Don’t Turn Bedrock Into a Money Pit

Careful planning with architects, engineers and designers is critical to making sure tricky systems in rocky soil remain profitable for the installer

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We’ve heard from readers who live in areas with shallow solids over bedrock. As you can imagine, they report that these conditions present many challenges for installing onsite systems. We have observed homesite preparation in areas where rock creates problems simply for creating an area to set the house. 

Techniques to prepare these areas include one or some combination, depending on the type of rock involved, of blasting with dynamite, using large jackhammer-type equipment on the end of an excavator arm to pulverize the rock into submission, and large bulldozers and earth movers to create an area to place the footings for the house.

All of this makes the job of evaluating the site and designing a sewage treatment system more difficult. It is important that handling sewage is part of initial planning for the house and not simply an architect’s simple drawing of line from the house to a septic tank and a few lines to represent the sewage treatment trenches.

Planning teamwork

If there are locations on the lot where soils are suitable for installation of the soil treatment unit, these need to be identified and protected through the lot preparation process. How sewage is going to get to these areas needs to be factored into the house layout and plumbing plans. A note here on site evaluation in these areas: Solid bedrock is identified in the field by whether a shovel or knife can penetrate the rock.

In one instance, we observed that the only area available for soil treatment was all the way on the opposite side of the house and about 200 feet away. This creates a challenge just to run the supply piping. All members of the house design, construction and installation team need to have continuous communication to make it all work. 

Without proper planning, activities that are normally easy for installers become much more difficult. In soils with minimal rock, digging holes for the sewage tanks and trenches to run the supply piping between components can be done in a few hours at most. Depending on the bedrock outcrops, there might be need to route piping to avoid those areas or do some blasting or jackhammering to provide an area for the tanks and run supply pipes. 

Proper bedding material to set the tanks and piping will need to come from offsite. When these systems in rocky conditions are in a cold climate, tanks and piping will need to be near the surface and will require insulation. Tanks may need insulation on the top and maybe the sides as well, depending on the exposure. Insulated piping is available, usually consisting of a smaller pipe inside a larger pipe and with foam insulation in between. 

Design problems and solutions differ in different types of bedrock and other rock problems. Of course, our goals are always the same — to have the amount of effluent generated from the house be accepted and treated before it is released to the environment. 

Creviced limestone

From a bedrock perspective, a primary concern is treatment if the bedrock is creviced limestone. If the bedrock is solid, impermeable granite, we worry about treatment and where the effluent goes after it leaves the system. For soils where rocks are more than 35% by volume, we worry about rapid movement of water through the soils resulting in a lack of treatment. 

In areas of creviced limestone bedrock, the shallow soils over the top and the soil material filling the crevices is usually permeable and can be a good treatment media. However, if the sewage is applied directly to just one or a few crevices, the effluent can quickly move through the crevices to contaminate groundwater and water wells located far from the sewage source.

The solution is to utilize whatever thickness of natural soil is over the bedrock and applying effluent in a matter that does not concentrate in one or a few parts of the soil treatment area. 

Pressure distribution is a must, and the more uniform distribution is over both time and space the better. This means controlling doses by using a timer. In an interesting piece of onsite history, the current version of onsite sewage treatment mounds came about due to problems in areas of creviced limestone bedrock in Wisconsin. Installers have more options these days, including an at-grade system or drip distribution if the soil over the bedrock is deep enough.

Similarly, where the soils contain a large percentage of rock, spreading out flow over time and space allows effluent to infiltrate over the entire area providing treatment. Solutions would be like the limestone situation — with mounds, at-grades or drip systems. 

In addition to acceptance and treatment, we worry about where the effluent may go in the solid or “impermeable situations.” Part of the design needs to factor in whether the effluent will move along the top of the bedrock and then outlet somewhere nearby as it passes through the treatment media and hits the bedrock surface. If so, it needs to be treated before it outlets in the drainageway. Some additional water management is required where the water comes in to the drainageway. Additional pretreatment components may be required to ensure treatment. 

Estimate carefully 

Bedrock problems are some of the most challenging conditions an installer will face. In addition to the problems discussed above, these areas also often have significant land slope. This makes moving equipment in and out and getting the components into place very difficult. As an aside, it also makes estimating the job difficult and any mistakes very costly. 


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