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PDFWAC 246-272B-06700

Sand-lined trenches and beds.

(1) Sand-lined trenches or beds may be used to meet Treatment Level B.
(2) The minimum dosing frequency is:
(a) Twelve equally spaced timed doses per day for coarse sand; or
(b) Six equally spaced timed doses per day for ASTM C-33 sand.
(3) The maximum allowable hydraulic loading rate is one gallon per square foot per day.
(4) The design engineer shall:
(a) Use coarse sand that meets the following specifications:
(i) Particle size distribution per Table 8 of this section;
(ii) Effective particle size from 0.3 to 0.5 mm; and
(iii) Uniformity coefficient less than four; or
(b) Use sand that meets the ASTM-33 specification, Table 9 of this section and:
(i) Has no more than forty-five percent passing any one sieve and retained on the next consecutive sieve, of those listed in Table 9 of this section; and
(ii) Has a fineness modulus of not less than 2.3 or more than 3.1. The fineness modulus is calculated by adding the cumulative percents of samples retained on the sieves listed in Table 9 and dividing the sum by 100; or
(c) Verify that the sand material meets the desired specification and provide to the department a particle size analysis of the sand material.
(5) The minimum depth of sand media is twenty-four inches.
(6) The design engineer shall:
(a) Design the LOSS to meet the drainfield requirements in Table 3 in WAC 246-272B-06050 for the minimum horizontal setback from the edge of the sand-lined trench or bed;
(b) Specify that for installations in type 1 soil, trench and bed widths be increased by one foot from the calculated minimum absorption area based on design flow; the additional volume must be filled with sand media in order to prevent effluent from bypassing filter media by flowing out the sidewalls; and
(c) Specify monitoring ports as required in WAC 246-272B-06400.
Table 8: Coarse Sand—Required Particle Size Distribution
Sieve
Particle Diameter, mm
Percent Passing by Weight
 
3/8 inch
 
9.50
100
 
No. 4
 
4.75
95 to 100
 
No. 8
 
2.36
80 to 100
 
No. 16
 
1.18
45 to 85
 
No. 30
 
0.6
15 to 60
 
No. 50
 
0.3
3 to 15
 
No. 100
 
0.15
0 to 4
Table 9: Fine Aggregate—ASTM C-33 Sand—Particle Size Distribution
Sieve
Particle Diameter, mm
Percent Passing by Weight
 
3/8 inch
 
9.50
100
 
No. 4
 
4.75
95 to 100
 
No. 8
 
2.36
80 to 100
 
No. 16
 
1.18
50 to 85
 
No. 30
 
0.6
25 to 60
 
No. 50
 
0.3
5 to 30
 
No. 100
 
0.15
0 to 10; prefer < 4
 
No. 200
 
0.075
0 - 3; prefer 0
[Statutory Authority: RCW 70.118B.020. WSR 11-12-035, § 246-272B-06700, filed 5/25/11, effective 7/1/11.]
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