HomeMy WebLinkAbout10.0 Geologic Evaluation CTLThompsonGEOLOGIC EVALUATION
SPRING VALLEY RANCH P.U.D.
GARFIELD COUNTY, COLORADO
Prepared For:
SPRING VALLEY DEVELOPMENT, INC.
411 East Main Street, Suite 206
Aspen, CO 81611
Job No. GS -2936
March 13, 2000
CTL/THOIVIPSON, INC.
CONSULTING ENGINEERS
234 CENTER DRIVE • GLENWOOD SPRINGS, COLORADO 81801 • (970) 945-2809
TABLE OF CONTENTS
INTRODUCTION 1
SITE DESCRIPTION 1
Lower Area 2
Ranch Lots Area 2
PROPOSED DEVELOPMENT 3
Lower Area 3
Ranch Lots Area 4
GEOLOGIC SETTING AND STRUCTURAL GEOLOGY 4
GEOLOGIC CONDITIONS 5
Bedrock Units 6
Surficial Deposits 7
POTENTIAL GEOLOGIC HAZARDS AND DEVELOPMENT CONSIDERATIONS 10
Potentially Unstable Slopes 11
DebrislMud Flows 12
Rockfall 13
Ground Subsidence 13
Radiation 15
Regional Seismicity 15
Development Plan Review 16
LIMITATIONS 18
FIGURES 1 AND 2 - GEOLOGIC MAP
FIGURES 3 AND 4 - GEOLOGIC HAZARDS MAP
FIGURE 5 - GENERALIZED STRATIGRAPHIC COLUMN
TABLE 1 - LOTS IDENTIFIED WITH BUILDING ENVELOPES LOCATED IN
GEOLOGIC HAZARD AREAS
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INTRODUCTION
The purpose of this report is to define general geologic conditions, identify
potential geologic hazards and establish preliminary hazard mitigation concepts for
the Spring Valley Ranch P.U.D. in Garfield County, Colorado. The information
presented herein is intended to assist the client in development of the property
during the master planning phases. Preliminary foundation recommendations, other
geotechnical criteria and recommendations and construction criteria for grading and
infrastructure installation are presented in our Preliminary Geotechnical
Investigation (CTL/Thompson, Inc. Job No. GS -2956, dated March 1, 2000). This
report was prepared based on a review of available published geologic literature and
maps, analysis of aerial photographs, reconnaissance level geologic field mapping
of the site and our experience.
SITE DESCRIPTION
Spring Valley Ranch P.U.Q. is an approximately 6,000 acre parcel above and
to the northeast of the Roaring Fork River Valley in the central part of western
Colorado. The City of Glenwood Springs and confluence of the Roaring Fork and
Colorado Rivers are approximately 5 miles to the northwest. Topographically, the
property extends from higher ground to the northeast with elevations ranging up to
approximately 9600 feet down to lower areas on the floor of Spring Valley to the
southwest with elevations of approximately 6900 feet. Ground surfaces generally
"stair step" down to the southwest via a series of topographic "benches" that trend
northwest -southeast. Ground surfaces of the top of the "benches" slope at
comparatively gentle to moderate gradients with the ground surface slopes between
the benches sloping at moderate to steep gradients. This general "stair step"
topography has been deeply incised by Landis Creek and numerous intermittent
drainages that generally trend down to the southwest.
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The Lower Area is located on the lower elevations in the southwest and west
portion of the ranch property. The Ranch Lots Area is located at higher elevations
northeast of the Lower Area. The following paragraphs describe the Lower Area and
Ranch Lots Area.
Lower Area
The LowerArea can be visualized topographically as the floor of Spring Valley
and an upper "bench" area separated by approximately 500 to 700 vertical feet of
moderate to steep southwest facing slopes. The long dimension of the bench area
is in a northwest -southeast direction generally parallel to the gently rolling floor of
Spring Valley. The upper bench is a moderately rolling surface with gently sloping
meadow areas and low lying basins separated by low ridges. Topographic relief on
the upper bench is approximately 200 vertical feet. The northeast boundary of the
area is along the toe of steep slopes that extend up to the northeast approximately
1,200 to 1,400 vertical feet. Vegetation on the Lower Area consists of open meadows
with grasses and weeds and scattered areas of oak brush on [ow lying areas and on
the floor of Spring Valley. Much of the flatter ground has been cleared of brush and
cultivated. Sloping ground is generally vegetated with sage brush, grasses and
weeds with areas of dense scrub oak.
