require proof rolling, and any soft spots or voids should be backfilled with compacted fill material. Figure 47 shows the preparation of the RSF cut.

Photo showing the cut preparation of the reinforced soil foundation (RSF). The base of the excavation is being compacted before construction of the RSF with a 30-inch sheepsfoot roller compactor.

Figure 47. Photo. RSF cut preparation.

The RSF should be encapsulated in geotextile reinforcement placed perpendicular to the abutment face to protect it from possible erosion (see figure 48 ). The reinforcement sheets should be measured and sized to fully enclose the RSF on three sides: the face and the two wing wall sides. If the GRS abutment is adjacent to water, the reinforcement sheets should overlap, starting with the first layer on the upstream side of the RSF. All overlapped sections of reinforcement in the area of the RSF should be oriented to prevent running water from penetrating the layers of reinforcement. The first layer of reinforcement should be placed on the upstream side of the abutment with subsequent layers, if needed, overlapped a minimum of 3 ft on the downstream side. This prevents water from infiltrating the RSF. The wrapped corners of the RSF need to be tight and without exposed soil within the RSF to complete the encapsulation.

Photo showing the encapsulation of the fill in the reinforced soil foundation (RSF). The workers are compacting the crushed, well-graded gravel fill with a vibratory roller before folding the geotextile over the top of the RSF.

Figure 48. Photo. Encapsulation of fill in RSF.

Typical reinforcement spacing in the RSF is 12 inches. The reinforcement should be pulled taught to remove all wrinkles prior to placing and compacting the structural backfill. Fill should be placed from the face to the back to roll folds or wrinkles to the free end of the reinforcement layer.

The RSF should be constructed with structural fill, as specified in chapter 3. The structural fill is to be compacted in accordance with section 7.5 in compacted lifts not to exceed 6 inches (with two compacted lifts per each 12–inch layer). The first course of wall block sits directly on the RSF, as shown in figure 49 , so it is important that the fill material is graded and level before encapsulating the RSF.

Photo showing the placement of the first course of solid concrete modular block directly on the geotextile-encapsulated reinforced soil foundation (RSF). The solid blocks are red and will be embedded within the riprap. There is an assortment of tools to spread the gravel and a vibratory plate compactor shown in the photo.

Figure 49. Photo. Placement of wall block on wrapped RSF.

While the base of a typical GRS abutment is built with solid CMU, damage can occur during the placement of channel rock protection or from other large pieces of concrete rubble that extend above the solid block zone. Riprap protection should be placed in a manner to prevent damage to the CMU wall face. Impact of large rock or concrete fragments during placement can crack the CMU block. Larger rocks should be uniformly distributed and placed firmly in contact with each other, with smaller rocks and fragments filling the voids between the larger rocks. This procedure often requires hand placement of smaller rocks to fill the voids. If any CMU block is damaged, refer to chapter 8 for repair procedures.

 

7.5 COMPACTION

Compaction of the backfill should be to at least 95 percent of maximum dry density according to AASHTO T–99. Backfill material containing fines should be compacted at a moisture content close to optimum (±2 percent). Lifts of 8 inches should be compacted using vibratory roller compactionequipment. The facing blocks provide a form for each lift of fill. Other stiffness–based compaction control methods can be used. For open–graded fills, compact to non–movement or no appreciable displacement and assess with visual inspection.

Since the facing elements are not rigidly connected to the reinforcement, hand–operated compaction equipment (e.g., a lightweight mechanical tamper, plate, or roller) is required within 1.5 ft of the front of the wall face. It is very important for adequate GRS performance that the backfill is properly compacted. The top 5 ft of the abutment should be compacted to 100 percent of the maximum density according to AASHTO T–99.

Onsite compaction equipment should be selected to achieve the required density of the fill materials. Considering that compaction is critical to the success of the project, compaction equipment should be in good operating order for efficient use. In addition, backup equipment should be available to provide quality construction throughout the project and to avoid construction delays.

7.5.1 Compaction Procedure

Once fill is placed at the required thickness and graded, all areas behind the CMU block should be compacted to the required density. Any depression behind the facing block should be filled level to the top of the CMU block prior to compaction.

