recent advances in soil nailed earth retention

Earth Retention Systems 2003: A Joint Conference Presented by ASCE Metropolitan Section Geotechnical Group, The Deep Foundations Institute, and ADSC: The International Association of Foundation Drilling May 6 and 7, 2003, New York City

RECENT ADVANCES IN SOIL NAILED EARTH RETENTION Tom A. Armour, P.E., DBM Contractors, Inc., Federal Way, WA., USA David M. Cotton, P.E., Golder Associates, Redmond, WA., USA Abstract: Different variations of soil nailing techniques have been used in the United States since the late 1970s. Such techniques owe their origins to French developments, which began in the early 1970s as a spin-off from European tunneling excavation support practice. The growing demands of urban infrastructure development and rehabilitation and environmental protection have created an active and expanding soil nailing market demand for earth retention and slope stabilization in the United States since its early developmental years. There is a clear need for a fundamental review of the large number of soil nailing techniques that are currently being used in the United States. Following a brief summary tracing the historical development of soil nailing and a generic classification of applications, this paper provides a review of the different recent advances in the design and construction of soil nailed systems. Discussion topics include nail design advances, permanent top-down, structure support/underpinning, alternative face stability measures, composite systems, hollow bar and fiberglass nails and architectural reinforced concrete/shotcrete facings. Case histories will be presented to illustrate each of these advances.

I Introduction In the last two decades or so in the United States, there has been an increasing use of small diameter cast-in-place bored inclusions for in-situ ground reinforcement. By far the greatest number have been designed and installed to act as soil nails reinforcing the ground, creating an in-situ coherent gravity structure, ideal for earth retention and slope stabilization. This increased use can be attributed to the growing demands of urban infrastructure development and environmental protection as well as the development and use of alternative project procurement methods during the growth of innovation. However, the expanded soil nail use and acceptance over the years is due primarily to the many technological advances developed as a result of a better understanding of the behavior of soil nailed structures and the use of improved construction materials and techniques. In general, the most significant advances in soil nail design and construction during the last 15 years have included the following:

A. Design

1. Development of permanent soil nail wall systems, 2. Increased understanding in soil nail loading and stress distribution, 3. Increased understanding in grout to ground bond stress values, 4. Increased understanding in wall facing pressures, 5. Use of strut nails to carry vertical loads, 6. Use of vertical support members to provide additional face stability, 7. Composite soil nail systems, 8. Use of splayed nails to support re-entrant corner alignments, 9. Waterproofing design alternatives, 10. Improvements to drainage design details, 11. Use of Permanent Nails to carry vertical loads, and 12. Use of Permanent Nails to support major structures.


B. Construction

1. Advances in construction materials and drilling and grouting techniques, 2. Top-down permanent wall construction, 3. Structural support/underpinning of major loads and structures adjacent to soil nail supported

excavations, 4. Alternative means and methods for constructing permanent architectural wall facings, and 5. Installation below the water table.

The new challenges facing soil nailing designers and specialty geotechnical contractors of today come in two major categories: 1.) Designing for difficult or unusual loading conditions and 2.) Designing and building in difficult ground using alternative stabilization technologies in concert with soil nailing to make this system viable in areas not previously felt to be feasible for in-situ ground reinforcement techniques. II Historical Development Soil nailing, an extension of the New Austrian Tunneling Method, is an in-situ reinforcement technique that has been used during the last three decades, primarily in Europe and the United States, to retain excavations or stabilize slopes. The fundamental concept of soil nailing is reinforcing the ground with closely spaced passive inclusions to increase the shear strength of the in-situ ground, to restrain its displacements, and to limit its decompression during and after excavation. Implementation of soil nailed earth retention and slope stabilization technology followed the same research and development course in the US as in Europe, although Europe always seemed to lead the US until the early 1990s. In general, in both Europe and the US, the private sector developed the technology, with government agencies subsequently contracting to have the information codified and made available to the general public. In Europe, the official document was RECOMMENDATIONS CLOUTERRE 1991, and in the US it was the US Department of Transportation, FHWA 1996 document, Manual for Design & Construction Monitoring of Soil Nail Walls. Prior to the publication of these documents, the predominant use of soil nail retention systems was for temporary excavation support and almost exclusively used only in the private sector. Since the early 1990s millions of square feet of temporary and permanent soil nail structures have been built in both public and private sectors. Earth retention and slope stabilization applications for highways, power plants, commercial buildings and developments, water treatment plants and pump stations, tunnel portals, bridge abutments, and landslide remediation are common uses. The possibilities continue to grow for this in-situ ground reinforcement technique, driven primarily by economics and the ease, speed and flexibility of construction over the more traditional earth retention systems. III Product Development After the completion of the French National Research project CLOUTERRE in 1991, significant product development occurred, and similarly, in 1996 after the FHWA completed its Manual for Design & Construction Monitoring of Soil Nail Walls. The FHWA research project organized all of the known soil nail design and construction research data, funded a major research project to develop a rational approach to soil nail wall facing design, and produced a user friendly manual of design and construction guidelines. Use of the manual would ultimately speed the cost efficient implementation of temporary and permanent soil nailed earth retention structures on all public and private projects. More recent significant product developments are discussed in detail below:


A. Design Advances The 1996 FHWA effort developed a rational limit equilibrium stability approach for load transfer and development of loads on soil nails (as it related to earth pressure), validated and based primarily on field data from instrumented structures funded from the private and public sectors. Instrumentation included strain gauges, load cells, and inclinometers. In addition, data has also been compiled on the numerous nail pull-out tests carried out by contractors. This database has made it possible to constantly improve designers estimate of the grout to ground bond stress values allowing improved designs with fewer contingencies, applications on sites once considered not prudent for soil nailing, and lower cost structures (as a result of the lower density of nails as well as shorter nails). FHWA also funded a research program executed by UC San Diego developing, for the first time, a rational approach to facing design that included the critical failure mode. The most important design advantage coming from this research was that the wall facing pressures for soil nailed earth retention systems were determined to be approximately one-third to one-half that of the more conventional tensioned ground anchor systems as shown in Figure 1. By reducing the concrete facing thickness and steel reinforcement content needed, additional cost savings would be apparent.

