HomeMy WebLinkAbout800587.tiff RESOLUTION
RE: APPROVAL OF GEOLOGIC HAZARD DISTRICT DEVELOPMENT PERMIT -
DALE HARRINGTON.
WHEREAS, the Board of County Commissioners of Weld County,
Colorado., pursuant to Colorado statute and the Weld County Home
Rule Charter, is vested with the authority of administering the
affairs of Weld County, Colorado, and
WHEREAS, Dale Harrington has requested a Geologic Hazard
District Development Permit for the following described real
estate:
Part of the North Half of Section 12,
Township 1 North, Range 68 West of the
6th P.M. , Weld County, Colorado.
WHEREAS, the Board of County Commissioners has studied the
testimony and statements of all of those present, has studied
the recommendations of the Colorado Geological Survey and the
recommendations of the Department of Planning Services and having
been fully informed, is satisfied that the request is in com-
pliance with the purpose and intent of the Weld County Geologic
Hazard District Regulations, and
WHEREAS, the Board of County Commissioners finds that the
request of Dale Harrington for a Geologic Hazard District Develop-
ment Permit should be approved conditional upon the applicant
submitting the construction and design criteria for structures,
improvements and utilities with the application for final platting
of the subdivision.
NOW, THEREFORE, BE IT RESOLVED by the Board of County Com-
missioners of Weld County, Colorado that the request of Dale
Harrington for a Geologic Hazard District Development Permit
for the abovedescribed real estate be, and hereby is, approved
conditional upon the applicant submitting the construction and
design criteria for structures, improvements and utilities with
the application for final platting of the subdivision.
_ .. _.. 800587
! ,,
A-0`SCI
The above and foregoing Resolution was , on motion duly made
and seconded, adopted by the following vote on the 11th day of
June, A.D. , 1980.
BOARD OF COUNTY COMMISSIONERS
- WELD COUNTY, COLORADO
'*/!�yy (Aye)
C. W. Kir y, CHairman
- 1 (Aye)
L onard L. Roe, Pro-Tem
//
C.e+ftf.-_ (Aye)
Norman Carlson"�y�(�i /Jt�l/ (Aye)
a un ar
ATTEST: //JJ .JL(� �'` frJ�al (Aye)
ll.vA;us- ctin/ ne K. Steinmark
Weld County Clerk and Recorder
and clerk to the Board
Bt: (e,Lt-
(;D putt' Co ntyClerk
APPR@�ED;AS 2;0 FORM:
County Attorney
DATE PRESENTED : JUNE 16, 1980
DEPARTMENT OF PLANNING SERVICES
PHONE 1303)3564000 EXT.404
915 10TH STREET
GREELEY,COLORADO 80631
June I I , 1980
COLORADO
Board of County Commissioners
Weld County, Colorado
915 IOth Street
Greeley, Colorado 80631
Re: Request for approval of Geologic Hazard District Development Permit
Dear Commissioners:
The attached report, letters and maps are in reference to a request by
Dale Harrington for approval of a Geologic Hazard District Development
Permit on property described as Part of the NI, Section 12, TIN, R68W
of the 6th P.M. , Weld County Colorado. The property is located I mile
west of the southern boundry of the town of Dacono.
Mr. Harrington has applied for a change of zone on a 5 acre parcel which
is a portion of this request. The remainder of the property is zoned
Industrial and Mr. Harrington has applied to subdivide the total parcel .
The Geologic Hazard Report by Chen and Associates outlines several alter-
natives for development of the property. Review of this report by the
Colorado Geological Survey and Weld County Health Department resulted in
the following recommendation.
I . Criteria for the design and construction of structures shall
be developed jointly by Chen and Associates and the Colorado
Geological Survey to be incorporated into the final plat
2. Sewage disposal shall be engineer designed septic systems or
a lagoon system with back-up systems in the event of failure.
Based upon the information submitted and the adopted policies in the County,
the Department of Planning Services Staff recommends the request be approved
for the following reasons:
Board of County Commissioners
June II , 1980
Page 2
I . The Colorado Geological Survey feels that the adverse effects
of possible subsidence can be minimized by proper design and
construction.
2. It is the opinion of the Department of Planning Services Staff
that the request is in compliance with the purpose and intent
of the Weld County Geologic Hazard District Regulations, which
state in part, that the adverse impacts of geologic hazards on
life and property shall be reduced by providing for geologic
hazard investigation and mitigation of adverse impacts of such
hazards at the time of initial construction.
The Department of Planning Services recommendation is conditional upon the
applicant submitting the construction and design criteria for structures,
improvements and utilities with the application for final platting of the
subdivision.
