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FHWA Home / OIPD / Accelerating Innovation / Every Day Counts / GRS-IBS FAQs

GRS-IBS FAQs

Category Question Answer
Construction What is a typical construction schedule? An abutment can be built in days, depending on the geometry, site complexity, and size of the abutment as well as the experience of the crew. Construction rates initially start slow but speed up as the crew gain experience. The construction method is very simple so even inexperienced people can become proficient quickly. The average construction rates can vary between 200 to 500 sf per day depending on the specific site conditions, geometry, and crew experiece.
Construction Can GRS be used as intermediate bent piers for a multi-span GRS-IBS? Yes, although construction of GRS piers is more difficult. A conventional pier could also be used in combination with GRS-IBS abutments.
Construction Have GRS walls been built as staged construction, rather than full road closure? Yes, GRS walls work very well in this type of construction. The construction is very similar to the current construction practice using MSE technology.
Construction Could GRS-IBS bridges be done as a design-build project? Yes, all contracting options can be utilized for GRS-IBS projects including design-build. For example, RI DOT contracted one project in 2012 using design-build. Value engineering proposals using GRS IBS have also been utilized to save time and money.
Construction How would existing utilities be maintained? The flexibility of the system accommodates construction around existing or planned utilities. It is best to plan for the future installation of the underground utilities through the placement of conduit during construction of the abutment. While it is not recommended, if a water line must be placed within the abutment, it is important to encase the water line within an outside pipe that exits the face of the wall. Try to move utilities outside of reinforced soil structure. Utilities can be sleeved or trenched within the approaches with block outs on the back wall to accommodate existing or anticipated future utilities. If several water bearing utilities are present and cannot be moved, we recommend another wall type which does utilize reinforced soil.
Construction How is the facing held in place during compaction directly behind the wall? There is no need to hold the block in place during construction. Use thin lifts with lighter equipment if the block moves. Refer to Section 7.5 of the GRS-IBS Interim Implementation Guide (FHWA-HRT-11-026).
Construction How does the rebar and concrete penetrate through the geosynthetic layers when tying the top three courses of the GRS abutment wall together? The top two layers of reinforcement are cut inside the block. The rebar and concrete flow only through the top three block layers and are retained on the third layer. Refer to Section 7.7.7 of the GRS-IBS interim implementation Guide (FHWA-HRT-11-026).
Construction How to ensure fixity between the superstructure and the substructure? The superstructure is not mechanically connected to the abutments. The resistance to displacement comes from friction and the passive resistance developed in the GRS mass. Typically, the superstructure is not anchored to the GRS abutment. The integrated approach and wing walls around the superstructure provide the support as the superstructure would need to shear the fill along a passive wedge in order to develop any displacement. Many of the superstructures supported on the IBS have been overtopped without adverse effects. A few of the bridges to date have added details to vent the superstructure to minimize the uplift forces as well as employed some tie down details to address this design consideration. If buoyancy is a concern, deadman anchors can be utilized to tie the superstructure to the abutment. Refer to Section 7.9.4 and Section 7.10 of the GRS-IBS Interim Implementation Guide (FHWA-HRT-11-026). Buoyancy should be considered in the design of any bridge where this may be a concern. If the superstructure is not heavy enough, such as with timber bridges, then anchors may be needed to tie the superstructure to a heavier concrete footing or into the GRS abutment itself. Design details for this case can be supplied upon request.
Construction Is staging the bridge construction a concern? Staging can be done with adequate project planning. Staging or phased construction is very similar to other abutment types.
Construction Does the orientation matter when placing geosynthetic reinforcement? The strong direction for the geosynthetic should be placed perpendicular to the abutment face. For geotextiles and biaxial geogrids, this is commonly the cross-machine direction; for uniaxial geogrids, this is commonly the machine direction. Biaxial reinforcement is recommended where the ultimate wide width tensile strength will be equal in both directions. Note that while the ultimate strength is equal for biaxial reinforcement, the strength at 2% strain will vary by direction; hence, we recommended properly placing the material with the strong direction perpendicular to the abutment face.
Construction What are reasonable construction rates? There is an initial learning curve, but when using a CMU facing, expect 400 blocks/day to start out. After gaining greater efficiency after practice, crews can lay about 600 to 800 block per day. There is a lot of variability depending on the geometry, structure design complexity, excavation, material selection.
Cost and Funding Is GRS-IBS significantly less expensive than traditional methods? Building a bridge with GRS-IBS is potentially 25 to 60 percent less expensive than traditional methods, depending on the standard of construction and the method of contracting (local forces versus a private contractor). Maintenance costs will likely also be reduced since the GRS-IBS has fewer parts than a conventional bridge system and is joint-less. Since GRS-IBS can be built in variable weather conditions, labor costs can also be reduced: few stoppages for weather are needed. Material costs are also reduced. The system is built with common materials and readily available equipment.
Cost and Funding How does the cost of a bridge using GRS-IBS compare to a bridge built on deep foundations? This will depend on the local commercial situation, because one cannot completely separate the deep foundations to make a one-to-one comparison. The foundation selection affects span length, excavation, pile cap, approach slabs, etc. Our experiences have shown overall cost savings from 25 to 60 percent.
Cost and Funding How does contracting the project out impact the cost? Generally speaking the costs will be higher when the project is contracted out. As the contractors gain experience they become much more efficient in construction and the costs should decrease. Several factors, such a risk perceived by the contractor, are difficult to quantify but will diminish with experience. Owner agencies have realized cost savings compared to traditional construction methods even when contracting the project out.
Cost and Funding What is the typical ratio of preconstruction engineering to construction costs (PE/CN) on this type of project? Engineering costs for the projects in Defiance County, St Lawrence County, and Huston Township were around $10k. Depending on the cost of the structure, the PE/CN ratio would typically be 10-15%. This will be highly variable depending on the complexity, size, and location of the structure.
Cost and Funding Have any GRS-IBS projects been constructed using federal funds? Yes, GRS IBS project have been constructed and are in design in many states using Federal Funds. Highway Bridge Program funds may be used if the existing bridge is eligible for the funds.
