320 Guidelines for Design & Installation 2002 COMMONWEALTH OF MASSACHUSETTS
EXECUTIVE OFFICE OF ENVIRONMENTAL AFFAIRS
DEPARTMENT OF ENVIRONMENTAL PROTECTION
ONE WINTER STREET, BOSTON, MA 02108 617-2825600
BOB DURAND
Secretary
LAUREN A.LISS
Commissioner
GUIDELINES FOR DESIGN AND INSTALLATION OF IMPERVIOUS BARRIERS AND
SLOPE STABILIZATION FOR TITLE 5 SYSTEMS
Effective date: March I,2002 Policv/SOP/Guideline#: BRP/DWM/WPeP/G02-1
Program Applicability: BRP,Watershed Permitting,
Title 5
Approved by: [signed] Regulation Reference: 310 CMR 15.211(1)4.
Cynthia Giles, 310 CMR 15.255
Assistant Commissioner,BRP
Supersedes Policy/SOP/Guideline: NONE
PURPOSE:
These guidelines supplement Title 5, 310 CMR 15.255, which addresses systems
constructed in fill. Specifically, this guidance contains design and installation criteria for
impervious barriers, and slope stabilization when either is required by 310 CMR 15.255 in
conjunction with a system constructed in fill. Additionally, this guidance supplements 310
CMR 15.211(1)[4], which requires slope stabilization for certain soil absorption systems
adjacent to naturally occurring downhill slopes.
APPLICABILITY:
This document is a guide for Massachusetts Registered Professional Engineers (PE)
and Massachusetts Registered Sanitarians (RS) who design Title 5 systems using
impervious barriers, and for PE's that design impervious barriers using retaining walls, or
structural barriers such as concrete retaining walls, and for boards of health and DEP staff
who review applications for Title 5 systems and who observe and inspect the installation of
Title 5 systems.
TNa Information Is cva0able In alternate format by eating our ADA Coordinator et(617)574-6871.
DEP on the World Wide Web: htpnwww.state.ma.undep
0 Primed on Recycled Paper
'guidelines for the Design and Installation of Impervious Barriers
NTRODUCTION:
Title 5, 310 CMR 15.255, addresses the construction of systems in fill. An
impervious barrier is required for systems constructed in fill when the designer can not
provide the minimum 15 foot horizontal distance between the edge of the top of the two
inch layer of washed stone (peastone) cover over the soil absorption system, and the top
of the sideslope. For a system constructed in fill, 310 CMR 15.255 requires slope
stabilization, such as a concrete retaining wall, when the sideslopes to a soil absorption
system are steeper than 3:1 (horizontal: vertical). Section 310 CMR 15.211(1)[4] requires
slope stabilization when systems are located in an area adjacent to a naturally occurring
downhill slope steeper than 3:1. When a naturally occurring downhill slope adjacent to a
system is steeper than 3:1, Title 5 requires stabilization in accordance with accepted
engineering practice which may include installation of a concrete retaining wall designed
in accordance with 310 CMR 15.255(2)(a)through(g),
TEXT:
Impervious Barriers:
Pursuant to 310 CMR 15.255(2), an impervious barrier is required for a system
constructed in fill when the designer can not provide a minimum distance of 15 feet
between the edge of soil absorption system and the top of the side slope. Impervious barriers
must also be incorporated into the design when a minimum 15-foot horizontal distance
cannot be provided between the edge of the soil absorption system and an adjacent naturally
occurring downhill slope. (see Figure 1)
3:1 slope
Less than 15'
Top Elevation of SAS
Impervious Barrier
Fill or Naturally Occurring
Pervious Material
Sail Absorption System
Finished Grade
Figure 1: Impervious Barrier
Page 2 of 2
iuidelines 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 harriers 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 durability and
resistance to the temperature and moisture conditions expected in the
subsurface environment and
3. 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;
3. be anchored or reinforced as necessary to maintain structural integrity; and
4. not allow seepage or deteriorate over time.
Page 3 of 3
iuidelines for the Design and Installation of Impervious Barriers
mpervious Barrier with Retaining Wall:
An impervious barrier, as an integral part of a retaining wall, is required when both the
iorizontal distance between the edges of the two inch layer of washed peastone and the adjacent
ide slope can not be met and the side slope is steeper than 3:1 (310 CMR 15. 255(2)) . The
criteria listed in this guidance for impervious barriers shall also apply to impervious barriers
(sing retaining walls.
The following conditions also apply to retaining walls used only for structural purposes:
I. Materials used shall be structurally sound,non-degradable and waterproof.
The structure shall have no weep holes when used as an impervious barrier.
2. It shall be designed by a Massachusetts Registered Professional
Engineer, consistent with the provisions in 310 CMR 15.255(2)(b).
3. The horizontal distance from the edge of the SAS to the retaining wall shall
be measured from the SAS to the closest edge of the footing if a footing is used.
4. In accordance with 31,0 CMR 15.255(2)(d), a Massachusetts Registered
Professional Engineer shall design and inspect the installation and
construction of the impervious barrier using a retaining wall or a retaining
wall, and shall certify, in writing,that the structure has been constructed
according to the approved design. The written certification shall be submitted
to the approving authority prior to the issuance of the Certificate of
Compliance.
Slope w/out
retaining
wall —y''
(steeper
than 3:1)
15'minimum'
Top Elevation of SAS
Fill
O O O
Finished Grades
Soil Absorption System
Naturally Occurring
Pervious Material
• •FootIng
`impervious barrier necessary if less than 15'
Figure 2: Typical Retaining Wall without Impervious Barrier
Page 4 of 4
Guidelines for the Design and Installation of Impervious Barriers
Construction
Construction of any impervious barrier, impervious barrier and retaining wall, or
retaining wall alone, shall follow accepted engineering practice. They shall not be
constructed during weather conditions that may prevent the proper installation of the
structure. Extreme temperatures may affect newly poured concrete, for example, and
may cause a membrane liner material to become overly inflexible, brittle or weak. Wet
conditions may prevent the application of waterproof coatings or the ability to properly
compact clays. Structures shall not be backfilled without inspection by the designer and
the approving authority. The owner shall not modify the impervious barrier, or retaining
structures without approval of the designer and without receiving appropriate approvals
from the Board of Health or DEP (when DEP approval is required).
Engineers, Registered Sanitarians and system installers should review and be fully
aware of state and local building codes that contain requirements applicable to retaining
walls and other structures used for slope stabilization.
In accordance with 310 CMR 15.255(2)(e), an as-built plan shall be prepared and
the designer shall certify that the impervious barrier, and/or retaining structure has been
constructed in accordance with Ale approved plan. These plans must be submitted to the
approving authority.
imp.barr.doc
Page 5 of 5