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Managing Burst Forces
14th In A Series
There is a basic formula to calculate the volume of lubrication required. By taking the inner diameter of the existing pipe and subtracting it from the outside diameter of the expander head, you are able to easily calculate the annular volume that will follow the burst head through the ground as it pulls the new pipe in place. Once that volume is calculated, potential fluid loss can be calculated based on type of soil and experience from a given geotechnical area. In some soils, a 1:1 ratio is used and in others a 2:1. Once volume in gallons per foot is determined, it can be multiplied by the total length of a burst to give a total volume in gallons for the burst which will need to be prepared and ready prior to installation.
Designing the mix
Once the volume is determined, the design of the mix must be planned. One of the most critical aspects of any lubrication mix is the makeup water which in many cases is pre-treated with soda ash or another additive. It is not common for salt water from coastal areas to be used in lubrication mixes; however, there are additives available that can be used with salt water. Based on soil conditions, the mix is made in most cases with a bentonite base and additives used to maximize the effects of the lube. The additives for unconsolidated soils, gravel, large cobbles and reactive clays will have contrasting materials which will affect the viscosity and ability of the lubrication to keep the hole stabilized. (See photo at head of article.)
In the field, a simple mud pump, mud mixer and supply system is attached to the new pipe just behind the expander head which injects the lube as the pipe is pulled in place.
One important factor to keep in mind is that, in many cases, some of the lubrication will flow to the insertion and/or receiving pit and may need to be vacuumed and removed from the site. Unlike with HDD where the majority of the lubrication is removed from the site, pipe bursting typically only requires enough lube to reduce friction as it is a one pass process. Disposal of the lubrication should be planned in advance and although the materials are safe for environmentally sensitive areas and drinking water applications, many people unfamiliar with the process will simply not understand that the material is not hazardous.