HomeMy WebLinkAboutB15-0199_Geotech letter_1435270380.pdf HHepworth-Pawlak Geotechnical, Inc.
5020 County Road of
CGlenwoodSprints.,Cuk,radu 81601
Phone:970-945-7988
HEPWORTH-PAW LAK GEOTECHNICAL
Fax:970-945-S454
email:hpgeuahpgeotech.com
May 29, 2015
KH Webb Architects
Attn: Debra Monroe
710 West Lionshead Circle, Unit A
Vail, Colorado 81657
(debra@khwebb.corn)
Job No. 115 211A
Subject: Recommendations for Boulder Retaining Wall Design, Proposed
Residence, Lot 8B, Filing 4, Lions Ridge, 1463 Aspen Grove Lane, Vail,
Colorado
Dear Debra:
As requested, we are providing recommendations for design of the proposed boulder
retaining walls at the subject site. The services were performed in accordance with our
agreement for professional services to KH Webb Architects, dated May 7, 2015.
Hepworth-Pawlak Geotechnical previously performed a subsoil study that included two
borings on the subject lot and submitted our findings in a report dated October 17, 2002,
under our Job No. 101 549.
The boulder retaining walls will retain cut along the uphill side and a limited area of fill
on the downhill side of the proposed building. The cut walls will be one and two tiers up
to about 8 feet high. The fill wall will be a single tier up to 6 feet in height. The near
surface subsoils encountered at the site consisted of clayey sand to sandy clay with
scattered rock fragments underlain a relatively shallow depths by hard, sandstone
bedrock. No free water was encountered in the borings when drilled in September 2002.
No indications of slope instability were observed on the hillside to be retained by the
boulder walls when we recently observed the site.
A two tier boulder retaining wall up to about 8 feet total height to retain the hillside cut
and a single tier wall up to 6 feet in height to retain cut or fill should be feasible with
proper design and construction and provided the walls are designed as gravity retaining
structures with adequate section for stability. The individual tiers should be limited to
about 4 feet for the two tier walls exposed height. The walls should be provided with an
underdrain system to prevent buildup of hydrostatic pressure. A typical detail for a two
tier boulder wall section is attached as Figure 1. For a one tier boulder wall, the
individual wall section shown on the detail should be used with a maximum exposed
height of 6 feet.
Parker 303-841-7119 • Colorado Springs 719-633-5562 • Silverthorne 970-468-1989
KH Webb Architects
May 29, 2015
Page 2
The boulders should be subangular shape(not rounded) and about 2 to 4 feet in diameter
with the larger boulders typically placed at the bottom of the wall. The base width of the
walls should be a minimum 2/3 the wall height. The base of the walls should be keyed
below the ground surface at least 1 foot. Loosened subgrade soils should be moistened
and compacted prior to the boulder placement. Fill placed below the upper tier wall can
consist of the on-site soils, excluding topsoil and oversized rocks, or a road base
compacted to at least 95% standard Proctor density at a moisture content within about 2%
of optimum. The face of the walls should be battered back at about 1/2 (horizontal) to 1
(vertical). An underdrain as shown on the detail should be provided behind each wall
tier. A filter fabric, such as Mirafi 140N, 160N or a geosynthetic drainage mat with filter
fabric such as Miradrain G200N, should separate the on-site soils from the boulders. The
space between the boulders can be filled with 3/ inch drain rock if desired. Graded slopes
between the wall tiers and above the walls should be 2 horizontal to 1 vertical or flatter
and revegetated to prevent erosion. If groundwater seepage is encountered in the
excavation for the walls, we should be contacted to review the design parameters prior to
the wall construction.
We should observe construction of the boulder walls on a regular basis to evaluate
compliance with our recommendations.
If you have any questions or need further assistance, please call our office.
Sincerely,
HEPWORTHp ,AK GE_QTCHNICAL, INC.
voiegfuilim
• • ' • a♦
t s
-.
cSY r•
David A. Young, P.E S.2_ 324164/41/41
DAY/ksw ♦ice + ....•• ��o`er
OWAL'�-����
attachment Figure 1, Typica���� `1'ier Boulder Retaining Wall Detail
Job No. 115 211A
CCc Piech