U.S. Department of Transportation
Federal Highway Administration
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Federal Highway Administration Research and Technology
Coordinating, Developing, and Delivering Highway Transportation Innovations
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Publication Number: FHWA-HRT-11-026
Date: January 2011
Geosynthetic Reinforced Soil Integrated Bridge System Interim Implementation Guide
CHAPTER 9. QUALITY CONTROL AND QUALITY ASSURANCE
Quality is everyone's responsibility. The quality of GRS–IBS begins with an understanding of the concept of wall strength and the interrelation of components of a GRS system and with sound construction practices.
QC consists of implementation, measurement, and enforcement of sound construction practices and field inspection procedures to ensure construction quality as outlined in this manual. QC also involves the selection of quality materials. The successful completion of a project is dependent on a proper monitoring program with necessary adjustments at each stage of construction. QC is the responsibility of the builder.
QA is necessary to ensure the finished product meets specifications through inspection, testing, and final acceptance. The process involves constant evaluation of the project activities related to planning, design, development of plans and specifications, construction, and all interactions associated with these fundamental activities. QA can either be the responsibility of the owner agency or a third–party agency.
9.2 ROLE OF THE CONTRACTOR
Since GRS is a nonproprietary generic wall system, the contractor building the wall can be responsible for developing and maintaining a QA/QC plan for project quality. Prequalification based on the procedures outlined in this manual should be a necessary requirement for this type of construction.
QC testing performed during construction mainly applies to onsite field testing of backfill material and associated laboratory tests.
9.3.1 Laboratory Testing
Gradation and moisture–density tests (e.g., Proctor compaction test) will be required for field monitoring of the backfill material. The classification tests and moisture–density tests should follow AASHTO standards for aggregate sampling and testing.
Large–scale direct shear tests or triaxial tests are the most effective methods for determining the friction angle for coarse–grained backfill aggregates. These methods of testing are preferred over the standard direct shear test (AASHTO T236) or smaller diameter triaxial tests that are performed on the minus No. 10 material.
9.3.2 Field Testing
Fill placement and compaction is the predominant construction activity that needs to be monitored in a GRS–IBS project. Field density tests should be performed on each layer. The field test method should be applicable to the aggregate type that is used for the backfill material.
Dense–graded backfill, which consists of State transportation department crushed base course, can be tested with a nuclear gauge. State transportation department density testing procedures can be used. Moisture content should be monitored and controlled prior to fill placement for an effective compaction process.
A procedural (method–based) specification is preferable for the compaction of open–graded fill material, which exhibits a high percentage of void space. Open–graded gravels are not conducive to in–place nuclear density testing procedures as the direct transmission nuclear gauge procedure is difficult to perform (the transmission hole will typically not stay open), and nuclear backscatter testing is not effective due to poor soil/gauge contact.
In lieu of density testing for open–graded gravels, maximum density can be achieved with a recommended procedural specification. The procedure can specify three to five passes with a walk–behind vibratory plate compactor near the wall face. Larger ride–on vibratory rollers with greater frequency and efficiency can be used in the core of the GRS mass with fewer passes 3 ft from the wall face.
9.4 CONSTRUCTION INSPECTION
Thorough inspection before and during construction will ensure the GRS structure is built in accordance to the plans and guidelines. Inspection requires an understanding of GRS design and methodology. Familiarity and understanding of the drawings is necessary. It is important to have firsthand knowledge of the GRS construction processes. A properly implemented field inspection program provides an opportunity to take corrective action during the construction process.
A critical component of construction is compaction behind the facing element followed by placement of the geosynthetic reinforcement. Those responsible for performing these construction activities are best suited for maintaining the quality of each GRS wall layer. Note that in the RSF and the integrated approach, a geotextile must be used to prevent migration of fill material and erosion.
Once materials are delivered to the site, they should be inspected for compliance with the guidelines and project specifications. Materials should be visually inspected for quality, damage, and defects.
