HomeMy WebLinkAboutTimber Ridge Rockfall Meeting 082510
Meeting Minutes
Alpine Eng. Inc.
George Ruther, Rick Pylman
To:
CC:
Glenn Palmer AEI, 970-926-3373ph. 970-926-3390(fax)
From:
August 25, 2010
Date:
Timber Ridge Rockfall
Re:
__________________________________________________________
The following was discussed with Todd Pyle and Stephanie of Yenter on August 24, 2010:
1. Yenter has design build capacity (Colorado PE) where HP Geotech could review the design.
2. Typically, solid rocks do not need to be stabilize
evaluation outcrops, estimating a risk for falling, and then determining an
appropriate mitigation. The mitigation can vary from
with anchors), to various sizes of netting covers over risky rocks which are then anchored down. Todd
thought that after the assessment stabilized
and not all of the stabilization area would need to be netted.This would help break up the visual
impact.
3. Most CDOT projects use a 20 to 30 year design period for rockfall abatment. We should probably use
a higher design period (determine the industry standard, probably like a 100 year period).
4. The netting can be painted different colors, and different size of net (openings) are available. Larger
rocks could use larger net opening, which allows vegetation and trees to grow through it better than the
smaller Tecco Mesh (chain link fence size.) They try to set the net close to the ground so it is less
visible.
5. The hand drills can drill 6 feet deep. If higher resistance is needed, they drill more anchors. In
sandstone, the mechanical drill can drill 800 to 900 lf per day, the hand drills can only do 80 to 90 lf
per day. The crane required for the mechanical drills can span about 300 feet max, where the bottom of
the rock outcropping is about 375ft (so a crane probably cannot be used, thus most if not all drilling,
will need to be hand drilled (with 6 foot max depth).
Helicopter cost $3500 per hour, crane cost is about $200 to 300 per hour. Todd thought they might be
able to crane DOWN to the site from Buffer Creek Road, but the crane would need the entire road to
set up (probably not ok with emergency vehicles).
6. Yenter felt that Phase 2 (future mitigation) could be accomplished in a similar format as phase 1.(Risk
assessment, then cable lash/strapping or netting, or leave stable rocks alone.)
7. Temporary fencing:
it was discussed that rocks could move (and fall) during field/risk assessment and stabilization work,
and that either a temp fence should be provided on the south side of Lionsridge Loop, or evacuate
If a helicopter was used for the assessment , there would be much less risk for rockfall.
8. Todd said that they usually close a road or highway for a day or two, and then roll all the loose rocks
off the hill FIRST. This eliminates the highest risk rocks, as well as reduces the amount of drilling to
anchor the rock. Rolling off loose rocks is a much cheaper and more reliable method of mitigation.
buildings
9. Schedule/sequence:
August 25, 2010
a. Obtain property owner poll to determine if they would consider any rockfall mitigation to
occur.
(I would assume that property owners will require some sort of indemnification from any
damage/injury which might occur as a result of rockfall , rockfall mitigation, or rockfall
mitigation construction. My experience is that the legal review of documents and agreement
of terms and conditions could take some time).
b. Provide fieldwork for risk assessment videotape (helicopter 1 hour, or rock climbers 2 to 3
days).
c. Provide risk assessment and proposed stabilization method from video. (1 week to 1 month).
d. Determine work schedule (need risk assessment first).
i. Set Temporary netting for stray rocks.
ii. Set pins above rock outcrop for rock climbers to attach to.
iii. Roll off loose rocks 2 to 3 days
iv. Drill bolts/anchors
v. Lay netting (helicopter or crane).
vi. Wire netting to bolts
vii. Anchor top and bottom.
10. Noise impact.
An air compressor (as is used to blow out sprinkler systems) is needed to power the drills. The drilling
is noisy (jack hammering).
While we were above the rock outcrop on Buffer Creek Road, it was noticed that the I-70 traffic noise
was very loud, and that it could be measured for decibels. There are noise consultants that could
estimate the loudness of drills at various distances.
We also noticed that the I-70 noise was much quieter even 6 feet back from the edge of slope down to
I-70. It is probable that the same would occur with the noise from drilling.
11. Visual Impact from houses above.
The rock outcrop was almost entirely NOT visible from above (on Buffer Creek Road) so that the
netting would also likely not be visible from most houses above.
From below, the rock outcrop is a fairly long distance from the nearest housing or roadway, where
distance decreases impact.
12. Other impacts:
a. Concrete/grout mixers will be needed above the site, to pump down to fill anchor/bolt holes. It
would be easiest to have them placed on Buffer Creek Road.
b. The air compressors would probably mostly be located on Buffer Creek Road.
c. Rick to check with David Johnson if any EIR implications.
13. Yenter was asked to provide a proposal for the field video for risk management and evaluation for
mitigation for phase 1 and 2 of Timber Ridge. Two proposals were requested : with helicopter and with
rock climbers.
If you have any questions, please call.
Glenn Palmer, PE
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