Figure 1: Wall Facing Pressure Comparison (Soil Nails vs. Anchors)

B. Construction Advances

In addition to the widely accepted approach for the soil nail facing designs, developments such as double corrosion protection (DCP) and durable epoxy coatings for the soil nail materials and more environmentally sensitive reinforced concrete/shotcrete facing treatments have promoted the more frequent use of soil nailing for permanent structures. Once thought to be applicable only for temporary structures due to their rather homely appearance, soil nail walls have become increasingly popular for permanent use because of the advent of more environmentally sensitive facings. Cast-in-place reinforced concrete facings can be structurally attached to a soil nail wall system providing a wall that matches an existing bridge or sound wall. Pre-cast concrete panels developed for mechanically stabilized earth (MSE) wall systems have been used frequently to improve the aesthetics of a shotcrete faced soil nail wall. More recently, in mountainous areas, the use of reinforced sculpted shotcrete, (Figure 2) has been utilized to help blend the soil nailed structure with the existing geology.


Figure 2: Reinforced Sculpted Shotcrete Faced Soil Nailed Wall Other important product developments also seemed to be fueled by the accepted use of permanent soil nailed structures. The top-down wall facing method was developed in the private sector for the construction of permanent basement walls. These permanent wall facings are typically 10-12 inches thick, with temporary soil nails providing the temporary excavation support and the majority of the vertical wall load support. The reinforced concrete floor levels constructed as the basement structure is brought to grade support the permanent lateral earth pressures. Where utility interference did not allow the installation of the upper row of nails and/or permanent wall facings got to be over 12-inches thick for a typical soil nail layout spacing, additional support was needed to preclude lateral and vertical movements. This led to the addition of strut nails typically 10 feet long and placed at every second or third nail depending on the walls cantilever height or added facing thickness, (Figure 3). Following the successful completion of several top-down permanent walls with strut nails there was a logical progression to the idea that all permanent soil nail walls have the added capacity of vertical load resistance for design. In several projects using permanent nails, the traditional footing at the base of the wall was eliminated due to the load carrying capacity of the soil nails and ground exceeding the vertical load requirements, (Figure 4).

Figure 3: Permanent Top-Down Method with Strut Nails


Figure 4: Soil Nail Vertical Load Resistance

Strut nails also led to the development of more complex composite soil nail systems used in congested urban environments where overhead clearance restrictions, deep utility conflicts, and adjacent structures are sensitive to the lateral movements induced by conventional excavation support installations. These composite wall systems utilize steeply inclined pre-tensioned soil nails and sub vertical compression soil nails to suspend a shotcrete facing that can be pre-stressed against the soil to produce near at-rest horizontal earth pressures and restrain lateral movement, (Figure 5).

Figure 5: Composite Soil Nail Wall (Typical Section)

Another important development has been the use of splayed nails, nails having an angle of less than 90-degrees to the face of the wall. This has allowed for a substantial increase in the use of multiple bends in wall alignment and a simple solution to the reentrant part of that bend as shown in Figure 6.


Figure 6: Splayed Nail Layout Example

Numerous projects over the past decade have had strict requirements for water proofing. Development of new concrete admixtures and the advent of flexible bentonite membranes that could be physically attached to a vertical soil face prior to shotcrete application resulted in a new soil nail market application. For sites where subterranean walls had to be built below the permanent water table, the soil nail wall was (and is) now an option, (Figure 7a &7b). Drainage design details also improved from a single strip of geocomposite drain material 6 feet on center to a vertical and horizontal system 6 feet by 6 feet, and a full face system, (Figure 8).

(A) (B) Figure 7: Water Proofing Attached to Soil Face (A) and Attached to Temporary Shotcrete Facing (B)


Figure 8: Geocomposite Drainage Detail

C. Material Advances

In the late 1990s, the use of hollow injection bars (or what suppliers call self drilling bars) wasintroduced. These are high strength hollow steel all thread bar that can be used as a grout injecting, self boring soil nail. The nails are installed as the drill rods with a sacrificial bit. Cement grout is injected through the nail while drilling the hole to the design depth. Depending upon the ground type this method of installation not only solves the potential problem of hole stability in loose overburden soils, but provides for unit pullout capacities two and three times higher than those typically achieved using conventional drill and grout installation methods. Hollow injection bars are installed at much higher production rates with smaller equipment and less labor than conventional bar systems. Even though using hollow injection bars for soil nails results in higher material costs, they are typically lower in cost than more conventional bar systems requiring casing installation. In some urban areas, public agencies have restricted the use of buried steel members in public street right-of-way leading to the use of fiberglass tendons in soil nail construction. Developed for the tunneling industry, fiber reinforced composite soil nails offer a viable solution where excavation must occur through a reinforced soil mass. However, due to their poor shear capacity, the weight of the shotcrete wall limits the use of these types of materials to temporary walls, and the cost and availability of particularly high strength deformed tendons can also provide a challenge in their use.

IV Recent Design Innovations

Design advances recently have resulted from the increased application of soil nail wall design and construction with difficult or unusual design load requirements and by expanding soil nailing into ground conditions previously not considered typical for proper application. The use of vertical and sub-vertical elements have provided for the advanced use of soil nailing in these two areas. In general, the use of vertical elements using larger diameter drill holes (12-16-inches) with high strength steel pipes or small steel wide flange sections to cantilever the first excavation step and then transition to nailing has been used to handle high loads from adjacent structures and to get below deep utilities. Vertical elements using smaller diameter drill holes (6-8-inches) with normal No. 4-6 reinforcing bars, have been used to provide improved face stability measures on projects with a significant layer of fill or clean sand unit overlying the site (8-15 feet). In some instances this has been successfully done up to depths of 25 feet. A proper mix and use of the grout is essential to the success of this application. The grout strength needs


to be adjusted so it is easily excavated and the fluidity is important to restrict or encourage penetration of the grout.

