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D-1 APPENDIX D Recommended Practice for CLSM 1. SCOPE 1.1. This Recommended Practice is focused on criteria, specifications, and guidelines for the use of controlled low-strength material (CLSM) for backfill, utility bedding, void fill, and bridge approach applications. 1.2. The Recommended Practice describes methods, limits, and issues related to the successful application of CLSM. 1.3. The Recommended Practice is based on the research results described in the Final Report for NCHRP Project 24-12(01), including the content of the appendices. It should be used in conjunction with the findings, test methods, and specifi- cations described therein. 2. PREAMBLE 2.1. Transportation agencies face challenges related to the use of traditional backfill material (soil) or higher strength cemen- titious materials with respect to the combination of timely construction, ease of construction, quality control, and ease of removal, if required. 2.1.1. CLSM has attributes that make it a potentially effective option for state departments of transportations (DOTs) and other agencies involved with backfilling or void-filling operations. 2.1.2. CLSM is a material with variable properties that can result in poorer than expected performance without proper guidance for the user. 2.2. Careful consideration of CLSM constituent materials, mixture proportioning, testing, handling, and excavation is required for CLSM to be an optimal alternative to the use of traditional materials. 2.3. Where guidance related to properties, specifications, or other items cannot be specific, ranges of values are shown. Where appropriate ranges have not yet been identified, no values are shown. Anywhere that brackets  are indicated, the user is cautioned to make a reasoned judgment as to values to be included. The use of ranges in this Recommended Practice is not meant to indicate the appropriateness of those ranges for all applications. 3. USAGE DECISIONS AND PREPARATIONS FOR CLSM 3.1. As noted in the sections that follow, CLSM serves as a substitute primarily for structural fill. Thus, CLSM should be con- sidered for projects where its advantages (e.g., flowability and related construction time savings) outweigh any potential disadvantages (e.g., additional materials costs). 3.2. Agencies should consider preparing or recommending training to contractors who may undertake CLSM projects in their jurisdiction. 3.2.1. CLSM, as a material that differs notably from structural fill and ready-mixed concrete, requires familiarity with its purposes, mixture design, testing, and installation to help ensure a quality project. A project with a contractor who takes on a first CLSM job with inadequate preparation may result in loss of anticipated advantages of the CLSM and in concerns about future uses of the material. 4. APPLICATIONS AND RELATED GUIDANCE 4.1. BACKFILL 4.1.1. Definitions and Types 188.8.131.52. Backfill as intended in this Recommended Practice relates to the infill material to cover pipes (in trench applications) up to a specified grade (usually equal to the grade of undisturbed earth on either side of a trench wall) or to the horizontal-reactionproviding infill adjacent to retaining walls and other wall
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D-2 structures. The CLSM is the alternative to the infill material that is typically a compacted granular structural fill. 184.108.40.206. Backfill is not the same as utility bedding, although it can be contiguous with such bedding. Backfill also is not the same as void fill; the primary difference is that backfill is placed against a structure with the purpose of providing at least some structural resistance to loads. 220.127.116.11. Figure D.1 indicates two common backfill applications for which CLSM may be a candidate. 4.1.2. Criteria of Importance 18.104.22.168. Backfill generally must fill an open space of some sort, usually a space accessible from above, and it must provide some sort of structural support for the object that is being backfilled. In the case of a trench, the backfill may provide structural support for part of the pipe and the trench wall. For bridge abutments, retaining walls, and other walls, the backfill is providing support for the wall, usually acting as a bridge between the wall and the area of unexcavated, natural earth. 22.214.171.124. Flowability is important to both trench and wall applications. CLSM must flow from its point of deliv- ery to a reasonable distance, such as along a trench floor or to the wall. A mixture that is too stiff will not allow the material to reach all necessary locations without the application of additional equipment and labor. A mixture that is too liquid (no severe segregation) is generally not a problem, if all other material properties discussed below are met. "Runny" mixtures may cause difficulties if there are small gaps in sandbag, bulkheads, or similar retaining structures. 126.96.36.199.1. A flow resulting in a circular-type spread with a diameter of [175 to 250 mm] as measured by ASTM D 6103 is considered an appropriate criterion for backfill applications. 