Soil nail field inspectors manual

Typical PVC centralizers attached to a solid bar. The spacing between centralizers depicted is for illustration only; wider spacing is used in practice. Photograph courtesy of Williams Form Engineering Corp. Solid tendons with gray and purple epoxy coating left and partially encapsulated by corrugated sheathing right. Curing of a shotcrete test panel on site.

Shotcrete test panel cored for compressive testing of recovered specimens. Strip drain prior to initial shotcrete placement. Excavation of soil lift during construction of soil nail wall. Photograph courtesy of Moretrench American Corporation. Soil nail support of excavation with stabilizing berm.

Drilling nail holes. Solid nail tendon installation. Initial shotcrete application. Shotcrete reinforcement including WWM and waler bars. Photograph courtesy of Ryan R. Cast-in-place facing.

Precast panel facing. Photograph courtesy of Texas DOT. Sculpted wall facing. Photography courtesy of The Collin Group, Ltd. Preliminary layout of borings. Modified after Sabatini et al. Estimation of soil unit weight. Modified after U. Navy , Kulhawy and Mayne Friction angle of cohesionless soils as a function of normalized overburden Modified after Schmertmann Relationship between friction angle and plasticity index after Terzaghi, Peck, and Mesri Potential slip surfaces and soil nail tensile forces.

Potential critical stability case during construction Stress-transfer mechanism in soil nails: a basic soil layout; b schematic distribution of shear stresses at the grout-soil interface; c schematic distribution of resulting tensile forces Location of maximum tensile forces in soil nails. Modified after Byrne et al. Summary of maximum nail tensile forces measured in soil nail walls.

From Byrne et al. Summary of facing tensile forces measured in soil nail walls. Limitations to tensile forces in nails: a pullout resistance controls, b tensile resistance controls.

Potential limit states in soil nail walls: a internal stability slip surface intersecting soil and nails ; b global stability slip surface not intersecting nails ; c global stability: basal heave; d geotechnical strength: lateral sliding; e geotechnical strength: pullout; f structural strength: nail in tension; g facing structural strength: bending; h facing structural strength: punching shear; i facing structural strength: headed stud in tension Simple wedge for overall stability verification.

Basal heave for: a deposit of soft-fine grained soils, and b deposit of soft fine-grained soil underlain by stiff layer. Modified after Terzaghi et al. Bearing capacity factor Nc for determining basal heave. Lateral sliding of a soil nail wall. Limit states in soil nail wall facings. Modified after Lazarte Bending mechanism and nail force in facing. Section showing bending mechanism in initial facing.

Deformation of soil nail walls. Modified after Clouterre and Byrne et al. Soil nail patterns on wall face. Varying nail patterns: a example of nail arrangement for non- horizontal ground, b varying nail inclination and lengths around utilities, and c nail splaying at corners. Overall and compound slip surfaces in complex soil nail walls. Single nail stress-transfer mode: a soil nail layout, b distribution of mobilized bond stresses, and c hypothetical distribution of loads along the nail.

Reinforcement nomenclature for facing in bending. Soil pressure distribution behind facing. Limit states for punching shear in facing — horizontal cross sections: a bearing plate connection, b headed-stud connection. Geometry of headed studs. Forces acting behind a soil wedge Mononobe-Okabe Method.

Seismic active pressure coefficients: a horizontal backslope, and b correction for non-horizontal backslope. Effect of cohesion on the seismic active earth pressure coefficient. Drainage of soil nail walls. Typical drain pipe details.

Example of stepped soil nail wall. Source: Byrne et al. Composite wall structures: a cross section, and b soil nail walls with concrete piles. Strut-nail concept. Examples of frost protection: a first example, and b second example. General corrosion of a tendon.

After Hamilton et al. Procedure for selecting soil nail corrosion protection level. Class A corrosion protection: encapsulation. Modified from Byrne et al. Class B corrosion protection: galvanizing or epoxy-coating. Class C corrosion protection: bare steel tendon. Soil nail load testing setup. Source: Cadden et al. Soil nail load testing setup against shotcrete facing. Hydraulic jack used for soil nail load testing. Typical data sheet for soil nail load testing Example of data reduction from soil nail load testing.

