Design and Construction Guidelines for Geosynthetic-Reinforced Soil Bridge Abutments with a Flexible Facing (2006)

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A-1 APPENDIX A Review of Construction Guidelines for GRS Walls [Note:This appendix has not been edited by TRB.] The guidelines for construction of segmental Geosynthetic- Reinforced Soil (GRS) walls have been provided by various agencies. Most of these guidelines should be equally applica- ble to segmental GRS abutments. This Chapter summarizes the construction guidelines provided by: (1) American Asso- ciation of State Highway and Transportation Officials, AASHTO (1998), (2) National Concrete Masonry Associa- tion, NCMA (1997), (3) Federal Highway Administration, FHWA (Elias and Christopher, 1997), (4) Colorado Trans- portation Institute, CTI (Wu, 1994), (5) Swiss Association of Geotextile Professionals, SAGP (1981), and (6) Japan Rail- ways, JR (1998). American Association of State Highway and Transportation Officials, AASHTO (1997) Reinforcement and – Geosynthetic reinforcement and connection hardware shall be of the type and size designated Reinforcement on the plan or the approved working drawings and shall conform to the specified material Placement and manufacturing requirements. – All geosynthetic reinforcement shall be uniformly tensioned to remove any slack during placement. Backfill – Structure backfill material shall consist of material free from organic material or other unsuit- able material as determined by the engineer. – Grading shall be as follows unless otherwise specified: 100% passing 100 mm (4 in.) sieve, 0-60% passing No. 40 (0.42 mm) sieve, and 0-15% passing No. 200 (0.074mm) sieve; plasticity index (PI) as determined by AASHTO T90, shall not exceed 6. The maximum soil particle size shall generally be 20 mm (0.75 in.). – The backfill shall exhibit an angle of internal friction of not less than 34 degrees, as determined by the standard direct shear test on the portion finer than 2 mm (No.10) sieve, using a sample compacted to 95% of AASHTO T-99 at optimum moisture content. – The backfill shall be substantially free of shale or other soft, poor durability particles, and shall have an organic content not larger than 1%. For permanent applications, the backfill shall have a pH between 4.5 and 9. The pH limits may be increased to 3 to 11 for temporary applications. Backfill Placement – Backfill shall be placed and compacted simultaneously with the placement of facing and soil reinforcement. – Placement and compaction of backfill shall be accomplished without distortion or displacement of the facing or soil reinforcement. – Sheepsfoot or grid-type rollers shall not be used for compacting backfill within the limits of the soil reinforcement. – At each level of soil reinforcement the backfill material shall be roughly leveled to an elevation approximately 30 mm (0.2 ft) above the level of connection at the facing before placing the soil reinforcement. Facing – Masonry concrete blocks used as wall facing elements should have a minimum compressive strength of 28 MPa (4,000 psi) and a water absorption limit of 5%. – Facing blocks may be tested for freeze-thaw resistance and survive 300 freeze-thaw cycles without failure per ASTM C666. – Facing blocks should also meet the requirements of ASTM C90 and C140. – Facing blocks directly exposed to spray from deiced pavements shall be sealed after erection with a water resistance coating or be manufactured with a coating or additive to increase freeze- thaw resistance. – Facing blocks shall be placed and supported as necessary so that their final position is vertical or battered as shown on the plans or the approved working drawings with a tolerance acceptable to the engineer.