Compaction directly behind the CMU block should be performed in a manner that maintains wall alignment while improving the density of fill behind the block. This can be achieved in the following ways:

  • Placing a fill lift directly behind the CMU block face and rodding or foot tamping along the row of CMU block while exerting downward pressure on the block to prevent lateral movement. For multiple lifts, the top lift height is slightly higher than the block to compensate for compression of the fill during compaction.

  • Using a lightweight vibratory plate compactor directly behind the CMU block while exerting downward pressure on the block to prevent lateral movement.

  • Using larger vibratory compactors for the remainder of the fill area 3 ft from the face of the GRS wall. Check for outward block movement and adjust accordingly.

The most common compaction QC tool is the nuclear density gauge. Other instruments are also available for compaction control such as the Clegg hammer, the soil stiffness gauge, or the falling weight deflectometer. These devices are typically used by correlating their measurements to soil density and moisture content. Method–based compaction specifications can also be used. For open–graded fills, compact to non–movement or no appreciable displacement and assess with visual inspection.

 

7.6 REINFORCEMENT

Generally, the length of the reinforcement layers will follow the cut slope, as shown in figure 50. While the reinforcement layers in the GRS abutment can be any geosynthetic, the RSF and integrated approach should be constructed and encapsulated with a geotextile to confine the compacted granular fill. The geosynthetic should be placed so that the strongest direction is perpendicular to the abutment face, as shown in figure 51. Where the roll ends, the next roll should begin. Overlapping between sheets is required Overlapping between sheets is not required. The geosynthetic reinforcement should extend between layers of CMU block to provide a frictional connection. The geosynthetic reinforcement should cover a minimum of 85 percent of the top surface of the CMU block; any excess can be removed by either burning with a propane torch or cutting with a razor knife.
AMENDED May 24, 2012

Sketch of a typical geosynthetic reinforced soil integrated bridge system (GRS-IBS) built with a truncated base. The sketch shows the primary elements of the GRS-IBS, with highlights on the bearing reinforcement and the integration zones. The bearing reinforcement zone is directly beneath the bridge beam seat, consisting of 4-inch-thick secondary reinforcement layers at the top five courses of facing block. The integration zone is located directly behind the bridge beams. These 12-inch-spaced layers are wrapped behind the beams to encapsulate the fill and placed to blend the approachway with the existing road with layers extending over the cut-slope. Three zones of five layers each are marked and labeled zone 1, zone 2, and zone 3.

Figure 50. Illustration. Typical reinforcement zones.

 

Photo showing the method of cutting a predetermined length from a roll of reinforcement and rolling it out parallel to the face of wall. This method should only be used where the reinforcement can extend all the way back to the cut-slope or in situations where secondary reinforcement layers are needed.

Figure 51. Reinforcement rolled out parallel to wall face.

After the geosynthetic is rolled out, it should be laid so that it is taut, free of wrinkles, and flat. The geosynthetic can be held in place with the fill. Placement of fill should be from the wall face backward to remove and prevent the formation of wrinkles in the geosynthetic. A conscious effort should be taken during placement of fill to prevent the development of wrinkles.

Splices of reinforcement can occur without overlap. Splice seams should be staggered to avoid a continuous break in the reinforcement throughout the GRS structure. All splice seams should run perpendicular to the wall face.

Overlaps of adjacent geosynthetic should be trimmed where they are in contact with the surface of the CMU block to avoid varying geosynthetic thicknesses between the CMU block. Any seams in the geosynthetic should be staggered with each successive layer of the GRS abutment. All seams between adjacent sheets of geosynthetic located in the area beneath the footprint of the bridge seat should be perpendicular to the abutment wall face.

