PERMFILE72922 - Laserfiche WebLink

Home Browse Search

• exits the drain over a period of one month, an average flow rate of 0.2 cfs is calculated. This entry/exit rate is considered a conservative assumption. We used a method of calculating the capacity of gravel/rock drains presented by Leps (Reference 2). Using the lowest gradient in the drain area {about 2 percent) with 3/4-inch drain rock, a drain size of 9 square feet has a capacity of 0.6 cfs. This configuration provides a capacity with a factor of safety of 3 compared to the estimated flow. Providing three outlets increases the factor of safety to about 9. The location of the proposed toe drain is shown on Figs. 2 and 3 and typical sections of the drain, filter layer and outlets are shown on Fig. 4. We recommend that the outlet sections be provided with a solid PVC outlet pipe for the last 100 feet of the outlet (4-inch, solid PVC pipe, SDR 35). The processing method to be used will • separate the coal waste into two paths; a coarse waste consisting of all material larger than about the No. 28 sieve, and a fine waste (or cake) consisting of material ~~ finer than the No. 28 sieve. The coarse waste without the fines meets filter ~~ ~ ~~~ requirements from NAVFAC DM-7 (Reference 11) as shown on Fig. 4. The drain VGA within the toe buttress should be covered with a minimum 2 foot thick filter layer. ~(, c±~~ ,~(1' The filter can consist of suftably graded coal waste or imported material. Outlet ~ ~a`~`"` sections should be provided wkh filter fabric as shown on Fig. 4. We recommend the drain consist of a minimum of 9 square feet of rock ~~ meeting ASTM C33, No. 67 grading (3/4 inch to No.4) with a Slake Durability Index ~~ `^^~ (ASTM D-4644) of at least 90 percent. ~1.,W .X~"~~ According to state regulations (Reference 17) "Underdrains shall consist of non-degradable, non-acid or toxic-forming rock such as natural sand and gravel, • sandstone or limestone. Other durable rock may be used, if it has been 18