Nicholson Construction Company 12 McClane Street Cuddy, PA 15031

Telephone: 412-221-4500 Facsimile: 412-221-3127

Practical Soil Nail Wall Design and Constructability Issues

by

Walter G. Kutschke, P.E. URS Corporation, Pittsburgh, Pennsylvania

Fred S. Tarquinio, P.E.

Nicholson Construction Company, Cuddy, Pennsylvania

William K. Petersen, P.E. URS Corporation, Fort Washington, Pennsylvania

Presented at:

The Broadmoor DFI’s 32nd Annual Conference on Deep Foundations

Colorado Springs, Colorado October 11-13, 2007

07-02-157

PRACTICAL SOIL NAIL WALL DESIGN AND CONSTRUCTABILITY ISSUES

Walter G. Kutschke, P.E., URS Corporation, Pittsburgh, Pennsylvania, USA Fred S. Tarquinio, P.E., Nicholson Construction Company, Cuddy, Pennsylvania, USA William K. Petersen, P.E., URS Corporation, Ft. Washington, Pennsylvania, USA

Four significant soil nail wall projects in the eastern United States were recently completed with a combined area of 175,000 square feet of finished shotcrete surface. These projects required the use of innovative design and construction methods in order to address various challenges, including slide-prone back slope materials, perched water, highly erodible rock materials, curved wall alignments, very tight construction tolerances and unexpected subsurface conditions. Special focus is given to issues such as bench excavation, soil nail installation methods, shotcrete mix design, anticipated shotcrete quantities, shotcrete nozzlemen qualifications and weather conditions, in order to provide a lessons-learned database for future soil nail wall design and inspector considerations. These projects also underscore the importance of design engineer and soil nail wall contractor qualifications as well as effective communication with the owner.

Introduction The intent of this paper is not to reiterate the design and inspector guidelines presented in FHWA (2003) and FHWA (1994), but rather to present issues that occurred during the construction of four significant soil nail wall projects in the eastern United States. Although each project is different with regard to geologic conditions and design, these projects each experienced some similar situations. These similarities indicate a trend in design, specification and construction of soil nail wall projects. It is in these situations that lessons are learned and presented herein. The first soil nail wall project referenced herein was part of a larger project involving the construction of a new railroad alignment in the relatively mountainous terrain of western Pennsylvania. This project involved the construction of over 9,000 square feet of soil nail retaining structure with a total of 13,800 lineal feet of soil nails and 428 cubic yards of shotcrete. In addition, the project also required construction of a shotcrete slope protection system which used very similar construction techniques as those employed for soil nail walls. This effort required the placement of approximately 90,000 square feet of slope protection with a total of 34,000 lineal feet of rock anchors and nearly 2,800 cubic yards of shotcrete. This system is believed to be the largest known application of such a system to

date. Refer to Kutschke et al. (2007) for further details regarding this work. The second project involved the construction of a soil nail retaining wall used for the support of excavation for the new Chinese Embassy building in Washington, D.C. This project consisted of the installation of over 50,000 square feet of exposed shotcrete wall surface, approximately 1,600 soil nails and over 1,600 cubic yards of shotcrete. The third project involved the construction of a soil nail retaining wall for the support of excavation for a new retail development in eastern Pennsylvania. This project involved 10,000 square feet of exposed shotcrete surface requiring 400 soil nails and 400 cubic yards of shotcrete. The last project also involved the construction of a shotcrete wall used for support of excavation for a new retail development in southwestern Pennsylvania. This project consisted of 16,000 square feet of soil nail wall with approximately 600 soil nails and 800 cubic yards of shotcrete. These projects were all successfully completed and are in service. Subsurface Conditions Soil nail wall construction is not appropriate for all soil conditions (FHWA, 2003). In the authors’ opinion, well-drained cohesive soils, such as the