188.8.131.52. The CLSM mixture should have a setting time consistent with the overall construction schedule, because backfilling is an interim operation in construction. 184.108.40.206.1. For trenches under roadways or similar applications, fast setting times may be desirable so that a pavement layer may be placed on top of it. 220.127.116.11.2. For wall backfill, or in other trench applications, a general surrogate measurement as to whether the CLSM has sufficiently set is "walkability," that is, when a person of average weight and shoe size can walk on the surface of the CLSM without creating significant (greater than 3 mm) indents in the material. The CLSM mixture should set in such a time, consistent with walkability needs and other measurements, so that it does not unduly delay subsequent or con- current construction practices. 18.104.22.168.3. Pavements generally can be placed over CLSM when the CLSM has reached a strength of [0.2] MPa or a penetration resistance of [2.8] MPa according to the recommended test method in Appendix B. 22.214.171.124. Long-term strength issues should be considered, so that backfill may be removed later, such as when a pipe requires repair, or when additional future construction is performed. 126.96.36.199.1. CLSM should have some predetermined maximum strength to ensure its future remov- ability. 188.8.131.52.1.1. Laboratory-cured samples behave differently than field applications. Field cur- ing test samples may be useful, especially if the application will occur at high temperatures and/or will include fly ash. CLSM Pipe or utility Earth Bedding A. Trench Backfill B. Wall Backfill Figure D.1. Common backfill applications.
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D-3 184.108.40.206.2. For hot-weather construction, CLSM mixtures containing fly ash may obtain higher strength in the field than estimated in the laboratory. The actual value of that strength may depend on whether removal is anticipated using manual equipment or machinery. 220.127.116.11.2.1. The use of a dynamic cone penetrometer (DCP) test on site prior to excava- tion may allow for obtaining different equipment than originally anticipated if the actual in-situ strength is higher than anticipated. 18.104.22.168.2.2. Including aggregates in CLSM mixtures will generally result in more diffi- cult excavation (than CLSM without aggregates). Most CLSM does contain fine aggregate, and, in rare instances, CLSM can also contain coarse aggre- gate. For CLSM mixtures with similar compressive strengths, CLSM without aggregate is easier to excavate than CLSM containing fine aggregate, which is easier to excavate than CLSM containing coarse aggregate (with or without fine aggregate). 22.214.171.124.2.3. Correlation tests for CLSM mixtures and their excavatability may be useful to anticipate long-term strength gain for future excavation. 126.96.36.199.2.4. The removability modulus predictor (as developed by Hamilton County, Ohio), when used in conjunction with unit weight values, provides good estimates of excavatability. 188.8.131.52.2.5. If feasible, core cylinders should be kept at the site (perhaps buried in a nearby area) and then tested shortly before an excavation operation. This test may provide the best measure of excavatability of the actual application. 184.108.40.206.3. Backfill must provide some structural resistance to loads. A minimum strength must be spec- ified that is appropriate to whatever the structural needs (e.g., traffic loads) of the specific application may be. 220.127.116.11. Impermeability of CLSM may lead to difficulty in locating gas leaks in pipelines (when using typical leak detection equipment used for conventional backfill). This concern applies only to trench backfill applications. 18.104.22.168. Water permeability may be an issue to both trench backfill and to wall backfill. A barely permeable CLSM mixture may cause leaking water to travel along a pipe length until it reaches a suitable fissure in the CLSM. Thus, the location of evidence of water leakage (bubbling or ballooning of the ground surface, for example) may not coincide with the actual location of a pipe leak, causing difficulty in determining the exact location of the damaged pipe. For wall backfills, a nearly impermeable CLSM mixture may lead, depending on the application design, to excess water being unable to flow through or around the CLSM, which may lead to a buildup of water pressures against a wall or to washouts at the CLSM-soil interface. For applications where pipes are located near the foot of abutment walls, the locations of leaks may become difficult to ascertain in much the same way as they may be in trenches. 22.214.171.124.1. A minimum permeability is established based on the water permeability coefficient k. The min- imum k should be [1 × 10-4 mm/s] unless permeability is deemed not to be an issue. The per- meability coefficient can be measured using the recommended test method in Appendix B. 126.96.36.199. Proper air content will provide for the durability of backfill material in freeze-thaw conditions. A mini- mum air content of  percent is required unless otherwise specified or unless needs suggest a different limit. Also, for a given CLSM mixture, the higher the strength, the better the resistance to frost-induced damage. 