Cost and Funding Why is FHWA showing cost comparisons for a technology that does not comply with AASHTO’s Standard Specifications for Highway Bridges? The GRS-IBS is a proven, market-ready technology FHWA is rapidly deploying through the Every Day Counts program to bring it into widespread use. Innovation doesn’t begin with code. The IBS was initially developed in the mid-1990s at FHWA based on 20 years of research and experience with GRS walls. Since then, it has been implemented across the country. FHWA has published design and construction guidelines for the GRS-IBS (FHWA-HRT-11-026). Since the guidelines are relatively new, the code has yet to catch up. Generally, State DOT’s follow AASHTO as much as possible, but many have their own specifications to address state specific needs. AASHTO allows this with engineering justifications for the changes. In many cases, State DOTs use technology completely outside of AASHTO, such as soil nail walls. Although AASHTO does not address the GRS-IBS, FHWA has tested this technology demonstrating it can provide excellent performance. Cost comparisons are justified and reasonable showing the GRS-IBS can provide significant construction time and cost savings.
Cost and Funding Can other facing materials provide cost savings over CMUs and better aesthetics over shotcrete? When CMU are available locally meeting the required specification for durability and aesthetics they are the most efficient facing element with regards to cost and constructability. However, if the CMU are not readily available locally then other facing types can be selected to meet the project requirements. It is important for the project that the contract documents be flexible enough to obtain the most economical facing available that meets the project requirements.
Cost and Funding How does FHWA participate in projects where the state/county uses its own resources? The state or local agency needs to receive approval from the local FHWA Division office in order for state or local agencies to use federal funds to construct any project using their own forces.
Design / Engineering How long is the integrated approach behind the beam ends? The length of the integrated approach behind the beam ends depends on the site, but the layers in the upper 2 ft should extend to the cut slope, if applicable. Per the GRS Implementation Guide, the top layer should extend beyond the cut slope, approximately 3-feet, to tie into the existing embankment and minimize any issues at the transition between the existing embankment and newly constructed fill.
Design / Engineering Have projects been adjusted in the field due to encountering unforeseen field conditions (utility lines, etc.)? Yes and due to the modular nature of construction those changes are easy to make.
Design / Engineering What is the highest acceptable ADT for a GRS-IBS project? The GRS-IBS can accept any level of ADT. It can be used on local roads or on highways. ADT is not a design factor or limitation for the IBS. Although the majority of the GRS bridges have an ADT of less than 1000 there are several bridges with an ADT greater than 5,000 vehicles per day and a few with greater than 10,000. There is one which has an ADT greater than 40,000.
Design / Engineering Is there any gradation tolerance to the granular material? For the reinforced backfill, the upper grain-size limit is 2 inches for efficient compaction . The lower limit, or percent fines less than No. 200, is 12% . The recommended grain size limits aid in the constructability of abutments by 1) Allowing for easy placement and grading with hand tools 2) Allowing the use of smaller compaction equipment helps: a) Reducing movement of the blocks during compaction b) improves compaction in tight acute corners, near obstructions, and near the face. An important property of the material to be tested is the friction angle. The minimum required friction angle for the backfill is 38 degrees. Refer to the GRS-IBS Interim Implementation Guide, report FHWA-HRT-11-026, Section 3.3.
Design / Engineering The joints between the blocks do not appear to be sealed. Is there a potential to lose fines in the backfill during flood events? draining so as not to build up hydrostatic pressures behind the abutment wall. The resulting gap between the joints when constructed is small relative to the aggregate size when an open graded fill is selected. Chapter 3, Section 3.1.1, of the GRS-IBS Interim Implementation Guide (FHWA-HRT-11-026) specifies the use of open graded gravels to facilitate the flow of flood waters through the abutments. The open graded gravels are without appreciable percent fines (<5% passing the No 50 sieve) which are larger than the gaps between the facing blocks. No problems have been reported of fines escaping between the facing blocks. The GRS-IBS Interim Implementation Guide limits the amount of fines in the backfill to 12%. If flood-type events are anticipated, it recommends lower fine content, preferably an open graded aggregate. Proper alignment and end-to-end placement of the facing block also confines the fill. Refer to Section 8.3 of the GRS-IBS Interim Implementation Guide (FHWA- HRT-11-026).
Design / Engineering Do measures need to be taken to prevent the coarse material from being clogged by fines (sand)? Only if groundwater will enter the fill from the excavation. If so, a filter geotextile may need to be placed to prevent loss of material from the retained soil. This would typically be the case if there is seepage coming from the excavation.
Design / Engineering Can the GRS-IBS tolerate settlement and/or differential settlements? A GRS wall is a flexible structure that can withstand differential settlement along the wall face, particularly if is built with modular blocks. However, it is advisable to design and construct the wall for even, uniform settlement. Differential settlement issues are more important to the superstructure. Section 10.5.2.2 in the AASHTO LRFD Bridge Design Specifications (2010) outlines tolerable movements and movement criteria. Commentary in the code suggests that for simple spans, the angular distortion should be less than 0.008 rad. The abutment tolerance can be managed by appropriate selection of the facing type and construction sequence.
Design / Engineering How does the GRS-IBS accommodate lateral forces? With respect to external stability, GRS resists lateral earth pressures like that of conventional gravity structure resisting active pressures. Refer to Section 4.3.6.1 in the GRS-IBS Interim Implementation Guide (FHWA-HRT-11-026). GRS is also a ductile material and can absorb high energy lateral and vertical impacts from rocks falls. Refer to Section 8.4 in the GRS-IBS Interim Implementation Guide (FHWA-HRT-11-026) and in the conclusion presentation during the EDC exchange webinar for examples.
Design / Engineering What engineering is performed prior to construction? While the GRS-IBS is very simple to build, it is important that the GRS-IBS is properly designed prior to construction. The design process starts with establishing the project requirements from which the preliminary geometry of the GRS-IBS will be determined. Next, perform site and subsurface evaluations to assess soil, groundwater, drainage, and hydrological conditions at the site. Based on this, conduct a feasibility study to evaluate GRS-IBS as an alternative. Once GRS-IBS has been selected, the applicable loads should be identified and an external stability analysis performed. This includes evaluating the GRS abutment for direct sliding, bearing capacity, and global and compound stability. An internal stability analysis is then performed to ensure the GRS is stable in terms of capacity, deformations, and required reinforcement strength. An iterative process is used to assess the geometry and make adjustments as necessary to facilitate construction and assure long-term performance. Please refer to the GRS-IBS Interim Implementation Guide (FHWA-HRT-11-026) for details on design.