Example of data reduction from a soil nail creep test. Typical comprehensive monitoring instrumentation. Strain gauge attached to a sister bar. Definition of reinforcement cross-sectional area per unit width in the horizontal direction at the nail head. B C2L Correction coefficient of nail length for cohesion App. B C3L Correction coefficient of nail length for factor of safety App. B C1F Correction coefficient of nail load for drill hole diameter App.

B C2F Correction coefficient of nail load for cohesion App. This document is also intended for management, specification and contracting specialists, as well as for construction engineers dealing with soil nail systems. This document only addresses permanent walls and includes a design framework implemented in LRFD for permanent structures. The information contained herein is aimed at producing safe and cost-effective soil nail designs for roadway projects, and to help Owners to identify and manage the risks associated with soil nail wall projects.

This manual focuses solely on soil nail systems providing long-term support of excavation of a permanent structure. This manual does not specifically address the use of soil nails as temporary structures for providing temporary support of excavation or to stabilize landslides. Reinforcing elements that are post-tensioned, even if installed adjacent to conventional soil nails, are referred to as ground anchors, which are not addressed in this manual.

Information on ground anchors can be found in Article As stated in the definition, load transfer to and from the surrounding ground develops through shear stresses acting along the grout interface of the soil nail. As the reinforced-soil block deforms, shear stresses develop at the grout-ground interface. Because the retained soil deforms toward the excavation, soil nails undergo extension resulting in axial tensile forces in the soil nail tendon.

The axial tensile load in the tendon increases from the nail head to a maximum value; then decreases as the soil nail transfers load to the surrounding ground.

The tensile resistance of the soil nail tendon and the pull-out resistance of the soil nail are the main resisting mechanisms.

Various soil-reinforcing techniques involving the insertion of bars or rods in soils may bear some similarity to the soil nails described in this manual. Some of these alternative techniques, which do not meet the definition of soil nails, are described in Appendix G. This section also includes brief discussions of their differences with soil nails.

The passive steel bars are known as rock bolts in the tunnel industry. The concept of combining passive steel reinforcement and shotcrete extended subsequently to rock-slope stabilization projects Lang The use of this method was later expanded to stabilize slopes and excavations in soil. In soil nails were used to stabilize an approximately ft high cut-slope in sand for a railroad-widening project near Versailles, France Rabejac and Toudic The method proved to be cost-effective and the construction faster than other conventional support methods.

The use of soil nails became common in France and other European countries since the completion of the Versailles project. The first use of soil nails in earth-retaining systems in Germany took place in Stocker et al. The French Clouterre research program involving private and public participants started in That research involved full-scale testing, monitoring of in-service soil nail walls, field testing, and numerical simulations Schlosser , Clouterre and It was estimated that this system was completed in nearly half the time and at about 85 percent of the cost of conventional excavation-support systems Byrne et al.

In FHWA funded a demonstration project for the installation of a prototype, ft high soil nail wall near Cumberland Gap, Kentucky Nicholson Since its introduction in the U. This increase can be attributed to the technical feasibility and cost-competitiveness of soil nailing. Easements tend to be smaller for soil nail projects because soil nails are shorter than ground anchors, for example, given the same wall height.

Additionally, as the use of soil nailing has grown, the number of qualified, soil-nail specialty contractors has increased. The project experience gained among engineers and Owners, especially state transportation agencies, has also increased over the years. The first FHWA- published document for the design and construction of soil nails was authored by Elias and Juran FHWA also published the previous version of this manual Lazarte et al.

In , FHWA developed a series of multimedia presentation modules to: i describe the use of ground anchors and soil nails in roadway construction; and ii introduce this technology to various users utilizing a modern digital medium Smith The objectives of the pullout testing program were to: i develop estimates of bond stress of HBSNs as measured at four test sites; ii assess whether correlations existed between published values of bond resistances and conventional pre-drilled soil nails with solid bar tendons; and iii provide recommendations for practical, standard procedures for performing pullout tests of HBSNs.

Therefore, the soil nail wall design presented herein relies on ASD-based stability calculations to quantify soil nail loads and slip surface geometries, and then uses these results to perform LRFD checks. The chapter also presents a description of favorable and unfavorable ground conditions for soil nailing, guidelines for conducting feasibility evaluations of soil nailing, and the risks for Owners using this technology. Finally, it lists factors affecting costs and construction schedule.