A-2 Others – A minimum bench width of 4 ft is recommended at the base of walls constructed on slopes. – When required, a cast-in-place or precast reinforced concrete leveling pad shall be provided at the foundation level. Prior to placing the leveling pads, the foundation material shall be properly treated. National Concrete Masonry Association, NCMA (1997) Site Preparation – Excavation shall be carried out to the lines and grades shown on the project grading plans. Over-excavation shall be minimized. – A minimum 0.5 m of wall embedment is required. – Foundation soils not meeting the required strength should be removed and replaced with soil meeting the design criteria. – A minimum 6 in. thick layer of compacted granular material shall be placed for use as a leveling pad. The granular base shall be compacted to provide a firm and level bearing pad on which to place the first course of facing blocks. Compaction should be performed using a light-compactor to obtain a minimum of 95% of the maximum standard Proctor density (per ASTM D698). Reinforcement and – Geosynthetic reinforcement shall consist of high tenacity geogrids or geotextiles manufactured Reinforcement for soil reinforcement applications. Placement – Geosynthetic reinforcement should be installed under tension. A nominal tension shall be applied to the reinforcement and maintained by staples, stakes or hand tensioning until the rein- forcement has been covered by at least 6 in. of soil fill. – The geosynthetic reinforcement perpendicular to the wall face should consist of one continuous piece of material. Overlap of reinforcement in the design strength direction shall not be permit- ted. Adjacent sections of geosynthetic should be placed in a manner to assure that horizontal coverage shown on the plans is provided. – Tracked construction equipment shall not be operated directly on the geosynthetic reinforce- ment. A minimum backfill thickness of 6 in. is required prior to operation of tracked vehicles over the geosynthetic reinforcement. Turning of tracked vehicles should be kept to a minimum to prevent displacing the fill and damaging or moving the geosynthetic reinforcement. – Rubber-tired equipment may pass over the geosynthetic reinforcement at slow speeds less than 10 mph. Sudden braking and sharp turning should be avoided. Backfill – Reinforced backfill shall be free of debris and consist of one of the following inorganic USCS soil types: GP, GW, SW, SP, SM, meeting the following gradation: 75% to 100% passing 4 in. sieve, 20% to 100% passing No. 4 sieve, 0 to 60% passing No. 40 sieve, and 0 to 35% passing No. 200 sieve. The maximum size shall generally be limited to 3⁄4 in. – The plasticity of the fine fraction of the reinforced backfill shall be less than 20. – The pH of the backfill material shall be between 3 and 9 (per ASTM G 51). Backfill Placement – Reinforced backfill shall be placed as shown in construction plans in maximum compacted lift thickness of 10 in. – Reinforced backfill shall be compacted to a minimum 95% of standard Proctor density at a moisture content within 2% of optimum. – Backfill shall be placed, spread and compacted in such a manner that eliminates the develop- ment of wrinkles or movement of the geosynthetic reinforcement and the wall facing units. – Only hand-operated compaction equipment shall be allowed within 3 ft of the front of the wall face. Compaction within 3 ft of the back face of the facing units shall be achieved by at least three passes of a lightweight mechanical tamper, plate or roller. Soil density in this area should not be less than 90% standard Proctor density. – At the end of each day’s operation, the last level of backfill should be sloped away from the wall facing to direct runoff of rainwater away from the wall face. In addition, surface runoff from adjacent areas to enter the wall construction site should be avoided.

A-3 Facing – Concrete segmental units shall have a minimum 28 days compressive strength of 3,000 psi and a maximum absorption of 10 pcf per ASTM C140. For areas subject to detrimental freeze-thaw cycles, the concrete shall have adequate freeze-thaw protection. – All facing units shall be sound and free of cracks or other defects that would interfere with the proper placement of the unit or significantly impair the strength or permanence of the construc- tion. – Facing units dimensions shall not differ more than ± 1/8 in., except height which shall not differ more than ± 1/16 in. – If pins are used to interconnect the facing units, they shall consist of a non-degrading polymer or galvanized steel and be made for the expressed use with the facing units. – The cap block and/or top facing units should be bonded to the units below using cap adhesive that meets the requirements of the facing unit manufacturer. – The overall tolerance relative to the wall design verticality or batter shall not exceed ±1.25 in. maximum over a 10 ft distance; 3 in. maximum. Drainage – Drainage aggregates shall be a clean crushed stone or granular fill meeting the following grada- tion criteria: 100% passing 1 in. sieve, 75% to100% passing 3⁄4 in. sieve, 0 to 60% passing No. 4 sieve, 0 to 50% passing No. 40 sieve, and 0 to 5% passing No. 200 sieve. – Drainage collection pipes shall be perforated or slotted, PVC or corrugated HDPE pipe. – Drainage collection pipes shall be installed to maintain gravity flow of water outside of the rein- forced soil zone. The pipe should daylight into a storm sewer manhole or along a slope that an elevation smaller than the lowest point of the pipe within the aggregate drain. – The main collection drain pipe, just behind the block facing, shall be a minimum of 3 in. in diameter. The secondary collection drain pipes should be sloped a minimum of 2% to provide gravity flow into the main collection drain pipe. Drainage laterals shall be spaced at a maximum of 50 ft spacing along the wall face. – Drainage pipes and drainage aggregates may be wrapped with a geotextile that will function as a filter. Federal Highway Administration, FHWA (Elias et al. 2001) Site Preparation – Foundation should be prepared by removal of unsuitable materials from the area to be occupied by the retaining structure, including all organic matter, vegetation, and slide debris, if any. – Where construction of reinforced fill will require a side slope cut, a temporary earth support sys- tem may be required to maintain stability. Cautions should be exercised for excavation of utili- ties or removal of temporary bracing or sheeting in front of completed MSE structures. – Construction dewatering operations should be required for any excavations performed below the water table to prevent a reduction in shear strength due to hydrostatic water pressure. – A concrete leveling pad should have minimum dimensions of 150mm thick by 300mm wide and should have a minimum 13.8 MPa (3,000 psi) compressive strength. Cast-in-place pads should cure a minimum of 12 hours before facing panels are placed. A vertical tolerance of 3 mm (1/8 in.) to the design elevation is recommended for the leveling pad. Gravel pads of suit- able dimensions may be used with modular block wall construction. Reinforcement – Reinforcements should generally be placed perpendicular to the back of the facing panel. In Placement specific situations, e.g., abutments and curved walls, it may be permissible to skew the rein- forcements from their design location in either the horizontal or vertical direction. – Overlapping layers of reinforcements should be separated by a 75mm (3 in.) minimum thick- ness of fill. – Flexible reinforcements, such as geotextiles and geogrids, usually require pre-tensioning to remove any slack in the reinforcement or in the panel. The tension is then maintained by staking or by placing fill during tensioning.

