REV07474

• Permit No: C1982057
• IBM Index Class Name: Revision
• Doc Date: 7/26/2006
• Doc Name: Request for Review Memo
• From: Byron Walker
• To: Dan Mathews
• Type & Sequence: TR55
• Media Type: D
• Archive: No

STATE OF COLORADO
DIVISION OF MINERALS AND GEOLOGY
Department of Natural Resources
1373 Sherman St., Room 215
Denver, Colorado 80203
Phone: [phone]
FAX: [phone]
COLORADO
DIV7SlON OF
MIN sRALS
GEOLOGY
0.EC LANATION•NINING
SAFETY•SCIENCE
Interoffice MEMORANDUM gillowens
Govenwr
Russell George
t0: Dan Mathews Ececu[ive Director
cc: Sandy Brown, Dan He dez, Mike Boulay (e-mail) Ronald W. Cattany
from: B ron G. Walker Division Director
y Natural Resource Trustee
subject: Request for Support, TR-55 Review, Seneca II West, C-82-057
date: July 26, 2006
This review is in response to your e-mail request of July 17, 2006, forwarded by Sandy on July
17, 2006. Materials reviewed were those provided in the TR-55 Revision application. No site
visit was made by me (which I really prefer). Questions I have relate to the configuration of the
contributing watershed to PM-4D. Is the access road permanent? Is there no cut-slope ditch in the
vicinity of PM-4D (is the azea upslope of PM-4D to the west of the road contributing flow to
PM-4D by overflow of the road)? The review continues as if the quantity and flow rates ofwater
to PM-4D are appropriate.
The general approach is appropriate. From the information in the application, I presume the
original slide occurred in undisturbed ground below the road. As a reconstructed road
embankment, the safety factor of 1.3 is appropriate [Rule 4.03.2(3)(e)(ix)], as the embankment
slope is steeper than 5 horizontal to 1 vertical [Rule 4.03.2(3)(e)(viii)]. This really is a minimum.
Note that a factor of safety of 1.5 is required for excess spoil fills [Rule 4.09.1(7)] and a factor of
safety for backfilling is 1.3 [Rules 4.14.1(2)(f)(ii) and (g)(ii)]. I recommend acceptance of the
stability analysis included in the application. The design provides a factor of safety of 1.72.
As envisioned in the application, water is a key factor. The proposed plan controls drainage (see
pages 1 and 2 of the slope stability analysis). I agree with you that the drainage PM-4D is a
permanent diversion. Descriptions of channel configurations or methods of protection (liners) are
not described in Table 20-2. They may be described elsewhere, but such descriptions were not
included in this review. As proposed in the application, a 10-year precipitation event is
appropriate [Rule 4.05.3(3]. The 59% slope (Table 20-2 of the application) precludes any type of
stabilization by vegetation. Alternatives, with approval by the Division, are acceptable [Rule
4.05.3(3)]. In effect, diversion PM-4D is a drop structure. As you mentioned, I do not consider
metal half-pipes (see page 2 of the stability ana-ysis) or culverts appropriate to permanent
diversions. Such permanent diversions are not subject to the maintenance requirements for
permanent (metal) road culverts and impoundment primary spillways. Please ask the applicant
to submit a design for a durable rock, riprap-lined ditch with a geotextile underliner for
drainage PM-4D [Rule 4.05.3(7)(a)]. Note that a 0.3-foot freeboard would be required [Rule
4.05.3(7)(b)]. Grouting the riprap or shotcrete would greatly enhance permanent stability (both
from gravitation forces and seepage). The Division would be approving a diversion on a
Office of Office of Colorado
Mined Land Reclamation Active and Inactive Mines Geological Survey