HomeMy WebLinkAboutBELLA VIRA - PDP - 36-05A - CORRESPONDENCE - UTILITY PLANS (6)5.0 REFERENCES
Earth Engineering Company, Inc. November 17, 2004. Preliminary Subsurface Exploration Report,
Minatta Property, Fort Collins, Colorado, EEC Project No. 04-01-288.
Earth Engineering Company, Inc. February 2, 2005. Groundwater Level Observations, Minatta
Property, Fort Collins, Colorado, EEC Project No. 04-01-288.
Fetter, C.W. 2001. Applied Hydrogeology. Prentice -Hall, Inc., New Jersey.
Ayres & Associates, Inc. September 25, 1995. Groundwater modeling and assessment of
groundwater control options, Two Ponds at Overland Trail PUD, Fort Collins
Riverside Technology, Inc. September 1995. Ground Water Investigation, Ponds at Overland Trail
Property, Fort Collins, Colorado.
Waterloo Hydrogeologic. 2005. Visual MODFLOW, Waterloo, Ontario, Canada.
bella vira underdrain report.doc
14
ANd ERSON CONSU[TiNG ENGINEERS, INC.
FORT COLLINS ONLY
Subdrain Main
Sanitary
Sewer Main -FE Sanitary Service
Subdrain Service
(No Perforations)
Reducing Tee
PLAN VIEW Stake Identifier
(Sanitary Sewer)
Stake Identifier
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Trench
Sanitary
Sewer Main
Pipe Sizes to be specified
by the approved plans.
Bedding
Subdrain Main
CROSS SECTION
SUBDRAIN SERVICE WYE DETAIL
LARIMER COUNTY CONSTRUCTION REVISION NO: DRAWING
URBAN AREA DRAWINGS
STREET STANDARDS DATE: 10/25/O1 713.1F
Fi,ure 5. Subdrain Service Wye Detail
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SUBDRAIN MAIN - INSTALLATION DETAILS
::LAR'MER COUNTYCONSTRUCTION REVISION NO: DRAWING
URBAN AREA DRAWINGS
REET STANDARDS DATE: 10/25/01 713.2E
Figure 3. Cleanout Detail and Bedding/Filter Fabric Detail
ll
4.0 UNDERDRAIN DESIGN RECOMMENDATIONS
In consideration of the estimated underdrain flow and anticipated maintenance the following
recommendations are made.
A six-inch diameter, perforated, PVC pipe meeting AASHTO standard M278 should
be used as the underdrain main throughout the development except at the intersection
of Banyan Drive/Fiore Court where a eight -inch diameter, solid, PVC pipe will be
used as the outfall pipe. Six-inch pipes should have a minimum slope of 0.003 ft/ft.
The minimum slope of the eight -inch pipe should be 0.001 ft/ft.
2. The proposed design includes connections to individual house underdrains. Four -
inch diameter, non -perforated PVC or HDPE pipe should be used as service to
individual residences. Each residence should have a perforated foundation drain
connecting to the four -inch service line, which connects to the main underdrain
system. Typical details for the house underdrain and service line can be found in
Figure 4 and Figure 5.
3. Solid cleanouts should be provided at bends in the subdrain system and at intervals
no greater than 1,000 feet. A vertical sweep bend coupling should connect the
cleanout with the subdrain main for ease of maintenance tool access. A typical
cleanout detail is shown in Figure 3. The diameter of the cleanouts should match the
diameter of the pipe.
_I
4. The Developer's agreement with the City should include a provision, which identifies
maintenance responsibilities with regard to the Bella Vira development
5. The bedding and filter fabric design for the underdrain construction is as detailed in
Figure 3. For the configuration shown the filter material should be a CDOT Class 6
coarse aggregate.
6. The underdrain service to each residence should be clearly marked and identified
iwithin the service trench prior to burial. Service connections to each residence
should be inspected to ensure proper identification at the time of connection by the
City inspector or onsite engineer. Perimeter foundation drains around individual
j residences should be installed according to the details shown in Figure 4.
