Aggregate processing and handling facilities, mining sites, and related industrial operations often need to transport aggregate and ore across various points on their properties. The need to maximize surface area on such sites necessitates the use of tunnels for transporting bulk materials under roadways and parking areas and processing these materials. Some applications involve tunnels complete with hopper openings and feeder chutes located under bulk material piles. These are used for receiving and processing the aggregate and ore material and then transporting this material to other locations on site.
One of the logical options available that offers economic advantages in the design, installation, operation, and maintenance phases of these projects involves the use of large corrugated metal pipes (CMP) or corrugated structural plate pipes (SPP) and corrugated structural plate arches (SPA) for these tunnel structures. Advantages include reduced design time and related costs; simplicity of construction; the option to utilize site personnel for the assembly and installation phases of tunnel construction; reduced installation costs; and lower maintenance costs.
While CMP, SPP, and SPA are viable and attractive options for the construction of such tunnel structures, they must be properly designed and installed to provide the desired level of structural performance and service life. Understanding the basic principles of flexible metal pipe design and installation is a critical requirement. The need for appurtenances such as escape pipes, end walls (bulkheads), utility connections, hopper and feeder chute openings, floor slabs, lighting, and other features unique to such tunnel applications must be addressed while keeping the key structural aspects of flexible Corrugated metal pipe design in mind.
This article will discuss the general design, installation, and other specific considerations for aggregate and ore tunnel applications involving the use of CMP, SPP, and SPA. The details associated with special appurtenances will also be addressed.
CMP aggregate tunnels are constructed using round CMP, while structural plate tunnels are either round or arch shapes (see Figure 1). Arches typically are half-circle, singleradius shapes that are placed on reinforced concrete slab foundations or raised, pedestal-type footings where added interior clearance is desired. Two radius arch shapes (commonly referred to as horseshoe arches) may be used when high vertical clearance is needed and if the site conditions allow their use.
Normally, the interior of the tunnel has a concrete floor slab to provide a reasonably level and uniform working surface. For full-round pipe shapes, the floor is constructed by placing a granular bed on top of the pipe invert, and then a concrete surface is poured on top of the bedding. Flexible metal pipes will experience slight deflections and shape changes during installation, backfilling, and under loads. Therefore, it is recommended to construct concrete floors after the pipe has been backfilled and initially loaded to prevent problems with spalled concrete due to natural shape changes. Shrinkage is expected with curing concrete and any gaps at the edges of the concrete floor can be sealed with mastic or another type of suitable sealant material after the floor slab cures.
CMP, SPP, and SPA are classified as flexible pipe structures. They are by design less stiff than the surrounding soil backfill envelope. As such, they behave as soil-structure interaction systems that rely on the stiffness and density of the surrounding backfill for support under applied loads. Such structures installed in a competent backfill envelope that has been properly placed and compacted can carry significant amounts of overburden loads. The proper design and installation of these CMP and structural plate structures are covered in nationally recognized specifications such as American Association of State Highway and Transportation Officials (AASHTO) and ASTM, as well as industry-related references such as those provided by the National Corrugated Steel Pipe Association (NCSPA).
The design premise of CMP is that these pipes carry loads in ring compression, with the pipe supported by the soil envelope and the pipe wall carrying the applied loads in compressive thrust. The total load pressure over the pipe — consisting of both the dead loads, represented by the applied soil and aggregate material overburden weights, and the live loads, represented by any temporary construction traffic or more permanent vehicle live loads operating over the tunnel — is calculated. This combined load pressure is used to determine the ring compression force acting within the pipe wall, using Equation 1:
Equation 1:
Ring Compression Force (Wall Thrust) = Pt x Span/2
where:
Pt = Pll + Pdl
Pll = live load pressure
Pdl = dead load pressure
Span = maximum horizontal measurement (width) of the pipe tunnel
The unit weight of the aggregate material, along with the height of the pile above the pipe, are critical factors in determining the dead load acting on the tunnel. Many of these tunnels have aggregate piles above them that rise and fall with storage volume needs. Because the loading is cyclical in nature, the tunnel is carrying significant dead load near maximum cover heights as well as often carrying live loads at near minimum cover heights. Live load pressures are governed by factors such as cover height, load distribution at the travelling surface, and load dissipation through the fill zone above the crown of the pipe tunnel. The types of vehicles being considered, the tire footprint, and the load pattern are all relevant. Adequate minimum cover consisting of properly placed select backfill must be in place and well maintained for vehicle and equipment loads operating at low cover heights.
After the ring compression force value has been determined, the pipe is checked for wall buckling, required wall area, required installation stiffness, and seam strength (if applicable).
These tunnels, as with any buried flexible structure, require proper installation to perform structurally. This starts with installing such structures on a competent foundation — and this applies to the adjacent side fill embankments as well as to the pipe. The side fill zones must be supported to prevent differential settlement and loss of stable soil support for the pipe. Competent select fill must be used in the backfill envelope. Ideally, this should be a clean, well-graded, angular road base fill material with maximum particle size of 3 inches and a maximum of 15 percent passing the #200 sieve. The aggregate processed at the site may meet the requirements for select fill and, if used, the separator or indicator layer that denotes the top of the minimum select fill envelope must not be compromised, encroached upon, or removed. Backfill material should be placed in balanced, 8- to 10-inch loose lifts and compacted to 90 percent Modified Proctor Density (AASTHO T180). Side fill embankments are to be level and extend out far enough to provide stable, lateral resistance for the pipe tunnel. See Figure 2 for depiction of a proper backfill envelope for these structures.