Respectfully,
TI
Vickie Traxler
Assistant Zoning Administrator
VT:rg
SECTION II
GEOLOGIC HAZARD AREAS
A. Purpose and Intent ,
The Board of County Commissioners finds that there are, within the
County of Weld, various areas subject to unstable geologic conditions
which may cause serious damage to properties and subject the safety
of residents of such areas to hazards, and that the imprudent use and
occupation of these areas will pose a continuing danger to life and
property, unless appropriate land use measures are implemented concerning
the use and occupation of such hazardous areas . The .2E 22!e and intent
of these Geologic Hazard Regulations shall be to:
1 . Minimize hazards to public health and safety or to property in
regulated geologic hazard areas ;
2. Promote safe use of geologic hazard areas;
3. Reduce the adverse impact of geologic hazards on life and property by;
. a. Requiring land uses permitted in geologic hazard areas to be
protected from geologic hazards by providing for geologic
hazard investigation and the mitigation of the adverse impacts
of such hazards at the time of initial construction;
b. Regulating the manner in which structues designed for human
occupancy may be constructed so as to prevent danger to human
III life or property within such structures ;
4. Protect the public from the burden of excessive financial expend-
itures caused by damage from geologic hazards by regulating land
uses within geologic hazard areas.
I 9
I
,rair � o
S ,W o coAty, op U7)(
Norma N.se
rem z90
RICHARD D. LAMM * ,\L. * * JOHN W. ROLD
GOVERNOR yrt Director
1876
COLORADO GEOLOGICAL SURVEY
DEPARTMENT OF NATURAL RESOURCES
715 STATE CENTENNIAL BUILDING-1313 SHERMAN STREET
DENVER,COLORADO 80203 PHONE (303)839.2611
June 2, 1980
Ms. Vickie Traxler
Weld County Department of Planning Services
915 10th Street
Greeley, CO 80631
Dear Ms. Traxler: RE: PANORAM INDUSTRIAL PARK
WELD COUNTY
Regarding our conversation concerning Panoram Industrial Park on May 29, 1980;
we feel that adverse effects of possible subsidence can be minimized by
proper design and construction; however, there is some potential for structural
damage to buildings constructed in the area.
This factor as well as minimum building design criteria and individual owner
options will be submitted to you for incorporation into notes to be added to
the final plat. These notes will be submitted once the Chen report has been
reviewed by the Colorado Geological Survey.
If we can be of further assistance, please contact our office.
Sincerely,
Julia E. Turney
Engineering Geologist
:'tC
5\, ` 6>
JUN 1580
RECEIVED
Wild Coal ^{
en Mailing Commission �4�J
GEOLOGY
STORY OF THE PAST . . . KEY TO THE FUTURE
IC
Fe cc
O
RICHARD D. LAMM *R * JOHN W. ROLD
GOVERNOR 4*
Director
* 1876
COLORADO GEOLOGICAL SURVEY
DEPARTMENT OF NATURAL RESOURCES
715 STATE CENTENNIAL BUILDING-1313 SHERMAN STREET
DENVER,COLORADO 80203 PHONE (303)839-2611
May 22, 1980
Ms. Vickie Traxler
Weld County Department of Planning Services
915 10th Street
Greeley, Colorado 80631
Dear Ms. Traxler: RE: PANORAM INDUSTRIAL PARK
GEOLOGIC HAZARD
DEVELOPMENT PERMIT #2
WELD COUNTY
We have reviewed the Chen & Associates Inc. Report (Job No. 21 ,142) on subsidence for
the Harrington property. We also met with Mr. Ed Jennings of Robinson Engineering and
representatives of Chen & Associates on May 20, 1980. At the time of the meeting the
Colorado Geological Survey detailed its concerns regarding possible subsidence hazards
at the site:
1 . Determining how severe the hazard is over the site.
2. What kinds of structures will be built?
3. How will convential structures be damaged by subsidence?
4. How would ',special,' structures be damaged by subsidence?
5. Could subsidence cause human injury?
The result of this meeting was an agreement by the party representing Mr. Harrington and
the Colorado Geological Survey to develop anticipated nominal strain and tilt values
for the site. These values would be used in developing foundation and building design
parameters with which all site construction would comply. These criteria would appear
on the final plat. This would not preclude the lot developer making a site-specific
investigation to develop exact design criteria or to demonstrate the absence of significant
subsidence potential .
On the basis of this understanding with the developer, we recommend conditional approval
of the preliminary plat, subject to the development of a model for building construction
to be used on the final plat.