V Case Studies

A. Komo Television Project Background The original KOMO television station and parking lot encompassed an entire city block in downtown Seattle, Washington. A site plan reproduced in Figure 9 illustrates the project site is approximately 360-ft by 240-ft in plan, and bounded by John Street, on the East by 5th Avenue on the South by Denny Way and on the West by 4th Avenue. The original KOMO television station occupied the southwest quarter and a portion of the northwest quarter of the site. The remainder of the site was L-shaped and occupied by a parking lot and a 16-ft wide alley along the east side of the building.

Figure 9: KOMO Building Site Plan, Seattle, Washington

The proposed development involved construction of a below-grade parking garage throughout the L-shaped portion of the site, and subsequent expansion of the existing KOMO television building above grade over the new parking garage. The development would require 41,000 ft2 of temporary excavation support with up to 47-ft. deep vertical excavation cuts immediately adjacent to the existing television station and the surrounding city street. Along the east wall (5th Avenue), foundation piers for the elevated City of Seattle Monorail transit system were located within about 8 feet of the proposed excavation. Because the existing KOMO television station and the monorail foundation piers were sensitive to movement, the owners representatives determined that lateral movements due to excavation should


be kept below 0.5 inches, or 0.1% of the height of the excavation. The strict deflection criteria, coupled with the fact that there was no precedent for a soil nail wall built to support such high monorail pier loads or with a sensitive building located on a reentrant corner, initially led the owners consultants to conclude that an anchored soldier pile wall system would be the most appropriate excavation support system. However, the general contractor, Sellen Construction, allowed a Design-Build team comprised of DBM Contractors and Golder Associates to demonstrate and convince the project team that a less expensive soil nail wall could be designed and constructed to meet the project requirements. Subsurface Conditions In general, the site is underlain by a variable thickness of fill overlying glacial till. The fill was variable in character but generally consisted of 7-12 feet of loose to dense sand and silt with little to some gravel, cobbles, and building debris. The glacial till was generally characterized as very dense to hard sandy silt with little to some gravel and occasional cobbles and boulders. Small localized pockets or zones of low plasticity clay were also encountered, but these did not influence the overall character of the glacially overridden materials. Depending on the time of year, the geotechnical investigation found the groundwater table to exist slightly above, at, or below the base of the excavation. During the summer months when construction occurred, the groundwater table was below the base of the excavation, and only localized perched pockets of water were encountered above the base of the excavation. This groundwater did not pose any particular difficulties and was managed through the use of vertical geocomposite drainage board strips applied in a consistent pattern directly to the excavation face. Soil Nail Wall Design In general, the soil nail wall design consisted of six to nine rows of nails on a 6-by 6-foot nail pattern. The upper two rows, which were within the fill soils, were typically spaced at no more than 5 feet vertically. The typical wall section included a 4-inch-thick, wire mesh reinforced, shotcrete facing and Grade 60 nails that were generally 24 to 38 feet long in the top four to five rows and 16 to 28 feet long in the bottom rows. The nails were inclined at 15 degrees except where utilities or other obstructions were anticipated and steeper inclinations were required. Strut nails were used to assist in lateral support where utilities were in conflict with the upper row of nails and at the location where a crane was set adjacent to the wall to service the excavation. At the existing KOMO building, splayed nails (Figure 10) were used to support the corner of the structure. An additional 4.5 to 8.5 kips per lineal foot (klf) vertical surcharge was incorporated into the wall design to simulate the existing KOMO building footings. The typical KOMO nail wall section included a 4-inch-thick, wire mesh reinforced, shotcrete facing and Grade 60 nails that were 24 to 36 feet in the top 3 rows and 16 to 28 feet long in the rows below. A 5-inch-thick shotcrete facing was used near the building corner. These nails were on a 6-foot by 6-foot pattern.


Figure 10: Soil Nail Splay Plan, KOMO Building, Seattle, Washington For the wall design in front of the monorail piers (Figure 11a and 11b), a soil reaction pressure was obtained using the computer program LPILE by applying the original design monorail loading of 45.8 kips horizontal, 494 kips vertical, and 1,700 kip-feet of moment at the bottom of the capblock. A uniform lateral surcharge loading pressure of 4 ksf was determined based on the computed LPILE soil reaction; the uniform lateral surcharge was applied on the top three nail rows and over a width of the wall face. The 4 ksf applied over the drilled shaft width, along with the proposed soil nail design pattern, was also incorporated into a FLAC analysis. The FLAC analysis indicated that about 1.45 ksf lateral pressure would be transmitted to the wall face; the decrease in lateral pressure (from 4 to 1.45 ksf) reflected the arching affects of the soil nails. The designer anticipated movements of 0.5 inches or less based on FLAC and LPILE results.


Figure 11a: Monorail Pier Loading Diagram, KOMO Building, Seattle, Washington

Figure 11b: Monorail Pier Loading Diagram, KOMO Building, Seattle, Washington At the monorail piers, the nail pattern directly in front of the pier foundations was spread horizontally to straddle the existing foundations. At Piers 13 and 14, the first row of nails was horizontally spaced 14 feet apart to avoid the cap block; the next three rows were spaced at 9 feet to avoid the drilled shafts. The vertical spacing was 6 feet. Additional nails were installed on each side of the drilled shafts; the nail pattern beside the shafts was 2.5 feet horizontal by 6 feet vertical. This enhanced nail pattern covered 24 feet of soil nail wall in front of the monorail piers (Figure 12). At Piers 15 and 16, which are supported on spread footings and were spaced 6 feet vertically. Nails installed beneath the footings were on a 4-foot by 4-foot pattern. The typical monorail enhanced nail wall section included


an 8-inch-thick, reinforced, shotcrete facing for the top three rows. Nail lengths were unchanged, only the nail pattern and shotcrete facing were enhanced.

