HomeMy WebLinkAboutMiscellaneous - Exception (471)Guidelines for the Design and Installation of Impervious Barriers
Swales or grading shall be incorporated into the design to prevent ponding by
directing storm water away from the system. The grading shall be designed so that the
stormwater is not diverted onto abutting properties in accordance with 310 CMR 15.255 (2).
Any impervious barrier may be designed by a Massachusetts Registered
Professional Engineer. When the system has a design flow of less than 2,000 gallons per
day, a Massachusetts Registered Sanitarian can design impervious barriers that do not
include a retaining wall that provides any structural support for the slope.
Impervious barriers shall have the following design characteristics:
1. A top elevation that is at least as high as the top of the two-inch layer of
peastone covering the pipes in the soil absorption system, or at least as high as
the top of the chambers when leaching chambers are used.
2. The bottom of the impervious barrier shall extend to a minimum depth of at
least one foot below the existing natural ground elevation.
3. There shall be at least a 10 -foot horizontal distance between the bottom of the
impervious barrier and the edge of the adjacent finished side slope.
4. In remedial situations where there is less than four feet of vertical distance
between the bottom of the soil absorption system and ledge or other
impervious strata, (consistent with the requirements for the granting of a
variance specified in 310 CMR 15.415), the bottom of the impervious barrier
should be at least six inches above the impervious strata to avoid impounding
the treated effluent.
Clay barriers shall be:
1. constructed of compacted clay (conforming to NRCS classifications for
clay, with a hydraulic conductivity no higher than 10-7 cm/sec);
2. at least two feet in width; and
3. covered with at least six inches of cover material (soil) and stabilized
(e.g., vegetated) to prevent erosion of clay due to weather conditions.
Plastic membrane barriers shall be:
1. of sufficient tensile strength to withstand perforation, including
cracking, tearing and breaking,
2. at least 40 mils in thickness, and have significant durabilit}, and
resistance to the temperature and moisture conditions expected in the
subsurface environment; and
installed without holes or gaps and so that perforations do not develop
after installation.
Additionally, as specified in 310 CMR 15.255(2):
Materials for impervious barriers shall:
1. have no weep holes;
2. have a waterproof surface on at least the upgradient side, consisting of a
durable waterproof lining or applied waterproof coating;
be anchored or reinforced as necessary to maintain structural integrity; and
4. not allow seepage or deteriorate over time.