Ranch Lots Area
The Ranch Lots Area is adjacent to and above the Lower Area to the
northeast. Topographically, the Ranch Lots Area can be visualized as an upper
terrace to the northeast which "stair steps" down to the Lower Area over a series of
benches that trend northwest -southeast and generally slope down to the southwest.
Ground surface elevations vary from approximately 7,600 feet to approximately 9,600
feet. At lower elevations vegetation consists of sage brush, scrub oak and some
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pinion/juniper with an undergrowth of grasses and weeds. At higher elevations
vegetation grades into aspen and conifer forestation with broad areas of open
meadow grasses, weeds and wild flowers.
PROPOSED DEVELOPMENT
Lower Area
An existing high voltage electric transmission line has been relocated along
the northeast boundary of the Lower Area. Approximately 10 miles of interior
roadways are planned. Water and sewer systems will be centralized.
We understand plans are to develop the area with a 36 hole golf course and
single family and multi -family residential units. The golf course and 171 "Golf Lots",
varying from 1 to 3 acres each will be on a large bench area at lower elevations
occurring in the central part of the ranch. The golf course will include a clubhouse
of approximately 40,000 square feet. At the west end of the ranch, 134 "Estate Lots"
ranging from 3to 11 acres each are planned. Each "Estate Lot" will have a building
envelope of 1.75 acres or less. Fifteen duplex buildings for a total of 30 dwelling
units will be on a separate Duplex Parcel. Seventy-five cooperative ownership
"Wilderness Cabins" are planned. The existing ranch facility will be designated as
an "Equestrian Estate". The southwest part of the site on the floor of Spring Valley
(approximately 597 acres) will be maintained as open space/irrigated pasture. A
Village Center, Firehouse and an Equestrian Center are planned in the southwest
part of the property. The Village Center, will be mixed use commercial/residential
including 75 "affordable housing" units.
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Ranch Lots Area
The Ranch Lots Area is to be developed with 91 lots ranging from 16 acres
to approximately 60 acres each. Each Ranch Lot will have a building envelope of 3
acres or Tess to be developed as a "homestead" which will likely include a main
residence, possiblya guest house and/or caretakers residence, barn and associated
out buildings. Access will be provided by constructing approximately 15 miles of
roadway. Utilities such as electric and centralized water will be provided. Sewer
service will likely be Individual Sewage Disposal Systems (ISDS).
Initial construction will be to install utilities and roadways. We have assumed
that residential buildings will be one or two stories tall and wood framed. Partial or
full basements were considered likely for most structures. We anticipate foundation
loads will be comparatively light and range from 1,000 to 3,000 pounds per lineal foot
of foundation wall. Interior column loads may be 15 to 30 kips. Road and utilities
will be constructed to provide access and services to the development. We
anticipate the clubhouse and Village Center buildings will be comparatively large
structures with moderate foundation loads.
GEOLOGIC SETTING AND STRUCTURAL GEOLOGY
Spring Valley Ranch is located in an area of complex regional geology at the
conjunction of several structural geologic elements. The White River Uplift is to the
north. The Sawatch Uplift is to the east, with the Elk Mountains to the south and the
Grand Hogback to the west. These are large scale features related to the continental
tectonic setting.
The degree of geologic activity at Spring Valley Ranch is primarily influenced
by a smaller scale localized feature, the Carbondale Collapse Center, The
Carbondale Collapse Center is a collapse feature with an aerial extent of
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approximately 200 square miles. Glenwood Springs, Carbondale and the lower part
of the Roaring Fork and Crystal River Valleys are within the boundaries of the
collapse feature. It is generally accepted that the collapse feature formed as the
result of evaporate minerals in the Pennsylvanian aged Eagle Valley Evaporite being
dissolved and removed by circulating ground water. Where this "dissolution"
process undermines overlying bedrock units and surficial deposits, collapse of the
overlying materials has resulted in ground subsidence.