Design / Engineering What are the geotechnical needs for the GRS-IBS? As with any project, there are geotechnical requirements starting with a subsurface evaluation to determine the soil stratigraphy and project feasibility. Refer to AASHTO’s Standard Practice for Conducting Geotechnical Subsurface Investigations or the FHWA’s Soil and Foundations Reference Manual for detailed guidance. Perform soil testing on the foundation, reinforced, and retained soils. FHWA recommends at least 1 boring per abutment although different states may have different requirements. The gradation and strength properties are needed in design. For the reinforced backfill, a large scale direct shear test is recommended to determine the friction angle since the aggregates are larger than what can be tested in standard devices. For well graded reinforced backfill materials, use proctor compaction tests to determine the maximum dry density and optimum moisture content. In the field, perform QA/QC on each lift of compacted backfill to ensure proper compaction.
Design / Engineering What bearing capacity is needed under the reinforced soil foundation? The required bearing capacity is a function of the demand imposed by the GRS mas and the bridge load. The calculated bearing capacity is dependent on the subsurface soil conditions, water elevation, and geometry of the structure. It is also dependent on the settlement limits for the specific structure.
Design / Engineering What are the height limitations for the abutment walls? The GRS-IBS Interim Implementation Guide (FHWA-HRT-11-026) limits the height of the abutments to 30-feet; however, this is based on the tallest GRS abutment to date. There are no technical reasons why a taller abutment cannot be built; the increased weight due to the added materials would just need to be taken into account in the design. FHWA is interested in instrumenting a GRS-IBS with abutment walls taller than 30-feet to expand the limits. If you have, or are considering, a project that meets this criteria, please contact Daniel Alzamora (Daniel.alzamora@dot.gov) or Mike Adams (mike.adams@dot.gov).
Design / Engineering What is the maximum load limit for the IBS? The GRS-IBS is extremely strong; however, a combined bridge dead and live load of 4,000 psf. (unfactored) is recommended as a limit for serviceability. Performance tests conducted at FHWA’s Turner-Fairbank Highway Research Center have measured ultimate capacities much greater than 20,000 psf, which is 5 times greater than the required 4,000 PFS. The GRS-IBS is designed to accommodate the design loads of the road. Most of the structures have been designed for HL-93 loads. No load limits on the bridges are needed.
Design / Engineering What are the failure modes for a GRS abutment? Direct sliding, bearing capacity, and global and compound stability are the three failure modes that must be checked with respect to external stability of a GRS abutment. For internal stability, capacity and required reinforcement strength must be checked at the strength limit state. Refer to Sections 4.3.6 and 4.3.7 in the GRS-IBS Interim Implementation Guide (FHWA-HRT-11-026) for guidance on how to conduct these analyses. For GRS abutments designed and built as outlined the guidance, however, serviceability limit states will likely govern the design, not the strength limit.
Design / Engineering Is the GRS-IBS limited to single-span structures? The GRS-IBS Interim Implementation Guide (FHWA-HRT-11-026) limits the GRS-IBS to single-span structures based on the experience to date. Multi-span structures are possible with the GRS-IBS and have been built; the loads (and subsequent deformations) at each support would need to be accounted for in the design of the superstructure.
Design / Engineering Are sample design calculations available? Yes, a design example is presented in Section 4.4 in the GRS-IBS Interim Implementation Guide (FHWA-HRT-11-026). FHWA is also available for technical assistance on the design and/or construction of the GRS-IBS.
Design / Engineering What gap is left between the ends of the superstructure and the soil to allow for longitudinal expansion, and how is it typically sealed off? No gap between the ends of the superstructure and the fill is required. The IBS is tightly compacted directly against the back wall. There have been no issues associated with the thermal cycles of the superstructure. In fact, they have performed very well, with no bulging of the girders or deck or buckling of the approaches. The joint between the approach and superstructures on over 40 structures have not opened up or developed a crack. The ride-ability across the joint has remained as smooth as when it was constructed. Refer to Section 7.10 of the GRS-IBS Interim Implementation Guide (FHWA-HRT-11-026).
Design / Engineering What is the typical design load for these bridges? The bridge is typically designed for an HL-93 load and then the abutments are designed to account for the total dead and live load. The bearing area is sized to limit the combined dead and live loads to 4,000 psf unfactored.
Design / Engineering Are battered abutments appropriate for use on a GRS-IBS project? Battered facing systems have been used and they do work well. This has been done before on several projects.
Design / Engineering How much compression do you get in the GRS abutment? Depends on the fill and geosynthetic used; however 0.5 percent vertical strain is typical. For a 25 ft. high wall, this equates to about an inch of movement.
Design / Engineering Where can one find more design examples? A design spreadsheet is available, upon request.
Design / Engineering Where can one find design calculations for GRS? FHWA recommends the GRS-IBS Interim Implementation Guide (Publication No. FHWA HRT-11-026) for the design of GRS abutments. In addition, a design spreadsheet is available, upon request.
Design / Engineering Where can one find standard specifications for GRS? Sample guide specifications are available through FHWA Publication No. FHWA-HRT-12-051 (http://www.fhwa.dot.gov/publications/research/infrastructure/structures/12051/).
Design / Engineering How does the GRS-IBS accommodate expansive soils? The GRS-IBS is tolerant to these types of differential movements. Settlement measurements of several IBSs in Defiance County, OH show seasonal fluctuations with the desiccation and hydration of the clay foundation. Both abutments are supported on the same foundation material. No problems have been observed, often with no cracking in the asphalt pavement at the bridge/approach transition.
Design / Engineering How deep are the abutments constructed to account for frost? There is no design embedment requirement related to frost depth. GRS abutments are flexible structures; small movements of heave due to frost will not be transferred through the abutment to the bridge structure. If frost depth needs to be accounted for, excavate to the frost depth and use the geosynthetic reinforced foundation using non-frost susceptible soil to fill in the excavation back to the bottom of wall elevation. There is no need to the bury wall facing lower than the bottom of wall elevation.