Backfill: In addition to the quarry material certificate showing the gradation of the aggregate, a visual inspection should be performed to verify maximum grain size, amount of fines and grain shape (angular or rounded), excess fines, moisture content, and durability.
Facing block: As outlined in chapter 3, the facing block should be inspected for integrity, consistency, and dimension tolerances. Confirm that sufficient quantities and proper block type (e.g., solid block, corners, and face block) are present onsite and ready for use.
Geosynthetic reinforcement: Verify that the specified type and strength of geosynthetic is correct along with the required roll dimensions. Chapter 3 provides detailed tests that should be documented for each roll of reinforcement.
Compaction of the backfill in a GRS wall or abutment is a critical construction activity. It should be confirmed that the compaction equipment onsite is compatible with the selected backfill material.
Verify that the required hand tools are onsite for spreading and grading aggregate, maintaining the facing alignment, and sweeping the top of the CMU facing block.
9.4.3 Project Layout
Verify that all layout reference points are established, with particular emphasis on the location of the following areas:
9.4.4 Construction Activities
GRS is built from the bottom to the top. Those responsible for inspection need to make certain each layer is constructed and tested in accordance with the contract drawings and specifications before proceeding with subsequent layers. Inspection and QC/QA activities are discussed in this section.
Working bench: Before excavation, the working bench/platform needs to be inspected for stability with consideration for drainage. Any movement should be controlled.
Foundation excavation: The foundation should be cut as outlined in the plan and inspected for any soft areas before compaction and proof rolling.
Geotextile–wrapped RSF foundation: For encapsulation of the RSF, it should be confirmed that the open edge of any overlap is facing downstream and the three sides (two wing walls and one abutment face wall) are contained by a layer of geotextile to prevent erosion.
Leveling course: In order to set the first course of facing block level and plumb, the top elevation of the RSF should be as close to grade as possible. Often, a thin (0.5–inch) leveling layer of aggregate is placed under the first course. Inspection of this leveling layer should be performed to determine its thickness or the need to replace it with a low slump wet concrete/grout mix.
Compaction of backfill: Inspection of backfill operations should verify compliance with the construction guidelines outlined in chapter 7. Compaction behind the wall face and within the bearing area is important. Inspection should confirm that each lift never exceeds the specified thickness.
Compaction control should be maintained through field density tests or other soil stiffness–based methods. For backfill material containing fines (minus No. 200), the moisture content should be within the specified range (±2 percent). This improves the compaction process. Compaction of open–graded aggregate should be observed to ensure nonmovement of material under the compaction equipment; this observation is an indication of compaction or stiffness and is dictated by the number of passes.
Reinforcement installation: Inspect the installation of each reinforcement layer to ensure it is properly placed, has adequate facing element coverage for the frictional connection, and is free of wrinkles. Anticipate the location and placement of the bearing bed reinforcement layers, particularly in situations when the bridge is superelevated.
Facing block placement: Prior to placement of the reinforcement layer over the facing block, the block should be inspected to verify a clean surface. This is essential in maintaining wall alignment and avoiding block cracking due to point loads. The inspection process should ensure that there is no rocking motion when setting the block, which can be indicative of point load bearing.
Wall alignment: Visual inspection should be performed at regular intervals during construction. This will help ensure that the wall is within vertical and horizontal tolerances for alignment. QC should be performed on block alignment using a string line on at least every third course. Vertical alignment can be checked with a plumb bob.
Wall termination: Make sure that all wing wall terminations are sufficiently embedded to prevent undermining from erosion. A terminated course needs to be founded on a stable compacted layer of granular fill material as outlined in chapter 7 or on an excavated cut into native soils.
Fill slope side: The fill slope at the wing walls is usually built with native soil as the GRS wall advances upward. The fill slope should be constructed with a drain path that leads away from the wall face. Surface runoff should be diverted to prevent saturation of the soil fill slope. Temporary drainage may need to be installed to preserve the integrity of the cut slope.
