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The materials for construction of a soil nail wall will vary with local practice and site conditions. The following section provides some guidelines on the materials for construction, how they can be installed, and how installation can be monitored. 4.1 Materials A plan view of a SOIL SCREW® Retention Wall System with a permanent shotcrete facing is provided in Figure 4.1.1 and shows the components of the SOIL SCREW™ Retention Wall System required to build the wall. Each of the components is discussed below. 4.1.1 Screw Anchors - SOIL SCREW® Retention Wall System anchor lead sections and extensions are manufactured in standard lengths of 5 feet and 7 feet. The lead section has a bolthole on the top and a pilot point on the bottom. The extension has a bolthole at the top and a socket with a bolthole on the bottom. Both lead sections and extensions have 8-inch diameter helices spaced on approximately 2.5-foot centers. Each installed anchor will consist of one lead section and may have one or more extensions as well. If additional anchor length is required for making attachments to facing systems, this can be provided with short extensions without compromising the soil nail design. These extensions are shown on Figure 4.1.2 and include a 20-inch long threaded adapter, an 11-inch long threadbar adapter and several lengths of plain extension. These are all standard products used for both soil nails and tieback anchors. Screw anchor leads and extensions come in either galvanized or black steel, depending upon the project requirements. Threaded adapters and thread bar adapters are all galvanized. Typical spacing for screw anchor soil nails varies from a four-foot center-to-center pattern, to a 5 by 6 foot pattern, depending upon wall height and soil conditions. 4.1.2 Wall Connectors - Connection of the screw anchor to the shotcrete facing can be designed and constructed in several ways. The cost of the connector is a function of the performance desired and the design life. For temporary soil nail walls, the connection needs to be of sufficient durability to last until the end of the project. Stress transfer to the face occurs over an extended period of time in a soil nail wall, depending on the shear strength of the reinforced soil. Connectors utilizing a 3/4 inch thick plate and #4 rebar, as shown in Figure 4.1.3, are typically sufficient. The shotcrete thickness will vary from 3 to 4 inches and should completely cover the connector. For permanent walls, the ability to positively tension the connector is desired. The use of a threaded connector facilitates tensioning the anchor by torqueing the nut, thus reducing the potential for localized movement of the face. Figure 4.1.1 shows an example of a typical permanent connector using a threaded adapter that is bolted to the bearing plate. For this application, a bearing plate with studs is used for increased bond between the concrete face and nail head. 4.1.3 Shotcrete - The function of the shotcrete in a soil nail wall is to create a barrier to prevent deterioration of the excavated soil face and to provide a structural facing to transfer soil stress to the nail heads. Shotcrete is applied as soon after excavation and nail placement as possible. In some cases it may be applied immediately after excavation if poor ground is encountered. In this case, its function is to stabilize the excavation face so nails can be installed. Shotcrete is defined as concrete or mortar that is projected at high velocity onto a surface. Shotcrete is comprised of cement, water and aggregate typically less than a 1/2 inch sieve size, with the majority of the aggregate classified as sand size particles. Wet-mix shotcrete is normally used since it is usually less expensive, easier to install and has a higher throughput. The properties of the shotcrete that are critical during installation are its ability to be pumped and its adhesion. These are controlled by the mix design, cement water ratio, air entrainment, the pumping system used and other variables. A detailed discussion of shotcrete mix design and placement properties is provided in the FHWA-SA-96-069 Manual for Design and Construction Monitoring of Soil Nail Walls, November 1996. The critical properties of finished shotcrete are its shear and flexural strength to prevent punching failure at the nail head and support the retained soil, and its bond with the steel reinforcements to transfer stress. Wire mesh or rebar used with shotcrete should be spaced so the openings between bars are greater than 4 inches to be sure that bond is obtained with the soil face and around the reinforcing bars. The required strength of the shotcrete will be determined as part of the design process. However, as a minimum to provide adequate strength and durability, the shotcrete will need to have a minimum 28-day compressive strength of 4000 psi, and a water/ cement ratio of less than 0.5. Detailed specifications for shotcrete construction are provided in Appendix B. 4.1.4 Drainage Materials - Drainage behind the wall face is typically provided by placement of strip drains as shown in Figure 4.1.1. Strip drains are placed between the columns of nails, however, if localized seepage zones are encountered during construction, additional drains may be needed to help relieve pore pressure buildup. It is important to control water during construction to develop a good bond between the excavated soil and the shotcrete. However, adequate space for the soil/shotcrete bond is recommended, typically 50% or more, so that shotcrete does not delaminate, and voids are not created behind the wall face. Strip drains should provide a continuous drainage path from the top of the wall to the bottom interceptor drain or weep hole. Normally, a single drain can be unrolled from the top down as excavation progresses. However, very special care must be taken during subsequent excavation to protect the strip drain from damage during excavation. If splicing of drains is required, care should be taken to make sure that the drain cores are all covered with a geotextile to prevent clogging. Drains should be spliced or "shingled" in a manner as recommended by the manufacturer to assure that water from the upper drain flows readily to the lower drain. 