Durability How durable are the facing elements used in the wall? The most commonly used facing element, the split-face CMU facing unit, should have 4000 psi compressive strength and 5% water absorption (Section 3.2). This combination of properties seems to provide adequate durability for dry cast CMUs. (See FHWA-HRT-07-021 Durability of Segmental Retaining Wall Blocks) In colder climates, perform a freeze-thaw test (ASTM C1262-10) and follow ASTM C1372 - Standard Specification for Dry-Cast Segmental Retaining Wall Units. Other durable facing elements have also been used.
Durability What is the design life of the geosynthetics? The design life of geosynthetic reinforcement is much greater than the required design life of 100 years. For more detailed information, refer to the FHWA Tech Brief, Durability of Geosynthetics for Highway Applications (http://www.fhwa.dot.gov/publications/research/infrastructure/structures/01050/01050.pdf).
Durability Can the wall face withstand debris impact? GRS walls are specifically used as rock fall barriers for both lateral and vertical protection. The military also uses reinforced soil for blast protection. GRS abutments have been subjected to log impacts during floods without damage. To date, impact due to ice flow has not been observed with any in-service GRS-IBS. Ice striking the abutment wall may crack some of the blocks. The abutment will still be stable and the bridge will remain safe. Depending on the damage, repair may be needed. Refer to the GRS-IBS Interim Implementation Guide (FHWA-HRT-11-026) for more information on repair of the facing elements.
Durability What is the durability of the bridge system? With proper maintenance, life expectancy of a GRS-IBS will be equal to or excee that of a traditional bridge. Like traditional bridge systems, the main durability concern for the GRS-IBS is associated with the superstructure; however IBS creates a joint-less, smooth transition, dramatically reducing the combined effects of corrosion and fatigue of the superstructure. In terms of the substructure, the GRS abutment is comprised of quality granular fill material and polymer reinforcement. Both of these main ingredients are durable and easily capable of producing a substructure with a useful life of more than 100 years. In fact, some users of the technology propose constructing these abutments with plans to accommodate larger superstructures for future bridge widening.
Durability What are the biggest concerns with durability in the GRS-IBS? The oldest production GRS-IBS structure was built in 2005 and shows no indication of durability issues. In fact, it still has no “bump at the end of the bridge.” As with any structure, measures can be taken to ensure durability and design life of the bridge system. For the GRS-IBS, and in the case of precast concrete superstructures, the beam ends embedded in GRS mass should be coated with bitumen or another protective layering system. Also, while the facing blocks are not considered a structural element, they should be manufactured according to the specification in the design guide. The GRS-IBS Interim Implementation Manual provides additional guidance. Chapter 8 of the IBS manual covers aspects of in-service inspection, maintenance and repair.
Durability What are the potential problems with GRS to be addressed in order to extend the life of the structure? The only component of the GRS mass that could degrade within the design life is the facing element. Using a facing meeting the requirement specified in the implementation guide is important to minimize the need to maintain the facing element. Refer to Chapter 8 of the GRS-IBS Interim Implementation Guide (FHWA-HRT-11-026).
Durability How is long-term settlement accounted for? Long term settlement of the foundation is calculated using conventional geotechnical practice. The abutment settlement is calculated based on the stress strain curves for GRS IBS.
Durability What is the long term durability of the geosynthetic reinforcement? The geosynthetic reinforcement is manufactured of engineered products that have been tested for the long term durability under various environmental conditions. This data is described in the Geosynthetics Design and Construction Guidelines, publication No. FHWA NHI-07-092, dated August 2008.
Durability Can the facing blocks be repaired? Yes, they can be repaired. One of the methods to repair the blocks is to chip out the damaged facing and grout a new facing element in its place.
Durability What data supports the projected 100-year service life? The structural components (aggregate and buried geotextile) have extremely high expected service lives. The cosmetic face may need cosmetic repairs over 100 years.
Durability How have these abutments performed during and after floods, especially streams carrying silts and debris? Many GRS-IBS projects cross water with sequential flooding. The abutments have performed very well, with debris removal on the superstructure as the only maintance needed. For projects crossing waterways, it is important to evaluate scour, stream instability, and countermeasures. Refer to Chapter 5 in the FHWA-HRT-11-026.
Durability How durable are the dry cast blocks considering road salts and freeze/thaw? The quality of dry cast product has improved considerably during the past decade. Information about the durability of SRW block is available here: http://www.fhwa.dot.gov/publications/research/infrastructure/structures/07021/. In addition some state DOTs (New York) have developed both performance and material specifications for dry cast blocks. The National Concrete Masonry Association (NCMA) also has additional information about the durability of dry cast blocks.
Design / Engineering Can the bridge be widened in the future? Yes, use standard phased construction techniques to widen the roadway. Use a minimum increased width of 30% of the wall height. We also recommend extending the reinforcement in the upper 2 ft portion of the wall a minimum of 3 ft, to allow integration of a future widening effort with the GRS-IBS.
Extreme Events How does GRS-IBS design address scour? FHWA guidance HEC-18, HEC-20, and HEC-23 address the design of an IBS for scour, stream instability, and abutment countermeasures, respectively. Chapter 5 of report FHWA HRT-11-026, Hydraulic Design of the GRS-IBS, summarizes the requirements for hydraulic considerations. Consult a hydraulic engineer in the design of any bridge structure over water.
Extreme Events What erosion and scour protection is used for the GRS-IBS? Riprap is the most common scour countermeasure that has been used for water crossing projects to date. It has performed very well. Figure 36 on page 72 of FHWA-HRT-11-026 illustrates a typical cross-section for the scour countermeasure. More information about the design of a proper countermeasure can be obtained from Design Guideline 18 Riprap Protection for Bottomless Culverts ( http://www.fhwa.dot.gov/engineering/hydraulics/pubs/09112/page18.cfm). Additional information about bridge scour and stream instability and countermeasure design can be found in HEC-23 (http://www.fhwa.dot.gov/engineering/hydraulics/pubs/09111/index.cfm) Also, design a GRS abutment to facilitate surface drainage along the wing walls to prevent erosion and undermining similar to traditional structures. A variety of mitigation strategies exist: Set a foundation depth below the calculated scour for our soils or below the channel bottom, armor the foundation with appropriately sized rock channel protection, and use pigmented, solid CMU's behind the rock channel protection so that any movement or loss of rock channel protection can be easily monitored and repaired.
