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Review of the Elizabeth
Mine EE/CA:
October 23, 2001 |
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Richard N. Downer Re: Maintenance of Riprap on the Face and Top of TP-1 At the October 10th meeting I suggested that the face and top of TP-1 could be covered with rip rap which might be found near the South Cut. Thus, cutting down on the number of trucks passing through the village. Since the meeting I have spoken with Robert Finucane, 802-241-3454, Deputy Director of Dam Safety for the State of Vermont and Mark Rosenthal, Manager of the Union Village Dam. The following was garnered from them. Robert Finucane reported that his office is responsible for the maintenance of three riprapped-faced dams on the Winooski River: (1) Little River in Waterbury, (2) Wrightsville in Wrightsville on Route 12 and (3) East Barre in East Barre on Route 302. Annual maintenance includes cutting the grass all around the pools on the upstream sides and on the riprapped downstream faces once a year. The annual contract for these three dams is less than $10,000. The riprapped faces of each of the dams is as large as the proposed faces on TP-1 and 2. I estimate the proposed area of the two faces (TP-1 and 2) not to exceed 4 acres. Although the landscape contract is not broken out for just the faces, Robert Finucane estimated the cost of trimming the face of a single dam at less than $2,000 per year. This number seems to correlate well with the independent estimate from Mark Rosenthal at the Union Village Dam. He told Bob Walker $1,500 per year as the cost of trimming the face. For trimming the riprapped faces the State uses handheld string trimmers. On the less steep slopes the contractor uses a walk behind wheeled flail trimmer. Robert Finucane reported that by trimming every year they were able to keep ahead of the young trees. They had not resorted to hand pulling or to the use of chemicals. Mr. Finucane personally felt the use of chemicals (Roundup) would be a more cost-effective way of maintaining the faces. However, he noted that because his Department was a part of the Agency of Environmental Conservation he was precluded from authorizing the use any chemicals. As currently proposed, the tops of TP-1 and TP-2 will be graded at 5 % slopes. If they are seeded to grass, then conventional agricultural mowers can used. If they are covered with riprap only, then a small tractor hauled flail trimmer (brush hog) can be used or a walk behind wheeled trimmer, like the "Dr-Trimmer" can be used R S Means 2000 gives a price of $112.80 per acre for rough terrain cutting. Assuming 30 acres for TP-1 and 5 acres for TP-2 this translates to $3,933 per year. So $5,000 per year is a reasonable estimate for the annual maintenance costs for TP-1 and 2. The following are my questions to Mark Rosenthal at the Union Village Dam with his responses in italics. I have annotated some of his responses. 1. What is the slope of the Union Village Dam downstream face? The average slope of the dam is 33%. 33% is equivalent to 18.26 degrees above the horizontal or 1V : 3H.
Option 2C proposes to maintain the face of TP-1 at its present slope of 34 degrees or 1V : 1.48H. Option 3B proposes to regrade the face of TP-1 to a 1V : 3H or 18.26 degrees or 33%. Thus, the regraded slope will look very much like the Union Village Dam. 2. How high is the Union Village Dam downstream face? The height of the dam is 170 feet. Scaled from the drawings the height of TP-1 is approximately 150 feet. 3. What is the approximate area of the Union Village Dam downstream face? The approximate area of the downstream face is 6 acres. 4. What is the approximate size of the rip rap stones on the face of the Union Village Dam? The size of the rip rap is small due to weathering, about 7" minus. At new projects the size is often classed as blasted ledge.
5. Is it easy to walk on? The rip rap at Union Village Dam is easy to walk on because it is weathered. Blasted ledge is much more difficult to walk on. North Hartland Dam is a better example of what a slope covered with blasted ledge looks like. 6. How thick is the rip rap on the downstream face? I don’t know the thickness of the rip rap. 7. How do you maintain (annual cutting) the downstream face? For the past 11 years we’ve had the woody stems hand pulled. This can work well if you stay at it. I’ve also used fire (prescribed burning) on part of it that is between 75% and 100%. The use of Roundup as a spot spry is also done at some dams. 8. What is the approximate annual cost of maintaining the downstream face? Current contractual cost are between $1,000 and 2,000. I’ve no figures on the admin costs. 9. Finally, based on your experience with the maintenance of a rip rap faced dam do you have any recommendations or suggestions? As far as recommendations go I’d suggest that annual maintenance is the key. Keep the woody vegetation off and inspect the face annually.
Here are my responses to John Freitag questions. 1. Is it conceptually possible to just have an 18" rock cover over TP1 and TP2 on top of a fabric cover? It is possible to place large rip rap directly onto a geotextile. There are several companies that make geotextiles with very strong physical properties. For example, AMOCO makes a 16 ounce per square yard material with a grab tensile strength of 350 pounds for a 1 inch wide test piece and a Mullen Burst of 750 pounds per square inch. These are very strong materials. 2. Or does there need to be a cushion/protection layer of some kind to protect the fabric cover from the rip-rap rock? It is customary, but not necessary, to place a cushion layer on top of the geotextile before dumping large rip rap. In this case, it might be possible form a cushion using the fine material (less than 4 inched in diameter) that would result from the blasting process. Blasting produces a lot of fines that could be screened out for use in the cushion layer. 3. What kind of slope is needed on the sides of the tailings, can it be on the grade of Union Village Dam? What is the slope of the Dam? The average slope of the Union Village Dam is 33%. 33% is equivalent to 18.26 degrees above the horizontal or 1V : 3H.
Option 2C proposes to maintain the face of TP-1 at its present slope of 34 degrees or 1V : 1.48H. Option 3B proposes to regrade the face of TP-1 to a 1V : 3H or 18.26 degrees or 33%. Thus, the regraded slope will look very much like the Union Village Dam.
4. Will a fabric liner be damaged under rip-rap on the slopes? No. The same installation techniques will be used on the slopes and the tops.
Cindy Cook asked the effect the freeze and thaw cycle has on the membranes and other capping options. The following is a quote from Koerner, Robert M., "Designing with Geosynthetics", Pentice-Hall, 1998, page 630.
"Freeze-Thaw. The central property of a hydrated GCL (Geosynthetic Clay Liner) insofar as freeze-thaw behavior is concerned is its permeability. Daniel et al, used a rectangular laboratory box and subjected the entire assembly to 10 freeze-thaw cycles. The permeability showed a slight increase from 1.5 x 10 ^-9 to 5.5 x 10 ^-9 cm/s. Krus et al. Report no change in flexible wall permeability tests of the specimens evaluated after 20 freeze-thaw cycles. While the moisture in the bentonite of the GCL can freeze, causing disruption of the soil structure, upon thawing the bentonite clay is very self-healing and apparently returns to its original state." From page 450 of Koerner - ". . . Cold will generally not degrade the geomembrane in any appreciable way, at least under the temperatures normally encountered. Furthermore, tests on a variety of different seam types have shown no adverse effects to cyclic cold temperatures for 500 cycles (see Hsuan et al.)."
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