Figure 12: Partial Wall Elevation View at Monorail Piers, KOMO Building, Seattle, Washington Performance and Conditions A total of nine borehole inclinometers were installed around the site perimeter, one in front of each of the four monorail piers, one centered along each of the two walls adjacent to the existing KOMO building, one at the reentrant corner, and one centered along both the north and south walls. In addition, optical survey monitoring points were established in sets and spaced 50-ft apart around the excavation perimeter. For each set of monitoring points, one was placed at the top of the soil nail wall, one was placed 20-ft behind the wall, and several were placed on 20-ft centers over the height of the wall. All of the inclinometer and optical survey monitoring data indicated that lateral wall movements were less than 0.25 inch. Therefore, the soil nail wall performed as intended by its design. Photographs of the completed excavation are shown in Figure 13. It is concluded that in glacially-overridden soils of the Pacific Northwest, properly designed soil nail walls may be used to resist heavy surcharge loadings, and existing building surcharges at reentrant corners, while limiting lateral wall displacements to relatively low levels (0.05% of the wall height).


(A) (B)

Figure 13: Photographs of Reentrant Corner (A) and Monorail Support Wall (B), KOMO Building, Seattle, Washington

B. Portland Suspension Wall Project Background Rouse Management Inc., a nationwide developer and real estate manager, contracted Golder Associates, Inc. (GAI) in January 1998 to provide geotechnical engineering and temporary excavation support wall design services for a new retail center located in downtown Portland, Oregon. The development consists of one level of below-grade retail shops and a multi-level above-grade retail center. The project site comprises Block 50, bounded by S.W. Morrison Street on the South by S.W. Yamhill Street on the West by S.W. 4th Avenue, and on the East by S.W. 3rd Avenue. The existing grades vary from EL.+38 feet in the northeast corner, to EL.+44 feet in the southwest corner. The excavation base was at EL.+24 feet, resulting in excavation depths ranging from 14 to 20 feet. The north and south sides of the site were immediately adjacent to a critical metropolitan light-rail system. Figure 14 shows the project site plan. The site is approximately 230 ft. by 230 ft. in plan dimension. The below-grade excavation extends beyond the property lines into the street on all sides of the site. Along the east side of the site, the permanent below-grade wall is located in the middle of the nearest traffic lane, 26 feet beyond the property line. Along the north and south sides of the site, the permanent below-grade walls are located within one feet of the curb line, 5 feet from the nearest track rail, and 16 feet beyond the property lines. Temporary excavation support walls were required along the north, south, and east sides of the site.


Figure 14: Site Plan, Rouse Project, Portland, Oregon Subsurface Conditions In general, the subsurface conditions at the project site are comprised of three distinct soil layers. The surface soil layer is characterized as fill, reaching depths of 2 to 12 feet, and is comprised of loose to medium dense silt and sand with intermixed gravel, boulders, and rubble. Underlying the fill layer, firm silts with varying amounts of fine to medium sand, extend to depths of 13 to 25 feet. These two upper units are underlain by dense to very dense gravels and cobbles in a matrix of silt and sand. The top of the dense gravels varies from EL.+19 feet in the southwest corner of the site, to EL.+25 feet in the north end of the site, and to EL.+ 27 feet in the southeast corner of the site. Therefore, the less competent soil units controlled the lateral earth pressure wall loads. For the purposes of design, the upper soil units were combined into one design soil unit. The temporary wall design parameters for the upper unit, including nail adhesion for the lower gravels, are summarized below:

At rest horizontal earth pressure coefficient, K0 = 0.45 Total soil unit weight, g = 125 pcf Soil friction angle, f = 34 Soil cohesion , c=0 psf Design nail adhesion in upper soils (silts & sands), ADU = 2 k/ft Design nail adhesion in lower soils (gravels), ADL = 5 k/ft

The Willamette River, located several blocks north of the site, controls the groundwater level at the project site. During the summer months when the excavation support walls were built, the water level consistently occurred at about EL.+10 feet or 14 feet below the base of the excavation. Therefore, groundwater was not a design issue for the temporary excavation support wall system. Wall Issues and Design Constraints The east wall along S.W. 3rd Avenue is 15 to 18 feet high (Figure 15a). The main design constraints for the east wall consisted of a shallow gas line, and a 2.5 feet diameter high-pressure water line


located roughly 18 feet behind the wall and 10 feet below grade. Because the adjacent roadway was not considered sensitive to movement, a conventional soil nail wall was constructed along the east side of the site, with slightly steepened Row 1 nails to avoid the water line combined with strut nails to provide vertical support. In firm silts and compact sands, this type of excavation support system would be expected to experience lateral movements on the order of 0.2 to 0.4% of the wall height, or 0.5 to 1 inches. The north wall, along S.W. Morrison Avenue, is 14 to 15 feet high. The south wall, along S.W. Yamhill Avenue is 18 to 20 feet high (Figure 15b). Both walls are located within 5 feet of the nearest train rail, and within 2 feet of the track slab. The light-rail system is owned and operated by the Tri-County Metropolitan Authority (Tri-Met). Tri-Met specified that the construction-included lateral and vertical movement of the light-rail system must be essentially zero. Therefore, soil nailed walls or conventionally-designed anchored soldier pile walls would not be adequate. It became evident that the selected excavation support system would have to prestress the adjacent ground to at-rest levels of horizontal stress in order to limit wall movements. The design uniform apparent earth pressure was selected at 35H psf, plus any additional surcharge loading.

(A) (B)

Figure 15: Typical Cross Sections at Rouse Project, Portland, Oregon. (A) East Wall, (B) South Wall. Overhead high-voltage DC power lines were located roughly 20 feet above each track and were horizontally about 8 feet from the shoring walls. Therefore, vertical soldier pile drill rig leads would fall within about 5 feet of the power lines. The distance between the power lines and drill rig leads would require the power to be shut down during the installation of the soldier piles. A train passed by the site about every seven minutes during the day. Tri-Met would only agree to shut the power down for a relatively short period of time from 2 am to 6 am. Therefore, although piles could be installed during the 4-hour power-off-window, the slow progress would severely impact the construction schedule and project budget. Another method of ground support was needed that did not require a shut down of the light-rail power lines. In addition to the above constraints, a 3 feet diameter combined sewer was located behind each of the north and south walls. Each sewer was generally 14 to 15 feet deep and located roughly 13 to 14 feet behind the excavation support walls. The location and size of the sewer made the installation of traditional sub-horizontal ground anchors impossible. The anchor declinations would have to be at least 45 to provide the minimum 3 feet clearance specified by the City of Portland.