Extreme Events Can I use GRS-IBS in locations with high scour risk? The following excerpt from the GRS-IBS Interim Implementation Guide (FHWA-HRT-11-026) addresses scour (http://www.fhwa.dot.gov/publications/research/infrastructure/structures/11026/11026.pdf). “When bridges are constructed to span a waterway, their foundations must be designed, detailed, and constructed in compliance with section 2.6 (Hydrology and Hydraulics) of the AASHTO LRFD Bridge Design Specifications or an FHWA Division Office-approved drainage or bridge manual. These provisions apply equally to both shallow and deep foundations. GRS-IBS has been successfully used to build abutments near rivers and streams. However, assessing the potential impact of stream instability, scour, and adverse flow conditions is a vital consideration in the decision to use this technology. The potential for issues with stream instability, scour, and adverse flow conditions can lead to deep foundation bottom elevations or expensive countermeasures that could reduce the cost-effectiveness of GRS-IBS abutments. If the potential for abutment scour, contraction scour, long-term degradation, or channel migration is high, costly design considerations or countermeasures could be required. Other factors, such as channel instability and adverse flow conditions (skewed approach flow, highly contracted flow, high velocity flow through the bridge opening, etc.) at the bridge, could also result in costly design considerations or countermeasures to stabilize the channel against further instability. Any of these conditions might make it advisable to select an alternative bridge abutment technology. A thorough hydraulic analysis, scour evaluation, and assessment of channel stability of a bridge design will include an appropriate estimate of the design flow, development of water surface profiles through the proposed opening, assessment of scour (abutment, contraction, and long term degradation), and if necessary, the design of countermeasures to protect the bridge or stabilize the channel. FHWA and others have developed procedures to assist the engineer in performing these analyses, and these procedures should be followed for GRS-IBS design.” The decision to use GRS IBS should be based on the constructability and cost of making the required excavation to set the faing below the scour depth.
Extreme Events Can GRS IBS be used in seismically active regions? How do GRS-IBS structures perform during seismic events? Yes, GRS-IBS technology can and have been utilized in seismic regions such as UT, CA, WA, and HI. GRS abutments perform very well in seismic events as has been observed with all bottom-up reinforced soil walls.
Extreme Events How is the GRS-IBS designed for seismic loading? A GRS structure should be designed for external stability with respect to seismic loads as with any other gravity wall., The National Cooperative Highway Research Program (NCHRP) recently completed a study, 12-59(1), The Seismic Performance of GRS Abutments. This study showed, through full scale shake table tests up to 1g and numerical modeling, that a GRS abutment could withstand ground these types of loading conditions. For these reasons, a no seismic design requirement is suggested for the internal stability of a GRS-IBS provided it is built as outlined in section 5.4, page 73 in the Implementation Guide (FHWA-HRT-11-026). For single span bridges, the loads from the superstructure due to seismic shaking needs to be addressed in the design to make sure the bridge does not slide off of the bearing areas. With the GRS IBS, the bridge is restrained longitudinally and laterally by the passive resistance from the GRS integrated approach. In the design of a single span bridge, the main concern is ensuring that the bridge will not shake off of its support. This is achieved either by anchoring, developing a sufficient bearing area to accommodate the anticipated displacement, or a combination.
Extreme Events Have any tests compared the performance of GRS versus MSE walls under seismic loads? There has not been any specific test that addresses this question but both have had shake table tests with blocks and both have demonstrated the excellent performance of a reinforced soil system.
Extreme Events Regarding AASHTO LRFD Bridge Design Specification Appendix A11 - Seismic design of abutments, how are Monoonobe-Okabe Analysis parameters modified by GRS? Either M-O or generalized limit equilibrium methods are used to satisfy external stability under seismic conditions. No special modifications are needed for the design of GRS specifically. Note that for GRS, there are no special internal stability requirements for seismic conditions.
In-Service Performance What is the recommended post-construction maintenance for GRS? Refer to Chapter 8 of the GRS-IBS Interim Implementation Guide (FHWA-HRT-11-026). Inspect the abutments on the standard bridge inspection cycle. Keep vegetation off of the facing. Also deck washing, scour and erosion monitoring, abutment wall settlement monitoring, approach settlement monitoring. Not much different than other abutment walls.
In-Service Performance How would the GRS-IBS perform if the approach guard rail is hit? The approach rail on the first GRS built in St. Lawrence County, NY was hit and it reacted as anticipated They had to pull 2 posts that were bent at ground level and replace them. Note the GRS-IBS Interim Implementation Guide recommends a setback distance of 4 ft. or the deflection distance (whichever is greater) for guardrail posts.
In-Service Performance How do GRS-IBS structures hold up to flooding? Most of the GRS-IBS projects have been built over water. Many of these have experienced frequent flood events without adverse performance. However, it is important to design the abutment in accordance to the Interim Implementation Guide which covers the use of the FHWA Hydraulic Engineering Circular (HEC) manuals, linked below. For abutments crossing water, an open graded aggregate (e.g. AASHTO No. 89, No. 8, No. 78, etc.) is recommended for the select backfill material. HEC-18 HEC-20 HEC-23 Hydraulic Design of the GRS-IBS (FHWA-HRT-11-026)
In-Service Performance Have any GRS-IBSs been submerged? If so, how did they perform? A project constructed in ME is subject to several feet of tidal fluctuations on a daily basis and during construction the entire site would be under water .
In-Service Performance What is the age of the oldest GRS-IBS in-service? The oldest production IBS was built in 2005.
In-Service Performance Have there been issues related to moisture/seepage behind and through the masonry blocks? Based on the in-service performance of numerous GRS-IBS case histories, moisture/seepage between blocks has not presented problems and can be considered a positive attribute by mitigating hydrostatic pressure with the GRS mass. Efflorescence of the facing element could occur as result of moisture reacting with dry cast modular block product, but it is primarily a cosmetic problem.
In-Service Performance How does GRS-IBS respond to thermal cycles? The response of this system to thermal cycles has been great. No cracks in the pavement where the bridge meets the approach has been observed in the in-service GRS-IBSs, even the 140-feet steel girder bridge in Defiance County, OH.
