1992
6650.003
McCombs, William H.
The American City Magazine
Collection
Buttenheim Publishing Corporation
1972
August
2 pages
N
Unfavorable soils make the construction of sewers, forcemain, and other underground utilities expensive and risky. If firm soil underlays unstable soil within five to six feet below the pipe, the unstable soil can be removed and replaced with washed rock in sizes from 3/4 to 2 inches. But this method becomes too costly and ineffective when the unstable soil extends beyond the five to six foot depth. Also, the use of rock can result in a “sloughing off” since it is appreciably heavier than the surrounding soil, permitting the pipe to settle and crack.Pile foundation may also be used, but increases the cost approximately four to six times that of pipe laid in good soils. Also, the piling must be constructed and located where a subsequent load placed on the pipe by traffic or other changes in surface conditions will not result in the vertical failure of the pipe or the shifting of soils which could produce lateral movement of the pipe.
A third method employs lightweight pipe such as corrugated metal and plastic that floats on the unstable soil bedding. The theory here is that the lightweight pipe would stay in place making removal of the unstable soil unnecessary. However, construction of this type has generally resulted in failure of the pipe and subsequent replacement.
Minnesota is particularly plagued with areas of unfavorable soil conditions, especially adjacent to our numerous lakes and low swamp areas, where it is most often necessary to construct the sanitary and storm sewers. Our office has encountered several projects where the soils made the construction so expensive that it was not economically feasible to construct the facilities. Thus we had to find a better method of construction.
Our investigation centered around finding a material that could be placed under the pipe similar to the rock method of construction, and yet would be lighter than the existing soil. It would not experience the sloughing problems and in some cases could serve as a bridge across some marginal soils. On the job in question, located in the Village of Medicine Lake, the major type of soil in the construction area was peat having a weight of 55 to 65 pounds per cubic foot. Thus we needed a material that would be less than this weight, yet would be strong, inert, insoluble, and non-corrosive to the pipe.
A lightweight aggregate similar to that used in the manufacture of lightweight concrete and concrete block seemed promising, so we contacted North Central Lightweight Aggregate Co., Inc., a manufacturer of such material. We inspected the material and had laboratory tests made. These tests showed the “Aglite”, an expanded clay aggregate, to be light in weight, hard, durable, inert, and insoluble–all the properties needed for a good foundation material for underground piping.