7. The system should be designed such that clay cutoff walls are provided at boundaries
of the development to preclude hydraulic communication with offsite utility trenches
either upstream or downstream.
bella vira underdrain report.doc 10 ANdERSON CONSutiiNG ENGiNEEaS, INC.
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from Fiore Court intersect with the Banyan Drive collector (underdrain invert el. = 5112.8 ft). At
this point an eight -inch solid outfall pipe will break slope with the sanitary sewer and follow a
parallel alignment with a slope of 0.0011 ft/ft south to a point on Banyan Drive approximately 5 feet
from the southern property boundary. The underdrain pipe will then turn roughly east and run
approximately 470 feet at a slope of 0.0011 ft/ft, parallel to the property boundary, within a 10-foot
wide drainage easement. At this point the outfall pipe will bear southeast 45' and continue 45 feet at
the same slope, to a point where it bears south 45°. From this point the pipe runs an additional 20
feet at a slope of 0.0011 ft/ft to the outfall point in the bottom of the Minatta Pond (underdrain invert
el. = 5112.0 ft). The outfall point shown on Figure 2 is approximate; the actual location will need to
be located by survey to verify the elevation.
An alternative alignment for the 8-inch underdrain outfall pipe is also shown on Figure 2.
The alternative outfall alignment will begin at the intersection of Banyan Drive and Fiore Court
(underdrain invert el. = 5112.8 ft); from this point the pipe will follow the alignment of the sanitary
sewer 275 feet east along Fiore Court at a slope of 0.0012 ft/ft opposite the slope of the sanitary
sewer. At the top of Fiore Court, where the sanitary sewer terminates, the outfall pipe will turn 90'
to the south and extend 44 feet, from whence it will turn southeast 30' and proceed 300 feet,
maintaining the previous slope along both legs. At this point the alignment will turn 45'to the south
and proceed 50 feet at a slope of 0.0012 ft/ft to the outfall point at the bottom of Minatta Pond
(underdrain invert el. 5112.0 ft).
The 100-year water surface elevation in the Minatta Pond is 5115.5 feet. The underdrain
outfall will be submerged by approximately three and one-half feet of water during the 100-year
event. This pending will surcharge the underdrain street collector pipes up to West Elizabeth Street.
However, the lowest elevation of a house perimeter drain within the subdivision is approximately
5117.7 feet, more than two feet above the 100-yr flood water surface elevation. Thus the hydraulic
gradient within the system is toward the outfall during the 100-year event. A plan and profile of the
proposed underdrain system will be shown on the Bella Vira utility plan, which will be prepared by
the site civil design engineer.
i
_J
bella vira underdrain report.doc 8 ANd ERSON CONSULTING ENGINEERS, INC.
reduced by 60%. The underdrain configuration was also common to both scenarios, with drain
locations and elevations input to the MODFLOW drain package based on design data presented on
the grading and sanitary sewer plans and profile produced by Jim Sell Design, the civil design
engineer for the project.
3.2 Underdrain Design Flow
The MODFLOW simulations produced steady-state discharge estimates corresponding to the
expected long-term discharges from the underdrain system. The discharge under typical irrigation
and precipitation conditions is estimated to be 100 gpm. The discharge under high water, i.e. 100-
year rainfall, conditions is expected to be 200 gpm. Initially following installation of the underdrain
the discharges from the system will exceed these estimates while the water table adjusts to the
presence of the underdrain.