Composite Wall System Based on the three constraints imposed on the north and south shoring walls, a new type of excavation support wall, referred to as a suspension wall system was developed. The wall design constraints are summarized as follows:

Limit wall movements to essentially zero. Avoid overhead power obstruction. Avoid deep combined sewer obstruction.

Steeply inclined, pre-stressed tension anchors were utilized to provide the high lateral pre-stress and avoid the deep combined sewers. Sub-vertical compression anchors were utilized to resist the vertical component of load from the tension anchors, while maintaining more than 9 feet of horizontal clearance from the overhead power lines. A reinforced shotcrete facing, constructed in staged vertical lifts, was suspended by the group of tension and compression anchors, and provided the means to transfer the resultant horizontal force to the retained soil. A typical cross-section is presented in Figure 16. Each shoring wall contained three 5-ft high lifts of structural shotcrete. For areas where the wall height exceeded 15 feet, the upper portion of the wall was sloped back and flash coated. The first lift contained both tension and compression anchors, while the second and third lifts contained only tension anchors. Because of the differing declinations, the tension and compression anchors were staggered along the wall.

Figure 16: Typical Cross Section Suspension Wall System All tension anchors were installed at a 45 declination from horizontal except for the Row 1 anchors of the north wall which are installed at a 60 declination. All compression anchors were installed at a 75 declination. Note that the tension and compression anchors were located vertically so that their lines of action coincided within the wall facing, so as to not impose significant rotation on the first lift of shotcrete during pre-stressing. The tension and compression anchors may also be referred to as soil nails mainly because the density of the drilled elements is more in line with a soil nail system. Except for the fact that the elements are pretensioned, and include a sheathed no-load zone, the details of the element are very


much like that of soil nails. However, the tension elements function more like anchors than soil nails. Both terms may be used to refer to the tension and compression elements of the suspension wall system. The relative mix of silt and sand in the soils was thought to be unpredictable and quite variable, so construction plans showed the shotcrete facing as a two-application system. The intended construction sequence for areas of poor face stability is described as follows. After all of the anchor installations for the lift were completed, the wall face was cut to the neat-line in roughly 20 feet wide alternating slots. The first application of shotcrete consisted of a 2 inch thick flashcoat, and would commence as soon as the geocomposite drain board and mesh were placed against the soil, and the compression nail cushion, nut, and plate were installed. A small bearing plate and nut were wet-set and attached to the tension nail to secure the flashcoat. After a period of at least four hours, the intermediate slots were cut and flashcoated as well. The following day, the second and final shotcrete application would take place. The main mat of deformed reinforcing bars would be placed at the center of the full wall, the first nut would be sleeved, and the remaining 4 to 6 inches of shotcrete would be applied. The final bearing plate and nut were wet-set at an angle appropriate for stressing. Initially, the slot-cutting and flashcoating procedures were considered mandatory by Tri-Met for the first lift along the north and south walls, because of the risk of movement associated with loss of ground if caving conditions were encountered. However, after about 75 percent of the north wall lift 1 and 50 percent of the south wall lift 1 were constructed, GAI and Tri-Met were convinced that because the ground was mostly a silt, the stand-up time was excellent and slot-cutting and flashcoating were not necessary. Performance and Conclusions Along the north and south walls, where the suspension wall system was installed, the measured lateral wall movements and vertical track slab movements were less than the 0.06 inch accuracy of the optical survey. Therefore, the suspension wall system performed as expected with essentially no wall movements. Along the east soil nail wall, both lateral and vertical wall movements ranged from 0.2 to 0.8 inch, with an average of about 0.5 inch. This magnitude of wall movement was anticipated in the design of a conventional soil nail wall for the firm silts and compact sands at the site. Therefore, the east wall demonstrated that the lack of wall movement observed along the north and south walls can in fact be attributed to the use of the Suspension Wall System.

C. Bellevue Technology Tower

Project Background The proposed Bellevue Technology Tower excavation encompassed an area slightly smaller than a city block, (194 feet by 167 feet) in downtown Bellevue, Washington. A site plan is shown on Figure 17, illustrating the project site is bounded on two sides by city streets and existing buildings on the other two sides. The shape of the excavation also includes a reentrant corner. The Key Bank Tower located on the west boundary of the excavation has three levels of below grade parking garage and a 14-story tower on top of the garage set back from the excavation face approximately 20 feet. The planned development was to include a 20-story building with eight stories of below grade parking. However, due to an economic downturn in the local market the project construction temporarily ceased following the excavation and foundation construction. Therefore, an evaluation of the soil nail system for an extended period beyond the 12-month planned design life needed to be completed after construction ceased.


Figure 17: Site Plan, Bellevue Technology Tower, Bellevue, Washington The base of the excavation was constructed to a depth of 86.5 feet, below 108th Avenue N.E. It is believed that this is the deepest soil nail wall, built to date in the US, with a top-down construction procedure. The top-down building procedure includes building the permanent 12-inch thick shotcrete basement wall, with a double curtain of steel reinforcing, as the excavation progresses, installing the temporary nail system to a maximum depth of 86.5 feet. A seven foot thick mat foundation was excavated approximately 20 feet inside the excavation line extending the total depth of the excavation to approximately 93.5 feet below street grade. The excavation required approximately 54,500 square feet of permanent 12-inch shotcrete wall construction. During the excavation and shotcrete placement of the last level of nails, at an average depth of 74.5 feet, the 2001 Nisqually Earthquake occurred with an Mw 6.8 event. No ground loss was experienced, and only nominal permanent lateral displacements were measured after the earthquake. DBM Contractors and Golder Associates worked together during construction to assure a successful project, economically, and in terms of wall performance. Subsurface Conditions In general, the site is underlain by variable, minor amounts of fill, overlying dense till, dense advance outwash sands and gravels, and glaciolacustrine deposits of silt. The fill varied in thickness from less than one foot to 14 feet in the northwest corner of the property, and consisted of loose to dense gravel and sand on the west side of the site, and soft to hard silts with sand and gravel on the east side. The native glacial till is generally a dense sandy silt, with little gravel, and ranged from 30 feet thick on the east side of the site to zero feet on the west. The advance outwash gravel was generally a very dense gravel, with some fine to coarse sand and trace silt. Below the advance outwash gravels were the advance outwash sands, consisting of a very dense fine to coarse sand, with trace silt and some gravel. The advance outwash sand and gravels extended to an average depth of 72 feet where the glaciolacustrine deposits were encountered. The glaciolacustrine silts were generally hard and very dense silt and clayey silt with a trace of very fine sand and subrounded gravel. Groundwater was encountered at or near the base of the excavation, at an average depth of 72 feet below street level. Groundwater appears to be perched on top of the glaciolacustrine deposits. Soil Nail Wall Design In general, the soil nail wall design consisted of eleven to thirteen rows of nails on a 6-by 6-foot nail pattern. The upper row of nails included a strut nail system used to control deflection and ground loss