Basalt Flows at, and in the vicinity of Spring Valley Ranch range in age from
approximately 3 million years to 22 million years old. Based on radiometric dating
by the Colorado Geologic Survey and elevation differences in numerous Quaternary
to Tertiary aged Basalt Flows, total subsidence in the Carbondale Collapse Center
has been estimated on the order of 3,000 to 4,000 vertical feet. Based on the
radiometric dating of basalts and salinity output of various hot springs in the
Roaring Fork and Colorado River Valleys, it is estimated that the subsidence has
occurred over a time span of at least 3.7 million years. Assuming 4,000 vertical feet
of uniform subsidence over a time span of 4 million years, theoretical subsidence
rates are on the order of 1 foot per 1000 years. Although the collapse mechanism is
active in a geologic time frame, in the context of time frames relevant to human
activity the collapse mechanism can be considered to have a low degree of activity.
GEOLOGIC CONDITIONS
Bedrock units at Spring Valley Ranch that influence geologic conditions
include the Pennsylvanian aged Eagle Valley Evaporite and Eagle Valley Formation,
the Pennsylvanian to Permian aged Maroon Formation and Quaternary to Tertiary
aged Basalt Flows. These bedrock units have been fractured and faulted by regional
collapse originating in the Eagle Valley Evaporite. The Maroon Formation and Basalt
Flows are nearest to the ground surface and are the only bedrock units that outcrop
over most of the site. The Eagle Valley Formation is exposed at the ground surface
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in a comparatively small area at the northwest corner of the property. A generalized
stratigraphic column showing the sequence of geologic units is shown on Figure 5.
Bedrock Units
The Eagle Valley Evaporite is a heterogeneous rock unit consisting mostly of
evaporite minerals interbedded with siltstone, sandstone and limestone. Under
lithostatic pressure the evaporite material undergoes plastic flow deformation which
has resulted in a high degree of bending and contorting of the original generally
horizontal bedding. The result has been a rock unit with random pockets of highly
soluble evaporite minerals intermixed with pockets of low solubility sandstone,
siltstone and limestones. Where the soluble minerals are overlain by consolidated
material (bedrock, basalt flows, etc.), small to large blocks have been down faulted
by collapse into the voids. Where the soluble minerals are overlain by
unconsolidated material (stream gravels, soils, etc.), sinkholes can occur. Depending
on the amount of soluble minerals, ground water circulation and strength and
thickness of overburden materials, varying amounts of surface subsidence has
occurred. In the locale, the amount of circulating groundwater which "drives" the
subsidence mechanism is much greater in the Roaring Fork River Valley
approximately 2 miles from Spring Valley Ranch. We believe that the degree of
potential activity of the Carbondale Collapse Center is lower at Spring Valley Ranch
than in Roaring Fork River Valley.
The Eagle Valley Formation (map unit Pev) is a transitional unit that contains
increasing amounts of siltstone, sandstone and limestone interbedded with
decreasing amounts of evaporite beds.
The Maroon Formation (map unit P -Pm) consists of interbedded
conglomerates, sandstones, siltstones and claystones. Maroon Formation outcrops
at the northwest part of the Lower Area and at numerous locations in the north part
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of the Ranch Lots Area. Characteristic of the Maroon Formation and its derived
surficial deposits is a red color due to oxidized iron content. Much of the surficial
deposits to the north of Landis Creek on the Lower Area and north part of the Ranch
Lots Area appear to have been derived from the Maroon Formation.
The most widespread rock outcrop at the site are the Basalt Flows (map unit
QTb). Basalt Flows appear to have occurred sporadically since late Tertiary time,
throughout formation of the Carbondale Collapse Center. The Basalt Flows appear
to occur over down faulted blocks of Maroon Formation and may be interlayered with
surficial deposits.
Surficial Deposits
Geologic conditions at Spring. Valley Ranch have produced a varied and
complex assortment of Quaternary aged surficial deposits. The development of the
surficial deposits has been primarily controlled by the collapse process and
simultaneous weathering, erosion and mass wasting.
During collapse, small to large blocks of bedrock were down faulted along
steeply dipping faults that probably terminate at depth in the Eagle Valley Evaporite.