Sincerely,
(y-d'u�l"i'a"E. Turney
Engineering Geologist _#1\222324?S6
JET/9P .„a;L �► �c',
MAY 1980
cc: Ed Jennings w,
Ralph Mock RECEIVED 81
LUC �f WIN County
GEOLOGY \e7 NMnin;Cnalnlisslot
STORY OF THE PAST . . . KEY TO THE FUTURE ��l )(ICY'
• ( / v.., BOARD OF HEALTH
r�'�CY County Ieaztn .epartm, David Werking, DDS, Greeley
1516 HOSPITAL ROAD William Slick, Roggen
Charles W. Judie, M1, MPHFran Christensen, Eaton
Director GREELEY, COLORADO 80631 A.M. Dominguez, Jr., JD, Greeley
(303)353-0540 Annette M. Lopez, Greeley
Herschel Phelps, Jr. MD, Greeley
Kathleen Shaughnessy, PHN, Ault
May 23, 1980 Arthur G. Watson, Platteville
John M. Wheeler, Greeley
TO: Vickie Traxl
FROM: John G. Hall w
SUBJECT: Geologic azard Development Permit Dale Harrington
The following comments are submitted regarding the above referenced
permit:
1. Recommend applicant pursue a lagoon system for sewage disposal .
2. All septic systems, should this method be chosen, will
require an engineer design.
3. It is strongly suggested that additional space be allowed
for back-up septic systems in the event of failure of the
engineered systems.
4. An Emission permit for fugitive dust shall be applied for
through this office or the Colorado Department of Health.
•
ti026 2718
r%4`� MAY 1980 t
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WIIDCTo Vickie Traxl er Date May 13, 1980
COLORADO From T1rmur Srheltinaja. Engineering Manager
Subject: fat e Harri nrltnn renl ogi r Hazard Deve1 oprrnst Perini t
I have read the itpurt by Chen and Associates dated April 16, 1980. In
regard to the crag mine subsidence problem, our office does not have the
expertise to ==rent on the report or make suggestions. The subsidence
issue should be referred to the appropriate State and Federal Agencies.
Drew Scheltinga
Engineering Manager
DS:sad
ez:1.31'415 itd;
MINN 1S8o `''o
F- RECEIVED N
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t� W.eldConetlF Plaooia¢W®einsiw
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chen and associates, inc. =
.7. CONSULTING ENGINEERS ✓`• �r`~
SOIL E FOUNDATION
ENGINEERING
96 SOUTH ZUNI STREET • DENVER, COLORADO $0223 • 303/7444105
PRELIMINARY ENGINEERING GEOLOGY, SOILS,
AND COAL MINE SUHSIDflcE EVALUATION FOR
THE HARRINGPCN PROPERTY
WELD COUNTY, COLORADO
Prepared for:
RBINSai SURVEYIIC ADD ENGINEERING COMPANY
2950 SOUTH FOX STREET
ENGLESYOOD, COLORADO 80110
Job No. 20,142 April 16, 1980
OFFICES: COLORADO SPRINGS, COLORADO / GLENWOOD SPRINGS, COLORADO / CASPER, WYOMING
TABLE OF =CENTS
CONCLUSIONS 1
SCOPE 2
PROPOSED DEVELOPMENT 2
SITE CONDITIONS 2
GEOLOGIC SETTING 3
GENERAL SOIL AND FOUNDATION CONDITIONS 5
GEOTECINICAL CONDITIONS AFFECTING THE
PROPOSED DEVELOPMENT 6
ABANCONED COAL MINES 7
POTENTIAL SURFACE EFFECTS ARID RISKS ASSOCIATED
WITH THE ABANDONED MINES 9
ALTERNATIVES FOR SITE DEVELOPMENT 14
Utilities 15
Buildings 15
Air Shaft 16
SITE GRADING 16
ASPHALT PAVEMENTS 17
ON-SITE SEWAGE DISPOSAL SYSTEM 17
ECONOMIC MINERAL IEPOSITS 18
ADDITIONAL INVESTIGATION 19
FIG. 1 - SURFICIAL GEOLOGY MAP AND
EXPLORATORY HOLE LOCATIONS
FIG. 2 - LOGS OF EXPLORATORY HOLES
FIGS. 3 through 5 - SWELL-CONSOLIDATION TEST RESULTS
TABLE I - SUMMARY OF LABORATORY TEST RESULTS
TABLE II - PERCOLATION TEST RESULTS
PLATE I - ABANDONED CXXIL MINES
CONCLUSIONS
(1) The Harrington pLuperty is underlain by the abandoned workings
of the Graden Coal Mine which are from 256 to 327 feet below the
surface.
(2) It is likely that much of the subsidence potential associated
with the abandoned mine has already occurred; however, there is
a possibility that additional subsidence may occur in the
future.
(3) It is not technically or economically feasible to do a study
that could predict if additional subsidence will occur during
the service life of the proposed development; however,
preliminary calculations indicate if subsidence were to occur
during the service life of the project, it would be damaging to
conventional type construction.
(4) Alternatives for site development have been given. The choice
of alternatives is largely a matter of economic considerations.
- 2 -
SCOPE
This report presents the results of a preliminary engineering
geology and soils investigation for the Harrington property located
about 1 mile southwest of Dacona, Colorado. The general geologic and
soil conditions at the site are described and their anticipated
effects on development to an industrial park are discussed. The data
and recommendations presented are preliminary and are suitable for
evaluating project feasibility. Additional studies are necessary
prior to establishing design criteria.