188.8.131.52. Corrosion issues should be considered for trench applications when pipes run transversely through a back- filled area. The soil-CLSM interface can cause an electrochemical potential leading to corrosion of metal- lic pipe in this area (this potential can occur when different soils interface without the presence of CLSM also). CLSM generally is a better environment than soil backfill with respect to corrosion problems. 184.108.40.206.1. Wrapping or coating pipe in the interface region may be an effective corrosion mitigation approach. 220.127.116.11.2. Whenever such an interface exists, it is important to specify either a cathodic protection scheme or a physical protection scheme, such as coating or covering the pipe with a protective layer in this interface region.
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D-4 18.104.22.168. Where interim and final grades of construction materials are important, such as in a trench transverse to a roadway (where subsidence could cause a dip in the final roadway surface or cracking in an asphalt or portland cement concrete or chip-seal surface because of uneven support conditions), it is impor- tant to limit or take into account the subsidence of CLSM. Typically, CLSM may "shrink" approximately 6 mm for every 300 mm of depth. Thus, layers above the CLSM, or an additional thin lift of CLSM, may be required after any initial subsidence. Because overfilling trenches is impractical (because the CLSM would simply run over the edges), the proper planning related to subsidence must be undertaken. 22.214.171.124. Because pipes may exhibit buoyancy ("pipe floating"), weighting or securing of pipes in CLSM appli- cations may be required. 126.96.36.199. Case-by-case analyses of special conditions requiring special criteria should be conducted. 188.8.131.52.1. Thermal properties of the CLSM backfill may be important for a utility application in which hot or cold water is being piped. 184.108.40.206.2. For roadway supportrelated applications, the California Bearing Ratio (CBR) or resilient modulus (MR) of the in-place CLSM may be critical. Performance criteria related to these and other items should be specified by the engineer, with the appropriate test methods indi- cated as described in Appendix B. 4.1.3. Specifications Issues 220.127.116.11. Agency construction and other documents should consider appropriate descriptions when specifying CLSM as backfill. 18.104.22.168. A sample work description could state: "Furnish and install backfill to provide necessary structural support for utilities, trench walls, retaining walls, abutments, and other applications." Material specifications should note that CLSM may be composed of some or all of the following com- ponents and their associated specification or test method: Aggregate AASHTO M 6 or as approved by the engineer. Water Water used in mixing and curing of CLSM shall be subject to approval and shall be reasonably clean and free of oil, salt, acid, alkali, sugar, vegetable, or other substance injurious to the finished product. Water shall be in accordance with AASHTO T 26. Color agent ASTM C 979 Cement AASHTO M 85 Mineral admixtures AASHTO M 295 or as approved by the engineer. Chemical admixtures AASHTO M 194 or as approved by the engineer. 22.214.171.124. Backfill should not contain any material deemed toxic or hazardous. Material Safety Data Sheets (MSDS) must be available for any component of the mixture upon request. Backfill shall be compatible with bed- ding materials, electrochemically and otherwise if used as a metal pipe backfill application. 126.96.36.199. Proportioning of CLSM mixtures should be the responsibility of the contractor or the contractor's supplier. The mixture should be rejectable for failure to meet, or to sustain, the mixture's consistency and all prop- erties specified herein. 188.8.131.52. Construction specifications should include guidance on batching, mixing, and transportation, such as the following: CLSM may be produced on site or batched at a remote facility and appropriately mixed and transported to the site. If transported, an appropriate transit-mix truck shall be used. [End plugs or lower transport volumes shall be required for mixtures of extreme flowability or as required by the engi- neer.] Hauling and dumping using a conventional open-haul unit is allowed if approved by the engineer. No blade mixing shall be allowed. 184.108.40.206. Guidance on sampling and testing should be included in project specifications, such as the following: All CLSM shall be accompanied by a batch ("delivery") ticket that certifies the content of the material and the data on the following items: (a) project designation; (b) date; (c) time; (d) compressive strength, fc ; (e) yield and unit weight; (f ) flowability; and (g) removability modulus (optional). 220.127.116.11.1. The following tests should be performed for each  cubic meters of material delivered and used on the project site.