ACE recommends underdrain pipe sizes based on the estimated discharges and the design
slopes of the sanitary sewer system as well as reliability and serviceability concerns. Due to the
variability of soil conditions, and the susceptibility of underdrain systems to clogging from the
infiltration of fine soil particles, the minimum recommended collector pipe size is six -inches placed
at a minimum slope of 0.003 ft/ft. A six-inch pvc pipe will convey a maximum of 162 gpm at a
slope of 0.003 ft/ft (Manning's n = 0.011). It is ACE's understanding that the minimum slope within
—# the proposed sanitary sewer system is 0.004 ft/ft. A six-inch pvc pipe will convey a maximum of
188 gpm at this slope. If, for any reason, the slope of an underdrain pipe is adjusted to any slope
shallower than 0.003 ft/ft the pipe diameter should be increased from six inches to eight inches. At
the location where the perforated collector pipes discharge to a solid outfall pipe it is recommended
that this outfall pipe be eight -inches in diameter at a minimum slope of 0.001 ft/ft. The six-inch and
eight -inch diameter lines will be of sufficient size and durability to permit the use of tools necessary
to perform regularly scheduled maintenance or to remove possible pipe blockages.
3.3 Outfall Design
A schematic of the proposed underdrain system is presented in Figure 2. The system will
consist of six-inch diameter perforated collection pipes running underneath the sanitary sewer lines
in the street. Individual 4" diameter house services will connect into the perforated street collectors.
The underdrain will follow the alignment and slope of the of the sanitary sewer lines on Pratolina
Court, Fiore Court, Sunflower Drive, West Elizabeth Street, and on Banyan Drive until the collectors
bella vira underdrain report.doc 7 ANdERSON CONSUMING ENGINEERS, INC.
3.0 GROUND WATER FLOW ANALYSIS
ACE used a computer simulation of the proposed drain system to evaluate anticipated pipe
flows and the resultant water table beneath the development. This section summarizes the model
inputs and results, and discusses the outfall requirements of the proposed underdrain system.
3.1 Method of Analysis
ACE elected to use the Modular Three -Dimensional Finite -Difference Ground -Water Flow
Model (Visual MODFLOW v4.1) to determine design inflow rates and the impact of the proposed
underdrain on the water table at the Bella Vira site. MODFLOW was originally developed by the
USGS and has since been adapted for a Windows computer environment by Waterloo Hydrogeologic
(Waterloo, 2005) to simulate steady state or transient flow under a variety of aquifer conditions.
A model grid (20 row by 54 column) was established with a grid spacing of approximately 50
feet by 50 feet. The grid rows were oriented east to west, perpendicular to Overland Trail. Flow
within the study area is west to east generally following the slope of the surface topography. Given
the characteristics of the geologic and soil conditions from the boring logs, detailed in Section 2.0 of
this report, the model grid was constructed of two vertical layers with two separate hydraulic
conductivity values of 7.0 and 0.4 ft/day. The boundaries of the site were modeled as four separate
general head boundaries indexed to the water levels in College Lake to the west and the adjacent
detention ponds to the east.
These initial model inputs were calibrated to the water levels recorded in the piezometers,
College Lake, and the detention ponds by ACE on August 14, 2006. To calibrate the model to pre -
development site conditions, a reasonable recharge value of 7 inches per year of resultant percolation
from irrigation and rainfall was assumed.
Upon the completion of calibration, the model was rerun under two post -development
scenarios with the completed underdrain in place. The first scenario was an analysis of underdrain
discharge based on the boundary heads for College Lake and the detention ponds used for
calibration, which is considered representative of the typical conditions the system will operate
under. The second scenario was an analysis of the potential discharge of the underdrain when the
boundary heads in College Lake and the detention ponds are at sustained high water levels,
synonymous with a 100-year flood event. Under both scenarios the recharge due to deep percolation
from irrigation and precipitation was maintained at 7 inches per year. The rationale for this
assumption is that while lawn irrigation results in an increased application depth of irrigation water,
the efficiency of application has increased and the pervious area within the development has been
bells vira underdrain report.doo 6 ANd ERSON CONSoITINq ENGINEERS, INC.
installed by EEC at the site there were 4 registered wells within 600 feet of the proposed underdrain.
These wells include Permit Nos. 26300, 26301, 901668, and 1995174. Permit Nos. 26300 and
26301 were monitoring holes completed as part of the 1995 RTi study which, by rule, would have
been abandoned a year following their construction. Permit No. 901668 belongs to a cancelled well
permit. The well with Permit No. 1995174 has been recorded as abandoned. Given that none of
these 4 wells is active, no notification of the well owners will be required.