This faulting has reduced some of the bedrock to rubble and generated an
underlying "stair step" structure with down faulted basins. Several mechanisms
have interacted to develop the present day surface topography. Throughout the
down faulting process, weathering and erosion have acted on the Maroon Formation,
Basalt Flows and their rubblized derivatives to produce deposits of slopewash,
colluvium (map unit Qc) and alluvium. Colluvium grades into slope failure
complexes (map unit Qsfc) where the colluvium has been draped over down faulted
bedrock blocks. Slope failure complex deposits are along the east parts of the
Lower Area and Ranch Lots Area. Some areas have been identified as landslides
(map unit Qls) and rockslides (map unit Qrs). Within down -faulted basins and along
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major drainages, alluvium and colluvium are interfingered producing undifferentiated
deposits (map unit Qac). Lacustrine deposits (mapped unit Qlc) have accumulated
in some of the down faulted basins. Debris flow fans are found at the mouths of
several drainages. Some of the debris fans are recent and may be active (map unit
Qdf), other debris fans are ancient and appear dormant (map unit Qdfo). Interpreted
geologic units are shown on the Geologic Map, Figures 1 and 2 . Surficial deposits
are described below.
Map Unit Description
Qc Quaternary aged colluvium consisting of silty to clayey gravels
and cobbles with boulders and blocks. Some deposits contain
blocks of bedrock. Generally gray and brown where derived
from Basalt Flows and reddish where derived from Maroon
Formation. Includes talus, vegetated talus and boulder fields.
Generally occurs on and along the base of slopes and below
outcrops. Colluvial deposits grade into bedrock at outcrops
and into slope failure complexes in down faulted areas.
Qac
Qdf
Quaternary aged alluvium and colluvium (undifferentiated).
This unit consists of colluvium interbedded and interlayered
with fine to coarse grained alluvium. The colluvial component
is described above. The alluvium consists of stratified layers
of silts, sands and gravels. Generally occurs in the lower parts
of down faulted basins and along drainage floors.
Quaternary aged debris fan (young and potentially active).
Stratified deposits made up of silty to clayey sand and gravel
with cobbles and boulders. Deposits tend to contain higher
amounts of larger material near the upper parts of the fan and
are increasingly finer grained toward the distal end of the fan.
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Qdfo
Qsfc
QIc
Qls
The fans are potentially active and subject to periodic alluvial
events ranging from flash floods to mud flows and debris flows.
Quaternary aged debris fan (old and apparently dormant).
Similar to the active debris fan deposits (Qdf), however, they
have been down cut by their drainages or their drainage basins
have been changed by erosion and deposition such that they
are no longer subject to alluvial events. The mapped debris fan
at the northwest part of the site appears to consist of sands
and clays and may be a delta that developed in an ancient lake.
Quaternary aged slope failure complex. This unit has similar
composition to colluvium (Qc) but has moved as a result of a
complex system of slumps, landslides and debris avalanches.
Slope failure complex deposits appear stable, however, some
areas may be near stabilitylfailure equilibrium. Slope failure
complex areas should be evaluated for slope stability prior to
construction.
Quaternary aged lacustrine deposits. These deposits
accumulated in down -faulted basins that contained ancient
lakes. Lacustrine deposits consist of interlayered clays, silts
and sands. Some layers may contain significant amounts of
organics.
Quaternary aged landslide deposits. This unit is similar in
composition to colluvium (Qc) and slope failure complex (Qsfc)
deposits. These deposits have an identifiable lobate boundary
and evacuation area. Depending on site specific conditions
these deposits may have a higher or lower degree of slope
stability than surrounding areas.
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Qrs
QTb
Quaternary aged rock slide deposits. This unit is a special case
of landsliding in which the failing material is coarsely granular
colluvium, the result of bedrock that rubblizes during failure.
The deposit consists of cobbles and boulders with little matrix
material and resembles a talus or boulder field.
Quaternary to Tertiary aged Basalt Flows. This unit occurs as
bedrock, however, in places it likely overlies and is interlayered
with ancient surficial deposits. We believe some of the lobate
surface topography that has been interpreted by others as
landslides and slope failure complexes may actually be Basalt
Flows that occurred over terrain that exhibited topographic
relief at the time the Basalt Flows occurred.