Select backfill should be free of materials that are potentially corrosive or harmful to the pipe. If the ore or aggregate material is corrosive in nature, then the material must be isolated from the pipe. Additional protection is provided by using protective spray on coatings. Impermeable barriers such as plastic membranes combined with subdrains can be installed within the top fill envelope to prevent corrosive material or leachate from reaching the pipe surface.
Mine Safety and Health Administration (MSHA) regulations typically require escape or outlet pipes to provide emergency evacuation. Such outlet pipes and stubs for escape tunnels are treated as typical CMP fittings and will likely need to be framed with suitable reinforcing members designed to carry the ring compression loads around the pipe opening. Industry guidelines and specifications are available for reference in the design and detailing of such reinforcement. However, a simplistic approach is to use the ring compression force in the pipe tunnel wall as a bending load applied to the reinforcing members spanning across the opening. Simple span beam formulas can be used to determine the bending moment:
Equation 2
M = C x L2/8
where:
C = ring compression force (wall thrust)
L = length of opening
M = bending moment
The ends of the beam members are tied to compression members (curved to fit the pipe tunnel wall) that carry the end thrust from the longitudinal beams. The reinforcing framework can be comprised of standard structural shapes for the smaller size openings. Longer openings in larger-diameter tunnels under high fills may require a reinforced concrete collar framework to carry these loads.
Conveyor tunnels often are installed under aggregate and ore piles and have hopper and feeder mechanisms penetrating through the crowns of the tunnels. Typically these heavy machine components are supported independently of the pipe tunnel by either an internal, floor-mounted support system or an external, footing-supported collar system above the pipe. The opening in the pipe tunnel is still reinforced, but this reinforcement is intended to carry the ring compression loads around the opening, not to bear the weight of the hoppers, feeders, and material inside of those structures.
In situations where the tunnel is expected to carry the loads from the hoppers and feeder chutes, a suitable heavy structural framework that carries the loads well beyond the opening and provides shape control and added stiffness to the tunnel structure would be required.
Flashing around such openings may be incorporated to provide a reasonably soil-tight seal; however, such flashing is not intended to carry the full weight of the hoppers and feeders and material transported inside.
A fairly common scenario is to locate hopper penetrations and feeder chutes at reinforced concrete junction structures, with the corrugated metal or structural plate tunnel stubbing into the concrete structure and thus eliminating some of the above concerns.
Conveyors should be supported on floor-mounted columns when possible. In the event that conveyor support framework is intended to be mounted or suspended from the crown of the pipe tunnel, there will need to be additional structural members that distribute point loads from the conveyor supports up to the conveyor tunnel crown to help retain shape of the pipe tunnel. These tunnels may experience slight shape changes and deflections during installation and under load cycles. Mechanisms to adjust the conveyor hanging system and conveyor belt components should be used to ensure smooth operation.
End walls can be constructed from reinforced concrete or from metal end panels. Reinforced concrete end walls are formed, poured, and cured similarly to standard headwalls. Typically, prefabricated steel end walls are detailed as reinforced diaphragms. End walls are normally fabricated from flat steel plates or corrugated plates that are reinforced with welded channels. Such end walls may have an escape pipe through them for evacuation.
Light fixtures and small service conduits are readily mounted to the CMP, SPP, or SPA using self-tapping or blind hardware. Heavy utilities such as steam pipes and cable trays require special detailing similar to the approach used for conveyor trusses. Catwalks and platforms should be supported by the floor
CMP and SPP tunnels with coatings compatible with the installed environment usually require little maintenance. Sealing or caulking at the juncture between the floor slab and the tunnel wall may be required occasionally to seal any openings caused by curing of the concrete or by normal pipe movement and shape changes.
Tunnels can be extended in length if future needs dictate. The juncture between old and new sections of tunnel can be accomplished using reinforced concrete headwalls or through the use of bands or similar connections. Care must be taken during any future modifications to keep loads and soil envelope support for the tunnels balanced and uniform. Any tap-ins or similar modifications must be done in a manner to keep the entire tunnel periphery symmetrically loaded and supported and to maintain stability of the soil envelope.
CMP, SPP, and SPA tunnels offer an affordable, practical, and low-maintenance alternative to other means of aggregate tunnel construction. The combined strength of the pipe product and the surrounding select backfill envelope functioning in a soilstructure interaction system provides the load-carrying capability and structural performance required. Such pipe tunnels, installed properly, provide years of service and offer the flexibility of future modifications. Attention to details such as fittings, openings, conveyor support, hopper and feeder chute requirements, and utility needs ensures a functional, safe, and practical tunnel structure.
Jim Noll, P.E., is the director of Engineering Services for CONTECH. He has more than 30 years experience in the corrugated metal pipe industry and is an active member of various technical organizations, including ASCE, AREMA, and ASTM.
Steve Tysl, P.E., is a civil engineer for CONTECH and an active member in ASTM and SEAoO. He has more than 18 years experience in civil engineering and materials/construction engineering.
Matt Westrich, P.E., is a civil engineer for CONTECH. He has more than 12 years experience in the civil/structural engineering field.