PROPOSED DEVELOPMENT
We understand that the 68-acre parcel will be subdivided into an
industrial park. Forty-seven lots are proposed, ranging from 1 acre
to 6 acres. At this time, specific site facilities and building types
have not been determined. We anticipate that only light industrial
structures and associated facilities will be placed on the sites.
Several access roads are planned, and a railroad spur will be
constructed from the existing Union Pacific line which lies just to
the east of the property. It is proposed to use on-site sewage
disposal systems of the standard leach field type.
SITE CONDITIONS
The site is located in the northern one-half of Sec. 12,
T. 1 N., R. 68 W., in Weld County, about 1 mile southwest of Dacona,
Colorado. The majority of the ground surface across the site slopes
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down towards the northwest. The slopes are very gentle and generally
are from 1% to 2%.
Major drainages do not pass through the site. A minor northwest
trending intermittent drainage crosses the southwest corner of the
property. The drainage was dry at the time of our investigation.
Near the southwest portion of the site are three stock watering ponds
impounded behind small earthen dams. These reservoirs are relatively •
small as shown on Fig. 1. Embankment heights are on the order of 4 to
6 feet.
Near the west central portion of the site are several existing
structures, also shown on Fig. 1. These structures are of
single-story, wood frame and masonry block construction. Building use
is agricultural and industrial. Foundation cracking was noted in some
of the structures.
The parcel is underlain by the abandoned workings of the Graden
Mine, see Plate I. The main shaft of the Graden Mine is situated near
the southeast property corner. Currently, the majority of the
property is being utilized for agricultural purposes. The northern
one-half and eastern one-half have been cultivated. The southwestern
one-quarter of the site is being used as pasture with vegetation
consisting of native grasses.
GEOLOGIC SETTING
The site is situated on a rolling upland surface between Boulder
Creek to the west and the South Platte River to the east. The upper
soils at the site are relatively uniform and consist of several feet
- 4 -
of sandy clay, see Figs. 1 and 2. These soils are eolian deposits.
The eolian deposits are underlain by a few feet of residual soils
which result from the weathering of the tipper surface of the bedrock.
Bedrock underlying the site is the Laramie Formation of Late
Cretaceous age. The Laramie Formation consists predominantly of
sandstone and claystone. Claystone in the Laramie is generally medium
to dark gray and is mast predominant in the upper part of the
formation. The sandstone is light gray and usually medium grained.
Lenticular coal beds are present in the lower 200 to 300 feet of the
formation. Seven coal beds of minable thickness have been identified
within the general area. Mining has occurred under the site in Coal
Bed No: 3, also referred to as the Goeham Seam.
The site is located on the northwestern flank of the Denver
structural basin. Regional bedrock dips in the area are to the
southeast with strikes toward the northeast. This part of the Denver
Basin is complexly faulted into a series of narrow horsts (upthrown
blocks) and grabens (downthrown blocks) by a system of northeast
trending faults. These faults are not discernable on the surface and
their recognition is based on subsurface data, including mine mapping
and exploratory holes. Displacement along these faults usually does
not exceed 200 feet, and major faults act as a natural barrier to
mining. Faults with large offsets have not been mapped at the site
and the continuity of the mining verifies this. Faults have been
mapped 2,000 feet to the east of the site and 3,000 feet to the west.
These faults were formed contemporaneously with the deposition of the
--\
- 5 -
Laramie Formation and are not considered as potential sources of
earthquake activity.
GENERAL SOIL AND MUNE:SCION CONDITIONS
The general subsoil conditions at the site were investigated by
drilling 5 exploratory holes at the locations shown on Fig. 1. Logs
of the holes are shown on Fig. 2.
Samples of the upper soils and bedrock were obtained and
returned to our laboratory for examination and testing. The samples
were tested to determine their general engineering properties. A
summary of test results is shown on Table I.
In general, the subsoils consisted of 4 to 8 feet of sandy clay
overlying 1 to 3.5 feet of weathered claystone and sandstone. A thin
veneer of fill was encountered in Hole 5. This fill consists of sandy
clay with scattered pieces of shale and coal. Bedrock was encountered
in all the test holes at depths ranging from 6 to 9 feet. The
majority of the bedrock encountered in the exploratory holes was
claystone. Sandstone was encountered in Hole 3 from 6 to 11 feet.
Samples of the sandy clays were moderately to slightly plastic,
medium moist, brown to light brown, with occasional calcareous lenses.
Based on the penetration test results, these soils are stiff to very
stiff in consistency. Swell-consolidation test results indicate that
the sandy clays have low to moderate swell potential, as shown on
Figs. 4 and 5.
- 6 -
The claystone and sandstone bedrock is medium hard to hard,
brawn to gray and slightly moist to moist. The claystone bedrock
exhibits moderate to high swell potential, as shown on Figs. 3 and 5.