POTENTIAL GEOLOGIC HAZARDS AND DEVELOPMENT CONSIDERATIONS
Our geologic study identified several geologic conditions that need to be
considered during the planning and development phases of this project. The
geologic conditions identified will not prevent the development of the property for
the intended uses, however, geologic hazards must be planned for and mitigated
during the proposed phases of site development. Interpreted geologic hazards are
shown on Figures 3 and 4.
Geologic hazards at the site include potentially unstable slopes, debris/mud
flows, rockfall and ground subsidence. Other concerns related to geologic
conditions include regional issues of radiation and seismicity. Concerns related to
the engineering characteristics of soils include soil consolidation and expansion
and are addressed in our Preliminary Geotechnical Investigation (CTLlThompson Job
No. GS -2956, dated March 1, 2000). The following sections discuss geologic hazards
and development concerns.
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Potentially Unstable Slopes
The most widespread potential geologic hazard identified at the site is
potentially unstable slopes. Erosion has deeply incised slope failure complex
deposits and other surficial deposits which suggest stability during the last several
thousand years. In our opinion, most slope failure deposits are ancient, dormant and
are stable. We have delineated potentially unstable slopes into two qualitative
categories, Pus, and Pus2. Potentially unstable slope areas are generally designated
as slopes steeper than approximately 30 percent. Where the slopes are underlain by
slope failure complex deposits (Qsfc), we believe it appropriate to consider the
slopes to posses comparatively less inherent stability. Those slopes are mapped as
Pus, on the Geologic Hazards Map. Where the steeper slopes are not underlain by
slope failure complex deposits or the slope failure complex deposits appear to be
underlain by comparatively large blocks of intact bedrock, we believe it appropriate
to consider the slopes to posses comparatively greater inherent stability and are
mapped as Pus2.
Within the Ranch Lots Area, three landslides, two located adjacent to the
northwest of Landis Creek and one at the northwest corner of the site exhibited
evidence of recent and possibly active movement such as "pistol grip" deformation
of tree trunks and several small, bare ground "scarp like" areas that could be related
to slope failure. If development will be in these areas then detailed geotechnical
investigations will be needed to address slope stability prior to each construction
phase. Construction which disturbs the landslides will need to be evaluated on a
case by case basis to ensure that existing slope stability is not adversely effected.
Development in these areas may require mitigation such as slope retainage systems
and subsurface drainage systems as each phase of construction occurs.
From the standpoint of slope stability, development will need to focus on
enhancing and not decreasing the existing slope stability and possibly avoiding
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some unstable areas that may be identified during more detailed geologic
evaluations. Common methods of enhancing slope stability include:
1. Avoiding or minimizing removal of the natural soils at the base of
slopes that would remove materials supporting the natural slopes and
therefore reduce the stability of these slopes;
2. Avoiding or minimizing fills at the tops of the slopes that would add
weight and therefore increase driving forces that could reduce stability
of the natural slopes;
3. Improving site drainage to reduce water infiltration into the ground at
questionably stable areas which would reduce total shear strength and
"lubricate" slope materials; and
4. Where development will encroach on areas near stability/failure
equilibrium, proactive mitigation such as slope retainage (soil nails,
tieback anchors, retaining walls) and/or subsurface drainage systems
can be used to increase hillside stability.
Slope stability concerns can be effectively mitigated by considering the
effects of siting of structures, site grading and surface drainage during the planning
of each development phase.
Debris/Mud Flows
Debris/mud flow hazards are limited to debris fans (Qdf) and drainages. In
theses areas surface runoff is concentrated into narrow drainages where turbulent
flow can be confined, allowing sediment loads to achieve sufficient concentrations
to produce debris/mud flows. The more significant potential hazard is associated
with Landis Creek and several of its tributary drainages. A second area of potential
hazard is associated with the drainage at the south end of the site. Other areas of
potential hazard are along the east part of the Lower Area along the base of the
Ranch Lots Area. In our opinion, the potential debris flow hazard can be mitigated
by incorporating into the site drainage plan a combination of detention structures,
conveyance channels and energy -dissipating or impact/diversion structures.
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Roadways and infrastructure sited in the hazard area will likely require a low degree
of mitigation. Residences sited in hazard areas should be evaluated for the potential
debris/mud flow hazard on a site specific basis prior to construction.
