06-03-2013, 10:01 AM
Analysis Design and Construction of Soil Nail Walls
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INTRODUCTION
PURPOSE
This document provides state-of-the-practice information for the selection, analysis, design, and
construction of soil nail walls in highway applications using the Allowable Stress Design (ASD)
procedure. The intended audience for this document includes geotechnical, structural, and highway
design and construction specialists involved in soil nail wall systems. The primary goal of this
document is to provide the practitioner sufficient information to facilitate the safe and cost-effective
use of permanent soil nail walls for a variety of transportation-related projects.
This document presents historical background information and a description of soil nail wall
systems, step-by-step design procedures, simplified design charts, and general construction
specifications. The document concludes with a detailed soil nail wall design example. This
document provides sufficient information to confidently design soil nail walls in a wide range of
ground conditions. Limitations related to the use of these systems in marginal ground conditions
are also provided. Information provided herein is not intended to represent a prescriptive
methodology; rather the information should be used in conjunction with good engineering judgment
for specific projects.
BACKGROUND
Origins of Soil Nail Walls
The origins of soil nailing can be traced to a support system for underground excavations in rock
referred to as the New Austrian Tunneling Method (Rabcewicz, 1964a, 1964b, 1965). This
tunneling method consists of the installation of passive (i.e., not prestressed as for ground anchors)
steel reinforcement in the rock (e.g., rockbolts) followed by the application of reinforced shotcrete.
This concept of combining passive steel reinforcement and shotcrete has also been applied to the
stabilization of rock slopes since the early 1960s (e.g., Lang, 1961). This ground-support technique
relies on the mobilization of the tensile strength of the steel reinforcement at relatively small
deformations in the surrounding ground. This support is enhanced by the continuity of the
shotcrete. The combination of passive reinforcement and shotcrete when applied to soil, in lieu of
rock, is termed soil nailing.
History of Use and Development in the United States
Pioneering applications of soil nail walls in North America were for temporary excavation support
in Vancouver, B.C, Washington, D.C. and Mexico City, Mexico in the late 1960s and early 1970s.
One of the first published applications of soil nailing in the United States was the support of the
13.7-m (45-ft) deep foundation excavation in dense silty lacustrine sands for the expansion of the
Good Samaritan Hospital in Portland, Oregon in 1976 (Byrne et al; 1998). The construction of the
retaining system was reportedly conducted in nearly half the time and at about 85 percent of the cost
of conventional excavation support systems. In 1984, a prototype soil nail wall 12-m (40-ft) high
was built near Cumberland Gap, Kentucky, as part of a demonstration project funded by the U.S.
Department of Transportation Federal Highway Administration (FHWA) (Nicholson, 1986). In
1989, the Oregon Department of Transportation built an 8-m (24-ft) high wall as the first
application of a soil nail wall used in a bridge abutment cut wall (end-slope removal). In 1988, a
12.2-m (40-ft) high, 2-tiered wall was built along Interstate Highway 78, near Allentown,
Pennsylvania. Each wall tier was 6.1-m (20-ft) high with a 3 m (10 ft) horizontal offset and was
built in colluvium and highly weathered rock. Other examples of early uses of soil nail walls
include those built along Interstate 10 in San Bernadino, California; Interstate 90 near Seattle,
Washington; and along George Washington Parkway (Interstate 495) in Virginia (Byrne et al.,
1998).
DOCUMENT ORGANIZATION
This document provides an update to the FHWA design manual (Byrne et al., 1998) and
incorporates recent trends in design methods, construction contracting, and construction monitoring.
The design principles presented in this document are based on the ASD procedure. This document
also presents new simplified charts that can be used in the preliminary design phase of a project,
and discusses advantages and limitations of two computer programs, SNAIL and GOLDNAIL,
developed for the analysis and design of soil nail walls.
COMPONENTS OF A SOIL NAIL WALL
The components of a soil nail wall installed using the techniques listed above vary from one
technique to another. As discussed in Chapter 2, most commonly in the United States, soil nails are
grouted nails consisting of a drilled hole in which a bar is placed and then grouted under gravity.
Driven soil nails have been more commonly used in France and Germany than in the United States
and are only used for temporary applications. Jet-grouted and launched nails are not common in the
United States.
Although the concept of soil nail walls is not proprietary, several specialized components or
procedures are under U.S. or international patents. Patented components include, but are not
limited to, some threaded bars, corrosion-protection systems, and nail installation systems (e.g.,
self-drilling, jet-grouted, and launched soil nails). Hollow bars are proprietary systems and are used
only in temporary walls because of concerns regarding the consistency of the corrosion protection.
The remainder of this section presents a description of the main components of a typical soil nail
used in the U.S. practice (Figure 4.1).