at the face in the upper fill materials. Strut nails were also used to support the weight of a conveyor about midway down the north wall, which was being used to remove soil from the base of the excavation. The platform load was approximately 150 kips, supported by 10 strut nails. The maximum wall height on the north wall, 86.5 feet, included nail lengths that ranged from a maximum for 66 feet in the upper eight rows to decreasing lengths of 54, 50, 45 and 30 feet. The bar sizes ranged from 1-1/4-inch 150 grade steel to 1-inch 150 grade steel. The bars were typically inclined at 15 degrees, except for the upper row which was inclined at 20 degrees to avoid utilities. Strut nails were inclined at 45 degrees. A typical section is shown in Figure 18. The splayed layout for the southwest reentrant corner is shown in Figure 19. During excavation and construction of the excavation support wall, optical survey monitoring was performed that recorded maximum horizontal displacements on the order of 0.001H to 0.002H, where H is the maximum wall height. This resulted in permanent displacements at the top of the wall ranging from 0.25 inches (west wall) to 2 inches (north wall). These displacements during excavation and construction of the wall are normal and within the range for the soil type at the site.

Figure 18: Typical Wall Cross Section, Bellevue Technology Tower, Bellevue, Washington


Figure 19: Splayed Nail Layout Plan, Bellevue Technology Tower, Bellevue, Washington

One of the unique features on this project was the use of vertical elements to stabilize a potential ground loss problem at the face of the excavation on the west side. The excavation section is shown in Figure 20, which illustrates the Key Bank Tower parking garage and tower footings. Anticipated lateral stresses at the face of the excavation combined with excavating 7-ft vertical faces in the relatively clean advance outwash sands and gravels, led to the decision to include the use of vertical face stabilization elements. These are shown in Figure 21 and consisted of 3-inch O.D. Schedule 80 pipe in a 6-inch diameter drill hole filled with lean grout and spaced 36-inches on center. Vertical elements were extended 18 feet below the basement footing for the bank tower, and included three rows of nails. It was felt that adequate confinement would be provided at this level so that no loss of ground support would be experienced below the footing.

Figure 20: Typical Cross Section West Wall, Bellevue Technology Tower, Bellevue, Washington Performance and Conclusions As noted above, the soil nail system performed well during the Nisqually earthquake of February 28, 2001, which occurred during excavation of the deepest lift of the excavation with only nominal movements of the soil nail wall. Maximum movements of 0.25 inches were noted on the reentrant


corner of the excavation, typically regarded as the weakest point of the structure. This movement was well within the design criteria for the wall. Additionally, no structural damage to neighboring buildings was discovered. The peak ground accelerations produced by the Nisqually Earthquake in Bellevue were on the order of .11g, about one-third of the design earthquake for permanent structures in the Seattle area. Finally, it is worth noting that an extended life study for the system has been completed since the project was put on hold prior to completing the floor slabs for the garage. A temporary soil nail system is only designed for the life of the excavation which in this case was approximately one year. Therefore the owner requested an evaluation of the soil nail system for an estimated life of reasonable performance. A temporary wall is not designed for earthquake resistance, although the static factor of safety does provide for some inherent resistance. This was certainly the case for the resistance provided during the Nisqually earthquake. In addition, the soil nails are not designed with any corrosion protection for extended life performance. Based on the results of an extensive seismic analysis of the as built condition it was determined that the existing temporary excavation support system is adequate to resist the UBC design earthquake over the next 4 to 8 years. D. Provo Canyon Project Background

In 1995, construction began on a slope stabilization project along a 1.5-mile stretch of SR-189 in Provo Canyon, Utah. A portion of the scope of work included the construction of approximately 170,000 square feet of permanent soil nails and rock dowels. The project plans also detailed a reinforced concrete cast-in-place form lined finish wall facing for these structures.

Upon completion of the rock stabilization system, construction began on the concrete facing. Almost immediately, residents of Provo Canyon began to voice concern and displeasure regarding the aesthetics of the concrete facing. This concern, coupled with other contractural issues terminated the project in 1997. In July 2001, Utah Department of Transportation (UDOT) issued a Request For Proposal (RFP) to select a Design/Build Team to design and construct the continuation of the rock stabilization required from Vivian Park to the Wasatch County Line on SR-189. The two phase technical cost proposal included the procurement of the best value contractor to design and construct the safe, efficient, cost conscious, environmentally sensitive and compatible soil nail stabilization, wall drainage, and cast-in-place, pre-cast or structural shotcrete slope face treatments. UDOTs main goal included the requirement of aesthetic slope stabilization structures that are responsive to public concerns and compatible with the existing geological features of Provo Canyon. Secondary goals included reasonable cost and a project completion within the construction period from April 2002 to October 2002. Included as part of the $6.5 million project were two interesting features. These two features included a contractor proposed Warranty Bond for the permanent wall drainage system where the proposing contractor specified the warranty duration, warranty criteria with measurable standards and remedial work plan. The second feature included a $200,000 Performance Incentive Fee awarded only if the contractor exceeded the minimum requirements specified for the aesthetic treatment of the slope faces. These requirements included blending the retaining structures into the natural canyon environment including color, texture, natural vegetation, relief, shadowing and pleasing transitions that match or enhance the surrounding physical features of the canyon. Award of the Contractor Performance Incentive was to be determined by the Provo Canyon Aesthetics Team. The team was composed of the UDOT Project Manager, three members of the public and three UDOT staff members, all appointed by the UDOT Project Manager.