Rockfall
Rockfall hazards are generally limited to the sides of the Landis Creek
drainage and below rock outcrops along and above the east boundary of the Lower
Area. We observed some boulders that have been dislodged by construction of
access roads. Individual site specific geologic hazard evaluations should address
natural as well as man made rockfall hazards for building sites that are below
roadways. We qualitatively rate the overall degree of rockfall hazard as low to
moderate. If rockfall areas cannot be avoided then mitigation will be needed.
Mitigation may consist of removal or stabilization of the rockfall source by scaling,
rock bolting, grading or construction of impact structures such as mechanically
stabilized earth walls or catchment structures. Rockfall mitigation design should be
based on detailed site specific rockfall hazard evaluations prior to each construction
phase.
Ground Subsidence
We did not observe any sink holes during our field reconnaissance. We
observed some low, basin areas that appear to be between down faulted blocks of
bedrock. Approximate fault locations are shown on the Geologic Map (Figures 1 and
2) and Geologic Hazards Map (Figures 3 and 4). We consider the area defined as
extending 250 horizontal feet away from the faults shown on our maps to be the
Potential Fault Influence Zone. These faults have likely moved in Quaternary time
(within the last 2 million years). We believe the possibility of fault movement at
Spring Valley Ranch to be low. The possibility of movement of the faults can be
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reduced by limiting the amount of water that could infiltrate the fault planes and
provide lubrication. Irrigation ditches or other waterways located within 250
horizontal feet of the faults should be Tined. All ponds at Spring Valley Ranch should
be lined with a man made, synthetic, impermeable liner to reduce the probability of
infiltration which could adversely effect the collapse mechanism and initiate fault
movement. Irrigation pipes should be constructed to automatically shut off should
a break occur. Surface drainage should be provided to prevent surface ponding and
cause rapid run off away from fault areas.
In our opinion, any regional collapse activity will be most evident across the
faults. We reviewed seismic data recorded by the United States Geologic Survey
(USGS) from the year 1960 to present for a 200 kilometer radius area that includes
the entire Carbondale Collapse Feature and surrounding area. The data indicates
that earthquake activity nearest to Spring Valley occurred south of the Town of
Carbondale. The earthquakes varied in magnitude from 2.1 to 3.2 on the Richter
Scale. The earthquakes were rated as very minor to minor by the USGS. No
significant damage to modern buildings is known to have occurred due to
earthquakes or fault movement in the area. As discussed in the seismicity section
below, a review of the Colorado Geologic Survey (CGS) Earthquake Potential in
Colorado (Bulletin No. 43, dated 1981) does not identify any potentially active faults
on the Spring Valley Ranch P.U.D. site. The faults at Spring Valley Ranch P.U.D. are
likely more complex than the approximate locations shown on our maps. We
envision the faults to be a complex array of shears and micro faults that result in a
fault zone on the order of 10's of feet in width. The potential for fault movement at
Spring Valley Ranch P.U.D. is similar to other developments in the area. Other
development has resulted in buildings and infrastructure over similar faults in the
locale such as the Colorado Mountain College Spring Valley Campus and has not,
to our knowledge, experienced any damage due to fault movement. In our opinion,
it is not unreasonable and is appropriate to site buildings in potential fault influence
zones at Spring Valley Ranch P.U.D. For buildings sited within 250 horizontal feet
of the faults, it may be appropriate to increase the steel reinforcement in
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foundations and foundation walls for buildings. The amount of reinforcement
needed should be determined by a structural engineer based on criteria presented
in site specific soils and foundation investigations.
Radiation
Elevated concentrations of radioactive materials are not generally associated
with the geologic conditions that occur at Spring Valley Ranch P.U.D.. We
performed a radiation survey by spot screening widely spaced locations across the
Lower Area for gamma radiation. Measurements were taken with a Ludlum
Instruments, Model 19, Micro -R -meter carried at arms length approximately 2 feet
above the ground surface. Our readings ranged from approximately 10 to 17 micro
Roentgens per hour. In our opinion, these levels of radiation are consistent with
normal background radiation in the area.
