Tere has long been a need for a reliable, simple-to-use means of determining the early-age rate of strength gain in shotcrete.During approximately the first 24 hours after shotcrete has been placed, its compressive strength is typically too low to measure using standard core-extraction and testing procedures. Monitoring the rate of early-strength development in shotcrete is important in tunneling, mining, and other applications such as the underpinning of structures. Recent studies by the authors have demonstrated that there is a simple, direct method for determining the early-age compressive strength development of shotcrete. It involves the shooting of a set of beams in a standard steel mold and testing the beams after stripping, using an adaptation of ASTM C 116, œStandard Test Method for Compressive Strength of Concrete using Portions of Beams Broken in Flexure. This œTechnical Tip describes a procedure for deter-mining the compressive strength of shotcrete beams and presents results of tests conducted with plain and accelerated shotcretes produced by both the wet- and dry-mix shotcrete processes.
Shotcrete Meets the Challenge of Huge Water Project in Ecuador
Shotcrete Meets the Challenge of Huge Water Project in Ecuador replacing traditional œform-and-pour reinforced concrete construction methods with high-production shotcrete, the massive Trasvases Manabi Water Project in Ecuador was finished months ahead of schedule. Contractor Norberto Odebrecht, in conjunction with Shotcrete Technologies, Inc., of Idaho Springs, Colorado; and Commercial Shotcrete, Inc., of Higley, Arizona, placed over 6000 m3 (7800 yd3) of shotcrete in less than half the time it would have taken by the specified œform-and-pour method. They put the project an entire rainy season (approximately four months) ahead of schedule.
Design Guidelines for the Use of FIber-Reinforced Shotcrete in Ground Support
Developments in Shotcrete in Hobart, Tasmania, Australia in April 2001. One of the outstanding papers presented at the Conference was a paper by Grant, Ratcliffe and Papworth on œDesign Guidelines for the use of SFRS in Ground Support. Frank Papworth was asked to submit an updated paper on the subject for publication in the ASA Shotcrete Magazine and so here it is. It is more technical than most of the papers published in the ASA Shotcrete Magazine, but was selected because it was considered that it would be of considerable value to designers of fiber-reinforced shotcrete linings for ground support in civil and mining applications.
Abstract: There are presently no design guide-lines based on toughness for the use of fiber-reinforced shotcrete (FRS) in ground support for underground mine development. Typically, in the Australian mining environment, the approach to the use of FRS has been one of borrowing experiences from other mines and a œtrial-and-error method of design, installation, and assessment. There is a need for a ground support design guide that can be simply applied by œfront-line personnel.
This paper provides an overview of the performance characteristics of FRS and how the various shotcrete guides specify its use. Practical experiences with the use of FRS in Australia and Canada in various applications and ground conditions are combined with existing empirically based ground support-design methods to develop a ground support guideline that incorporates the concept of toughness. An assessment of structural synthetic fibers shows that their low modulus makes their performance characteristics different from those of steel fibers, and that they are not likely to be economical in linings where crack widths are limited, but that they are preferable where large deflections are permissible.
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Fiber-reinforced shotcrete (FRS) has been used successfully for ground support for more than
Dealing with Reinforcing
Shotcrete Solution to Tricky Underpinning Problem
A downtown Vancouver excavation and shoring project involving underpinning of two adjacent structures took on a strange twist when it was discovered that the three-level, early 1920s building to the north of the excava-tion had a precariously attached brick wall founded on a rubble footing, which was required to be underpinned to construct the new building to the south. The struc-tural engineer, acting for the owner of the
Shotcrete Retrofil of a Mechanically Stabilized Earth Wall
The Municipality of Maple Ridge in British Columbia, Canada, commissioned a spe-cialty contractor to build a 9 m (29.5 ft)diameter culvert, incorporated in a mechanically stabilized earth (MSE) wall, to provide a street crossing for a stream. The culvert had been partly sunk into the stream bed to adapt to the site conditions. The MSE walls were a maxi-mum 8 m (26.3 ft) high above the ground and 70 m (230 ft) long. Figure 1 shows part of the North Face of the MSE wall.
The fill used in the construction of the MSE wall contained a fraction of fine dredged river sand. After construction, fine particles of the sand dried out and started to migrate through the galvanized metal screen that comprised part of the MSE wall. This resulted in voids at the surface of the MSE walls. The voids were a concern as they constituted a potential cause for future settlement of the sidewalks and asphalt pavement constructed between the MSE walls. The specialty contractor decided to remedy this situation by arresting the migrating fill before it came to its natural equilibrium, by refilling the surface voids and applying a shotcrete lining.
REPAIR SPECIFICATION
The design engineer recommended filling the voids with shotcrete and stabilizing the surface of the MSE walls with a permanent shotcrete lin-ing. The specifications called for the following procedures:
Setting the Standard with Soil Nail Technologies
Nicholson Construction Company was awarded the con- tract to build a reinforced shotcrete soil nail retaining wall along State Route 9900 in Blair County, Penn-sylvania (U.S.). The 330-ft-(100-m)-long, 35-foot-(10.7-m)-high permanent retaining wall was cut into the hillside to provide an access route to the new Blair County Convention Center (see Figure 1). The general contractor for this project was HRI, Inc. and the owner was PennDOT.
Soil Nailing is Designed to Fit
Soil nailing has become a popular method to shore exca-vations and to build retaining walls due to its versatility and cost effectiveness. Generally broken into two cat-egories, temporary and permanent ground retention systems, soil nailing has evolved into many variations. A range of dif-ferent means and methods have emerged, giving the construc-tion industry an incredibly adaptable shoring system. The im-portance of a well-seasoned soil-nail team, however, cannot be understated for the success of any project.
Temporary soil nailing is the system that utilizes the nail for a limited time during construction process for ground support. Once the permanent structure is in place, the nails are essentially aban-doned. By contrast, permanent soil nails remain in service for the life of the wall. The shotcrete application required during the shor-ing process can also be temporary or permanent, but not necessar-ily the same as the nail. Hence, a temporary facing may be con-structed with permanent nails or vice versa.
Typically, in basement construction, soil nailing is used as a temporary shoring system and is installed concurrently with the excavation for the basement walls. After the basement has been excavated and the soil nail system is in place, either a mat slab or footings are poured and a permanent cast-in-place con-crete or shotcrete wall is installed. If a shotcrete wall is cho-sen, it can be installed with many different finishes to fit the use of the structure. Typically, shotcrete basement walls are fin-ished with wood or rubber floats to create a sand finish. In tem-porary soil nailed walls, the structural floors and walls are in-stalled; the soil nailing retention system is no longer needed to support the walls. In some cases, the wall is installed in a top down manner concurrent with the soil nail anchor installation, in both temporary and permanent shoring systems.
In the past, the highway departments often utilized the per-manent soil nail wall system with shotcrete lagging, covered with a cast-in-place or precast facing to give the completed wall a cast-in-place look. Now, however, permanent soil nailed walls are being used extensively along West Coast highways and freeways.
Although materials, equipment, and methods vary greatly, soil nail retaining wall construction involves variations of the following very basic steps:
Designing and Installing a Shotcrete Strengthening Application on the Spokane Street Bridge
The Spokane Steet Bridge connects the community of West Seattle to downtown via Highway 99 and Interstate south of the city center. The bridge was not originally and constructed to handle is present-day usage consequently.
Shotcrete for Underground Support
In November 1999 I had the opportunity to attend the Third International Symposium on Sprayed Concrete for Underground Support.