In-Service Performance Have any GRS-IBS structures failed? As of April 2015, there have been no failures. The structures have performed as anticipated. Failures are not expected to occur if designed and constructed according to FHWA guidance (FHWA-HRT-11-026).
In-Service Performance Have any GRS-IBS projects had issues with the facing elements? No; the facing element is a façade and a form for each lift of fill. It is not a structural element. The GRS is internally supported. There have been no reports of major issues with the CMU facing elements used on GRS-IBS projects. Minor cosmetic problems have occurred with cracked blocks when the gravel is not swept from the tops of the block leading to point loads. Refer to Section 7.7.2 in the GRS-IBS Interim Implementation Guide (FHWA-HRT-11-026).
In-Service Performance Is data available regarding lateral movement of GRS abutments? Yes, FHWA has been and continues to monitor in place structures. There are several papers publishing this data. For more information, please contact Mike Adams at mike.adams@dot.gov.
In-Service Performance Have there been any issues with erosion/scour on water crossings? To date, there have been no issues with scour for in-service IBSs. Each bridge needs to be designed for the hydraulic conditions at the site, including estimating the scour depth and appropriately sizing the riprap.
In-Service Performance Is monitoring data being collected for the GRS-IBS bridges? Currently, FHWA monitors the performance of several bridges, with plans to instrument many others. Some of this information is available in the FHWA Synthesis Report (Publication No. FHWA HRT-11-027). The performance of the bridges is also recorded as part of the state and federal bridge inspection policies.
Key Concepts When did DOTs begin building GRS abutments? The first structure was constructed in 2005.
Key Concepts Will there be any more showcase or demonstration projects? Yes, we anticipate holding several showcase demonstrations around the country as projects go out for construction. Refer to the EDC website for a listing of scheduled events as they are announced.
Key Concepts What is an “Applicable Performance Test” in the GRS-IBS Interim Implementation Guide? The GRS-IBS Interim Implementation Guide (FHWA-HRT-11-026) recommends an “applicable performance test.” A performance test (or mini-pier experiment) is a large element GRS load test used to provide a designer with material strength properties of a particular GRS composite mass built with a unique combination of reinforcement, compacted fill, and facing elements. The procedure involves axially loading the GRS mass while measuring vertical settlement and lateral deformation to monitor performance. The resulting stress-strain curve can then be used to aid in the design process to predict performance of a full-scale GRS composite abutment. Refer to Appendix B in the GRS-IBS Interim Implementation Guide for more information.
Key Concepts How does the design differ from MSE? The design methodology for the GRS abutment, outlined in the GRS-IBS Interim Implementation Guide (FHWA-HRT-11-026), differs in ten major aspects from current practice outlined in AASHTO and GEC-11: (1) maximum reinforcement spacing, (2) minimum reinforcement length, (3) vertical capacity, (4) deformations, (5) reinforcement strength, (6) pullout, (7) connection requirements, (8) maximum friction angle, (9) minimum bridge seat width and (10) limiting eccentricity. These differences are primarily due to the close reinforcement spacing (≤ 12 inches) required for the GRS IBS. Increasing the frequency of reinforcement forms a more efficient composite material that behaves differently from the tie-back wall system assumed in the AASHTO design method. The design method used for a GRS-IBS should follow FHWA-HRT-11-026 to achieve the most economical bridge structure. For more specific information, please contact Jennifer Nicks (Jennifer.nicks@dot.gov).
Key Concepts Does this approach conflict with the typically conservative nature of bridge engineers? Where does FHWA envision this being used going forward? For counties, it makes sense where more risk can be taken, but can it be used on trunk lines? No. The GRS-IBS is a safe alternative depending on the site and project conditions, regardless of ADT. FHWA will continue to encourage its use where it is appropriate.
Key Concepts Is the geotextile connected to blocks? Geosynthetic reinforcement is connected to the facing through friction.
Key Concepts Do LTAP training materials exist? Yes; materials are already available to assist with training development. Contact Daniel Alzamora (daniel.alzamora@dot.gov) for details.
Key Concepts How well has the Interim Implementation Guide been received by AASHTO and designers? Designers are using the Interim Implementation Guide for design guidance. Working through the AASHTO review and approval process will take some time.
Key Concepts Is the performance the same between GRS and MSE? The close spacing of reinforcement improves the efficiency and redundancy of the soil/reinforcement interaction which reduces the loads on the reinforcement and on the connection. In addition, the close spacing improves the placement and compaction of the reinforced fill improving the overall performance of the structure.
Key Concepts What is Geosynthetic Reinforced Soil (GRS)? Geosynthetic Reinforced Soil (GRS) is an engineered fill consisting of alternating layers of compacted granular fill material and sheets of geosynthetic reinforcement spaced at a maximum of 12 inch vertical spacing.
Key Concepts What is an Integrated Bridge System (IBS)? An Integrated Bridge System (IBS) is a fast, cost-effective method of bridge support that blends the roadway into the superstructure using GRS technology. This creates a simple, joint-less interface between the bridge and the roadway. The IBS is typically built without many of the elements common to a conventional bridge abutment (e.g., the deep foundation, bridge seat, bridge bearings, deck joints, approach slab, end wall, and sleeper slab). It consists of (1) a reinforced soil foundation (RSF), (2) a GRS abutment, and (3) a GRS integrated approach.
Key Concepts What are the benefits of an IBS? The performance of GRS-IBS bridges to date has been an improvement on similar bridges built with conventional construction techniques. The GRS-IBS bridges have performed as well as the conventional bridges structurally and functionally in addition to eliminating the "bump at the end of the bridge" that often results from conventional construction. The suppression of the "bump at the end of the bridge" has been maintained to date for all of the GRS-IBS bridges in service. The first bridge constructed with this technology, the Bowman Road Bridge, has been in service for almost 5 years. In fact, this bridge has not even experienced cracking of the asphalt layer from the road to the bridge.
Key Concepts What is Accelerated Bridge Construction (ABC)? Accelerated Bridge Construction (ABC) is the term given to technologies and methods that fast-track bridge construction. In addition to saving time, these methods typically save money. GRS-IBS is a type of ABC.