Free water was not encountered in our test holes at the time of
drilling or when Checked 2 days later. An irrigation well has been
constructed in an abandoned air shaft in the southeastern corner of
the parcel. Static water levels in the well range from 113 to
125 feet below the surface, depending on the time of year.
GE0TECHNICAL CONDITIONS AFFECTING THE PROPOSED DEVELOPMENT
Of primary concern to site development is the potential for
differential ground deformations associated with the potential
collapse of the abandoned coal mine workings which underlie the site.
A preliminary evaluation of these conditions is discussed in the
following sections of this report. In addition, the upper sandy clay
soils ptinsess a low to moderate swell potential, and the underlying
claystone bedrock has a moderate to high swell potential. The swell
potential of the foundation material should be considered along with
potential surface deformations associated with the past mining in
selecting a foundation system. Foundation type and design parameters
should be site specific and will depend on building type as well as
foundation conditions.
The upper sandy clay soils and underlying bedrock have a low to
moderate erosion potential. A surface drainage plan should be made
for the development. Areas where concentrated high velocity flows are
anticipated should be protected from erosion. All stripped areas
r
- 7 -
should be revegetated or protected from wind and water erosion by
other means.
The site is located near the northern edge of the Denver Seismic
Zone. In the past, numerous small earthquakes and a few moderate size
earthquakes have occurred within the zone. The small earthquakes
resulted in no structural damage, even near their epicenters, and only
minor structural damage was caused by the moderate size earthquakes.
The northeast trending faults in the region are not considered to
present unusually high earthquake risk. Considering the type of
construction proposed, we recommend that all structures be designed
for seismic forces in accordance with the Uniform Building Code,
Seismic Zone I.
ABANDONED COAL MINES
The site is located near the northeastern end of the
Boulder-Weld Coal Field. Mine maps obtained from the U.S. Bureau of
Mines indicate that the site is underlain by the abandoned workings of
the Graden Mine, as shown on Plate I. The abandoned Boulder Valley
No. 3 Mine lies adjacent to the north property boundary, and the
abandoned Baum Mine lies adjacent to the east property boundary. All
of the nearby mines are flooded and are presently inaccessible.
Production records on file with the Colorado Division of Mines
indicate that the Graden Mine operated between 1935 and 1955. During
this period, 896,078 tons of coal was extracted fran a seam whidl
ranged from 7'10" to 6'8" thick. The mine was closed and the main
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shaft and air shaft were sealed in August of 1955. The last mine
operator was the W. E. Russell Coal Company.
Elevations on the mine map indicate that the depth to the coal
seam below the Harrington property range from 256 to 327 feet. The
layout of the mine workings, shown on the mine map, indicate that
mining practices used in the Graden Mine are similar to the mining
methods used elsewhere in the Boulder-Weld Coal Field. The mine map •
shows that a vertical shaft was excavated to the level of the mined
seam, which dipped about 1` toward the southeast. Double drift
haulage entries were then driven on a grid pattern in the coal seam.
The haulageways consist of two parallel 15-foot wide drifts on 50-foot
centers. Cross cuts were made along the haulageways on 50 to 80-foot
centers. Where coal was extracted, double drift panel entries of
similar dimensions as the haulageways were driven, generally at right
angles to the haulageways. Panel entries were spaced from about
170 to 200 feet apart with lengths from 400 to 800 feet. During the
first stage of mining, rooms were mined perpendicular to the panel
entries. The mine map indicates that during the first working, rooms
were about 20 feet wide and extended across the full width of the
panel. The rooms were separated by 20-foot wide pillars. The second
stage of mining consisted of removing pillars between rooms, and it
appears from the mine maps that some pillars within the haulageways
and panel entries were also mined upon retreat.
Studies of mining in the Boulder-weld Cnot Field estimate that
using this mining technique, from 70% to 95% of the coal in a worked
out area could be extracted, Lowrie (1966) and Amuedo & Ivey (1975).
- 9 -
Worked out areas, that is areas where pillars were removed during the
second stage of mining, in the Graden Mine and in adjacent mines are
shown by the shading on Plate I. Comparison of reported mined tonnage
from the Graden Mine to the estimated volume of worked out areas
indicates about 70% recovery. This recovery is comparable with
recovery rates reported from other mines in the Boulder-weld Coal
Field and also indicates that the worked out areas shown on the mine
map are probably reasonably accurate.
POTENTIAL SURFACE EFFECTS AND RISKS ASSOCIATED WITH THE ABANDONED
MINES
In coal mining districts throughout the world, it has long been
recognized that collapse of large mine workings results in surface
subsidence and differential ground movement above the collapsed
workings. Depending on the depth and the dimensions of the collapsed
area, these surface effects range from values which are insignificant
to surface structures to values where severe damage to surface
structures can occur. In the European coal fields, long experience
with the effects of mine subsidence has been gained, and methods have
been developed to predict the effects and estimate the risk of damage
to surface structures. Similar types of studies are currently being
developed for active underground mines in the U.S.; however, in areas
of abandoned mines such as those in the Boulder-Weld Coal Field,
little data is available which would allow accurate assessment of
surface subsidence effects and degrees of risk involved.