In December 2001, DBM Contractors, Inc. was awarded the Design/Build Contract. Included on DBMs Provo Canyon Project Team were Golder Associates (Design Engineer) and CemRock Landscapes, inc. (Shotcrete Subcontractor). Together the DBM team presented a detailed technical cost proposal that included a high quality simulated rock treatment to the slope faces. The objective of the sculpted shotcrete treatment proposed by DBM was to raise the bar as it related to the quality associated with simulated rock slope stabilization for UDOT and their stakeholders.

Design Considerations Based on the various existing conditions, three different slope treatment types were designed. In areas of competent rock with no shotcrete, minimal treatment was used. Treatment included scaling and installing additional rock dowels as required for stability of the rock face. New and existing rock dowels were trimmed and painted to preserve the aesthetics of the existing rock. In areas of soil nails and shotcrete, treatment consisted of a new six inch structural shotcrete facing with a 2-inch layer of sculpted shotcrete. Based on the RFP, the existing soil nail walls were analyzed at selected locations to determine that adequate static and dynamic factors of safety were present for a design life of 75 years. In areas of weak rock with existing rock dowels, treatment was the same as the soil nailed areas previously described. Permanent drainage considerations, where new shotcrete was been applied, were based on data collected from the previous design team and field observations. One foot wide geocomposite drain boards were used and covered between 20% to 60% of the treated surface. Water collected in the composite drain boards was released at the base of each wall through a standard 1.5 diameter weep pipe.

Besides providing for drainage behind the new shotcrete, the RFP specified that waterproofing be considered. DBMs design team worked with the local concrete supplier to develop a concrete mix that included waterproofing additives to reduce the permeability of the shotcrete and help seal potential small hairline cracks that might develop. This additive was used for both the structural concrete as well as the sculptured concrete.

For freeze/thaw durability of the sculpted shotcrete, consideration was given to the wide range of temperature differences that Provo Canyon experiences. DBMs design team worked with UDOT to develop a concrete mix that included air entrainment and synthetic fiber reinforcement. These synthetic fibers were used to control shrinkage and temperature cracking and provide additional durability and toughness in the shotcrete mix. Other projects with similar characteristics were studied to determine the weight of fiber reinforcement necessary per cubic yard of shotcrete this project. Consideration regarding the quantity of fiber reinforcement was also given to the application process (e.g. shotcrete).

Every simulated rock project has its own unique set of requirements and constraints. The Provo Canyon rockwork is a very large project relative to most artificial rockwork jobs. The natural setting is spectacular; its particular ecological nature stems from the singularity of its vegetation, elevation, topography, hydrology and wildlife. The canyon is a popular and notable recreation site. The aesthetic roadside rockwork treatments that DBM proposed had to be executed with an awareness of the environmental importance of the project. Within the constraint of a fixed budget, it was DBMs responsibility to produce a level of realism that maximized the aesthetic appeal of the finished product while minimizing its impact on the local ecology. In other words, the work had to be as transparent as possible.


To accomplish the above goal the DBM team had to develop a work strategy that balanced fixed cost with the different stakeholder demands for naturalistic realism. It was our opinion that the great majority of recreationalists will generally experience the project either from the window of a moving automobile or from the streamside opposite the treated slopes, and that the relative differences of viewing distances between the tops of the slopes and the bottom are minimal. This meant that if the rockwork we proposed had any variation in its level of realism, the changes would be very subtle with the level of realism decreasing only slightly for the work at the highest elevations (>60 above the roadway). Efflorescence or the buildup of white stains caused by leached calcium salts from the shotcrete would be minimized through the use of admixtures in the concrete that both waterproofed the shotcrete structure and lowered the production of calcium hydroxide during the hydration and curing of the concrete. Efflorescence cannot always be eliminated entirely, but through the use of these admixtures it can be reduced to an acceptable level. The aesthetic appeal of the many rockscapes on this project would also be enhanced greatly by the judicious use of vegetation. Most of the natural slopes in Provo Canyon are too steep to afford areas where plants can grow. Planter pockets would be placed in the slope faces, but presented difficulties related to drainage and naturalistic realism. For this reason planting within the rock faces would be minimal. Most vegetation designed for the project was placed either above of below the rock face with indigenous species of small form.

Construction Construction started in April 2002 and consisted of approximately 170,000 s.f. of permanent sculpted shotcrete soil nailed and rock doweled slope treatment. Slopes varied in vertical height up to 110 above the roadway surface. The westbound lanes of SR-189 were closed to allow access to the different slope faces. The project was divided into seven different wall locations (Wall A through Wall G) and included treatment to the existing tunnel portals and construction of an equipment enclosure that also had to blend with the Provo Canyon environment.

For connectivity between the existing rock dowels/anchors, soil nails and the new shotcrete surface, steel plates with headed studs were installed at each anchor location. To best illustrate this project please refer to the photographs in Figures 21-24.

Figure 21: The Existing Condition at Wall E, Provo Canyon, Utah


Figure 22: Looking South along SR-189 at Walls A-D, Provo Canyon, Utah

Figure 23: Completed Construction along Wall G, Provo Canyon, Utah


Figure 24: Construction Complete at Wall C, Provo Canyon, Utah Performance and Conclusions Construction was successfully completed in October 2002 on schedule and under budget. Included with the successful project completion was the payment of the full amount of the $200,000 Performance Incentive Fee. This acknowledged the full satisfaction of all of the project stakeholders regarding the aesthetic treatment of the Canyon Walls. As of the writing of this paper, the project is enduring its first winter season. Hopefully at the May presentation we will be able to illustrate the successful performance of the permanent drainage system and the wall facing treatment through its first spring thaw. UDOT awarded DBM Contractors, Inc. and Golder Associates with its 2002 Engineering and Construction Award of Excellence for their efforts on the beautification of Provo Canyon SR-189 from Vivian Park to the Wasatch County Line.