7.6.1 Operating Equipment on Geosynthetic Reinforcement

Driving should not be allowed directly on the geosynthetic reinforcement. Place a minimum 6-inch layer of granular fill prior to operating any vehicles or equipment over the geosynthetic reinforcement. In the bearing reinforcement zone, hand-operated compaction equipment should be used over the 4-inch lifts to prevent excessive installation damage of the reinforcement. Rubber–tired equipment may pass over the geosynthetic reinforcement at speeds less than 5 mi/h. Skid steers and tracked vehicles can cause considerable damage to the geosynthetic. On one occasion, a track hoe operating on a GRS structure turned and pulled the fabric causing deformation to the wall face. For this reason, it is recommended to restrict the use of these vehicles on GRS structures. If absolutely necessary, use may be permitted provided no sudden braking or sharp turning occur and a minimum 6–inch cover is placed.
AMENDED May 24, 2012

7.6.2 Bearing Reinforcement Bed

The bearing reinforcement bed provides additional strength in the upper GRS wall layers directly beneath the bearing area of the superstructure. These reinforcement layers are not sandwiched between two consecutive rows of block but are placed behind the CMU block at 4–inch spacing. This 4–inch reinforcement spacing is generally placed in the top five layers of the GRS abutment or as determined by design (see chapter 4).

Bearing bed reinforcement spacing in superelevated abutment walls requires additional planning. The 4–inch reinforcement spacing needs to be in place for the top five courses of block at the lowest elevation across the abutment wall (see figure 50 and figure 52 ). The reinforcement schedule will guide field personnel in the proper placement of the geosynthetic along a wall block course.

This sketch shows the layout of secondary reinforcement layer within the bearing bed with the termination of each layer of reinforcement across the abutment wall face from lowest to highest elevation.

Figure 52. Illustration. Superelevation reinforcement schedule.

7.6.3 Superelevation

The reinforcement layers become stair–stepped in the upper wall layers as the superelevation of the abutment is constructed (see figure 53 ). The reinforcement terminates along the angle surface of the superelevation. The GRS wall reinforcement schedule should show the termination of each layer of reinforcement across the abutment wall from low to high elevation (see figure 52 ).

The photo illustrates the placement of reinforcement layers for a superelevated abutment. The reinforcement is stair-stepped in the upper wall layers as the superelevation of the abutment is constructed. The reinforcement terminates along the angle surface of the superelevation.

Figure 53. Photo. Superelevation reinforcement layers.

 

7.7 WALL FACE

This manual focuses on the use of CMU for the wall facing; however since GRS is internally stable, any facing elements can be used in construction. For flexible facings other than the recommended CMU block (including wrapped, timber, natural rock, or welded wire basket formed facing), other construction guidelines may need to be followed. These are outlined by Wu et al.( 22 ) The design guidelines for GRS–IBS, however, remain the same as those in this manual.

7.7.1 Leveling Course

Setting the first course of facing block level and to grade is critical in maintaining wall alignment for the entire height of the abutment. Typically, the first course is placed on top of the RSF, directly on the geotextile. However, due to the large aggregate size of the RSF fill material, a thin leveling layer of fine aggregate can help set the CMU blocks to grade and prevent them from rocking. The leveling layer should be kept to a minimum thickness, no more than 0.5 inches. If the leveling layer exceeds this thickness and there is the potential for water to erode and undermine the aggregate, mortar or grout should be placed in the gap between the RSF and the first course.

7.7.2 Setting the CMU Block

CMU block wall construction should begin at the lowest portion of the excavation with each layer placed horizontally, as shown on the plans. Each layer should be constructed entirely before beginning the next layer. A stretcher or running bond should be maintained between courses of block so that the joints between the blocks are offset with each row.

If a designed scour countermeasure such as a riprap apron is used, a geotextile filter fabric should be placed under the apron and anchored between the first and second courses of CMU block.

Since the CMU blocks are dry stacked without mortar, it is important to avoid cracking the block and to maintain a horizontal uniform elevation by sweeping the top surface of the block clean of debris and fill material prior to the placement of the next layer of geosynthetic and CMU block. Gravel material between layers of block creates point loads that can cause cracks. Also, gravel material between the blocks causes them to rock, making it difficult to secure a good fit.

When setting a course of block, each block should be placed tightly against the adjoining block, preventing gaps from which fill material can escape. Before proceeding to the next layer, it is often useful to walk along the top of the blocks to easily identify a poorly seated block.

In order to avoid cutting a block when the CMU block schedule shows the wall terminating with half a block, a full CMU block can be turned 90 degrees, placing the 8–inch width toward the face. This typically occurs at the termination of a wing wall. The end block that forms the termination does not have to be a corner CMU (with two finished sides) because the ends of most wing walls are embedded into the fill slope.

