Mound Systems for On-site Wastewater Treatment

Siting, Design and Construction in Ohio

Bulletin 813

How Should Mound Systems Be Constructed?

Because a large portion of the mound system components are above-grade, mound systems are easier to construct than conventional soil absorption systems. The most important things for a contractor to remember are outlined in the following eleven-step procedures.

Recommended Construction Procedures for Mound Systems

Step 1

Lay out the proposed mound system along the contour of the lot in the area specified by the detailed soil and site evaluation. According to the designs, outline and stake three areas: distribution area, basal area, and overall footprint of the mound.

Step 2

Determine the septic tank and dosing tank locations based on the site layout. The tanks should be installed to the side or upslope of the mound.

Step 3

Prepare the site for the mound. Mow the grass to a maximum 2-inch height and remove the cuttings from the mound location. Construction should be delayed if the soil is too wet. Dig a trench from the dosing tank to the side or upslope of the mound. The trench is for the pressure pipe bringing septic tank effluent into the center of the mound. The pressure pipe should be installed with an adequate slope so that the septic tank effluent drains back to the dosing tank after the pump shuts off. The trench should be adequately bedded and filled with granular backfill to reduce settling.

Step 4

Prepare the soil surface in the basal area. A chisel plow on a small tractor or special teeth fitted to the backhoe bucket can be used to scarify the soil surface. Scarifying the soil surface and breaking up the sod growing in the basal area will improve the contact of the mound sand with the natural soil. Avoid any traffic and equipment on the basal area or downslope of the mound. The basal area will be covered with a layer of sand that protects the soil under the mound from compaction during construction.

Step 5

Bring in the mound sand to form the absorption area and cover the basal area. Two different methods can be used to bring in sand to shape the mound. The traditional approach uses a dump truck to deliver the sand to the upslope side of the mound. A backhoe moves the sand into the shape of the mound covering the entire basal area. The alternative method is to use a truck equipped with a conveyor to place the sand over the basal area to the appropriate depth. This approach is especially suited to limited access sites. Paint marks on the ground outlining the basal area and paint stakes indicating the desired depth of sand in the absorption area guide the conveyor operator in placing the sand to shape the mound.

Step 6

Level off the sand surface in the absorption area. Cut off the distribution main at least 4 inches above the sand surface and remove all bits of plastic and rough spots that can accumulate debris and clog the pipe. Place some sections of 6-inch pipes across the top of the sand to support the layout of the distribution pipes. Lay out the main, manifold, and laterals according to the designs. Cement all tees and joints to prevent leakages and pressure losses.

Step 7

The distribution orifices should be mechanically drilled at the shop and hand drilling is discouraged. Mark the hole positions evenly on the top of the laterals according to the design calculations. Drill 1/4-inch diameter holes with a sharp drill bit to help create clean holes and minimize rough edges. Drill 1/4-inch diameter drain holes in the bottom at the end of laterals. Slide a smaller diameter pipe back and forth inside the distribution pipes to knock off any pieces of plastic extending down from the drill holes. The distribution system is typically constructed with small diameter pipes with small holes spaced evenly along the top of the pipes. The pipe size, hole size, and hole spacing should be carefully calculated and linked. Consult with the designer before changes are made to make necessary adjustments during the construction.

Step 8

Install an observation port in the gravel down to the sand to serve as a window into the mound to check for ponding of wastewater on the sand surface. Three methods of stabilizing observation tubes are shown in Figure 4. Attach turn-ups to the end of each pipe with a removable cap to allow for flushing accumulated debris from the pipes. The turn-up is supported with gravel to accept the flushed wastewater and allow it to flow back into the mound for treatment. A valve box works well to house the turn-ups so they can be easily located and accessed for regular maintenance.

Figure 4. Three methods of stabilizing observation tubes. (After Converse and Tyler, 1990.)

Step 9

Cover the distribution system with clean washed gravel. The holes on the top of the pipe need to be shielded so that they are not restricted by the gravel cover. A 4-inch perforated pipe slid over the distribution pipe makes an effective shield. Cover the gravel with construction fabric before placing the insulating layer of soil on top of the mound. As an alternative to gravel cover, chambers can also be used to cover the sand and distribution pipes if the design specifies.

Step 10

Cover the sand and gravel with an insulating layer of soil. Deliver soil cover material to the upslope side of the mound and cover the mound working from the upslope side. Never allow any heavy equipment on the area downslope of the mound. The treated wastewater flows downslope through the soil. Heavy equipment will result in unnecessary soil compaction and lead to bleed out at the toe or downslope of the mound. Properly grade the upslope side of the mound to divert surface runoff around the mound. Stay on the mound while shaping the downslope side to limit soil compaction. A small bulldozer works well to shape the mound. Seed the soil cover of the mound to limit soil erosion as soon as the construction is completed. If it is late in the season, additional erosion control measures may be necessary.

Step 11

Preparation of as-built drawings is encouraged for good record keeping. The as-built drawings should include actual mound system layout, elevations, benchmark, and start-up date. The drawings should be kept for personal records with copies provided to owner and inspector.