A new twist in soil nailing for efficient and economic retaining wall construction

Located northwest of Portland, Ore., and owned by Lower Columbia River Water Conservation District, the Miller Creek Bridge project involved a culvert replacement which restored a Clatskanie River tributary back to its natural salmon spawning glory. The project encompassed two, 35 feet max-height Hilfiker Spiralnail true bridge abutment walls, which were constructed top-down in 5-foot-high wall steps, utilizing a 1-foot-wide Hilfiker Trinity Cage system filled with quarry spalls. While site analyses, pullout testing of two Spiralnails, evaluation of alternatives, and permit applications began in 2003, the project was completed in 2005.

The Spiralnail system with wire cage facing system was selected for the Miller Creek Bridge project for many reasons, including its ability to accomplish a shorter bridge span, accommodate rapid construction, minimize earthwork, use locally available quarry spalls, as well as the system’s overall economy and the easy removal of existing 84-inch-diameter concrete culvert segments. Other options that were considered include Spiralnail steepened slope abutments, traditional concrete abutments with 1.5:1 cut slopes, structural steel raised abutments, micropiles, and traditional soil nailing. The difference that resulted in the final decision was a savings of 6,000 cubic yards of in-place cut volume and the achievement of a 55-foot shorter bridge span. The project was completed in seven weeks by a young construction crew of four. Kynsi Construction of Clatskanie, Ore., was the contractor. AZIZ Engineering Company of Redmond, Wash., was the designer and engineer of record working in association with Pacific Forest Resources of Enumclaw, Wash. Miller Creek once again became a productive salmon spawning stream soon after completion of this project.

Background

Spiralnail slope stabilization and retaining wall systems have been used in the Puget Sound area since 1998. This soil nailing or structural earth reinforcement method has solved the traditional soil nailing problems in a wide range of applications. It’s simple, fast, and economical—easy enough for a typical earthwork contractor to install in difficult circumstances. The Spiralnail offers several wall facing options that include precast and cast-in-place concrete and non-concrete systems, as well as a particularly interesting non-destructive and invisible (yet structural) facing that preserves existing steep slope vegetation and large trees without grading. Such steep slopes can be natural or constructed. Several notable steep slope stabilization projects have been completed using the latter, environment-friendly approach in some of Washington’s most critical coastal areas.

Spiralnail compound systems, with existing structural-earth technologies, are now available for top-down, bottom-up, and other construction modes. These offer the combined advantages of widely used MSE systems and soil nailing for widening and hillside projects. At the same time, the Spiralnail has developed a piling method known as Spiralpile system for seismic-resistant trenchless foundations and articulated retaining wall applications. The Spiralpile offers considerably higher bearing capacity than a micropile or penpile. All manufactured components of Spiralnail and Spiralpile systems, including the specialized hydropercussion hammer and capacity tester, are supplied by Hilfiker Retaining Walls of Eureka, Calif. Technical assistance is available for planning, design, and construction phases, including engineering design and supervision by a licensed and experienced geostructures engineer, as required.

Concept and principles

A Spiralnail is a steel, square tube, twisted by a quarter-turn every foot and fully twisted every 4 feet. It is not, however, fabricated by twisting a square tube. The twisting must be extremely consistent, so that the Spiralnail does not shear the soil while driven or drill-driven. The Spiralnail is manufactured by cold forming from extra strong pipe, in some applications using standard weight pipe for larger sizes. The standard Spiralnail is 2.5 inches in diameter; and presently, larger sizes are available up to 4.2 inches in diameter. The original product was made by welding three, thick steel wires helically around a pipe. The former standard is easier and more economical to manufacture. Depending on design-life and use, Spiralnails can be hot-dip galvanized or used untreated. The nails are typically left hollow, but can be filled with grout with an internal reinforcement bar for double corrosion protection. The steel strength for Spiralnail is minimum 42-kips per square inch (ksi) yield stress (F_y) and minimum 58-ksi tensile stress (F_u). A special splicing method used during the cold-forming process allows manufactured Spiralnails to be infinitely long. Typical nail lengths used to date are 8, 12, 14, 16, 18, 22, and 24 feet. The longest driven nail used to date is 24 feet; longer lengths must be drill-driven to ensure nail alignment and penetration into very dense soils.

How does Spiralnail work? The traditional soil nailing in the United States relies on grout bonding with the soil, typically after drilling and cleaning a hole for placement of the reinforcing bars. The bond is achieved after the grout reaches minimal strength at least 72 hours later; with the full bond strength developing months later when the soil recovers from arching and normal stress is restored. Spiralnails are driven either by displacement, using a conical driving tip, or by minimal displacement without a driving tip that is also generated by drill-driving. The latter is achieved by simultaneously drilling a pilot hole at the cutting face of the Spiralnail while the nail is driven.

Displacement of the soil while the Spiralnail is driven takes advantage of in-place soil strength and normal soil stress. Moreover, since the nails are spiraled, the spiral ribs generate a mechanical bond with the soil, which develops passive resistance in the soil. The soil is reinforced in the same manner as MSE structures, and there is no reliance on skin friction. Therefore, Spiralnails go to work instantly reinforcing the soil.

The advantages are obvious: There is no 72-hour waiting time after each successive row of soil nails, no drilling and cleaning of holes, no grouting, and no concrete work with the Spiralnails special wall facing systems. Remove these specialty operations and any diligent earthwork contractor can accomplish soil nailing in record time. Hence, a soil nail wall can be built for the price of a welded wire wall in places and soils never imagined for soil nailing.

Is the Spiralnail system design any different from traditional soil nailing? Not really. The mechanical bond with the soil can be field-tested using one or several test nails for pullout to measure yield, creep, and failure load. Alternatively, the mechanical bond with the soil can be estimated by an engineer experienced in Spiralnail testing in a variety of soils, including fill or native. Further research work is necessary to gain better knowledge of behavioral mechanics in different soils for design estimating. In my opinion, pre-design field-testing of the mechanical bond strength has no better substitute. Of particular interest is the recovery of Spiralnail pulled to bond failure; such nails left in place in granular soils typically recover to full strength within approximately a week after the test. Preliminary tests show that the soil cylinder influenced by Spiralnail loading can have a diameter of 10 feet or more. The initial research work done on a model in the lab at the University of Washington tends to suggest this.

Once the bond strength is determined, a soil nailing design program such as Snail-win can be used to model the wall or stabilized slope or a combination, entering the actual Spiralnail diameter, soil strength, and condition parameters. This is an iteration process until the nailing density, lengths, and inclinations are optimized. Experience and special care is recommended using this model; it is a force-equilibrium model. For instance, proper use of soil-cohesion strength is important in design. The concern over the mathematical influence cohesion has in stability-analysis formulation (that it should be assumed zero and compensating for this by jacking up the internal friction strength of the soils) must be carefully balanced.

Based on my experience, the mechanical bond that Spiralnail develops with the soil must be minimally considered with cohesion influence. Over-conservative design can result in stiff walls, susceptible to local overstress and cracking. Meanwhile, it must be noted that Spiralnails can be slotted or perforated and driven with a negative inclination for sub-drainage purposes.

Enayat S. Aziz, L.E.G., P.E., is president of AZIZ Engineering Company located in Redmond, Wash. He can be reached at eaziz@aziznw.com or 425-898-1007, ext. 17. Hilfiker Spiralnails and its appurtenant components and methods are proprietary products of Hilfiker Retaining Walls and/or intellectual property of AZIZ Engineering Company, covered by U.S. and international patents and patents pending. Technical assistance during project design is available through Hilfiker Retaining Walls or AZIZ Engineering Company.