2.4 Permitting
ACE will prepare a well permit application for the new underdrain system, which will then be
submitted to the SEO. A blank copy of the SEO's well permit application can be found in Appendix
D.
bella vira underdrain report.doc 5 ANdERSON CONSULTING ENGINEERS, INC
On August 14, 2006 ACE conducted baildown tests on all four of the existing piezometers.
To facilitate these tests the owner of the property replaced the 1 '/2" casings with 2" casings that
could accommodate ACE's permeability testing equipment, i.e. a computer controlled data
logger/cable and a 1 ''/z" polyethylene bailer. Piezometers P-2 and P-4 yielded reasonable test results.
The Hvorslev Method (Fetter, 2001) was used to analyze the test data to determine a representative
hydraulic conductivity for the water bearing material. The resultant conductivities for P-2 and P-4
were 0.7 ft/day and 0.04 ft/day, respectively. These values fall generally within published ranges
expected of claystone in P-2 and clay soils in P-4. Given that P-2 was completed into bedrock and P-
4 was not, these two values were assumed to be representative of the difference between the
conductivities of the bedrock and the overlying soil. Appendix B.1 contains the field notes and
computer analysis output from the permeability testing.
The conductivities resulting from the permeability testing fell within published ranges for the
in -situ materials at the site, as previously noted. The groundwater investigation at The Ponds in
1995 produced estimated conductivity values of 8.5 ft/day and 0.14 ft/day for similar bedrock and
overlaying soils. These values were utilized in the groundwater modeling efforts completed by
Ayres Associates for that development (Ayres, 1995). The difference between the two sets of
conductivity values is one order -of -magnitude, i.e. a factor of 10. The larger values correspond more
closely to the values that ACE would expect to see in the types of materials found at Bella Vira. The
difference is likely due to the relatively poor quality of the wells tested at Bella Vira, as evidenced by
the 50% test failure rate during permeability testing. Consequently, for modeling purposes, the test
values obtained at Bella Vira were increased by a factor of 10 to 7.0 ft/day and 0.4 ft/day to more
closely estimate actual conditions. The Ayres report is included in Appendix B.3.
Ground water flow in this region is subject to several different sources and influences. The
primary influences on groundwater flow appear to be the pool maintained in College Lake to the
northwest of the site, and the pool maintained in the detention ponds along Overland Trail at The
Ponds development. Seasonally, the losses from the Dixon Canal and the losses/gains from the
Pleasant Valley and Lake Canal may also be affecting the water table. Given the relatively high
slope at the site, the shallow topsoil, and the tight clay soils beneath the topsoil it is not expected that
deep percolation from precipitation or applied irrigation water is a significant contributor to the water
table.
2.3 Registered Wells
The Colorado State Engineers Office (SEO) was contacted to determine whether there were
any registered ground water wells within 600 feet of the site. Other than the 6 monitoring holes
bella vira underdrain report.doc 4 ANdERSON CONSulriNq ENgiNEERS, INC.
2.0 SITE CHARACTERIZATION
Investigations have been ongoing at the Bella Vira Development since the Fall of 2004. This
section summarizes the results and conclusions of these investigations.
2.1 Soil Profiles
Subsurface soils at the site were characterized by EEC as brown colored low to moderate
plasticity lean clay and low plasticity clayey sand soils overlying highly weathered
sandstone/claystone bedrock. The bedrock was colored olive/brown/gray and rust was moderately
hard on the surface and became increasingly cemented with increasing depth (EEC, 2004). Out of 6
total borings, 5 penetrated bedrock at depths of 3 to 14 feet. Boring logs and the results of the
geotechnical laboratory tests on soil samples are included in the EEC reports which can be found in
Appendix A.