A-4 Backfill and – Backfill material should be reasonably free from organic or otherwise deleterious materials Backfill Placement and should conform to the following gradation limits: 100 % passing 102 mm sieve, 0-60% pass- ing No. 40 sieve, and 1-15% passing No. 200 sieve. The plasticity index should not exceed 6. – Backfill material should exhibit an angle of internal friction of not less than 34 degrees, as determined by the standard direct shear test AASHTO T-236 on the portion finer than the No. 10 sieve, using a sample compacted to 95% of AASHTO T-99, Methods C or D. No testing is required for backfills where 80% of sizes are greater than 19 mm. – The backfill shall be substantially free of shale or other soft, poor durability particles, and shall have an organic content less than 1%, and a pH between 5 and 10. – Reinforced wall fill material should be placed and compacted at or within 2% dry of the opti- mum moisture content. If the reinforced fill is free draining with less than 5% passing a No. 200 sieve, water content of the fill may be within ± 3% of the optimum. – A density of 95% of AASHTO T-99 maximum value is recommended for retaining walls and slopes, and 100% of AASHTO T-99 is recommended for abutments and walls or slopes sup- porting structural foundations abutments. A procedural specification is preferable where a sig- nificant percentage of coarse material, generally ≥ 30% retained on the 19 mm (3/4 in.) sieve, prevents the use of the AASHTO T-99 or T-180 test methods. For procedural specification, typ- ically three to five passes with conventional vibratory roller compaction equipment is adequate. The actual requirements should be determined based on field trials. – Reinforced backfill should be dumped onto or parallel to the rear and middle of the reinforce- ments and bladed toward the front face. At no time should any construction equipment be in direct contact with the reinforcements. – Soil layers should be compacted up to or even slightly above the elevation of each level of rein- forcement connections prior to placing that layer of reinforcing elements. – With the exception of the 1-m zone directly behind the facing elements or slope face, large, smooth-drum, vibratory rollers should generally be used to obtain the desired compaction. Sheepsfoot rollers should not be permitted. When compacting uniform medium to fine sands (in excess of 60% passing a No. 40 sieve) use a smooth-drum static roller or lightweight (walk behind) vibratory roller. The use of large vibratory compaction equipment with this type of backfill material will make wall alignment control difficult. – Within 1 m (3 ft) of the wall or slope face, use small single or double drum, walk-behind vibra- tory rollers or vibratory plate compactors. – Placement of the reinforced fill near the front should not lag behind the remainder of the struc- ture by more than one lift. – Within 1 m (3 ft) of the wall, quality control should be maintained by a methods specification such as three passes of a light drum compactor. High quality fill is sometimes used in this zone so that the desired properties can be achieved with less compactive effort. – Flooding of the backfill to facilitate compaction is not permitted. – Compaction control testing of the reinforced backfill should be performed on a regular basis during the entire construction project. A minimum frequency of one test within the reinforced soil zone per 1.5 m (5 ft) of wall height for every 30 m (100 ft) of wall is recommended. Facing – The compressive strength for facing units should be 28 MPa (4,000 psi) and the water absorp- tion should be ≤ 5% after 24 hours. – The compressive strengths and water absorption of dry-cast modular blocks should be carefully checked on a lot basis. – Dimensional tolerances of the blocks are typically ± 3.2 mm (1/8 in.) for overall dimensions and ± 1.6 mm (1/16 in.) for block height. Drainage – MSE structures should be designed to permit drainage of any seepage or trapped groundwater in the retained soil. If water levels intersect the structure, it is also likely that a drainage structure behind and beneath the wall will be required. Surface water infiltration into the retained fill and reinforced fill should be minimized by providing an impermeable cap and adequate slopes to nearby surface drain pipes or paved ditches with outlets to storm sewers or to natural drains.