7.7.3 Wall Face Alignment

When placing and compacting fill behind the CMU block, it is sometimes necessary to set the block back about 0.5 inches to allow for lateral outward movement of the CMU block during compaction. It should be noted that each combination of wall facing and backfill reacts differently during the compaction process, and adjustment of the setback distance between block courses should be performed as needed to maintain the necessary batter.

The vertical GRS wall should be checked for plumbness at least every other layer, and any deviations greater than 0.25 inches should be corrected. Before placement of the backfill, every other row of block alignment should be checked with a string line referenced off the back of the facing block from wall corner to corner (see figure 54 ).

The photo shows the alignment of the facing concrete masonry unit (CMU) block with a string line referenced off the back of the block.

Figure 54. Photo. Checking block alignment with string line reference from back of block.

If CMU blocks become displaced during construction, they can often be hammered back into position using a 3–lb sledgehammer and a block of wood as protection. If the CMU block is excessively out of alignment, the fill material needs to be excavated, the CMU block repositioned, and the fill material replaced and recompacted.

7.7.4 Block Alignment for Battered Walls

Block alignment for battered walls is similar to that for vertical walls. In abutment situations where the face wall turns to form the wing wall, however, it is necessary to trim blocks on either end to account for the reduced wall length. All cuts should be performed to maintain the standard running or stretcher bond between the rows of dry–stacked blocks, with the vertical joints of each course midway between those of adjoining courses.

In special situations, negative battered walls have been constructed when the top area needs to be greater than the bottom, as in the case of road widening shown in figure 55. The negative batter can be created by offsetting the CMU block by a measured amount in consecutive wall layers then filling and compacting as specified.

The photo shows a modular block geosynthetic reinforced soil (GRS) abutment wall built with a negative batter alignment. The negative batter is 15 degrees.

Figure 55. Photo. Negative batter wall face.

7.7.5 Superelevation

When the plan shows a superelevation for the bridge, the top courses of CMU beneath the superstructure should be trimmed to match the elevation difference and clear space across the abutment (see figure 56 ). This will produce a sloped face wall and aid in construction of the beam seat. One method is to snap a chalk line along the back of the block at the superelevation slope. A carpenter's angle finder can also be used to mark the cut.

The photo shows the face of a geosynthetic reinforced soil (GRS) abutment built with modular blocks. There is a slope across the face of the GRS abutment to create the superelevation for the bridge. The photo shows that the rectangular modular blocks were cut to match the slope of the superelevation.

Figure 56. Photo. Blocks trimmed to match superelevation.

7.7.6 Wall Corners

Right–angle wall corners, as shown in figure 57, are constructed with CMU corner blocks that have architectural detail on two sides, providing an aesthetic finish. Facing wall and wing wall courses should be staggered to form a tight, interlocking, stable corner.

The photo shows a modular block geosynthetic reinforced soil (GRS) abutment built with a right-angle wing wall. The corner is formed where the face and wing wall meet.

Figure 57. Photo. Right–angle wall corner.

Walls with angles larger or smaller than 90 degrees require additional effort. The corner blocks need to be cut to form the angled face. As a result, a vertical seam or joint is formed at the corner (see figure 58 ). Corners with vertical seams may have open block joints, making it prudent to fill the corner blocks with a concrete mix and install bent rebar to close and connect the seam at each course of block, as shown in figure 59. This procedure secures the two faces and prevents compaction–induced separation during construction of subsequent GRS layers. It may also be used wherever added strength at the wall corner is desired.

The photo shows a modular block geosynthetic reinforced soil (GRS) abutment built with a obtuse angle wing wall (greater than 90 degrees). The use of rectangular modular blocks makes it necessary to cut the blocks at angle, creating a vertical seam at the corner where the wing wall and abutment wall meet. The photo also illustrates the placement of the riprap at the abutment face and placement of the concrete box beams directly on the GRS abutment.

Figure 58. Photo. Vertical seam in wing wall.

 

The photo is a downward view at the corner where the abutment face and wing wall meet to form an obtuse angle (greater than 90 degrees). The photo shows that the corner blocks are cut to form the angle, and the inside web of hollow-core block is notched so that a bent piece of rebar can be inserted into the core of the adjacent block's segment.