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D-5 Strength Six (6) cylinders will be required, with three (3) cylinders tested according to the test method in Appendix B at 28 days and three (3) cylinders tested at 91 days. The contractor shall be responsible for the curing and protection of the cylinders until such time that they are ready to be tested or to be picked up by the testing agency. Note: For any project using less than  cubic meters of material, three (3) cylinders will be required for every  cubic meters of material, with two (2) cylinders tested at 28 days as noted above and the third tested at 91 days. Flowability Three (3) samples shall be tested according to ASTM D 6103 on site prior to installation of the material as backfill. The material must provide a flow diameter of no less than 200 mm, unless specified by the engineer. Air Content For jobs where long-term freeze-thaw durability has been indicated as a concern, the air con- tent of fresh CLSM will be determined using the test method in Appendix B prior to instal- lation of the material as backfill. The CLSM must have an air content no less than  percent by volume. 18.104.22.168.2. Site preparation guidance should consider inclusion of the following language. 22.214.171.124.2.1. If utility bedding is not already present, excavate to line and grade shown on the plans or described in the specifications. Excavate rock, hardpan, and other unyielding material to  mm below the designed trench grade. If utility bedding is present, ensure that the bedding is not covered by rock, soil, or dele- terious material. 126.96.36.199.2.2. Clear the trench or wall area of any deleterious material; soil clods; loose, sloughing, caving, or otherwise unsuitable soil; or other materials such that a reasonably clear and clean fill area is provided. 188.8.131.52.2.3. Cleanup and backfill of trenches for water mains shall begin immediately upon completion of the hydrostatic test (if necessary) or as directed by the engineer. 184.108.40.206.2.4. No placement of CLSM shall commence until all items have been inspected by the engineer and approved for backfilling. [Wait  days or meet a minimum compressive strength of  MPa before backfilling against newly constructed masonry or concrete structures.] 220.127.116.11.2.5. For trench applications, provide suitable vertical wall containment such as sandbag or soil bulkheads to limit the flow distance of the CLSM to no more than  m from the discharge location. For backfill applications, provide suitable vertical wall containment to ensure that the CLSM will not flow into areas beyond those specified on the plans. For steeply sloping trenches, pro- vide bulkheads at intervals as approved by the engineer. 18.104.22.168.2.6. If standing water exists, CLSM may be poured if the standing water represents no more than approximately  percent of the volume of CLSM to be placed in a single lift. If more water than this limit is present, it must be removed through appropriate water control measures. 22.214.171.124.2.7. Ensure that all sheeting and bracing, temporary formwork, and other items assisting with the construction can be removed after completion of the CLSM placement.