A-5 – Internal drainage of the reinforced fill can be attained by use of a free-draining granular material that is free of fines (less than 5% passing No. 200 sieve). Arrangement should be provided for drainage to the base of the fill to prevent water exiting the wall face and causing erosion and/or face stains. The drains should have suitable outlets for discharge of seepage away from the rein- forced soil structure. Care should be taken to avoid creating planes of weakness within the structure with drainage layers. Colorado Transportation Institute, CTI (Wu 1994) Site Preparation – Before placement of the reinforcement, the ground should be graded to provide a smooth, fairly level surface. – The surface should be clear of vegetation, large rocks, stumps, and the like. Depressions may need to be filled; soft spots may need to be excavated and replaced with backfill material; and the site may need to be proof rolled. – It is usually not necessary to sub-excavate the ground for embedment and frost heave protec- tion, as is commonly done in the construction of conventional reinforced concrete walls. – If site preparation involves excavation, the construction site should be excavated to the limits shown in the construction plans. The excavation width at any depth should be at least equal to the length of the reinforcement layer designed for that elevation. – A nominal thickness (about 4 in. to 6 in.) of granular soil should be placed at the base of the wall for drainage and leveling purposes. Reinforcement – Geosynthetics, especially geotextiles, should not be exposed to sunlight and extreme and Reinforcement temperatures for an extended period of time. Damaged or improperly handled geosynthetic Placement reinforcement should be rejected. – After the reinforcement is in place, it should be examined carefully. Damaged or torn materials should be replaced or repaired as prescribed in the specifications. – In no case should construction equipment be allowed to operate directly on any geosynthetic reinforcement before fill is placed. When using geotextile reinforcement, a minimum backfill cover of 6 in. should always be maintained between the geotextile and moderate size construc- tion equipment (e.g., Caterpillar D6 or 955). – Geosynthetic reinforcement should be unrolled in the direction perpendicular to the wall face whenever feasible. In which case, overlapping of adjacent geosynthetic sheets should be at least 12 in., and sewing or other connections are usually not necessary. If the reinforcement is not unrolled perpendicular to the wall face, joining adjacent geosynthetic sheets should be performed strictly according to the plan and specifications. Generally, overlapping the layers should be of a mini- mum of 3 ft; sewing of layers should be made on a minimum of 4 in. overlaps. – Wrinkles and folds in the geosynthetic reinforcement prior to placement of fill should be kept to a minimum. Slight pre-tension of geosynthetic reinforcement (e.g., by stretching) is beneficial. Backfill Placement – Backfill should be progressively dumped and spread toward the back of the wall to ensure that little or no slack is left in the reinforcement. – Special attention should be given to ensuring good compaction of the backfill, especially near the face of the wall. Otherwise detrimental settlements behind the face may cause a downward drag on the reinforcement, which might induce excessive tensile stress at locations where the reinforcement is attached to the facing. – Care should be taken not to allow heavy construction equipment to operate too close (within 1 ft to 2 ft) to the wall face. Otherwise undesirable bulging of face may result. – Each lift should not exceed 12 in. in loose thickness and should be compacted to achieve a mini- mum of 95% of the maximum dry density according to ASTM D698 or AASHTO T-99. It is recommended that the placement moisture content for a granular fill be ± 2% of the optimum, and within 4% wet of optimum (i.e., between the optimum and 4% wet-of-optimum) for a cohesive fill.