residual clays and weathered bedrock characteristic of the Piedmont and Appalachian regions, are ideal for soil nail wall construction. The importance of soil type cannot be overemphasized, as illustrated by the following examples. The ground condition had been severely altered at one of the referenced projects just prior to its construction. Following completion of the field survey obtained for design activities, the original property owner had excavated a haul road into the hillside for log truck operations, with the road alignment corresponding roughly with the planned alignment of the soil nail wall. In order to re-establish the original ground surface to preclude backslope failures, the general contractor backfilled this haul road cut with poorly-compacted material having significant fines content. As excavation for the soil nail wall proceeded through this loose fill, which reached a maximum depth of about 6 feet at the wall face, a large degree of sloughing occurred, particularly in response to disturbance from drilling the nail holes. This condition was further exacerbated by a heavy rainfall event from the remnants of Hurricane Katrina. The changed ground elevations and slope failures created numerous problems related to the as-designed wall geometry, and additional survey and design effort was required to adapt the wall to fit these changed conditions. Furthermore, a very large volume of additional shotcrete was required to fill in voids where sloughing had taken place at the wall face. Since the backfilling operation occurred before the soil nail wall contractor had mobilized to the site, there was nobody present on-site that was able to foresee the dire consequences of this decision. A detailed pre-construction meeting, involving the general contractor, subcontractor and the engineer may have prevented this occurrence. Groundwater, surface runoff or perched water in the wall excavation are likely to cause stability and drainage problems during construction. In addition, the shotcrete may have a problem adhering to the excavated face if surface water is present. This was a major problem in a small area of the wall in eastern Pennsylvania. As a result, large quantities of shotcrete were required in order to maintain the alignment of this permanent wall. Excessive seepage can be detrimental to newly placed shotcrete because it acts to wash the

cement off of the aggregate and to create additional load resulting in minor cracking and sloughing to complete loss of shotcrete adhesion to the wall face. The most effective means to address seepage at the wall face is to control and direct the groundwater flow. The placement of additional drainage geocomposite and / or the use of PVC drain pipe to capture and direct the drainage away from the newly placed shotcrete have both been effective, as displayed by Figure 1, where drainage in excess of 80 gallons per hour was occurring at select

Figure 1 – Additional Wall Drainage for Seepage Control locations along the shotcrete face. Although this is an extreme event, it demonstrates the effectiveness of this approach. The geocomposite drains and PVC drain pipe effectively collected and diverted the flow away from the slope face and allowed the placement of shotcrete in this instance. It is important to note that when these drainage measures are employed, they are self-supported by securing them to the slope face or steel reinforcement rather than relying on the shotcrete for support. Bench Excavation Bench excavation heights are not only limited to the stand-up time of the ground, but consideration must also be given to the nozzleman’s abilities. In order for the proper application of shotcrete, the nozzle must be perpendicular to the slope face. As the angle between the slope face and nozzle increases, the degree of compaction decreases with a corresponding increase in rebound. Bench heights beyond about 5 to 6 feet place additional burden on the nozzleman and can result in quality problems as the upper reaches are

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difficult to shoot and finish properly. As such, it is the author’s experience that limiting the bench height to 5 to 6 feet enables a nozzleman to properly and safely shoot the wall face and upper overlap area. Figure 2 displays a typical

Figure 2 – Challenging Application of Shotcrete due to Excessive Bench Height Excavation situation when the bench height approaches this upper limit. The congested reinforcement zone in the overlap area requires particular attention that is difficult and burdensome for this experienced nozzleman. Also the ability for him to blend in the final shotcrete layer is further exacerbated as the rebound will significantly increase at the upper reaches (note the significant rebound that has all ready accumulated at the base). Design must utilize appropriate vertical distances between lifts considering not only soil conditions, but also practical heights between lifts as well as address maximum permissible bench lifts in the specifications. For comparison purposes, Figure 3 indicates an appropriate bench excavation that will be much easier for a nozzleman to work with.

Figure 3 – Appropriate Bench Height Excavation

Nail Installation Air-track drilling is an economical drill method when drilling into materials that do not require casing to support the hole. Figure 4 illustrates a typical air-track drilling operation.

Figure 4 – Typical Air-Track Drilling Soil nail drilling production rates are highly dependent on equipment and driller, but rates of 1 to 2 feet per minute are typical values in hard clay or weathered rock. Nail hole diameters are generally limited to 4 to 5 inches with air-track equipment. Although these hole diameters theoretically provide sufficient grout coverage between the nail and bonding strata, the ability of the driller to consistently create a straight shaft is debatable. This consideration, combined with the natural sag of the bar as it deflects under self-weight between the centralizer support points, will significantly reduce grout coverage locally along the bar. Therefore, a centralizer spacing of less than the 10-foot industry standard is warranted in environments that require long-term corrosion protection. Furthermore, centralizers should be secured to the soil nails by tie-wire; methods such a taping do not properly secure the centralizer and can result in bunching of the centralizers as the nails are inserted into the hole. Although air-track drills are an economical means of advancing a soil nail drill hole, the drilling operation can create significant disturbance at the slope face resulting in sloughing and soil break-outs. If this condition persists, a drill berm is highly effective, as shown on Figure 5.