This information encompasses field and laboratory testing data and soil and rock parameters including bond resistance. The chapter also lists the information needed for assessing corrosion potential for soil nails, for designing soil nail walls under seismic loading, and for assessing frost action. The chapter also includes a detailed discussion on the process followed for calibration of the various resistance factors used in soil nail wall design.

The chapter presents guidance for the verification of various strength and service limit states. Additionally, the chapter presents supplementary, special considerations in design, including: i special wall geometries; ii drainage; iii effects of frost; and iv effects of creeping soil.

The step-by-step design procedure of this chapter is illustrated with a design example presented in Appendix C. It also contains criteria for selecting adequate levels of corrosion protection as a function of corrosion potential. The chapter introduces different systems to provide corrosion protection, including epoxy-coated bars, grout and other elements. The chapter also provides the key considerations for developing high-quality construction specifications. The chapter introduces requirements for performing soil nail load testing both proof and verification tests , and instrumentation and monitoring during construction, as well as for long-term monitoring.

It also contains a discussion about favorable and unfavorable subsurface conditions for cost-effective construction of soil nail walls to aid in evaluating their feasibility, and the main factors affecting the construction costs of these systems.

Soil nails and an initial shotcrete facing are installed at each excavation lift to provide support. Subsequently, a final shotcrete or cast-in-place- concrete CIP facing is installed. Nails are most often installed at a vertical spacing of 4 to 6 ft. The nail vertical spacing is comparable to the typical height of a stable, excavation lift, which is commonly 3 to 5 ft and could be more in some soils.

The horizontal spacing of nails is often also in the range of 4 to 6 ft. Figure 2. Both the soil nails and the initial and final facing contribute to the stability of the excavation. The soil nails support the soil and transfer loads to the soil mass behind the wall. The facing supports the soil between nails and immediately behind the face, provides structural continuity, and enables the soil nail wall to act as a unit. Soil nail walls can be more advantageous than other top-down retaining systems where the ground can temporarily sustain short, vertical or sub-vertical unsupported cuts.

Section 2. Soil nail walls are permanent earth-retaining structures in most roadway projects in the U. However, soil nail walls are also constructed as temporary structures i. More detailed descriptions of these components are included in Chapter 3.

These terms will be used interchangeably in this manual. Soil movement can occur during excavation, after excavation in the absence of external loads as a result of time-dependent deformations , or after excavation when external loads such as surcharge or traffic loads are applied. The tendons can be solid or hollow bars.

Solid bars are placed in stable drill holes and grouted in place. Hollow bars are fitted with a sacrificial drill bit and are used to drill the hole to then remain in place as the permanent soil nail reinforcement; they are described in detail in Chapter Both solid and hollow bars are typically fully threaded.

The grout functions to: i transfer shear stresses between the deforming ground and the tendons; ii transfer tensile stresses from the tendons to the surrounding stable soil; and iii provide some level of corrosion protection to the tendons. Grout is placed in the drill holes under gravity using the tremie method. The required level of corrosion protection is greater for soils with higher corrosion potential and for projects with lower risk tolerance.

The lowest level of corrosion protection in U. Encapsulation of the bar provides the highest level of corrosion protection and is achieved by adding a protective sheath and grouting the bars in a phased process. Corrosion protection of the soil nail tendon can also be provided by application of a fusion-bonded, epoxy coating, galvanization, or sacrificial steel. Chapter 7 contains detailed descriptions of the corrosion protection techniques available for soil nails.

Soon after excavation, the initial facing is applied on the exposed soil at each excavation lift before or after nail installation to provide temporary stability and protection.

The initial facing also receives the bearing plate of the soil nail. The final facing is constructed over the initial facing and provides structural continuity throughout the design life.

The final facing may also include an aesthetic finish. The initial facing most commonly consists of reinforced shotcrete. The final facing generally consists of CIP-reinforced concrete, reinforced shotcrete, or precast concrete panels. Other reinforcement options include the use of steel or synthetic fiber particularly for temporary facing in soft or weathered rock.

