Underground structures may be built by a variety of means including embankment construction, open-trench construction, jacking, tunneling, and microtunneling.
The proper design and installation of the foundation, bedding, and backfill for embankment and trench installations are critical to the performance of underground structures. They are also essential factors for achieving an accurate structural analysis of the system. The foundation preparation, bedding, and backfill of underground structures should be done in accordance with standards established by local and state transportation agencies. These standards vary from region to region, but the important aspects of typical practices are reviewed below.
Regardless of whether the pipe is installed in an embankment or a trench, the foundation must provide relatively uniform resistance to loads. If rock is encountered, it should be excavated and replaced with soil. If soft material is encountered, it should be removed for a width of three pipe spans and replaced with suitable material. Care must be taken to ensure that the foundation under the pipe is not stiffer than the adjacent zones, because this will attract additional load on the pipe.
The bedding is then placed above the foundation. Bedding thickness and material is contingent upon the type of pipe and the quality of the installation required. Pipe-arch structures require excellent soil support at the corners, because pressures are higher there. For most applications 3 to 6 in (75 to 150 mm) of bedding is sufficient. Some agencies require a shaped bedding for all pipe because of the difficulties in compacting the backfill in the haunch area. More recently, for most round pipes, in lieu of a shaped bedding, specifications call for the bedding under the middle one-third of the pipe diameter to be left uncompacted. This is so that the pipe can properly seat itself in the bedding, resulting in a greater length of support along the bottom circumference of the pipe. Pipe arches and large span structures should always be placed on a shaped bedding.
The backfill should be placed in 6- to 8-in (150- to 200-mm) compacted layers around the structure. Each backfill layer must be compacted to the minimum density required in the construction specifications. Densities less than 90 percent standard Proctor density should not be permitted. The backfill must be kept in balance on each side of the pipe. A granular material free of organic content and with little or no plasticity makes good backfill.
Complete installation requirements for the various pipe materials can be found in AASHTO, ASTM, and state DOT specifications.
