Subgrade Preparation for ACBs
Stable and compacted subgrade soil should be prepared to the lines, grades, and cross sections shown on the contract drawings. Termination trenches and transitions between slopes and embankment crests, benches, berms, and toes should be compacted, shaped and unfirmly graded to facilitate the development of intimate contact between the ACB and uniformly graded to facilitate the development of intimate contact between the ACB revetment systems and the underlying grade. Secure the revetments in a manner that prevents soil migration when the ACB matrix is terminated at a structure, such as a concrete slab or wall.
Subgrade soil should be approved by the engineer to confirm that the actual subgrade soil conditions meet the required material and compaction standards. Soils not meeting the required standards should be removed and replaced with acceptable material.
Care should be exercised so as not to excavate below the grades shown on the contract drawings, unless directed by the engineer to remove unsatisfactory materials and any excessive excavation should be filled with compacted backfill material as approved by the engineer. Where it is impractical, in the option of the engineer, to dewater the area to be filled, over-excavations should be backfilled with crushed rock or stone conforming to the grading and quality requirements of well-graded coarse aggregate in ASTM C 33 "Standard Specification for Concrete Aggregates", or as directed by the engineer.
When placing dry areas to receive the ACB system, the surface should be graded smooth to ensure that intimate contact is achieved between the subgrade surface and the geotextile and between the geotextile and the bottom surface of the ACB revetment system. Unsatisfactory soils, soils too wet to achieve desired compaction, and soils containing roots, sod, brush, or other organic materials, should be removed, replaced with approved material and compacted. The subgrade should be uniformly compacted to a minimum 90 percent of whichever is more stringent. Should the subgrade surface for any reason become rough, eroded, corrugated, uneven, textured, or traffic marked prior to ACB installation, such unsatisfactory portion should be scarified, reworked, re-compacted, or replaced as directed by the engineer.
Excavation of the subgrade, above the water line, should not be more than 2 inches (51mm) below the grade indicated on the contract drawings. Excavation of the subgrade below the water line should not be more than 4 inches (102mM) below the grade indicated on the contract drawings.
Where such areas are below the allowable grades, they should be brought to grade by placing approved material and compacting in lifts not exceeding 6 inches (152 mm) in thickness. Where such areas are above the allowable grades they should be brought to grade by removing materials or reworking existing material, and compacting as directed by the engineer. The subgrade should be raked, screened, or rolled by hand or machine to achieve a smooth compacted surface that is free of loose material, clods, rocks, roots or other materials that would prevent satisfactory contact between the geotextile and the subgrade. Immediately prior to placing the geotextile and ACB system, the prepared subgrade should be inspected and approved by the engineer.
Placement of Geotextile on Subgrades
The geotextile should be placed directly on the prepared area, in intimate contact with the subgrade, and free of folds or wrinkles. The geotextile shall be placed in such a manner that placement of the overlying materials will not excessively stretch or tear the geotextile, After placement of the overlying materials will not excessively stretch or tear the geotextile. After geotextile placement, the work area should not be disturbed so as to result in a loss of intimate contact between the articulating concrete block and the geotextile, or between the geotextile and the subgrade. The geotextile should not be left exposed longer than the manufacturer's recommendation to minimize damage due to ultraviolet radiation.
The geotextile should be placed so that the upstream strips of fabric overlap downstream strips, and so that up-slope strips overlap down slope strips. Overlaps should be in the direction of flow wherever possible. The joints should be overlapped a minimum 3 ft (1m) for below-water installations and a minimum 1.5 feet (0.5m) for dry installations in accordance with ASTM D6884 (ref. 16). When a sewn seam is used for geotextile seaming, the thread should consist of high strength, U.V. resistant polypropylene or polyester.
When a granular filter is used, the geotextile should be placed so as to encapsulate the granular filter. The distance between encapsulation points should not exceed 20 feet (6 m). The geotextile should extend to the edge of the revetment within the top, toe, and side termination points of the revetment. if necessary to expedite construction and to maintain the recommended overlaps, anchoring pins or 11 gauge, 6- by 1- inch (152 by 25 mm) U-staples may be used; however, weights (e.g., sand filled bags) are preferred to prevent creating holes in the geotextile.
Placement of ACB System on Geotextile
The articulating concrete block system should be placed on the geotextile in such a manner as to produce a smooth plane surface in intimate contact with the geotextile. For blocks within the mat and blocks that are hand set, the joint spacing between adjacent blocks is to be maintained so that binding of blocks does not occur and block-to-block interlock is achieved. In curvature and grade change areas, alignment of the individual block and the orientation of the neighboring adjacent block is to provide for intimate block to fabric contact and block-to-block interlock. Care should be taken during block installation so as to avoid geotextile is used, the ACB system placement should begin at the upstream end and option is to place a temporary toe on the front edge of the ACB system to protect against undermining when flows are anticipated. On sloped sections, where practical, placement shall begin at the toe of the slope and proceed up the slope. Block placement shall not bring block-to-block interconnections into tension. Individual blocks within the plane of the finished system shall not exceed the protrusion tolerance beyond that used in the stability design of the system. The typical protrusion tolerance is 0.5 inches (13mm).
Do not use the ACB revetment system as a road for heavy construction traffic unless designed as a flexible pavement that can handle the expected wheel loads. Light traffic, such as single axle trucks and mowing equipment, may operate on installed ACB systems.
NCMA Design Manual for ACB Revetment Systems, Pages 51-53 (Ref. 3).