Soil Considerations in Pipe Bursting
The soil and subsurface investigation includes collecting the necessary information to properly design the project. It assists the contractor in submitting a proper bid by selecting the appropriate bursting system (type and size), shoring of the pulling and insertion shafts, dewatering system, compacting backfill material, etc. This proper decisions and biding increase the chances of success during the construction phase of the project.
The soil investigation activities include soil borings, standard penetration tests, groundwater level determinations, trench geometry investigation, and native soil and trench backfill material classifications. If the presence of washouts or voids around the existing pipe is suspected, a Ground Penetrating Radar (GPR) survey may assist in determining locations and magnitude of these voids. Special attention should be given to the presence of major difficulties that may render pipe bursting not feasible such as the presence of rock, hard cemented dense soils, very soft or loose soils, reinforced concrete encasement, very narrow trench in hard soils or rock, or ductile point repairs. If contaminated soil is suspected, the type and extent of contamination should be identified and indicated in the contract documents. The contractor should be requested to take the necessary measures to handle and dispose of this contaminated soil.
The soil around the pipe (backfill and native soil) must be somewhat compressible to absorb the diameter expansion. Compressible soils are the ideal soils for pipe bursting because the outward ground displacements will be limited to an area surrounding the pipe alignment. Original backfill is the most suitable soil for bursting followed by (increasing difficulty) compressible clay, loose cobble, beach and running sand, densely compacted clay, and sandstone. Soils with long standup time allow the overcut (created by the expanded hole) to remain open for most of the bursting operation, thus reducing the friction force between the soil and the pipe. The overcut lowers the needed pulling forces and consequently the axial stress on the new pipe during installation. Somewhat less favorable ground conditions for pipe bursting involve densely compacted soils and backfills, soils below the water table and expandable soils. Special soils such as highly expansive soils or collapsible soils will also cause problems.
Pipe bursting below the groundwater table (GWT) increases the difficulty of the bursting operations because the groundwater flows towards the insertion shaft requiring dewatering of the shaft. Also, in very soft or loose soils, significant ground movements may take place causing significant sags in the new line and damage to nearby structures. In severe situations, the soils particles migrate to the old pipe converting the bursting operation into a piercing operation. If the groundwater is lowered via any dewatering technique such as deep wells, well-point system, or open sumps in the pulling and receiving shafts; the effective soil pressure will increase. This will increase the vertical loads on the pipe causing increased friction, bursting and pulling force, and tensile stresses in the PE pipe. On the other hand, the presence of water reduces the coefficient of friction between the pipe and the soil, somewhat offsetting the increase in necessary pulling force.
If the original soil borings (during the old pipe installation) are available, they should be reviewed and made part of the supplemental information available to the bidders. The determination of the trench geometry and backfill material and compaction is important for the designer and contractor.