Figure 5 – Prudent Application of a Drill Berm Figure 5 displays the slope face, in this case consisting of cohesive residual soil, after drilling and grouting. The dashed line noted in the figure represents the back-of-wall face. The use of a drill berm in this situation prevented the sloughing and general drill disturbance from impacting the back-of-wall face. There would have been substantial detrimental impacts to this structure had a drill berm not be used. Telehandlers or similar machines are commonly used to lift a bundle of soil nails for the labor force to insert them into the grouted drill hole. Although this practice is acceptable, care should be taken as the nails are lifted from the forks. At no time should the inspector allow the nails to slide against the fork and into the hole. This needlessly exposes the nail to abrasion that can create holes in the epoxy coating. Nails should be manually lifted and inserted into the hole. Structural Materials The most important aspects of material design and quality control with respect to soil nail walls are the nail grout and shotcrete. Typical nail grout consists of a cement and water combination with approximately 0.45 water:cement ratio, having 28-day compressive strength of at least 4,000 psi. However, it is crucial in the timing of most soil nail projects to have 3-, 2- or possibly 1-day strength results. Shotcrete is generally applied using the wet-mix process (FHWA 2003). This process generally results in a higher volume throughput with less rebound. Wet-mix application rates for these projects were typically about 6 to 7 minutes per cubic yard of shotcrete. Similar to the nail grout, it is critical to have shotcrete compressive

strength results at 3, 2 or 1 day(s) in order to maximize production without comprising the integrity of the wall. Proper mix design and adequate drainage are paramount to the longevity of the shotcrete face due to freeze / thaw cycling. Shotcrete slump is largely self-controlling; too wet and it will slough, too dry and it will not pump. A combination of proper air entrainment and a low water:cement ratio help provide adequate freeze / thaw durability. Published literature indicates that loss of entrained air during the pumping and spray application is typically 4-5% (FHWA 1998). Typical wet mix shotcrete designs require a water:cement ratio no greater than 0.45 with minimum air entrainment of 7 - 10%, measured at the truck. Pozzolans, such as fly ash, improve pumpability and will produce a more durable shotcrete by mitigating the alkali-silica reactions, increasing resistance to sulfate attack, and reducing ingress of potentially deleterious materials such as chloride and water. However, fly ash has the potential to impact air entrainment. Hill (2006) indicates that as the loss on ignition value of fly ash increases, the dosage of air entrainment chemical will generally increase. Suitable material selection is essential. Proper aggregate distribution is very important with regard to strength and durability of the finished shotcrete face, but also is very critical with regard to pumpability and the ability of the shotcrete mix to adhere to the excavated face. Figure 6 represents the recommended range of aggregate size distribution for a good shotcrete mix, which is in general conformance with FHWA (2003).

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Shotcrete reinforcement is based on the structural requirements of the soil nail wall. In addition to this reinforcement, an additional layer of wire mesh-type reinforcement can be added. The wire mesh opening should be no smaller than 4 inches, since smaller openings will generally act to interfere with shotcrete placement. It is suggested that this mesh provides additional confinement to minimize shotcrete sag as a greater thickness of shotcrete is placed; however, its benefit is questionable and lift thickness should be limited to 6± inches even where it is used. Wall drainage is paramount to the longevity of a soil nail wall system. Geocomposite drainage panels or strips are often used to provide drainage. These materials are generally tacked to the slope face with a reinforcing pin and installed in shingle fashion as the excavation is lowered. Drains are daylighted by means of a weephole, and care must be taken to avoid creating a low spot for water to collect. Weep holes must be covered during application of shotcrete to avoid clogging the drain. Extreme care must be taken by the nozzleman to avoid placing shotcrete behind the drainage panel and therefore render it useless. As such, it is extremely important that the drains are securely fastened against the slope face prior to shotcrete placement. Shotcrete Installation On most projects, the general contractor performs the bulk excavation, and therefore is required to provide the finished cut soil/rock faces onto which the specialty geotechnical subcontractor will apply the shotcrete. In most cases, the general contractors on these projects found it challenging to excavate the weathered rock to the planned angles without significant overbreak, as exampled by Figure 7. As a result, it was necessary to completely fill the overbreak pockets, in some cases as much as 3 feet deep, with shotcrete in order to leave a fairly uniform finished surface. From this experience, it is suggested that contracts include a pay item for excess shotcrete. However, it is also important to note that this item can be a source of contention, and thus pay items should be reviewed and accepted by the owner as readily as possible. A separate pay item for plain shotcrete is advantageous because it does not include incidentals such as reinforcement,