Figure 59. Photo. Rebar installed in vertical seam prior to grout.

7.7.7 Top of Facing Wall

The top three courses of CMU block in the abutment are susceptible to movement simply from not having the weight of successive layers holding them in place. To prevent displacement, the hollow cores of the top three courses of CMU blocks are filled with a concrete wall fill and pinned together with No. 4 rebar, preferably epoxy–coated and embedded with a minimum 2–inch cover (see figure 60 ).

The photo shows a laborer filling the top three courses of a modular facing block with concrete. In the bottom of the photo is a 0.5 inch rebar dowel, which is also placed in the hollow-core block as part of the grouting procedure.

Figure 60. Photo. Connecting the top courses of blocks.

To grout and pin the top of the wall, the reinforcement between the top two courses of CMU block needs to be removed to open the core for placement of concrete wall fill and a 20–inch–long No. 4 rebar dowel, preferably epoxy–coated with 2–inch cover (see chapter 3). This can be accomplished either by cutting the reinforcement with a razor knife or by burning the reinforcement.

The concrete wall fill is placed in two steps. After the block void is filled with concrete to the top of the block and the steel rebar is inserted, a thin layer of the same concrete mix is placed on top of the block to form the coping cap, as shown in figure 61 and figure 62. The coping is then hand–troweled either square or round and sloped to drain. A wet–cast cap is more durable than a dry–cast cap and eliminates the need to furnish and install a separate cap unit.

This photo illustrates the wet-cast concrete coping details used to the form the cap of the geosynthetic reinforced soil (GRS) abutment walls. The type of coping finish used is rounded.

Figure 61. Photo. Rounded coping cap.

 

This photo illustrates the wet-cast concrete coping details used to the form the cap of the geosynthetic reinforced soil (GRS) abutment walls. The type of coping finish used is square.

Figure 62. Photo. Square coping cap.

Once the top of wall has been grouted and pinned, care should be taken to avoid any construction activity that may pull on the top layer of reinforcement. The frictional connection between the block is strong, and when courses are pinned together, the entire grouted wall face can be pulled out of alignment.

 

7.8 BEAM SEAT

The beam seat is constructed directly above the bearing bed reinforcement zone. The superstructure is then positioned on top of the beam seat, as shown in figure 63 and figure 64. The purpose of the beam seat is to ensure that the superstructure bears on the GRS abutment and not the wall facing block and to provide the necessary clear space between the superstructure and the wall face. Typically, the clear space is 3 inches, or 2 percent of the abutment height, depending on the required design (see chapter 4).

This photo illustrates placement of the concrete box on the geosynthetic reinforced soil (GRS) abutment bearing seat. The photo is from afar, with two men standing on the box beam stretched across a gap.

Figure 63. Photo. Box beam placed on beam seat.

 

This photo illustrates placement of the concrete box on the geosynthetic reinforced soil (GRS) abutment bearing seat. The picture is a close-up to show the 8-inch setback created with the foam board and solid 4-inch concrete block.

Figure 64. Photo. Detail view of box beam placed on beam seat.

In general, the thickness of the beam seat is approximately 8 inches and consists of two 4–inch lifts of wrapped–face GRS. Remember, before construction of the beam seat, the cores of the CMU blocks on the abutment wall face must be pinned with No. 4 rebar and filled with concrete wall mix (see figure 65 ).

This photo shows the grout-filled facing block within the bearing area zone before construction of the setback and beam seat.

Figure 65. Photo. Bearing area block grouted prior to beam placement.

7.8.1 Beam Seat Procedure

Once the block elevation beneath the bearing area is established and the hollow cores are filled with grout, the beam seat is ready for construction. The following steps should be used:

  1. Place precut 4–inch–thick foam board on the top of the bearing bed reinforcement. Sometimes, a thin layer of backfill may be necessary beneath the foam board for grading purposes and to ensure the proper clear space height and drainage (crown in bridge) (see figure 66 ). The foam board should butt against the back face of the CMU block. The exposed edge of the foam board helps form the nose of the reinforcement wrap across the length of the bearing area.