Site drainage: The working platform should be compacted and graded to drain surface water away from the working area. Any pit excavation should be sloped to drain to a location that can be pumped.
Heavy equipment operation: It is beneficial to have construction equipment centrally located in the work area and to have materials strategically stocked near the equipment for efficient transfer to the labor crew. Equipment operators should take caution when working near large layers of exposed geosynthetic.
Beam seat: Construction of the beam seat should be inspected to confirm the use of methods described in chapter 7. It is important to verify that the beam seat is constructed at the correct elevation and grade to provide the specified clear space and setback.
CMU core grouting: The core of the top three courses of CMU blocks should be filled with concrete wall mix. Rebar dowels should be cut to length (20 inches) and inserted into the core of the top three courses. The concrete mix should be rodded with a rebar dowel before insertion to eliminate voids. Sufficient concrete should be available to form the coping cap during the same pour.
Wrapped integrated approach: Prior to placement of the geotextile reinforcement, it should be verified that the length is adequate to wrap the fill and extend back towards the road as shown in the one–sheet plan. Sufficient reinforcement width should also be available to laterally confine the approach fill if necessary.
The lift thicknesses for each lift should be checked to ensure that they do not exceed the maximum thickness and that secondary reinforcement is placed within fill layers that are greater than 8 inches. If a granular road base is used in the wrapped approach, verify that its compaction conforms to density requirements for the road as well as the GRS.
At the top of the integrated approach, verify that a 1– to 2–inch layer of aggregate is placed on the top reinforcement layer for protection from hot mix asphalt. Verify that the paving fabric, if used, bridges the interface from the deck to the approach as described in chapter 7.
9.5.1 Compliance Documentation
Field test results should be carefully measured and archived as a permanent part of the job record. This information can also be used to modify field (construction and inspection) practices.
The main field measurement, moisture density tests, should be documented during construction. Other documentation should include construction modifications, field changes, and daily construction reports.
9.5.2 Record Drawings
As–built plans should be prepared and provided to the owner upon completion of the project.
9.6 CONTRACTING METHODS
Of the two types of contracting methods commonly used for specialty construction, the procedural method and performance method, the preferred approach for the GRS–IBS is the procedural method. GRS performance–based methods can be developed when the technique becomes more widespread. The generic nature of GRS walls and abutments fits well with the performance–based method, which can advance the technology by creating an opportunity to develop new techniques, details, and equipment.
9.6.1 Performance Method
In a performance–based contract, the contractor can choose a GRS system based on its performance and constructability. The contractor should verify that the GRS–IBS is constructible and performs as outlined in the requirements. Careful attention should be placed on the compatibility between the backfill material, the wall facing, and the reinforcement to assure that the wall meets the necessary requirements.
Under this method, design and performance criteria should be based on the data provided in chapter 4 and chapter 6. Material and construction specifications can be based on the information in chapter 3 and chapter 7. This contract method requires that the reviewers have considerable knowledge in GRS technology to accept design submittals.
9.6.2 Procedural Method
In this contract method, the agency or owner provides a detailed set of design plans and construction specifications in the bid document. QA begins with an initial plan, design, and review of construction materials. Approval should be dependent on someone experienced in the design and construction of the GRS system. Also, the completed project should be in compliance with local agency building codes and regulations.
Fully detailed plans and items requiring review prior to initiating a GRS project should consist of the following:
9.6.3 Contractor Submittals
Materials used to construct GRS–IBS are readily available from a number of sources. The only requirement is that they meet the standards provided in this manual. The main materials that should be reviewed prior to construction are as follows (see chapter 3 for more details):
9.7 PROJECT DRAWINGS AND DOCUMENTS
Appendix D provides typical GRS–IBS working drawings that include a one–page plan sheet, estimated quantities and general notes, project plans and profile, GRS abutment details, and a site plan.