Key Concepts What is Mechanically Stabilized Earth (MSE)? Mechanically Stabilized Earth (MSE) uses alternating layers of compacted soil and reinforcement with a typical max spacing of 32 inches. The reinforcement can be manufactured from steel or geogrid material and serves to provide tensile resistance to the soil. At this spacing, MSE walls need a facing element to retain the soil and transfer the lateral earth pressures against the facing to the reinforcement.
Key Concepts How does GRS differ from MSE? A primary difference is that the reinforcement layers are spaced at a maximum of 12 inches. With MSE, layers of reinforcement can be typically spaced up to 32 inches vertically. The close spacing improves the efficiency and redundancy of the reinforcement interaction with the soil. This helps reduce the loading on the reinforcement and facing whith improved constructability and performance.
Materials Have tests been performed with reinforced backfills having a high percentage of fines? No. The fill used in large scale GRS tests contained fines within the specified limits for fill of 12% or less. Only quality granular structural backfill is recommended for load bearing applications to minimize compression of the fill under the bridge load. Other fill materials may be considered outside the influence from the bridge load. While not recommended, if no select fill types are used then the following should be considered: • Increased deformations that may be time dependent • More difficult constructability • More detailed drainage considerations • Reduced soil shear strength • Increased QC/QA
Materials Are the facing elements cinder blocks? The recommended Concrete Masonry Unit (CMU) facing units are not cinder blocks. However, the term “cinder block” is often used as a generic term for hollow core CMU’s. A true cinder block would not meet the recommended strength (4000 psi) for a CMU under Section 3.2 of the GRS-IBS guidelines. Cinder blocks are low density or lighter weight CMU blocks, consisting of Portland cement and cinders (coal burning waste / ash) and no aggregate. The compressive strength of a cinder block usually ranges between 2800 and 3800 psi and could be as low 1900 psi (the minimum strength for light weight blocks per ASTM C90). For more information, refer to the GRS-IBS interim Implementation Guide, report FHWA-HRT-11-026, Section 3.2. A cinder block weighs approximately 30 lb., whereas a split face CMU weighs 42-45 lb. This added weight aids in compaction of fill near the wall face and reduces movement of the block during compaction. CMU blocks can be produced to meet a variety of performance and material specifications.
Materials Are the facing blocks solid? Solid blocks have been used for the bottom courses – from the RSF up to the level of scour protection – then use hollow core units. Solid blocks in this zone add an extra level of durability in a difficult-to-monitor area, since it is obscured by the channel rock protection. Refer to the GRS-IBS Interim Implementation Guide, report FHWA-HRT-11-026, Section 3.2 and Section 7.4.
Materials Do the facing blocks interlock? There is no connection requirement between blocks or between blocks and the geosynthetic reinforcement. Blocks that have pseudo connections or alignment mechanisms have been used for GRS IBS projects but these are not a design requirement. Refer to the GRS-IBS interim Implementation Guide; report FHWA-HRT-11-026, Section 4.1 and Section 7.7.2.
Materials Have any GRS abutments been constructed with a facing other than modular blocks? Yes, GRS IBS can be constructed with just about any facing type. Other facing elements used in GRS-IBS: • Sheet pile • Pre-cast panels • Standard SRW blocks • Large wet-cast blocks • Wrapped face walls behind existing cantilever walls • Wrapped face walls covered with shotcrete • Wrapped face walls protected by gabion mattresses. Refer to the GRS-IBS Interim Implementation Guide, report FHWA-HRT-11-026, Section 3.2.
Materials What are the specifications for the geotextile fabric? An ultimate strength of at least 4,800 lb./ft. is used for GRS load-bearing applications. In some cases, it might be appropriate to specify stronger reinforcement strength depending on the design requirements. Chapter 4 of the GRS-IBS Interim Implementation Guide (FHWA-HRT-11-026) provides guidance regarding the required reinforcement strength for a particular application, which is a function of the lateral stress, reinforcement spacing, and backfill properties. For information about the calculation of the required reinforcement strength, refer to Sections 4.3.7.3 and 4.4.7.4. Another important consideration in the performance of GRS-IBS is the reinforcement strength at 2 percent strain. (See Chapter 4). Limiting the required reinforcement strength to less than the reinforcement strength at 2 percent strain will ensure long-term performance and serviceability.
Materials What are some common footings that have been used with the IBS? A separate footing is not required with the IBS when using adjacent box beams. They can be placed directly on the GRS abutment. With girders, a precast or cast-in-place footing can be used to distribute the load evenly to the GRS abutment. This footing only needs to be strong enough to evenly distribute the load from the girders to the GRS mass.
Materials Can the reinforced soil foundation be a solid footing or is it a bridging element? The RSF is recommended below the GRS abutment to better distrubute the abutment loads to minmize differential movements across the abutment wall. The foundation is a granular fill encapsulated in geotextile. It is an interface between the GRS abutment and the existing ground conditions.
Materials Can you build GRS with lightweight fills? Yes, as long as the fill meets strength (friction angle) and durability (survive placement and compaction) requirements.
Materials Can sheet pile be used for the facing material and if so, how deep it should be driven? Sheet piles could be an efficient material for GRS-IBS facing. Sheet pile could be more expensive than a CMU or other block types, however use of sheet pile in water crossings would allow the construction to work in the dry by designing the GRS abutment from above the water table. This would not only eliminate excavation below the water table but also reduce the height of the wall and overall volume of excavation/fill required. The GRS abutment could be designed as a wrapped face against the sheet pile thus transferring the lateral earth pressure lower in the wall reducing the moment arm being carried by the sheet pile. The sheet pile would also act as the scour countermeasure. The depth of the sheet pile would have to be designed for the site specific conditions but it should consider the height from the bottom of the GRS to the scour depth as the cantilever wall.
Other Bridge Components Can one use standard guardr ails with GRS supported bridges? Select the guard rail you would normally use for the bridge approach. Wooden guardrail posts will require pre-punching with a steel punch to install. Guard rail post penetrations due to their installation do not impact the stability of the GRS abutment.
Other Bridge Components Have existing superstructures been re-used for GRS-IBS projects? As of April 2015, we are not aware of existing superstructures having been reused on top of a GRS-IBS. Railroad cars have been used as superstructures for a GRS-IBS project in Iowa. That said, considerations for reusing superstructures on a GRS-IBS would be the same as with any other type of sub-structure.