The photo  shows a row of 4-inch foam beneath a row of solid 4-inch concrete blocks directly behind the facing block. Workers are compacting the geosynthetic reinforced soil (GRS) backfill to prepare the area to construct the beam seat.

Figure 66. Photo. Foam board and 4–inch block assembly to form beam seat.

  1. Set 4–inch solid concrete blocks on top of the foam board across the entire length of the bearing area (see figure 67 ). The back edge of the top CMU face block holds the 4–inch concrete block in place during compaction. Note that the distance between the top of the grouted CMU block and the top of the beam seat (the clear space) is the distance the beams can settle before bearing on the facing blocks.

The photo is a close-up of the 4-inch foam beneath the solid 4-inch concrete block. The 4-inch concrete block is set 0.25 inch beneath the height of the facing block to prevent it from shoving forward during the compaction process.

Figure 67. Photo. 4–inch concrete block on top of foam board against top CMU face block.

  1. Use the first 4–inch wrapped layer of compacted fill as the thickness to the top of the foam board (see figure 68 ).

The photo shows construction of the first 4-inch wrapped layer in the beam seat. A track hoe is spreading a 4-inch-thick layer of gravel from the face of the wall backwards. Laborers are smoothing the gravel layer level

Figure 68. Photo. First 4–inch wrap butted against foam board.

  1. Place the second 4–inch wrapped layer of compacted fill to the top of the 4–inch solid block, creating the clear space as shown in figure 69. The top of this layer controls the beam elevation, and should therefore be carefully compacted and graded.

The photo shows how the top 4-inch wrapped layer of the beam seat is compacted directly against the 4-inch thick solid concrete. The photo also shows skewed concrete beams placed on the completed beam seat.

Figure 69. Photo. Top 4–inch wrap butted against 4–inch solid block.

  1. Before folding the final wrap, it may be necessary to grade the surface aggregate of the beam seat slightly high, to about 0.5 inches, to aid in seating the superstructure and to maximize contact with the bearing area.

7.8.2 Setback

The setback is the distance between the back of the facing block and the front of the beam seat. This distance can be established during construction of the beam seat and placement of the block and foam board used to form the beam seat wrap. The setback distance is usually 8 inches but can be greater.

7.8.3 Aluminum Flashing

The aluminum flashing drip edge is installed prior to setting the bridge beams and is placed in between the bottom of the beams and the foam board. The flashing is held in place by the pressure of the beams on the compressible foam board (see figure 70 ). The length of the flashing should extend beyond the outside edge of the bridge beams and be trimmed to fit against the parapets.

The photo shows the aluminum flashing (drip edge) located in the clear space between the beams and top row of concrete masonry unit (CMU) facing block.

Figure 70. Photo. Aluminum flashing (drip edge) between beams and top of CMU block.

7.8.4 CIP or Precast Footing

For GRS–IBS built without adjacent concrete beams, a CIP or precast footing may be necessary, as with steel beams or spread girders (see figure 71 ).

The photo shows the placement of steel girders on a cast-in-place (CIP) footing bearing on the beam seat of a large geosynthetic reinforced soil (GRS) abutment.

Figure 71. Photo. Steel girder on CIP footing.

 

7.9 PLACEMENT OF SUPERSTRUCTURE

Prepare the beam seat as described in section 7.8.1. The grade of the beam seat will control the final elevation of the bridge.

7.9.1 Crane Position on GRS Mass

The crane used for placement of the superstructure can be positioned on the GRS abutment provided the outrigger pads are sized within the capacity of the GRS mass. The outrigger pads should be sized for 4,000 psf near the face of the abutment wall with greater loads able to be supported with increasing distance from the abutment face (see figure 72 ).

The photo shows a large crane set up at the face of a geosynthetic reinforced soil (GRS) abutment. The outrigger is supported on a bearing pad placed at the face of the wall.

Figure 72. Photo. Outrigger pads near wall face.

7.9.2 Reinforcement of Beam Seat

An additional layer of reinforcement should be placed between the beam seat and concrete or steel beams to provide additional protection of the beam seat (see figure 73 ). The additional layer of reinforcement may decrease the sliding resistance between the superstructure and the beam seat.