If the final facing consists of shotcrete, the reinforcement in the final facing is similar to that described for shotcrete in the initial facing. If the final facing consists of CIP or precast concrete, rebar mesh is typical. The headed studs are attached to the bearing plate and become embedded within the final facing as depicted in Figure 2. The drainage system commonly consists of composite, geosynthetic drainage strips, also referred to as geocomposite strip drains.

Step 1. The depth of the initial excavation lift unsupported cut may range between 2. The feasibility of this step is critical because the excavation face must have the ability to remain unsupported, until the nails and initial face are installed, typically one to two days.

The type of soil that is excavated may limit the depth of the excavation lift. The excavated platform must be of sufficient width to provide safe access for the soil nail installation equipment. Step 2. Drilling of Nail Holes. Drill holes are advanced using specialized drilling equipment operated from the excavated platform.

The drill holes typically remain unsupported. Step 3. A Nail Installation and Grouting. Tendons are placed in the drilled hole. A tremie grout pipe is inserted in the drill hole along with the tendon; and the hole is filled with grout, placed under gravity or a nominal, low pressure less than 5 to 10 psi.

If hollow bars are used, the drilling and grouting take place in one operation. B Installation of Strip Drains. Strip drains are installed on the excavation face, continuously from the top of the excavation to slightly below the bottom of the excavation.

The strip drains are placed between adjacent nails and are unrolled down to the next excavation lift. Step 4. Construction of Initial Shotcrete Facing. Before the next lift of soil is excavated, an initial facing is applied to the unsupported cut. The initial facing typically consists of a lightly reinforced 4-in. The reinforcement includes welded-wire mesh WWM , which is placed in the middle of the facing thickness Figure 2.

Horizontal and vertical bars are also placed around the nail heads for bending resistance. As the shotcrete starts to cure, a steel bearing plate is placed over the tendon that is protruding from the drill hole. The bearing plate is lightly pressed into the fresh shotcrete. Hex nuts and washers are then installed to engage the nail head against the bearing plate.

The hex nut is wrench-tightened within 24 hours of the placement of the initial shotcrete. The shotcrete should attain its minimum specified 3-day compressive strength before proceeding with subsequent excavation lifts. For planning purposes, the curing period of the shotcrete should be considered 72 hours. Step 5.

Construction of Subsequent Levels. Steps 1 through 4 are repeated for the remaining excavation lifts. At each excavation lift, the strip drain is unrolled downward to the subsequent lift. The temporary shotcrete is continued with the previous shotcrete lift. Step 6. Construction of Final Facing. After the bottom of the excavation is reached and nails are installed and tested, the final facing is constructed.

Final facing may consist of CIP reinforced concrete, reinforced shotcrete, or prefabricated panels. Weepholes, a foot drain, and drainage ditches are then installed to discharge water that may collect in the continuous strip drain. Variations of the steps described above may be necessary to accommodate specific project conditions. For example, shotcrete may be applied at each lift immediately after excavation and before drilling of the holes and nail installation, particularly where stability of the excavation face is a concern.

Another variation may be grouting the drill hole before placement of the tendon in the wet grout. These factors help to reduce the environmental impacts along the transportation corridor. The impact to traffic may also be reduced because the equipment for installing soil nails is relatively small. Variations in the details shown schematically in Figure 2. Some of these variations will be discussed in Section 6. While the cost of installing a soil nail wall under a bridge abutment may be comparable to that of other applicable systems, the advantage of soil nailing is that the size of the soil nail drill rig is relatively small.

Soil nailing equipment can operate within limited overhead, and traffic flow along the underpass road may not need to be totally interrupted during the widening. The manual provides information useful to both the experienced and inexperienced soil nail inspector. The manual is organized into two main parts: Preconstruction Preparation and Construction Inspection. The primary objective of this project is to demonstrate the cost-effective and technically correct application of soil nailing by the U.

The project is directed toward geotechnical, structural, and construction engineers who design and construct geotechnical facilities embankments, cut slopes, retaining walls, and structures. The demonstration project has four major elements: 1. Soil Nailing Field Inspector's Manual. Project-specific Technical Assistance, including: Technical assistance on the feasibility, design, and construction monitoring aspects of soil nail walls on specific projects.

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