- Trench Configuration
The trencg configuration INFLUENCES THE EARTH LOADING that pipes must endure after the construction program has been completed. The most important factor influencing the future earth loading is the WIDTH of the trench at the level of the pipe. Wider trench widths increase earth loads on pipe in proportion to the horizontal width of the trench. This principle is illustrated in the figure below.
The applied earth load on a pipe is a combination of two earth loads: 1) the applied earth load DIRECTLY over the pipe; and, 2) a portion of the earth load ADJACENT to the pipe. The portion of earth load transferred by the adjacent soil is limited to a maximum load for a trench width termed the TRANSITION WIDTH. Widths greater than the transition width do not increase the loads beyond the maximum loading created by the transition width.
As a general rule, trench width should not exceed the pipe diameter plus 60 cm (2 feet). If the trench width increases, the load that will be carried by the pipe will also increase.
- Bedding
Preparation and proper compaction of the pipe bedding is very important. The pipe's ability to support the overlying earth loads is influenced by the degree of support that the pipe has under it and at its sides. For FLEXIBLE pipes, such as PVC, the pipe chiefly relies on the support given by the bedding. If PVC pipe is improperly supported, the pipe will deflect vertically. Improperly supported PVC pipe may be either damaged or deformed possibly causing leakage at the pipe joints. The effect of bedding is illustrated in the figure above.
As shown in this illustration, the pipe is capable of resisting the earth loads and deflects only slightly when properly bedded. If improperly bedded, the pipe is unable to support the overlying load and deflects vertically until the downward earth loads are equalled. For rigid pipe such as concrete pipe and AC pipe, the pipe will break if unsupported since the pipe cannot flex in response to loads.
Depending on the application and pipe strength, various degrees or classes of bedding with a load factor of 2.2 can provide twice as much load carrying ability as Class D bedding with its load factor of 1.1 (i.e. 2.2 = 1.1 X 2). The figure below illustrates classes of bedding.
- Class D - Load Factor 1.1
Using this bedding, little care is exercised to shape the pipe foundation and to compact bedding material around the pipe. This bedding method is NOT RECOMMENDED as a bedding alternative.
- Class C - Load Factor 1.5
With 'C' bedding, ordinary care is used to shape the bottom of the trench directly under the pipe. The bottom is shaped to bed the pipe for a height equal to 1/2 of the pipe diameter. Joint or coupling holes are excavated at each pipe joint to permit assembly and to safeguard against the pipe being supported at the joint. The trench is backfilled to a height of 15 cm (6 inches) over the top of the pipe with selected earth containing no rocks or frozen material. The bedding material is placed by hand and tamped to ensure reasonable compactness around and under the pipe.
- Class B - Load Factor 1.9
Using this method, the pipe is placed on a granular material bed that is carefully shaped to fit the bottom of the pipe. The width is equal to at least 60% of the pipe diameter. Backfill is placed by hand in 15-cm (6-inch) layers and compacted for a depth of at least 30 cm (12 inches) over the top of the pipe. The bedding is tamped thoroughly to ensure good compaction around and under the pipe. Class 'B' bedding is the most commonly used bedding method.
- Class A - Load Factor 2.2 - 3.4
In this method, the bottom of the pipe is bedded in plain or reinforced concrete of suitable thickness. The concrete extends upwards on each side of the pipe for a height equal to but not less than 1/4 of the pipe diameter measured from the bottom of the pipe. Alternatively, a concrete arch over the pipe can be used.
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