A-6 – Care should be exercised when placing backfill over geosynthetic reinforcement. The backfill should be emplaced from the wall face to the back of the wall to ensure that no slack is left in the reinforcement. – At the end of each day’s backfilling operation, the last lift of fill should be sloped away from the wall facing to direct any possible runoff away from the wall face. Facing – For modular block faced walls, the alignment of the first few courses of blocks is critical to the alignment of the wall face. – The concrete leveling pad under the first course of modular blocks can be replaced with a level- ing pad of compacted gravel (or compacted in-situ soil). However, the use of a concrete leveling pad is recommended when the foundation soil is relatively incompressible and not susceptible to significant shrinkage and swell due to moisture changes. A properly poured and leveled con- crete pad will speed up construction, ease the leveling process, and facilitate the construction of a straighter wall. – Walls with curves along their length require that the leveling pad be poured to the proper radius. In general, a curve radius of 10 ft or greater is not a problem; however, tight curves of 3 ft to 6 ft radius require special consideration (Moreno, et al., 1993). In some cases, field modification of the blocks may be necessary for tight curves. – The blocks should be laid from one end of the wall to the other to preclude laborious block cut- ting and fitting in the middle. When curves are involved in a wall, the blocks on the curves should be laid first, as their alignment is more critical and less forgiving. Tight curves often require cutting blocks to fit or breaking off the block tail. A diamond-tipped blade saw is recom- mended for the cutting. – When shear pins are used, they should be tapped into well-seated position immediately after set- ting each block to avoid getting fill into the block’s pin holes. – Leveling of the first course of blocks is especially important for wall alignment. A string line set over the pins from one end of the wall to the other will help leveling the blocks. – Geosynthetic reinforcement should be placed up to front face of the blocks to ensure maximum interface contact with the blocks. – After front of the geosynthetic reinforcement is properly secured (i.e., after the hollow cores of the next course is filled and compacted), the reinforcement should be pulled tight and pre-tensioned while the backfill is being placed. – To avoid movement of blocks during construction, a hand-operated tamper should be used to compact the soil within 3 ft of the wall face, and no construction vehicles are allowed within the 1 m (3 ft) region. Control of Water – Surface runoff should be directed away from the site during construction. Also, surface runoff during Construction from adjacent areas should be prevented from encroaching on the site. – The simplest way to control surface water is to excavate a trench or construct a dike or curb around the perimeter of the site, and disposal of the water by gravity or by pumping from sumps. – For walls constructed below the ground water table, dewatering may be required to provide a working platform. Although there are many methods available for this purpose (e.g., well points, horizontal drains, etc.), the simplest technique is to construct perimeter trenches and connect them to sumps. This method is most effective when the excavation is in cohesive mater- ial and the ground water is not too high. The trench should be installed as far from the location of the wall base as practical to prevent disturbance due to ground water seepage. In certain cases, impermeable barrier to reduce or eliminate the inflow of ground water into the work site may be more effective than dewatering. Usually the selection of the method is left to the contractor. Control of Water – To reduce percolation of surface water into the backfill during the service life of a wall, the throughout Service crest should be graded to direct runoff away from the back slope. Sometimes interceptor Life drains on the back slope can be used. Periodic maintenance is also necessary to minimize runoff infiltration.

A-7 – Geosynthetic reinforcement provides inherent drain capability at their face. Therefore, subsur- face drainage at wall face is generally not necessary. However, when a cohesive backfill is used, measures should be taken to minimize wetting of the cohesive soil. This can be achieved by providing a combination of granular drain materials and geotextiles, or a geocomposite drain along the top, the back, and the base of the cohesive backfill. Swiss Association of Geotextile Professionals, SAGP (1981) Site Selection – A uniform subsoil constitutes an important factor for the durability of the GRS retaining wall. Reinforcement – The geotextile reinforcement must be placed exclusively in the direction of the primary load. Placement Joints in the direction of the load are unacceptable. – The wrapped return must be extended at least 0.3 m in the horizontal direction and anchored in at least 10 cm of fill material, which is usually half the thickness of a layer. Folding back the geotextile and placing the next sheet of geotextile directly on top of it is unacceptable, since this will create a greater risk of sliding. – The overlapping of geotextile in the longitudinal direction of the wall must be at least 0.3 m, or even 0.5 m if there are large surface irregularities. – The top layer of geotextile reinforcement must not be placed directly below the ballast of a rail- way. It must be covered with a layer of sand or gravel at least 10 cm thick. – The top two uppermost sheets of geotextile should be made of a continuous strip to avoid any problem due to an inadequate anchoring of the top fold. Backfill Placement – Fill material and those next to the covering of the face must be laid and compacted according to professional standards in layers not exceeding 0.40 m. This means that layers 0.5 m to 0.6 m thick must be laid in two stages. – Compacted lifts, not exceeding a thickness of 0.4 m, are built up between layers of reinforcement. – The required compaction of the fill material is 97% Proctor, or ME = 6 MN/m = 600 kg/cm, thus, a 5-ton vibrating roller is necessary. Drainage – Care must be taken to collect and evacuate water on the upside (road, railway, storage space, etc.). When this is not feasible, a drainage system must be installed behind the geosynthetic- reinforced soil wall. – Drainage is also necessary when there is slope water. – When there is a possibility of large quantities of water, or even of liquid chemicals flowing through the top of the soil wall, a waterproof liner must be provided above the top geotextile bed, with a 2% to 3% downslope towards the upside, in order to collect and evacuate the liquid. A drain pipe must also be provided in a channel leading towards a hydrocarbon separator. Japan Railways, JR (1998) Site Preparation – On-site surveying for confirmation of site conditions should be made. – Any buried structures and existing structures should be identified and removed if necessary. If the existing structure is located along the slope of excavation, its stability should be evaluated. – Plants and remains of trees, etc. if located at the foundation should be removed prior to con- struction. The ground should be cleared by digging about 0.3 m into the subsoil. – The ground surface of the foundations should be leveled for ease of geotextile installation. For foundation soils of low strength and high compressibility, such as peat, the soils should be replaced by the backfill soil up to a required depth so that subsequent unnecessary settlement and lateral flow will not occur. If the subsoil foundation is composed of undulating rocks or gravels, the backfill soil should be used to level off the foundation. The undulating ground surface may damage the geosynthetic reinforcement and lead to unwanted sagging during installation.