VI Future Developments

A. Retaining Wall Remediation Remediation of failed retaining wall systems is becoming more and more prevalent with the design life of first generation infrastructure coming to an end. Successful projects in the past have been completed without the use of vertical elements or the benefit of the hollow injection bar system. The standard procedure in the recent past has been to penetrate the existing wall with soil nails and stabilize the ground behind the wall. Then demolish the wall and construct a new face. In the case where the old face can be salvaged then a new wall facing is placed over the old wall. However, the preferred design approach is to demolish the old wall. When this is done, face stability is often times the predominant problem. Recently, the use of vertical elements and hollow injection bars were used to remediate two existing retaining walls requiring demolition in downtown Seattle. In urban environments where numerous utilities, adjacent buildings and marginal ground conditions exist, remediation of existing building basement walls are now feasible using soil nailing systems with these advances.


B. CASE 2 Micropile Networks The concept of CASE 2 reticulated micropile networks developed by Dr. Fernando Lizzi and illustrated in Figure 25 is very similar to soil nailing. It involves the use of an appropriately spaced, three-dimensional arrangement of vertical and inclined piles that encompass and reinforce the ground, and at the same time is supported by the ground. For earth retention and slope stabilization, Lizzi suggested that the reticulated network of micropiles creates a stable, reinforced soil gravity mass in which the reinforced soil supplies the essential resisting force, and the piles, encompassed by the soil, supply additional resistance to the tensile and shear forces acting on the structure.

Figure 25: CASE 2 Micropiles, Reticulated Pile Network with Reinforced Soil Mass Loaded or Engaged

While substantial research and field testing have been conducted to establish reliable design and construction techniques for axially loaded single piles the behavior of reticulated CASE 2 micropile networks needs to be further investigated in order to develop reliable design and procurement methods. A current research project, Micropile Interstate Cooperative Research On Foundation and SloOpe Reinforcement (MICROFOR) is under development by the International Center for Ground Improvement (ICGI), Polytechnic University, New York in conjunction with The Port Authority of New York and New Jersey, FHWA and the ADSC. The main objective of this program is to establish a relevant experimental database for the performance, assessment, evaluation and development of reliable design methods and procedures for the engineering use of CASE 2 micropile network systems with respect to lateral, static, cyclic and seismic loading. It will be interesting to find out the results of this full scale field load test research project.

VII Conclusions In summary, the recent development of vertical and subvertical elements to standard soil nail systems has significantly increased the use and application of soil nail earth retention and slope stabilization systems. In addition, new construction methods using the hollow bar injection system to handle difficult ground conditions has also had a significant impact on the use of soil nailing. Advances in the design and construction confidence of using these systems to carry significant loads from adjacent structures or


direct vertical loads has benefited numerous projects in cost savings and schedule. Further research and field testing of CASE 2 micropile networks will assist in the development of reliable design methods that will help speed the implementation of these composite soil nail systems. Finally, the development of the top-down permanent wall construction approach and more environmentally friendly sculpted shotcrete facings have multiplied the application and benefits for soil nail earth retention and slope stabilization design and construction. Definitely stay tuned! VIII References Abramson, L.W., (1994), In Situ Ground Reinforcement, Ground Control and Improvement (Xanathos, Abramson and Bruce, Eds), John Wiley and Sons, Inc., New York, NY. Byrne, R.J., Cotton, D.M., Porterfield, J.A., Ueblacker, G., Wolschlag, C.J., Manual for the Design ad Construction Monitoring of Soil Nail Walls. Technical Publication for Office of Technology Publication for Office Technology Applications, Federal Highway Administration, May 1997. Chassie, R.G., 1994, FHWA Ground Nailing Demonstration Project, Guideline Manual and Workshop, United States Federal Highway Administration Publication. Cotton, D.M., Byrne, R.J., Wolschlag, C.J., Soil Nailing: The Recent Development of Design Innovations and Cost Saving Ideas Using the Top-Down Method of Permanent Wall Construction. Presented at the University of Wisconsin Milwaukee short course on Specialty Geotechnical Construction in Urban Environments, San Francisco, California, February 1998. Cotton, D.M., Soil Nailing Design and Performance publication for University of Washington Short Course New Developments in Urban Geotechnical Engineering Processes. 1993 1998. Cotton, D.M., Soil Nailing: The development of the Top-Down Method of Permanent Wall Construction and Local Stability Problems and Resolutions in Fill Materials, Glacial Till, Out Wash and Lucustrine Deposits. Presented at 1992 ASCE Seattle Section Geotechnical Seminar, University of Washington, March, 1992. Federal Highway Administration. (2000). Micropile Design and Construction Guidelines, Implementation Manual, Principal Investigators T.A. Armour, P.B. Groneck, J. Keeley FHWA Publication No. FHWA-SA-97-070. French National Research Project CLOUTERRE (1993) Recommendations CLOUTERRE 1991 Soil Nailing Recommendations 1991 Presses de lEcole Nationale des Ponts et Claussees English Translation, July. Porterfield, J.A., Cotton, D.M., Byrne, R.J., Soil Nailing Field Inspectors Manual Soil Nail Walls. Technical Publication for Office Technology Application Federal Highway Administration, April 1994. Wolschlag, C.J., Cotton, D.M., Byrne, R.J., Suspension Wall Shoring System. Presented at the GeoInstitute Conference Champagne Urbana, Illinois, University of Illinois, 1998. Wolschlag, C.J., Singam, G., Byrne, R.J., Cotton, D.M., Soil Nail Wall Case History: KOMO Television Station Expansion. Presented at the 1999 ASCE Seattle Section Geotechnical Seminar, University of Washington, April 1999.

recent advances in soil nailed earth retention

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