Figure 7 – Overbreak bearing plates, drainage strips, etc. It is emphasized that the owner should periodically review the work conditions in order to gain a level of confidence that additional shotcrete is necessary, and that quantities are not unjustifiably increased. Bid quantities should include a reasonable contingent value in order to minimize financial impact to the project. It is suggested that this value is approximately 30% of the overall estimated neat shotcrete quantity. Experienced Nozzlemen For shotcrete installation, especially for permanent shotcrete walls or temporary walls with tight horizontal tolerances, it is extremely important to have experienced shotcrete nozzlemen. These individuals are ultimately responsible for the final product, and this work requires a high degree of craftsmanship. Pre-construction test panels are necessary to evaluate the nozzlemen qualifications, and the preparation of shotcrete test panels (Figure 8) is

Figure 8 – Nozzlemen Test Panels

a standard Quality Assurance practice carried out in order to evaluate the qualifications of each nozzleman prior to the beginning of production. It is important to note in Figure 8 that the panels are at the same approximate angle as the slope face. Both reinforced and unreinforced shotcrete panels are prepared using the shotcrete mix proposed for use on the project. The reinforced panels are cored for visual observation to assess whether the nozzleman’s technique results in uniform shotcrete distribution around the reinforcement. Figure 9 indicates shotcrete

Figure 9 – Shotcrete Test Panel Cores cores obtained from a test panel. Note that the left-most core exhibits a significant build-up of aggregate (rock pocket) behind the reinforcement. This test panel was created by an inexperience laborer and he was not permitted to serve as a nozzleman. The cores taken from the unreinforced panels are generally tested for unconfined compressive strength and boiled absorption. It has been observed that, even among personnel that have been approved for a given project, different nozzlemen can produce a wide range of shotcrete quality depending on their individual experience and technique. Therefore, it cannot be assumed that just because a particular nozzleman demonstrated adequate qualifications per the project specifications that he will consistently produce high quality shotcrete in production. Inspection staff should be aware of poor technique and the inferior shotcrete qualities that develop as a result. It should also be understood that even the best shotcrete nozzlemen will not produce a shotcrete face that looks like poured concrete. Shotcrete faces in general will be rough and

non-uniformly colored unless followed by floating and colored with pigmented sealers. The nozzleman and inspector must also pay close attention to the bearing plate area as this will act as a barrier if the plates are mounted prior to shooting. Figure 10 indicates an experienced nozzleman. Note how the nozzle is near perpendicular to the slope face and relatively low rebound.

Figure 10 – Experienced Nozzlemen Proper curing of the shotcrete during cold weather is extremely important. Shotcrete not cured properly according to project specifications can result in low compressive strength and surface deterioration. In addition to curing, the receiving surface must be free of ice or other deleterious elements. Figure 11 indicates one method to pre-heat a receiving surface during inclement weather.

Figure 11 – Cold Weather Operations Obstructed from view under the tarp are a series of torpedo heaters. Also note the insulation blankets, adjacent to the blue tarp, which was placed on relatively fresh shotcrete. Test panels

shot under similar circumstances confirmed the suitability of this approach. Quality Control/Quality Assurance There are two basic elements for quality control / quality assurance for a soil nail wall project, namely:

1. The soil nail elements, specifically unconfined compressive strength testing of soil nail grout cubes and proof/verification testing of the soil nails.

2. The soil nail face, specifically the unconfined compressive strength and boiled absorption testing of the shotcrete.

Compressive strength testing of grout is relatively straightforward. Figure 12 indicates a typical scatter of nail grout data. It is important to

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Figure 12 – Typical Grout Cube Test Data Plot note that some data points fall below the criteria line and could result in rejection of soil nails. Although this was cause for concern during construction, there is no trend to support low grout breaks, and the probable explanation for these outliers is a defective cube (i.e., improper curing, cracked cube, et cetera). Significant discussions could have been avoided had this cube been identified as defective and not suitable for testing. Handling, curing, storage and transportation of grout cubes is very important, and proper care must be adhered to for accurate results of test samples. Specific gravity testing of the mixed grout using a mud balance is important to confirm the mix design of the grout, especially when low compressive test grout break results occur.