– If the water table is close to the ground surface or if outflow is detected, drainage channels or blankets should be installed. – Any rocks extruding through the ground surface should be removed since they may damage geosynthetic reinforcement. – For rock foundation, concrete should be used to level the ground surface. Reinforcement – Geosynthetic reinforcement must be placed flat without slack. Placement – The strong axis of geosynthetic reinforcement should be aligned perpendicular to the wall face. – Overlapping of geosynthetic reinforcement in the direction parallel to the wall face should be avoided whenever possible. Otherwise, a 50 cm overlapping is required. The overlapped portion should be tied using wires or strings. – Overlapping of geosynthetic reinforcement in the direction perpendicular to the wall face is per- mitted. A 0.1 m overlapping is recommended. – Geosynthetic reinforcement should be secured using sand bags or pins until backfill is placed so that they will not be blown away or torn by strong wind. – During installation, the folding-back portion of geosynthetic layer should be laid to the outside of the wall. After placing the gabions (formwork for facing), this portion should be folded back/wrapped to give a fold-back length of 0.3 m. The overlapped portion should be tied using wires or string. – Geosynthetic layers should be cut into appropriate length at the construction site. A proper cut- ting tool should be used. – A layer of backfill over the foundation surface may be necessary to protect the geosynthetic from being damaged and to provide a flat surface. Backfill Placement – The machinery and construction procedure should be properly arranged so that geosynthetic reinforcement will not sag during soil spreading. The main reasons leading to sagging of geosynthetic reinforcement during soil spreading are the high water content and the use of improper machinery, among other factors. – Backfill soil should render stability to the wall by allowing good compaction, giving little resid- ual settlement, and exhibiting essentially elastic response. – For construction using heavy machinery, the backfill soil must be spread parallel to the wall face. The soil spreading should start from the wall face and proceed inward to avoid sagging of geosynthetic reinforcement and bulging of the wall face. It is recommended that soil spreading within a distance of 1 m from the wall face should be performed manually. – Sudden stopping or turning of heavy machinery should be avoided. – A “trial construction” involving less than 30,000 m3 of soil should normally be conducted to evaluate compaction characteristics of the backfill. The specified degree of compaction should be obtained in the field and should be uniform throughout each wall construction. – Compaction within 1 m from the wall face should normally be conducted using a light com- paction plant. – Compaction should be completed within each day. If the work is terminated after soil spreading, precipitation overnight may soften the soil and thus affect compaction in the following day. If precipitation is anticipated, the top surface of the compacted soil should be covered with water proof sheets. Drain channels should be provided so that precipitation will not percolate into the backfill. – The construction should not proceed during rainy days. During precipitation the soil will be under-compacted. The presence of high water content will be a source of future settlement. Facing – Gabions should be used to ensure stability of the wall face during construction. The gabions function as a drainage layer after completion of construction and also as a buffer at the interface between the highly rigid concrete facing and the deformable backfill. – Gabions should not be simply placed on the wall face. A vibratory compactor should be used so that voids in the gabions will be minimized. – Alignment of the gabions should be checked, but strict enforcement is not required since a cast- in-place concrete facing will be installed over it. Priority should be given to good compaction of the gabions. A-8