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D-6 126.96.36.199.2.8. Whenever excavation is made for structures across private property, the top- soil removed in the excavation shall be kept separate and replaced, as nearly as feasible, in its original position, and the entire area shall be restored to a con- dition acceptable to the engineer. 188.8.131.52.3. Placement guidance should include consideration of the following language: 184.108.40.206.3.1. Placement of CLSM shall be completed no more than  minutes after the end of mixing. For fast-setting CLSM mixture, the material shall be mixed on site and placed immediately. 220.127.116.11.3.2. Place the CLSM directly in the trench or excavation. 18.104.22.168.3.3. Place the CLSM using pumps, chutes, or any other method as approved by the engineer. Place the CLSM in lifts such that the hydrostatic pressures developed will not compromise the integrity of bulkheads, formwork, trench or other soil walls, or other temporary or permanent structures. 22.214.171.124.3.4. Placement shall bring the material up uniformly to lines or limits as shown on plans. 126.96.36.199.3.5. For cases in which subsidence effects on the final grade are critical, place a final lift that will account for estimated subsidence or otherwise ensure that the final grades on the plans can be achieved and maintained. 188.8.131.52.3.6. The CLSM shall be applied in such a manner that no labor is required in the trench or excavation. No compaction or vibration equipment shall be allowed. 184.108.40.206.3.7. The CLSM must have a minimum temperature of  °C at the time of placement. 220.127.116.11.3.8. Place CLSM only in conditions where the ambient temperature is greater than  °C. Do not place CLSM in contact with frozen soil or other mat- erial. Once placed, keep the CLSM from freezing for a period of no less than  hours. 18.104.22.168.3.9. CLSM may not be placed in conditions of inclement weather (e.g., rain) unless approved by the engineer. [CLSM may be placed in conditions of inclement weather (e.g., rain) as long as any rainfall does not result in ponding on the surface of the in-place material and that the requirements for minimal stand- ing water, noted above, are met.] 22.214.171.124.3.10. For projects in which no pipe bedding is in place, ensure and maintain the appropriate horizontal and vertical alignment of pipes and fixtures prior to and during the placement procedure, and until such time as the CLSM has set to sufficient strength to hold the pipes in place. Use straps, soil anchors, or other approved means of restraint. 126.96.36.199.3.11. Pipe or other items damaged by the contractor during construction shall be replaced at the contractor's expense or repaired to the satisfaction of the engineer. 188.8.131.52.4. Material acceptance guidance should consider inclusion of the following language: 184.108.40.206.4.1. Material acceptance shall be based on all criteria specified, plus local experi- ence with excavatability, including the following: 220.127.116.11.4.1.1. Strength: a 28-day compressive strength of no more than [1 MPa] and no less than [0.2 MPa]. 18.104.22.168.4.1.2. Flowability: a diameter of no more than [250 mm] and no less than [175 mm]. 22.214.171.124.4.1.3. Removability modulus: a value, calculated using in-situ density and [91-day] compressive strength, or as dictated by the antici- pated removal methods and as specified by the engineer. 126.96.36.199.5. Material measurement guidance should consider inclusion of the following language: 188.8.131.52.5.1. Measurement shall be based on the payment lines indicated on the plans. Pay- ment shall be based on the CLSM in its hardened state. No payment shall be made for additional material required by slips, slides, cave-ins, over-excavation,
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D-7 or other actions resulting from the elements or from construction activities. No payment shall be made for unused or wasted material. 184.108.40.206.5.2. Material payment guidance should consider inclusion of the following language: 220.127.116.11.5.2.1. Payment shall be per cubic meter of in-place material including all costs for furnishing all materials, equipment, labor, and inci- dentals necessary to complete this item. 4.2. UTILITY BEDDING 4.2.1. Definitions and Types 18.104.22.168. Utilities could include pipe, electrical, telephone, and other types of conduits. 22.214.171.124. Utility bedding relates to the preplaced or infill material to provide support strength for utilities (usually underground). CLSM is the alternative to a bedding material that is typically a compacted granular struc- tural fill. 126.96.36.199. Utility bedding is not the same as backfill, although it can be contiguous with such backfill in the case of encasing the entire conduit. Utility bedding also is not the same as void fill; the primary difference is that utility bedding is placed underneath the utility structure with the purpose of providing supporting strength to the utilities and distributing loads and reactions. 