The photo shows steel beams placed directly on a geosynthetic reinforced soil (GRS) abutment for a temporary bridge. An extra sheet of geotextile was placed between the steel beams and beam seat to prevent the edges of the beams from cutting into the beam seat.

Figure 73. Photo. Additional reinforcement under beam.

7.9.3 Setting Superstructure on Beam Seat (Without CIP Footing)

Since the bearing surface is aggregate under a layer of geosynthetic reinforcement, it is important to set beams square and level. They should never be dragged over the beam seat surface, which could create the potential for an uneven bearing area or a void under the beam, producing uneven bearing stresses between bridge elements.

7.9.4 Wing Walls and Parapets

Wing walls and parapets are constructed after the superstructure is set. The CMU block in the parapet wall should be trimmed or saw cut for a custom fit against the beam edge to prevent the loss of fill material. Figure 74 and figure 75 show the construction of the parapet against the superstructure. If the gap between the superstructure and the facing block is difficult to fill using thin slices of cut block, a mortar mix should be used to close the space.

This photo illustrates how the beams ends are embedded into the geosynthetic reinforced soil (GRS) abutment. The photo shows the parapet wing walls up against the side of the beams with several additional layers of GRS, depending on the thickness of the bridge girders.

Figure 74. Photo. Parapet and wing wall construction, view 1.

 

This photo illustrates how the beams ends are embedded into the geosynthetic reinforced soil (GRS) abutment. The photo shows the parapet wing walls up against the side of the beams with several additional layers of GRS, depending on the thickness of the bridge girders.

Figure 75. Photo. Parapet and wing wall construction, view 2.

 

7.10 APPROACH INTEGRATION

Proper approach construction at the road and superstructure interface is essential to minimizing settlement in front of the bridge beams and eliminating the bump at the end of the bridge. This is accomplished by compacting and reinforcing the approach fill in wrapped geotextile layers and blending the integration zone with the approach road base course. The material for the integration zone should be well–graded, as outlined in chapter 3.

Once the superstructure is in place, the approach to the bridge can be constructed using the following steps:

  1. Trim a geotextile reinforcement sheet to provide the planned length after it is wrapped, and place it behind the beam end (see figure 76 ). The width of the sheet should allow for wrapping of the sides after the fill layer is placed and compacted. Wrapping of the sides prevents lateral migration of the fill.

Figure 76 through figure 80 illustrate the process of forming the integrated approach by compacting layers of gravel directly against the ends of the beams. Each layer is wrapped with a sheet of geotextile. The propose of the wrapped layers is to prevent the migration of fill during thermal movement of the beams. The thickness of the each wrapped layer is about 12 inches. The 12-inch layer is compacted in 6-inch layers with small secondary sheets of reinforcement placed between each 6-inch layer at the beam ends.

Figure 76. Photo. Placing reinforcement.

  1. Place a 6–inch lift of fill and compact, per compaction specifications for road base (see figure 77 ). Add a secondary layer of reinforcement on top of the 6–inch lift, and then place another 6–inch lift of fill and compact (see figure 78 ). Fold back the reinforcement sheet to wrap the compacted fill layer and smooth wrinkles (see figure 79 ).

Figure 76 through figure 80 illustrate the process of forming the integrated approach by compacting layers of gravel directly against the ends of the beams. Each layer is wrapped with a sheet of geotextile. The propose of the wrapped layers is to prevent the migration of fill during thermal movement of the beams. The thickness of the each wrapped layer is about 12 inches. The 12-inch layer is compacted in 6-inch layers with small secondary sheets of reinforcement placed between each 6-inch layer at the beam ends.

Figure 77. Photo. First 6–inch fill lift.

 

Figure 76 through figure 80 illustrate the process of forming the integrated approach by compacting layers of gravel directly against the ends of the beams. Each layer is wrapped with a sheet of geotextile. The propose of the wrapped layers is to prevent the migration of fill during thermal movement of the beams. The thickness of the each wrapped layer is about 12 inches. The 12-inch layer is compacted in 6-inch layers with small secondary sheets of reinforcement placed between each 6-inch layer at the beam ends.

Figure 78. Photo. Secondary reinforcement sheet.