Other Bridge Components Can GRS-IBS work without an asphalt overlay? Yes, although most of the structures have used asphalt on top of the concrete box girders. Projects have been constructed with a CIP slab or overlay.
Other Bridge Components What are the considerations for a GRS-IBS project utilizing steel girders? Set steel girders on either a cast-in-place or pre-cast concrete footing to distribute the girder load to the GRS mass. Construct back and side walls to form the parapets, with the integrated approach around the superstructure.
Other Bridge Components What is the typical bearing area for the beams? The minimum bearing with is 2.0 ft for spans less than or equal to 25 ft and 2.5 ft for spans greater than 25 ft. The design width of the bearing area is calculated to limit the load from the bridge to 4,000 psf.
Other Bridge Components What is the detail for placing the bridge beams on the GRS abutment? Depends on the type of girder. Most of the GRS projects have used adjacent box beams placed directly on the top layer of reinforcement of the GRS abutment. Refer to Sections 7.8, and 7.9 of the GRS-IBS Interim Implementation Guide (FHWA-HRT-11-026). Additional information can be found in the standard plan sheets.
Other Bridge Components Without typical bearings (elastomeric pads, anchor bolts, etc.), how are longitudinal and transverse loads (wind, stream pressure, seismic) from the superstructure transferred to the GRS abutments? The loads are resisted by the friction between the footing and the abutment and the passive earth pressures from the integrated approach and the parapet walls, all made of the same GRS material.
Site Selection How are these projects typically environmentally classified? GRS-IBS projects will have all the same permits and clearances required of any similar projects. The type of project does not eliminate or reduce the required permitting.
Site Selection Is there a stream velocity restriction for the use of GRS abutments? No; however the stream velocity will feed into the scour analysis and scour countermeasure design. If the scour analysis shows that the excavation depths are not practical from a cost and/or constructability perspective then another substructure type should be selected.
Site Selection Have GRS-IBS projects been built on soft subgrades? Yes, GRS abutments and walls have been built on weak soils. Results from a subsurface investigation should be used for a settlement and bearing capacity analysis during design of the IBS. The GRS abutment should be built on a reinforced soil foundation (RSF) to ensure uniform even settlement. For many bridge replacement projects, there is no appreciable change in stress history between the old and new structures. The existing structure preloaded the foundation soil and any net change in the surcharge will be the added weight of the superstructure. This should be accounted for in the design.
Site Selection Have GRS-IBS projects been built in areas with high stream velocities (e.g. in mountainous areas)? GRS abutments have been built in mountainous terrain with stream velocities of up to 10 fps; however, they were designed to accommodate the potential for scour. In some cases, the GRS abutment was built directly behind the existing abutment.
Site Selection Has the IBS been used for bridges on a skew, grade, or super elevation? Yes, the IBS can accommodate any level of skew, grade, and super elevation. For a skewed bridge, it is important to maintain the minimum bearing width of 2.5-feet along the length of the abutment wall face. For a bridge with super elevation, it is important to ensure that the minimum number of bearing bed reinforcement layers beneath the beam seat are installed across the length of the abutment face. At this time, there are no special considerations for GRS abutments that support a bridge on a grade. The first IBS built, the Bowman Road Bridge in Defiance County, OH, had skew of 24 degrees, and a super elevation of 7.6 degrees.
Site Selection Would the railroad industry honor this type of technology over one of their railroads? Each railroad is different, so this issue needs to be discussed with the particular railroad involved in the project. GRS bridges have been built over railroads and can be designed to withstand impact forces (for example, from a train derailment). The technology has been successfully used to construct rock fall barriers for both vertical and lateral protection.
Site Selection Has a GRS-IBS project been built within the flood plain where it has been submerged - how did the abutment standup? Several have been submerged (as anticipated in design) with no issues. NY DOT had a GRS-IBS bridge that was submerged within a couple of months of completion. There was no movement or detrimental effects of any kind.
Site Selection Do you have any published guidelines for evaluating the characteristics of what sites are or are not suitable for GRS? Anything that has a shallow foundation is suitable. You have to consider scour depth on a case-by-case basis.
Site Selection Do GRS-IBS projects have to be single span? While FHWA primarily recommends its use for single span structures, there have been several multi-span structures constructed using GRS IBS. The primary consideration is the settlement of the structure which is composed of foundation settlement and fill compression.
Site Selection On what kind of roadways can GRS IBS be used? GRS-IBS can be used to build bridges on all types of roads, whether they are on the National Highway System (NHS) or the local system.
Site Selection In what situations is GRS IBS type not recommended? This abutment type is not recommended at sites with large scour estimates. This is a design consideration when evaluating the suitability of this type of structure from a constructability and cost perspective.
Site Selection Are site selection criteria available? Yes; refer to the GRS-IBS Interim Implementation Guide (Publication No. FHWA HRT-11-026) for preliminary site selection criteria. Specific questions can be directed to Daniel Alzamora at daniel.alzamora@dot.gov.
Site Selection What are the longest bridge spans that GRS abutments can support? The longest constructed so far used 140 ft girders. The GRS abutment doesn't “know” if the bridge is 200 ft or 50 ft. The bearing area is sized to limit the load to 4000 psf and this load is accounted for in the design of the GRS-IBS. The issue is not technical but a practical question of how deep the girders would need to be and how much the structure would cost vs adding a pier.
Site Selection Is GRS appropriate for soft foundation conditions? Yes. Design of a GRS-IBS on soft soils requires the same level of design as any other shallow foundation. The geotechnical engineer will calculate the estimated settlement and the associated time based on the subsurface investigation. A decision will be made whether ground improvement is needed and practical to implement on the specific project. Typically, single span structures handle total and deferential settlements without any damage, which is consistent with our experience. Ground improvement can be used to mitigate these issues. Finally if there is a concern with long term foundation settlement with a bridge on pile foundations then the bump at the end of the bridge becomes more of an issue, increasing maintenance at the bridge approaches.
Site Selection Can GRS abutments be constructed over liquefiable soils in seismic regions? Yes, although ground improvement would be recommended if constructing an abutment on liquefiable soil in seismic regions.
Page last modified on March 26, 2018