188.8.131.52. Figure D.2 indicates two common utility bedding applications for CLSM. 4.2.2. Criteria of Importance 184.108.40.206. Utility bedding generally must provide enough support strength for the utilities, usually by influencing the load and reaction distribution and the resultant lateral pressures. In the case of bedding only, the bedding may provide structural support for the utility and distribute the reaction. For encasing the entire conduit, the application is providing support for the conduit, distributing the reaction, and transferring the load. 220.127.116.11. When CLSM is used for utility bedding, the width of excavation ("trench width") shown on the plans may need to be changed so that the clear distance between the outside of the pipe and the side of the exca- vation, on each side of the pipe, is a minimum of [150 mm], except that [300 mm] should be required for pipes of [1,050 mm] and greater in diameter or span when height of cover is greater than [6.1 m]. 18.104.22.168. Because CLSM is in a liquid state during placing, it will exert flotation on structures. It is cautioned that such flotation force may cause damage to the structures, especially when the structures do not have ade- quate resistance. Generally, CLSM shall not be used with underground structures having a span greater than [6.1 m], unless otherwise approved by the engineer. 22.214.171.124. The criteria for backfill noted previously that are also applicable to utility bedding include the follow- ing, with the same guidance on tests and specification limits: 126.96.36.199.1. Flowability (so that the material may flow along a trench floor and fill all spaces beneath the conduit that it is supporting) 188.8.131.52.2. Subsidence (such that conduit that is entirely encased does not lose support) 184.108.40.206.3. Setting time 220.127.116.11.4. Strength 18.104.22.168.5. Permeability 22.214.171.124.6. Air content Pipe or utility Pipe or utility CLSM A. Bedding Only B. Encasing of Conduit Figure D.2. Typical applications of CLSM in utility bedding.
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D-8 126.96.36.199.7. Corrosion (with the understanding that conduits entirely encased in CLSM are unlikely to exhibit corrosion resulting from electrochemical potential differences) 188.8.131.52.8. Thermal properties 184.108.40.206.9. Roadway support properties 4.2.3. Specifications Issues 220.127.116.11. Specifications, construction, materials, measurement, and payment guidance for backfill should be con- sidered, plus the following issues: 18.104.22.168.1. No placement of CLSM shall commence until all items have been inspected by the engineer and approved for utility bedding. 22.214.171.124.2. Adequate conduit anchorage shall be provided to ensure the movement of supported struc- ture is within tolerance limits, as designated by the engineer. 126.96.36.199.3. For bedding applications, provide suitable vertical wall containment such as sandbag or soil bulkheads to limit the flow distance of the CLSM to no more than  m from the discharge location. For encasing applications, provide suitable vertical wall containment to ensure that the CLSM will not flow into areas beyond those specified on the plans. For steeply sloping trenches, provide bulkheads at intervals as approved by the engineer. 188.8.131.52.4. The time limit for placement of CLSM should be no more than  minutes after the end of mixing. 184.108.40.206.5. CLSM should be carefully placed to fit the lower part of the conduit exterior for a width of at least 60 percent of the conduit breadth. Make sure no voids exist underneath the conduit. Placement should bring the material up uniformly to lines or limits as shown on plans. 4.3. VOID FILL 4.3.1. Definition and Types 220.127.116.11. Void fill relates to the infill material to occupy empty spaces created by erosion, construction, abandon- ment, and other activities. 18.104.22.168. The use of CLSM is a unique solution for void fills that are difficult, if not impossible, to fill with a com- pacted granular fill. 22.214.171.124. Void fill is not the same as utility backfill, although it can be similar. The primary difference is that void fill is generally placed to occupy empty spaces rather than to provide a sort of structural support. 126.96.36.199. Figure D.3 illustrates a typical void where CLSM can be applied. 4.3.2. Criteria of Importance 188.8.131.52. Void fill generally must fill an open or covered space of some sort, usually a space accessible from above, and it must occupy the space with minimum large voids left behind. 184.108.40.206. Its application must not cause undesired movement or damage of adjacent structures. 220.127.116.11. Flowability must be considered. CLSM must flow from its point of delivery to a reasonable distance, such as reaching the other end of the void. A mixture that is too stiff will not allow the material to reach all nec- essary locations without the use of additional equipment and labor. A mixture that is too liquid is gener- ally not a problem, if all other material properties discussed below are met. 18.104.22.168.1. A flow resulting in a circular-type spread with a diameter of [175 to 250 mm] as measured by ASTM D 6103 is considered an appropriate criterion, or as decided by the engineer. CLSM Figure D.3. Typical applications of CLSM in void fill.