 

Figure 76 through figure 80 illustrate the process of forming the integrated approach by compacting layers of gravel directly against the ends of the beams. Each layer is wrapped with a sheet of geotextile. The propose of the wrapped layers is to prevent the migration of fill during thermal movement of the beams. The thickness of the each wrapped layer is about 12 inches. The 12-inch layer is compacted in 6-inch layers with small secondary sheets of reinforcement placed between each 6-inch layer at the beam ends.

Figure 79. Photo. Completed wrapped approach layer.

  1. Repeat these steps until approximately 2 inches from top of beam grade, as shown in figure 80.

Figures 76 through 80 illustrate the process of forming the integrated approach by compacting layers of gravel directly against the ends of the beams. Each layer is wrapped with a sheet of geotextile. The propose of the wrapped layers is to prevent the migration of fill during thermal movement of the beams. The thickness of the each wrapped layer is about 12 inches. The 12-inch layer is compacted in 6-inch layers with small secondary sheets of reinforcement placed between each 6-inch layer at the beam ends.

Figure 80. Photo. Second 6–inch fill lift.

Multiple sheets can be used along the width of the approach, as long as all seams are kept perpendicular to the beam ends. The typical wrap reinforcement spacing is 12 inches, with intermediate layers spaced at 6 inches and compacted in 6–inch lifts. However, in the case of beams with a reduced depth, the spacing of the wrapped layers may need to be reduced and the intermediate layers eliminated. The top wrap fold should increase in length with each successive wrapped layer until the fill is 2 inches below the bridge grade.

7.10.1 Wrapped Reinforcement Layers on Sides

If lateral spreading of the fill in the integrated approach will be an issue (e.g., wing walls are not sufficient to confine the fill at the sides), the reinforcement sheets comprising the wrapped layers should be folded over along the sides and perpendicular to the bridge (see figure 81 ).

Photo showing a side view of the complete integrated approach. The top wrapped layer is nearly at the same height as the concrete bridge beam.

Figure 81. Photo. Completed approach fill.

7.10.2 Preloading

In some situations, it might be beneficial to preload the abutment before paving to minimize postconstruction deformation or settlement within the GRS mass. A simple method of preloading can be achieved be parking fully loaded trucks on the bridge for several days before placement of the asphalt pavement.

7.10.3 Paving

The top layer of reinforcement should be kept approximately 2 inches below the beam grade. This will allow a layer of aggregate cover to be placed to protect the reinforcement from contact with hot mix asphalt.

When GRS–IBS is built with adjacent precast concrete beams, a layer of paving fabric is extended over the beams onto the approach way. Extending the paving fabric 3 ft over the beam approach interface is recommended. This is necessary to bridge the gap and provide an interface to accommodate thermal movement, minimize surface water infiltration, and prevent cracks in the road. Note that paving fabric is already used on top of the beams as a barrier to water infiltration and to absorb stresses to minimize reflective and fatigue cracking of the new asphalt surface layer.

7.10.4 Guardrail Post

Steel H posts are recommended for any railing that is driven through the reinforcement. It is also possible to drill through the GRS mass with an auger to set other types of posts.

 

7.11 SITE DRAINAGE

The GRS–IBS construction area should be protected from surface runoff during the project. Critical areas are behind the abutment wall at the interface between the GRS abutment and the retained fill, at the base of the abutment, and at any location where a fill slope meets the wall face. Design needs to include provisions for surface drainage along the fill slope adjacent to the wing walls. Provisions for drainage should also be included at the boundary of the wing walls and the fill slope. Long walls built along variable elevation or abutment wing walls are often stepped to reduce excavation. In these situations, the termination of wall steps should be sufficiently embedded to prevent problems with erosion. The drainage swell or channel should be separated from the wall to avoid flow directly against the wall face.

Site preparation for drainage should include the following:

  • Grading: The site should be graded every night in anticipation of precipitation to avoid saturation of soil.

  • Diversion trenches: An alternative to grading is placing diversion trenches around the perimeter to divert water.

  • Compaction of loose soil: Any loose soil placed to construct GRS should be graded and compacted before stoppage of work for the day. Also, onsite stockpiles of fill material containing fines should be protected from excess precipitation.

 

 

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United States Department of Transportation - Federal Highway Administration