4.2 Alternate Facing Materials As discussed in section 2.3, precast concrete panels, segmental retaining wall units and gabion or welded wire facings, can also be used for facings of soil nail walls. Each of these has specific attachment details essential to their performance and are best addressed on a project-by-project basis, and by the supplier of the relevant facing systems. Typical details from projects where some of these materials have been used are contained in Figures 2.3.1, 2.3.2 and 2.3.3. Common to each of these facings will be the need to: Provide a connection that will control wall face movements and has adequate tensile strength for the anticipated design life. Provide containment for the soil exposed during excavation. Provide the flexural stiffness needed to support soil between the soil nails. Provide a system that is economic and easy to install. 4.3 Installation Equipment Figure 4.3.1 shows some of the torque heads and drive tools that can be used to install the screw anchor soil nails. These tools can be used with most any construction equipment by coupling the torque head to the hydraulic unit of the equipment. The flow and pressure requirements of each torque head are provided in Figure 4.3.1. Typically, units require that the hydraulic system have a forward, neutral, and reverse setting to be able to back out anchors, if needed. A standard backhoe, trackhoe or even a skid loader, can be used for installation of the anchors. Screw anchors can also be installed with any drill rig that can provide the necessary torque. For these units, all that is needed are the Kelly bar adapter and drive tool shown in Figure 4.3.1. Installation rates of up to 50 anchors per day can be obtained using this type of equipment. 4.4 Installation, Monitoring and Testing 4.4.1 Installation - The installation steps for construction of a soil nail wall are shown schematically on Figure 1.2.1 and discussed below. Step 1 - Excavation - Soil Nail walls are built from the top down. Therefore, it is imperative to have a soil that will stand near vertical for the time it will take to anchor and shotcrete or stabilize the wall face. For permanent shotcrete-faced walls, excavation should be made as close to the plan grade as possible, as additional excavation will need to be replaced with additional shotcrete thickness. For temporary walls, excavation limits are not as critical. However, excavation control is needed during construction of a temporary wall to ensure that the ultimate wall face does not go beyond or inside the area where the permanent wall is to be built. Excavation should proceed to a depth below the nail elevation that will facilitate installation and maintain a stable ground condition. This is typically 2 to 3 feet below the nail elevation, but no more than 5 feet to satisfy OSHA regulations. Alternate for Unstable Ground - If soft or unstable ground is encountered during construction, there are several alternatives. If the ground is sloughing prior to nailing or shotcrete placement, a flash coat of shotcrete can be placed immediately after excavation to stabilize the face. Soil nails can then be drilled through the shotcrete and a final shotcrete coat applied after nail installation. If the ground will not stand vertically during excavation, then a berm and slotting technique can be used. The soil should be excavated to a stable angle of repose, such that ground at the top of the slope is outside the wall face. Then a vertical trench, wide enough to allow for soil nail installation, typically one or two backhoe widths, is excavated just below the elevation for the soil nail. After soil nail installation is complete for a specified length of wall, the berm is excavated vertically in short stable steps and flash coated. Once the entire step is excavated and flash coated, wire mesh and shotcrete are placed. While these methods may help to overcome isolated poor ground conditions on a project, these methods can be expensive and time consuming. Therefore, if these conditions are anticipated for all or part of a project, alternative means of support (i.e., Soldier beams with Chance Tieback Anchors) should be considered. Step 2 - Screw Anchor Installation - Once a stable excavation is constructed, the anchors can be installed. The anchors should be installed along the excavated face within 6 inches of their plan location. First, the lead section, either 5 or 7 feet long, is installed. The anchor should be aligned on the correct angle from the horizontal, as required on the plans. This can be monitored by placement of a level and protractor on the equipment being used to drill the anchors. Once aligned, the anchor should be pushed into the ground up to the first helix (approx. 