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D-9 22.214.171.124. In cases where subsidence could cause a dip in the final surface, it is important to limit or take into account the subsidence of CLSM, if necessary. Typically, CLSM may "shrink" approximately 6 mm for every 300 mm of depth. Because overfilling voids is impractical (because the CLSM would simply run over the edges), proper planning related to subsidence must be undertaken. 126.96.36.199. If void fill must provide some structural resistance to loads, a minimum strength must be specified that is appropriate to whatever the structural needs of the specific application may be. 188.8.131.52. If the weight of void fill may cause disturbance to foundations of adjacent structures, appropriate con- siderations to unit weight, air content, or other issues are essential. Performance criteria related to these and other items should be specified by the engineer, with appropriate test methods indicated. 184.108.40.206. The criteria for backfill noted previously are also applicable to void fill, with the same guidance on tests and specification limits. 4.3.3. Specifications Issues 220.127.116.11. Specifications, construction, materials, measurement, and payment guidance issues noted for backfill should also be considered for void fill, as appropriate. 4.4. BRIDGE APPROACHES 4.4.1. Definition and Types 18.104.22.168. Bridge approach fill relates to the infill material to work as embankment (in fill behind bridge abut- ments applications) up to a specified grade (usually equal to the grade of pavement) or to the infill for bridge abutment. 22.214.171.124. CLSM is the alternative of an infill material that is typically a compacted granular structural fill. 126.96.36.199. Bridge approach fill is not the same as utility bedding, backfill, or void fill; the primary difference is that bridge approach fill is placed against a structure with the purpose of providing adequate structural resistance to loads. 188.8.131.52. Figure D.4 indicates common bridge approach fill applications. 4.4.2. Criteria of Importance 184.108.40.206. Bridge approach fill generally must fill an open space of some sort, usually a space accessible from above, and it must provide adequate structural support or/and least density for least differential settlements of the bridge approach system. 220.127.116.11. In the case of embankment fill, the fill may provide structural support for pavement above and distrib- ute loads. Bridge Embankment Joint Bridge Deck CLSM Abutment Fill Pavement Shallow Foundation CLSM Approach Fill Compacted Embankment Deep Foundation Natural Soil (Optional) Source: Modified from Jean-Louis Briaud, Ray W. James, and Stacey B. Hoffman, NCHRP Synthesis of Highway Practice 234: Settlement of Bridge Approaches (The Bump at the End of the Bridge), TRB, National Research Council, Washington, DC (1997), p. 4, Figure 1. Figure D.4. Typical applications of CLSM in bridge approach construction.