•-
- 10 -
As an interim measure, it has been proposed, Yokel (1978) that
surface subsidence profile characteristics developed by the British
National Coal Board (1975) be used to quantify the effects of mine
subsidence in the coal fields in the United States until additional
research and field measurement can be obtained. This method of
analysis applies to individual rooms, extracted long wall panels, and
full extraction mom and pillar workings, Skelly and Loy (1976).
However, it should be pointed out that some of the mine conditions
encountered in the abandoned mines of the Boulder-Weld field do not
directly apply to the conditions of the National Coal Board model, and
there has not been an extensive experience base developed which
demonstrates the suitability of this model for predicting ground
surface deformations for the conditions in the Boulder-Weld Coal
Field.
The only area in the Boulder-Weld Coal Field where subsidence
measurements have been taken, to our knowledge, was where Interstate
Highway 25 crosses over the abandoned Eagle Mine workings. The
highway was constructed prior to undermining, and a resurvey of the
highway alignment after mining gives a rough estimate of the
subsidence profile which developed at the surface about 6 years after
undermining.
We compared the surface subsidence measured with those predicted
by the National ('nal Board model. The actual maximum vertical
subsidence was about one-third of the maximum vertical subsidence
predicted by the National Chat Board model. When corrections for
reduction in maximum surface subsidence were applied, maximum
- 11 -
horizontal strains and ground surface tilts predicted by the Coal
Board model were comparable with those of the subsidence profile
measured over the Eagle Mine. Coal seam thickness mined and the depth
of mining in the Eagle Mine where the subsidence profile was measured
are comparable to those in the Graden Mine underlying the Harrington
property. Based on the above comparison, it is our opinion that the
National Coal Board model can be used to predict the effects of mine
subsidence at the Graden Mine; however, the results of these
predictions should be applied with judgment and with a degree of
conservatism.
To fully assess the risk and surface effects of future mine
subsidence associated with the abandoned mine workings below the
Harrington property would require a rather complete knowledge of the
depth and configuration of mine voids still present below the site.
Considering the depth and extent of mining and the inaccessibility of
the workings, it is not technically and economically feasible to
obtain complete information. However, there is enough information
available to make some preliminary judgments on the risks and
potential effect of future subsidence.
The worst possible conditions would be that no collapse of mine
workings has occurred since the mine was abandoned and that collapse
will occur during the service life of the project. This condition is
unlikely. A more likely condition is that in areas of the mine where
pillars were removed, the shaded areas on Plate I, caving of the
workings and subsidence occurred shortly after mining was completed.
This judgment is based on the extraction ratio calculated for the
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worked out areas based on mine production records for the Graden Mine.
A similar conclusion as to the time of subsidence for worked out areas
was reached by Amuedo & Ivey (1975) in their general evaluation of the
Boulder-Weld Coal Field. In his review of the general mining
practices in the Boulder- Weld Coal Field, Lowrie (1966) indicates
that the roof in the mined out areas usually caved naturally. In some
cases, blasting was used to induce roof collapse. Although it appears
reasonable to assume that caving of the worked out areas of the Graden
Mine has already occurred, there is a possibility that some small
uncollapsed void spaces are still present in these areas. In
addition, portions of the mine not worked out consist of haulageways
and panel entries with relatively large support pillars in caparison
to the adjacent openings. In these areas, a slow deterioration of
pillars with time could result in the collapse of mine workings during
the service life of the proposed project.
,r Because of the lads of detailed information on existing
conditions in the mine, it is not possible to estimate the rate of
pillar decay or to predict the time of future collapse of possible
mine voids. Since surface effects of mine subsidence extend beyond
the limits of the mine workings, there are no portions of the site
which can be considered totally removed from the potential effects of
surface susidence.
We have made some preliminary estimates of the effects of
surface subsidence on conventional type construction based on ground
surface deformation predicted by the National Coal Board model and
damage thresholds given by Yokel (1978). The results of the analysis
V
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indicate that damage to conventional construction is likely to occur
if subsidence of uncollapsed mine voids were to occur during the
service life of the project.
In addition to the subsidence effects discussed above, another
potential hazardous condition exists in areas where open mine voids
lie above the water table. In these areas, there is a potential that
surface water infiltration can cause piping of the overburden soils
and weaker bedrock into the open mine workings. At the site, the only
place where this condition occurs is in the vicinity of the air shaft
shown on Plate I. This area was examined in the field and no evidence
of surface distress was observed; however, failure has occurred in the
vicinity of the main shaft, which is located about 200 feet south of
the southern property line. A large, steep sided pit about 40 feet
wide and 15 feet deep is present in this area. The main shaft was
sealed in 1955; however, the failure has extended beyond the limits of
the surface cap.