6 inches), and then drilled or screwed into the soil to a depth that allows the next extension to be bolted onto the lead. The extension should be bolted on with the nut tightened to 40 ft-lbs., and then screwed into the ground. Anchor installation should proceed until the depth required on the plans is achieved. Step 3 - Shotcrete Placement - Prior to placing shotcrete, drainage composites and welded wire mesh should be placed along the excavated wall face. Any loose soil in areas where sloughing has occurred should be removed. Drainage composite is placed in between the anchors, as shown on the plans, and in any additional areas where excess seepage is observed. The composite should be unrolled down the wall face, and the unrolled section at the bottom of the wall should be covered with plastic in a trench below the area to be shotcreted. The top of the drain should be secured against the soil face so that it will not move during shotcrete placement. In the same way, the welded wire mesh or rebar should be placed over the composite and tied off to the nail heads. The bearing plate should also be attached to the end of the nail and be placed at a proper distance into the wall face, as shown on the plans. This is often done between the application of an initial half-thickness layer of shotcrete and the final finish facing. Before shotcrete is placed, a small berm or wooden form board (typically 6 inch to 8 inch tall) should be placed at the bottom of the excavation (Figure 4.4.1) to facilitate construction of subsequent lifts. This berm will allow for lapping of the wire mesh and provides a joint under which subsequent levels of shotcrete can be placed. Quality shotcrete is best obtained by experienced nozzlemen directing the shotcrete perpendicular to the wall face. This will minimize voids and sand pockets around the rebar or mesh, and maximizes bond of concrete to soil. It also minimizes the amount of excess concrete used by reducing rebound. Step 4 - Subsequent Excavation and Screw Anchor Installation - Excavation for the next layer of anchors and shotcrete proceeds as in Step 1. Great care must be exercised to protect the work that is in place. In particular, the shotcrete, nail heads and the strip drains should be protected from damage by excavating equipment. Once the excavation is complete, construction proceeds as outlined in Step 2. Step 4a - Subsequent Shotcrete Lift Installation - The area beneath the cold joint of the first shotcrete layer should be cleaned, and any loose particles removed by compressed air. The strip drain should then be rolled down, as in Step 3 (if damaged, a new section is placed with the damaged section removed per the manufacturer recommendations). Wire mesh or rebar is lapped under the shotcrete lip and #4 bar waler placed behind the plates for the nail head. The berm at the bottom of the excavation (see Fig. 4.4.1) is applied beneath the joint and progresses in the manner described in Step 3. Step 4 is repeated until the required depth of excavation is obtained. If weep holes are designed into the wall face, these are installed, as shown on Figure 4.4.2, prior to shooting the shotcrete on the final lift. If a trench drain and footing is used, this can also be installed prior to placing the final shotcrete lift. Step 5 - Install the Permanent Face - If a permanent facing is designed, then the rebar for this system is placed over the temporary shotcrete face, and the final shotcrete layer is placed. Alignment for the wall plumbness should be monitored during installation of the shotcrete layer. Oftentimes a thin series of wires is used to show the location of the wall face during shotcrete placement. Shotcrete is then placed and struck off to the level set by the wires. 4.4.2 Monitoring and Testing - All excavations, whether permanent or temporary, should be monitored regularly for movement during construction. Monitoring should include a daily walking inspection of the wall by the superintendent or foreman and the Engineer. Indications of movement or distress in the wall can include: Cracks developing behind the wall face Settlement or distress of adjacent structures or soil Cracks in the shotcrete facing Heaving of the excavation in front of the wall Seepage through the wall face If there are signs of movement or distress to the wall, steps must be taken immediately to investigate and correct any problem. This may include revising the construction techniques (smaller excavation lifts, using berms and slotting during excavation, etc.), providing additional drainage if required, monitoring the wall using survey monuments for a period of time, limiting surcharge, or possible redesign of the wall by including additional nails. In any case, the cause of distress must be uncovered, and actions taken quickly, to prevent further wall distress. For walls that will be greater than 10 feet in height, or wall with structures located above the wall, surveying of the wall for monitoring wall movement should be performed in addition to the daily field inspections. Some wall movement is to be expected as the wall is built. Therefore, it is important to locate survey points on the wall as excavation proceeds and to establish benchmarks that will not be disturbed by construction or other work on site. Regular surveying does not need to be performed on a daily basis unless there are indications of excessive movement of the wall, or there have been significant events, such as heavy rains or earthquakes. Monitoring should be logged over time to track any trends in the wall movement that may not be visible in daily inspections. Anchors should be tested to confirm their capacity (bond stress) and long term serviceability (creep). Non-production anchors are used for testing. The testing loads should not exceed 80% of the ultimate strength of the screw anchor being tested, and therefore, this will limit the length of the screw anchor. The anchors used for testing should consist of a length compatible with the limits of 80% of the ultimate strength and include a length of plain extension bar. This will enable the anchors to be installed to the typical depths for the project behind a typical failure plane and have smooth extensions, which will not bond with soil in the active wedge. Test screws can then be tested for bond and for creep, as discussed in the specifications in Appendix B.