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Mathews, Dan From: Mathews, Dan Sent: Tuesday, October 17, 2006 9:53 AM To: Roy Karo (RKaro@PeabodyEnergy.com) Cc: Cramer, Johanna Subject: FW: II-W TR-55 Slide Remediation Plan Roy, the forwarded email below is our input on the TR-55 plan that you requested in the letter of October 6. I passed it through Sandy, Mike, and Byron to get their input yesterday, which is why I am forwarding it to you, rather than sending it directly to you. I'll fax the Enkamat brochure I have now. Dan From: Mathews, Dan Sent: Monday, October 16, 2006 4:44 PM To: Brown, Sandy; Boulay, Mike; Walker, Byron Subject: II-W TR-55 Slide Remediation Plan Roy, This email is in response to your letter of October 6, in which you requested that DRMS staff review the conceptual plan to utilize turf reinforcement mat product for permanent channel sfabilization, rather than riprap with geotextlle underliner. DRMS staff has reviewed the conceptual plan, and we believe this approach is likely to be approvable, and may be preferable to the riprap installation (due to safety hazard considerations and the extent of disturbance that riprap installation would likely entail). Our comments regarding the conceptual plan include the following: 1) The referenced amended plan view sketch "Remediation Plan View" depicting the various roadside ditches and culverts described will need to be provided. 2) Steve Renner used Enkamat 7020 "soil filling" installation option in channel installations at Coal Basin. The spec sheet you had provided to me during my September inspection was for Enkamat 7003. 7020 is a heavier, thicker mat; listed applications are for permanent erosion control for vegetated channels with expected shear stress of up to 10psf, and permanent erosion control for moderate to steep slopes (up to 1.5:1). 7003 specs are for channels with shear stress up to 6psf and slight to moderate slopes up to 3.5:1. This would seem to indicate that Enkamat 7D20 would be a better choice for the II-W site. Your letter did not indicate whether you were planning to use the "soil filling" option or "non-soil filling". The Enkamat brochure I have states that "soil filling accelerates performance because the Enkamat, soil and the new vegetation interact together to resist shear forces when water is flowing through the channel or on top of a slope. If soil filling is utilized, spread 1/2 to 3/4 inches of fine soil into the mat to completely fill it." f will FAX to you a copy of the brochure I am taking this information from. Seeding would be done either after soil fill, or both before and after soil fill (in either case, seed over the top of the installed mat). Non-soil fill is only recommended in situations where the mat would be expected to trap sediment from an up-gradient source. Even for the non-soil fill, seeding is to be done over the top of the installed mat (apparently, the effectiveness of the mat is dependent on roots growing down through the nylon matrix). Obviously, the soil fill option would be more labor intensive, but would appear to be more effective in this situation. In any event, the specs for the turt reinforcement mat to be installed will need to be specified and demonstrated to be adequate for the anticipated slope and channel conditions, and the installation procedure (e.g. "soil filled" or "non-soil filled") will need to be specified and justified. 3) There is reference to installation of a geotextlle liner beneath the turf reinforcement mat. This would not be advised, as it would defeat the purpose of the turf reinforcement mat, which is to establish a reinforced grass turf. Please remove the reference to use of the geotextlle liner. 4j Our understanding is that the half pipe will be retained for a period of time unfit vegetation has become well established. But at some point, the half pipe will need to be removed, to demonstrate that the turf-reinforced, vegetated channel will function effectively as the permanent solution. The channel will need to function effectively for several years prior to final bond release. Please include narrative to clarify that the half pipe is a temporary measure that will be removed at least three years prior to submittal of a final bond release request for the location.