Soil nail testing generally involves verification and proof testing as outlined in FHWA (2003). Generally, soil nail tests should be performed to assess the overall nail resistance. Separating and testing various geologic strata within the length of a single nail is not recommended because it can create unnecessary complications. The important parameter is the overall resistance offered by the installed soil nail as compared to the design resistance required by the soil nail load diagram developed for a particular design. Figure 13 displays a typical soil nail test set-up.

Figure 13 – Typical Soil Nail Test Set-Up It is important that the inspector and contractor coordinate test activities. Typically, an observant inspector will select test nail locations based on drill rig response, review of cuttings, or some other geotechnical concern. The design engineer, inspector and contractor must understand the type of test and test loads and then use this information to select an appropriate sized soil nail bar to avoid overstressing the nail during a test situation, as might happen if a production nail was tested under a verification test load. The review and interpretation of the nail test data is done in accordance with the project specifications. Typically, two plots are generated, namely a movement vs. load plot, as exampled by Figure 14, and a movement vs. time plot, as exampled by Figure 15.

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Figure 15 – Soil Nail Test, Movement vs. Time Summary and Conclusions Four significant soil nail wall projects were recently completed with a combined retained area of 175,000 square feet. The lessons learned from these projects were: 1. Bench Stability – Although soil nail wall

construction is extremely versatile, its use should be limited to appropriate soil types. Consideration must be given to soil stand-up time and groundwater conditions. Constructability reviews during design must consider nail spacing and address bench height limitations in the project specifications. Innovation is the key to success when encountering difficult conditions and several possible solutions were presented herein for difficult ground conditions.

2. Shotcrete Over-Runs – All soil nail wall

projects will experience shotcrete overruns if the neat area/volume is used in the bid tabulations. Voids and slope sloughing are inevitable. Paying for shotcrete overruns can become a source of great contention between the engineer, owner and

contractor. Project specifications should consider the use of a bid item with contingent quantities for extra shotcrete, with extra quantities in the order of 30% of the neat volume. The owner needs to understand and accept these quantities as they develop.

3. Nozzlemen Experience – Nozzlemen are

ultimately responsible for the overall quality of the finished shotcrete product. Their craftsmanship results in the final aesthetic appearance of a wall (when specifications require a gun finish) and their skill attributes to the structural continuity of the wall. From a contractor’s perspective, the nozzelmen are given significant financial responsibility and they rely on their skill to apply the shotcrete in accordance with the tolerance noted in the specifications. Establishing their qualification prior to production is an industry standard that should always be adhered to.

4. Experience and Communication – The

experience that each team member brings to the project is vital to the success of a project. An experienced design engineer and contractor understand issues that are important. It is this experience and communication that can maintain schedules and limit financial risk.

The issues presented in this paper are those of the authors based on the referenced project experience. Other soil nail and shotcrete projects may not have experienced similar issues. Acknowledgements The authors would like to thank the people from URS Corporation, Nicholson Construction Company and Weidlinger Associates, Inc. who were involved in the design and construction of the referenced projects. References BONITA, G., TARQUINIO, F. and WAGNER, L., 2006. “Soil Nail Support of Excavation System for the Embassy of the Peoples Republic of China in the United States”, Proceedings of the Deep Foundations Institute (DFI) 31st Annual Conference on Deep Foundations, October 2006, Washington D.C.

FHWA, 2003. “Geotechnical Circular No. 7. Soil Nail Walls”, Publication FHWA-IF-03-017, U.S. Department of Transportation, Federal Highway Administration, Washington, D.C. FHWA, 1998. “Manual for Design & Construction Monitoring of Soil Nail Walls”, FHWA-SA-96-096R. U.S. Department of Transportation, Federal Highway Administration, Washington, D.C. FHWA, 1994. “Soil Nailing Field Inspectors Manual – Soil Nail Walls”. FHWA-SA-93-068, U.S. Department of Transportation, Federal Highway Administration, Washington, D.C. HILL, R.L., 2006. “The Impact of Fly Ash on Air-Entrained Concrete”, High Performance Concrete Bridge Views, #43, National Concrete Bridge Council, Skokie, IL. KUTSCHKE, W.G., PETERSEN, W.K, AND MEYERS, J.R., 2007. “Rock Slope Protection System for Differential Weathering Materials”, Proceedings of Geo-Denver 2007, Embankments, Dams and Slopes: Lessons Learned from New Orleans Levee Failures and Other Current Issues, Geotechnical Special Publication No. 161 (CD-ROM), ASCE, Reston, VA.