In summary, it is our opinion that the potential for future
subsidence is present in all portions of the Harrington property. It
is not possible to predict if subsidence will actually occur during
the service life of the project; however, if subsidence were to occur
during the service life of the project, its effects are likely to be
damaging to conventional type construction. Possible alternatives for
site development are discussed in the following section.
- 14 -
ALTERNATIVES FOR SITE DEVELOPMENT
Since it cannot be predicted if surface deformations associated
with the mine subsidence will occur during the service life of the
project, two alternatives for site development present themselves:
(1) The property could be developed using standard construction
techniques, and if surface deformations were to occur during the
service life of the project, resulting damage to the structures
repaired at that time. This alternative has the advantage that
lower initial construction oasts would be incurred, and if
subsidence were not to occur during the service life of the
project, money spent in special construction techniques would be
saved. This alternative has the disadvantage in that if
subsidence were to occur during the service life of the project,
the cast of repairs would be higher than if special construction
techniques were initially used. Potentially hazardous
construction, such as subsurface gas lines, should incorporate
special construction techniques as discussed below.
(2) Special construction techniques could be initially designed into
the project to reduce potential damage caused by subsidence
should it occur during the life of the project. This
alternative has the advantage that if subsidence were to occur
during the service life of the project, damage would be
minimized and the cost of repairs, if necessary, would be less
expensive than if standard construction techniques were
initially used. 'Ibis alternative has the disadvantage that
higher initial construction oasts would be incurred.
- 15 -
The choice of alternative for site development is largely a
matter of economic considerations since the technical know-how to
build under very adverse conditions is available. Some of the
special construction techniques are expected to be much more expensive
than standard construction; however, some special techniques could be
relatively expensive depending on the design parameters. Additional
studies will be necessary to establish these design parameters.
Possible precautions whidi would reduce potential damage in the event
of subsidence during the service life of the project are discussed
below:
Utilities: When possible, subsurface utilities should not be used.
Underground pipelines should be provided with flexible, reinforced
joints. Flexible joints are inportant where pipes are connected to
rigid structures such as buildings. This detail is particularly
inportant for potentially hazardous construction such as gas lines.
Check valves to control leakage should be considered for pressurized
lines.
Buildings: Favorable building locations and geometries should be
evaluated. Rigid foundation systems, such as post-tensioned slabs,
should be considered along with the possibility of designing flexible
superstructures whidt would be more tolerant to differential movements
than rigid structural systems. If damage from subsidence is caught in
its early stages, it should be less costly and more effective to
repair the damage in its early stages. It is recommended that
consideration be given to installing monitoring systems which could
detect the onset of possible subsidence in its early stages.
- 16 -
Air Shaft: A minimum setback of 100 feet is recommended for all
structures or facilities adjacent to the air shaft in the southeast
corner of the property. This setback could be reduced if additional
studies indicate that there is not a potential for caving.
Close coordination between the geotechnical engineer, structural
engineer and architect will be necessary during planning and design of
the project.
SITE GRADING
Since the ground surface slopes over the majority of the
property are relatively gentle, it is not anticipated that extensive
site grading will be necessary for project development. Our
preliminary investigation indicates that the upper sandy clay soils
are more favorable foundation materials than the underlying claystone
bedrock. It is, therefore, recommended that planned cuts be kept to a
minimum. The on-site soils should be suitable for overlot fill if
properly compacted; however, these soils possess a low to moderate
swell potential, and we do not recommend that they be used for
structural underslab fill.
For preliminary planning, we do not recommend that cut slopes,
unless retained, be made steeper than 2:1 (horizontal to vertical).
All cut and fill slopes should be provided with good drainage and
revegetated. If it is necessary to use deep cuts or high fills in
excess of 10 feet, it is recommended that their stability be evaluated
on an individual basis.
- 17 -
ASPHALT PAVEMENTS
Samples of subgrade soils and bedrock were taken in the field
and classified according to their AASWIO classification. The majority
of the subgrade material at the site is classified as A-6 and A-7-6
with group indices from 12 to 20. These types of materials are
considered poor support of pavement section. The following
preliminary pavement sections may be utilized for planning. We •
recommend a final pavement design be cone when final road layouts are
established.
Automobile and Truck Traffic; The pavement in areas of combined
automobile and truck traffic should consist of 6 inches of high
quality base course and 2 1/2 inches of. asphalt concrete surface
course. An alternate full depth asphalt section of 5 inches may be
used.
Truck loading, dock areas and other areas where truck turning
movements are concentrated should be paved with 6 inches of portland
cement concrete.
The collection and diversion of surface drainage away from paved
areas is extremely important to the satisfactory performance of an
asphalt pavement. The design of surface drainage should be carefully
considered to remove all water from paved areas.