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D-10 18.104.22.168. For bridge abutment fill, the fill is providing support for the wall, usually acting as a bridge between the wall and the area of embankment. 22.214.171.124. Because of the varying nature of the bridge approach, not all criteria below may be important for all applications. 126.96.36.199. The criteria for backfill noted previously that are also applicable to utility bedding include the following, with the same guidance on tests and specification limits: 188.8.131.52.1. Flowability 184.108.40.206.2. Subsidence 220.127.116.11.2.1. Both interim and final grades of construction materials are important for bridge approach fill. 18.104.22.168.3. Setting time 22.214.171.124.4. Strength (By its very nature, bridge approach fill must provide adequate structural resistance to loads. A minimum strength must be specified that is appropriate to whatever the structural needs (e.g., traffic loads) of the specific application may be. In rare cases, bridge approach fill may be removed later, such as when additional future construction is performed. For this rea- son, a predetermined maximum strength is recommended to ensure future removability of the CLSM. The actual value of that strength may depend on whether removal is anticipated using manual equipment or machinery.) 126.96.36.199.5. Permeability 188.8.131.52.5.1. Water permeability is an important issue for bridge approach fill. A very impermeable CLSM mixture may lead, depending on the application design, to excess water being unable to flow through or around the CLSM, which may lead to a buildup of water pressures against the abutment or to washouts at the CLSM-soil interface. The installation of appropriate drainage system shall be carefully evaluated. 184.108.40.206.5.2. Minimum water permeability is established based on the permeability coefficient k. The minimum k should be [1 × 10-4 mm/s] unless permeability is deemed not to be an issue. 220.127.116.11.6. Air Content 18.104.22.168.7. Corrosion (only if pipes or other metallic components are installed in the fill) 22.214.171.124.8. Thermal properties 126.96.36.199.9. Roadway support properties 4.4.3. Specifications Issues 4.4.4. Specifications, construction, materials, measurement, and payment guidance similar to that of backfill are appro- priate for bridge approach applications, with some revisions. 188.8.131.52. For bridge abutment fill, ensure and maintain the appropriate horizontal and vertical alignment of abut- ment walls prior to and during the placement procedure, and until such time as the CLSM has set to sufficient stiffness to exert minimum forces on the walls. Use soil counter fill or other approved means of restraint. 184.108.40.206. A 28-day compressive strength of no more than [8.4 MPa] and no less than [0.35 MPa] should be considered. 220.127.116.11. A flowability test diameter of no more than [225 mm] and no less than [178 mm] should be considered. 5. METHODS FOR LONG-TERM IMPROVEMENT OF PRACTICE 5.1. CLSM is a product whose future performance is best predicted by past performance in similar situations for similar mix- tures. Accordingly, where practical, efforts should be made to include samples for short- and long-term testing of CLSM material for every job. 5.2. As noted previously, where practical, specimens of the CLSM should be buried adjacent to the project site, in as similar con- ditions as possible, so that long-term properties of the CLSM can be tested. The performance of CLSM is highly dependent on the curing, climatic, and local field conditions, and such specimens will allow for both better prediction of properties under those conditions, and for the specific probable excavatability of the CLSM at that site should it be required. 5.2.1. Such test cylinders should be tested for excavatability prior to establishing the contract documents and schedule for a follow-up project, because of the likelihood of the test results indicating long-term strength gains that may differ from anticipated ones.
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D-11 5.3. Agencies should consider developing performance-tracking methods for CLSM applications. Relevant material and proj- ect information should be recorded, including mixture design, constituent materials (and their sources), specified test methods (and their results), and actual follow-up performance data where available. 5.3.1. Follow-up performance data should include any site observations made by maintenance or construction crews. For example, on a trench fill application in a roadway, if noticeable subsidence of the CLSM occurs that results in roadway damage, note should be taken. If excavation of a utility trench backfill requires more powerful equip- ment than was anticipated based on the original project material and construction specification (e.g., if the remov- ability was designed for hand tools and requires power equipment), it should be noted. 5.4. Corrosion activity for utility or other applications should be tracked at sites that undergo later excavation. Because corro- sion tends to be a longer term phenomenon, laboratory or short-term predictive tests can provide only limited guidance. 5.5. Where actual field performance of CLSM differs dramatically from the specified or predicted performance, efforts should be made to engage forensic studies of the site by appropriate experts, either in house or under contract. Results of such studies should be shared both among all applicable portions of the agency and with the industry at large to help provide better understanding of the causes of variable CLSM performance. 5.5.1. Agencies should consider noting the major "repositories" of information related to CLSM, such as the American Concrete Institute Committee 229, the National Ready-Mixed Concrete Association, and others, so that all appro- priate information can be shared with those groups.