Regional Seismicity
Spring Valley Ranch P.U.D. is located in the Western Mountain Seismotectonic
Province. A group of faults in the Grand Hogback to the west have displaced
Quaternary aged pre -Bull Lake (at least 100,000 years old) glacial deposits and two
faults to the south in the Cattle Creek area have displaced Tertiary aged intrusives.
In addition, faults at Spring Valley Ranch P.U.Q. related to the Carbondale Collapse
Center have displaced Quaternary aged surficial deposits (up to approximately 2
million years old). In this time frame these faults are considered to be potentially
active. We believe the possibility of fault movement in a time frame relevant to
human activity to be low. In our opinion, the faults at the site are comparatively
shallow and probably bottom in the Eagle Valley Formation. We believe the faults
are not deep seated faults penetrating the basement complex that are related to a
continental tectonic environment or capable of large movements. Historic
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earthquakes have not observed that were caused by movement of the faults at
Spring Valley Ranch P.U.D.
One historic earthquake centered in the Glenwood Springs area west of the
site exhibited Modified Mercalli Intensity of V. A second earthquake with its
epicenter located between Glenwood Springs and Cattle Creek had a magnitude in
the range of 4.0 to 4.9. Several earthquakes have occurred with epicenters located
in the Thompson Creek area south of Carbondale with magnitudes of up to 4.9. In
our opinion, ground accelerations associated with an earthquake intensity of VI can
be expected at this site. An intensity VI earthquake would be felt by all and may
result in cracking of weak masonry structures but would cause negligible damage
to structures of good design and construction. The region is in the Uniform Building
Code Seismic Risk Zone 1, however, the Colorado Geologic Survey recommends
considering all of Colorado except the northeast corner of the state to be in a
Seismic Risk Zone 2. The potential for an earthquake of sufficient intensity to cause
significant damage to structures during a design serviceability of approximately 100
years at Spring Valley Ranch P.U.D. is low. Structures in the development should be
designed to withstand moderately strong ground shaking with little or no damage.
We recommend anticipating horizontal ground accelerations on the order of 0.035G
to 0.07G in the design of structures to be built at Spring Valley Ranch P.U.D.
Development Plan Review
We evaluated a development plan showing 538 building envelopes. Based on
our preliminary mapping we have identified 36 lots with building envelopes located
in geologic hazard areas on which we recommend site specific evaluations to
address geologic hazards and develop mitigation recommendations (see Table 1).
It should be emphasized that geologic hazard mapping indicates areas of potential
hazards. Identification within a geologic hazard area does not necessarily indicate
that a condition exists that will result in damage to buildings or that mitigation will
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be needed. Site specific evaluations will more accurately delineate hazard
boundaries and better define the degree of potential for damage and determine if
mitigation is warranted. Site specific evaluations should be performed during each
development phase, as homes or structures are planned, prior to construction.
Our geologic map identifies geologic conditions and is not intended to
address the influence of hazardous geologic conditions on the proposed
development. Our geologic hazards map was developed considering geologic
conditions identified on our geologic map. Our geologic hazard map relates existing
geologic hazards to the proposed development plan. Proposed building envelopes,
roads alignment and other development feature locations should consider
information shown on our geologic hazards map.
Parts of the proposed road alignments are located in areas mapped as
potentially unstable slopes. The majority of the proposed roadways have been
pioneered. We have observed the road cuts made to pioneer the roads and have not
observed any evidence of slope failure due to pioneering of the roads. The proposed
road alignments appear feasible and appropriate from an engineering geology and
geotechnical viewpoint. We recommend the effect of road building in areas of
geologic hazards be evaluated as part of geotechnical investigations for roadway
design prior to each construction phase. Some sections of the proposed alignments
may require mitigation of slope stability concerns or adjustments to road alignments
because of existing geologic hazards.
For road alignments in potentially unstable slope hazard areas, we believe
mitigation may involve one or a combination of the following:
1. Limiting excavation depths to a maximum of approximately 5 to 10 feet
deep;
SPRING VALLEY DEVELOPMENT, INC.
SPRING VALLEY RANCH P.U.D.