ON-SITE SEWAGE DISPOSAL SYSTEM
Percolation tests were performed adjacent to the five
exploratory holes shown on Fig. 1 to evaluate the general suitability
of on-site soils for leaching field type septic systems. All
- 18 -
percolation rates were in excess of 120 minutes per inch as summarized
on Table II. Based on these results, it doeq not appear that the
on-site soils are suitable for standard leaching field type septic
systems. Consideration should be given to alternative systems of
sewage disposal.
ECONOMIC MINERAL DEPOSITS •
Mineral resources in southwestern Weld County include sand and
gravel aggregate, coal, gas and oil reserves. rna1 deposits in the
area are found within the lower portion of the of the Laramie
Formation. The majority of the coal beds within the Laramie Formation
are thin and discontinuous. The most extensive of these coal beds has
been mined at the site. Because of the thinness and lads of lateral
continuity of the remaining coal beds, it is our opinion that the coal
under the site does not represent a potentially recoverable reserve.
Oil production in the area is from the upper Cretaceous Sussex
sandstone and sandstone within the Dakota Formation. Gas exploration
has also been productive in the Dakota Formation. Oil and gas
reserves may be present beneath the parcel.
Sand and gravel resources are of limited extent in the area and
are confined primarily to isolated deposits in the Arapahoe Formation
and pediment gravels. Our test holes show that potential sand and
gravel deposits are not present at the site.
- 19 -
ADDITIONAL INVESTIGATIONS
The findings presented in this report are preliminary and
suitable for project feasibility evaluation. Additional studies will
be needed to establish foundation design criteria for specific
structures and other soil-related construction conditions. Additional
investigations are also necessary to establish design criteria for
special construction techniques to reduce potential damage associated
with possible future surface subsidence. Since remedial measures will
be less costly and more effective if potential damage is caught in the
early stages, we recommend that additional studies be done to
establish an early warning system to detect the onset of subsidence
should it occur during the service life of the project.
If there are any questions, or if we may be of further service,
please let us know.
a="- , CHEN AND ASSOCIATES, INC.
�-- s
<,'�� f•
�� By .���
\I
� . v Rai 'G. Mock,
�, Engineering Geologist
a
• , Reviewed By
Richard C. Hepw�frth, P.E.
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;hen and associates, inc�
Moisture Content= 22,4 percent
Dry Unit Weight= 106.3 pcf
Sample of: Claystone bedrock
From: Hole 1 at depth 9'-0"
9
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APPLIED PRESSURE — kef
#20, 142 SWELL-CONSOLIDATION TEST RESULTS Fig. 3
CA-1-79
'-• chen and associates,
Moisture Content = 12.7 percent
Dry Unit Weight= 107.6 pcf
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From: Hole 2 at depth 4'-0"
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Moisture Content= 5.2 percent
Dry Unit Weight= 105.0 pcf
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From: Hole 3 at depth 4'-0"
0
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u N\ressurexrans on under corstant
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0.1 1.0 10 100
APPLIED PRESSURE — ksf
#20, 142 SWELL-CONSOLIDATION TEST RESULTS Fig. 4
CA-1-79
chen and associates, z.
Moisture Content= 3.3 percent
Dry Unit Weight= 107.8 pct
Sample of: Sandy clay
„ From: Hole 4 at depth 4i-0"
c
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APPLIED PRESSURE — ksf
Moisture Content= 17.8 percent
Dry Unit Weight= 109.5 pct
Sample of: Claystone bedrock
From: Hole 4 at depth 91-0"
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#20, 142 SWELL-CONSOLIDATION TEST RESULTS Fig. 5
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TABLE II
PERCOLATION TEST RESULTS
WATER DEPTH WATER DEPTH
HOLE HOLE LENGTH OF AT START AT END DROP IN AVERAGE
NO. DEPTH INTERVAL OF INTERVAL OF INTERVAL WATER LEVEL PERCOLATION RATE
(In. ) (Min. ) (Inches) (Inches) (Inches) (Min./Inch)
P-1 41 30 12.75 12.75 0 •
30 12.75 12.75 0
30 12.75 12.75 0
30 12.75 12.75 0 No Percolation
P-2 33.5 30 14.25 14.25 0
30 14.25 14.25 0
30 14.25 14.25 0
30 14.25 14.25 0 No Percolation
P-3 35.75 30 14.5 14.25 .25
30 14.25 14.25 0
30 14.25 14.25 0
30 14.25 14.25 0 No Percolation
P-4 29.25 30 13.25 13.00 .25
30 13.00 13.00 0
30 13.00 13.00 0
30 13.00 13.00 0 No Percolation
P-5 35.25 30 13.75 13.75 0
30 13.75 13.75 0
30 13.75 13.50 .25
30 13.5 12.35 .25 120
#20, 142
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