2014 New York GSF Design & Installation Manual 7 www.eljen.com
1.0 Basic System Design
Jurisdiction: Please contact the local health department, watershed agencies, or Code Enforcement
Officer for site specific information to determine if there are any additional local design requirements beyond
the requirements of Appendix 75-A.
Absorption Field Size: Eljen requires residential systems use a recommended minimum of 5 B43 modules
per bedroom in either a trench or bed configuration. Tables 1 and 2 on Page 10 define the minimum trench
system length and estimated number of modules respectively. The system size is based on dividing the
design flow by the application rates given in Table 4B of Appendix 75-A and then applying Eljen’s 6 square
foot per linear foot (6 sf/ft) rating from Appendix 75-A.8 (c)(3)(iii) and minimums set in this manual. This will
determine the minimum trench length needed for each system. Trenches provide the most efficient design
and minimize quantities of Specified Sand required for installation. In trench designs, a minimum of 6 inches
of Specified Sand is required underneath and around the perimeter of the module row in the trench. Page
11 of this manual contains step-by-step sizing procedures for trench systems.
Tables 3 and 4 on Page 13 of this manual define the required absorption bed bottom area and the number
of modules in a bed configuration respectively. The same minimum number of modules is required by Eljen
in beds as specified in a trench for a given site. Depending on the number of rows, the length and total
number of modules will frequently increase in order to maintain an equal number of modules in each row.
The footprint is based on the infiltrative area calculated by dividing the design flow by the application rates
given in Table 5 of Appendix 75-A. In bed systems, the spacing between module rows will vary depending
on system geometry. A minimum of 12 inches of Specified Sand is required around the perimeter of the
module rows and a minimum of 12 inches of Specified Sand separating module rows. Equal numbers of
modules are required in each row for bed applications. Page 14 of this manual contains step-by-step sizing
procedures for bed systems.
Proposed Repair and Replacement system sizing shown in Tables 5 & 6 on Page 17 represent the
recommended minimum number of units and system area. Proposed sizing for Repair and Replacement
bed systems must be approved by the local health official if required. Eljen recommends that Repair and
Replacement bed systems are installed as “long and thin” as possible when site conditions allow.
Vertical Separation: As required by New York rules, in-ground designs require a minimum of 4 feet of
useable soil above bedrock, unsuitable soil, and seasonal high groundwater. The highest limiting condition
on the upslope side of the trench shall be at least 2-feet below the 6-inch Specified Sand layer as shown in
Figure 6.
Specified Sand for Trench and Bed Systems: The first 6 inches of Specified Sand immediately under,
between rows, and around the perimeter of the GSF system shall be an ASTM C33 Washed Concrete
Sand With Less Than 10% Passing a #100 Sieve And Less Than 5% Passing a #200 Sieve. Please
place a prominent note to this effect on each design drawing. See Page 4 of this manual for more
information on the ASTM C33 sand specification.
Fill for Shallow Trench Systems: As specified in 75-A.8.e, Eljen GSF systems may be installed on a
plowed ground surface with 2-feet of suitable soil between the bottom of the 6-inch Specified Sand layer
and the identified limiting conditions or restrictive soils. Specified Sand is place in the trenches in contact
with the native soils. The total depth of fill is determined by the elevation of the base of the GSF Specified
Sand layer and should not exceed 24 inches above the ground surface Additional fill is placed between
the trenches to an elevation of 12 inches above the module by using a light weight track machine. The
cover fill must demonstrate a percolation rate similar to the in situ while not exceeding the in situ soils
permeability. The cover fill normally includes a 4-inch to 6-inch layer of top soil that supports growth of a
robust vegetative cover capable of withstanding dry periods. System design and layout follow the
requirements for level or sloping site trenches with sizing based on the receiving soil percolation rate and
6 square feet per linear foot module rating in a Specified Sand lined trench. Minimum trench center
spacing is 8 feet on center based on trench separation of 4 feet of native soil. Designers are
recommended to specify greater trench spacing in slower soils where they seek to insure hydraulic
independence of each row of modules.
Placing GSF Modules: Each row of modules is laid level and end to end on top of a 6-inch minimum layer
of Specified Sand. No mechanical connection is required between the GSF modules.
