2011-04-29_PERMIT FILE - M2011001 ‎(17)‎

• Permit No: M2011001
• IBM Index Class Name: PERMIT FILE
• Doc Date: 4/29/2011
• Doc Name: Adequacy Review Response- Att. 16- TowerCo- Structure Agreement
• From: Tetra Tech
• To: DRMS
• Media Type: D
• Archive: No

TETRA TECH
February 7, 2011
Ms. Anne Best Johnson, AICP
Lafarge West, Inc.
11409 Business Park Circle, Suite 200
Longmont, CO 80504
RE: Response to Stability Analysis comments made in TowerCo letter dated January 27, 2011
Dear Anne:
This letter is in response to the Stability Analysis comment made by TowerCo in their January 27, 2011
letter sent to Pam Hora regarding TowerCo Site Number C02049 near the Iverson Mine. In the letter
TowerCo states the following:
"In regard to the Stability Analysis prepared by Tetra Tech, we note that the resulting safety factor,
based on mine highwall slope failure, is estimated at 1.1. Please note that the 2006 International
Building Code requires a minimum safety factor of 2.0 for towers and the tower industry design
standard, TIA-222-F, specifies a minimum safety factor of 2.0 for foundation design with respect to
soils failure, however, as the site owner, please be advised that it is TowerCo's prerogative to
establish and enforce whatever it deems to be an acceptable safety factor for its communication
facility."
The Stability Analysis Report, prepared under my supervision, was prepared to satisfy Division of
Reclamation, Mining and Safety (DRMS) standards which require us to use conservative parameters.
According to the DRMS, they have never had a report of any structure stability problems for structures
adjacent to a mining cell when their conservative model has been followed and a minimum safety factor
of 1.0 has been maintained. In following the DRMS standards, when we ran the slope stability analysis
model, we assumed that the underlying bedrock strength was weak, that the mining cell would be more
than 35 feet deep near the TowerCo cell tower, and that the Iverson Mine would have vertical mine
walls. The results of this model did in fact show a safety factor of 1.1, which is greater than 1.0 which
means that the structural stability of the TowerCo cell tower will not be affected by the Iverson Mine.
However, due to the comments made by TowerCo, we adjusted the stability analysis model to use more
realistic parameters. The bedrock strength was adjusted to correspond to a more realistic strength, the
mining cell depth was adjusted to its actual proposed depth of 19' near the TowerCo tower, and a more
realistic slope of .75:1 (Horizontal:Vertical) at the mine's high wall was used. The revised model
indicates a factor of safety of 2.8 for the cell tower. Attached is an illustration from the revised model.
In addition, in response to TowerCo's comments regarding the 2006 International Building Code and
tower industry design standards which require a minimum factor of safety of 2.0, this safety factor
relates to the bearing capacity of the underlying soils not the slope stability of adjacent mining.
Therefore, the factor of safety information they reference is unrelated to the Stability Analysis report
that we prepared.
3900 S. Sunset Street, Suite I-F Longmont. CO 80501
lei 303.772.5282 Fax [phone] vnwvuetratech.corr