CTUT GS -2936
17
2. Laying back excavated slopes to a maximum inclination of 2 to 1 or
flatter (horizontal to vertical);
3. Retaining excavations into potentially unstable slopes with soil nails,
tieback anchors or conventional concrete retaining walls; and/or
4. Installation of subsurface drainage systems.
Our preliminary geotechnical investigation (Job No. GS -2956, dated March 1,
2000) discusses general engineering characteristics of the various soils and bedrock
and provides preliminary geotechnical recommendations for site grading, slope
retainage, foundations and infrastructure.
LIMITATIONS
The discussion of geologic conditions presented herein is generalized and
preliminary. Geologic mapping and delineation of potential hazards is an
interpretation based on what could be seen during our field study. More detailed
studies need to be performed as development plans become finalized to better define
geologic conditions, develop mitigation recommendations and develop geotechnical
criteria for the actual construction. We need to be informed as plans are
conceptualized and further developed to allow continued geologic and geotechnical
input. We recommend detailed, site specific geologic hazard evaluations where
development will be in potential geologic hazard areas prior to each construction
phase. Further, we recommend that detailed, site specific geotechnical
investigations be performed for buildings and roadways to be built in this
development prior to each construction phase.
This investigation was conducted in a manner consistentwith the level of care
and skill ordinarily exercised by engineering geologists and geotechnical engineers
currently practicing under similar conditions in the locality of this project. No other
warranty, express or implied, is made.
SPRING VALLEY DEVELOPMENT, INC.
SPRING VALLEY RANCH P.U.O.
CTLJT GS -2936
18
We appreciate the opportunity to work with you on this project. If we can be
of further service or if you have questions regarding this report, please call.
CTL/THOMPSON, INC.
Wilson L. "Liv" Bowden, C
Engineering Geologist
(6 copies sent)
SPRING VALLEY DEVELOPMENT, INC.
SPRING VALLEY RANCH P.U.D.
CTLJT GS -2936
19
QUATERNARY
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Job No. GS -2936
SPRING VALLEY RANCH
EXPLANATION:
• ...... .
lc+
LJ
SURFICIAL DEPOSITS
(COLLUVIUM, ALLUVIUM, etc.
BASALT FLOWS
SILTSTONE
SANDSTONE
CLAYSTONE
CONGLOMERATE
EVAPORITE
(GYPSUM, HALITE)
IRREGULAR BOUNDARY,
INDICATES RELATIVE
RESISTANCE TO EROSION
AND WEATHERING
Generalized
Stratigraphic
Column
Flg. 5
TABLE 1
LOTS IDENTIFIED WITH
BUILDING ENVELOPES LOCATED IN GEOLOGIC HAZARD AREAS
tl
................................
OivThents•
G-89
Pus,
G-90
Pus,
G-94
Pus,
G-95
Pus,+ Rxfl
G-96
Pus,+ Rxfl
G-97
Pus,+ Rxfl
6-104
Pus,
6-106
Dbf,
6-107
Pus,+ Dbf,
G-108
Dbf,
G-109
Dbf, + Pus,
G-110
Pus,
G-111
Pus,
G-115
Pus,
G-116
Pus,
G-117
Pus,
G-118
Pus,
E-7
Pus,
E-16
Pus,
E-17
Pus,
E-18
Pus,
E-53
Pus,
E-55
Pus,
E-72
Pus,
E-74
Pus,
Recommend site specific geologic hazard
evaluation.
Explanations of geologic hazards symbols are shown on the attached Figures
3 and 4 and discussed in this report.
SPRING VALLEY DEVELOPMENT, INC.
SPRING VALLEY RANCH P.N.D.
CTLJT GS -2936
TABLE 1
LOTS IDENTIFIED WITH
BUILDING ENVELOPES LOCATED IN GEOLOGIC HAZARD AREAS
(Continued)
aolcgtc-tfzarils'
E-75
Pus, + Rxfl
E-79
Pus,
E-84
Pus,+ Dbf,
E-89
Dbf,
R-8
Dbf,
R-11
Pus,
R-12
Pus,
R-13
Pus,
R-27
Pus,
R-68
Pus,
M-1
Dbf,
Recommend site specific geologic hazard
evaluation.
Explanations of geologic hazards symbols are shown on the attached Figures
3 and 4 and discussed in this report.
SPRING VALLEY DEVELOPMENT, INC.
SPRING VALLEY RANCH P.U.D.
CTL!T GS -2936