2.2 Hydrogeologic Properties
Four piezometers were installed at the site in November of 2004 by EEC (Figure 1). The
—, piezometers consisted of 1 ''/z" diameter field slotted PVC casings installed into the 4" diameter
boring. Two of the four piezometers, P-1 and P-3 were dry during drilling. Water was encountered
in piezometers P-2 and P-4 during drilling. Water was encountered in all four piezometers in
subsequent measurements, and at levels higher than those initially encountered in P-2 and P-4 (EEC,
2005). The tendency of water to rise within the piezometers following the initial measurements at
construction is an indication of an artesian condition. It is possible that the overlying clayey soils are
acting to partially or fully confine water contained in the slightly more porous sandstone/claystone
bedrock. It should be noted that piezometer P-4 was not necessarily indicative of this condition,
since it was not completed into bedrock.
ACE obtained a copy of the Ground Water Investigation for The Ponds at Overland Trail
Property completed by Riverside Technology, Inc. (RTi) in September of 1995. The Ponds
development is directly south of the Bella Vira property along the foothills. This report indicated
that similar water level behavior in the borings completed for the Ponds. This report concluded,
similarly, that, in some areas, the less permeable overlying soil was confining ground water
contained in the bedrock (RTi, 1995). The RTi report is included in Appendix B.2.
bella vira underdrain report. doe 3 ANdERSON CONSUtiiNg ENgiNEEas, INC
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The Bella Vira Development is a proposed single- and multi -family residential development
in western Fort Collins, Colorado. Figure 1 shows the location of the site and the surrounding
vicinity. The western border of the site is adjacent to the foothills of Colorado's front range. The
site is bounded on the north by Colorado State University's West Campus, on the south by The
Ponds subdivision, and on the east by Overland Trail. The total site area is approximately 31 acres.
Earth Engineering Company, Inc. (EEC) conducted a geotechnical investigation on the site
in November of 2004 for the purpose of characterizing the subsurface soil conditions prior to
development (EEC, 2004). EEC drilled 6 borings to depths ranging from 15 to 25 feet below present
site grades. The site is underlain by siltstone, sandstone, and claystone bedrock at depths ranging
lfrom 3 to 14 feet. In general, the overburden soils are composed of either a sandy lean clay or clayey
sand with scattered gravel. During drilling, water was encountered in two of the boreholes (B-3 and
` B-6) at depths ranging from 10 to 14 feet. Four of the boreholes (B-2, B-3, B-5 and B-6) were fitted
with piezometers to allow for long term monitoring, thus becoming piezometers P-1, P-2, P-3 and
P-4. Approximately one week after drilling, all four boreholes showed evidence of a ground water
table 6 to 12 feet below site grades with a general gradient indicating groundwater flow from west to
east. EEC monitored the groundwater conditions one additional time in January of 2005. At that
I time all four boreholes showed evidence of a water table 8 to 12 feet below site grades (EEC, 2005).
In August 2006, Anderson Consulting Engineers, Inc. (ACE) was contracted by OFP
—� Development Company (Developer) to design an underdrain system to lower the ground water table
below the level of proposed basements. This design effort included a determination of the amount of
ground water inflow to be conveyed, the impacts to the water table, and the engineering details of the
underdrain system, including potential maintenance requirements. ACE based design
recommendations on the rules and regulations enacted by the City of Fort Collins that govern the
design, construction, and maintenance of underdrain systems. These rules and regulations can be
found in the 2001 edition of Larimer County Urban Area Street Standards.
bella vira underdrain report.doc I ANdERSON CONSULTING ENGINEERS, INC.
TABLE OF CONTENTS (Continued)
Appendix C.1: Pre -Development Condition
Appendix C.2: Post -Development Scenario 1, Typical Conditions
Appendix C.3: Post -Development Scenario 2, High Water Conditions
Appendix D: Well Permit Application
bella vira underdrain report.doc
ANd ERSON CONSUITINC{ ENGINEERS, INC.
TABLE OF CONTENTS
1.0 INTRODUCTION............................................................................................................... 1
2.0 SITE CHARACTERIZATION........................................................................................... 3
2.1 Soil Profiles............................................................................................................. 3
2.2 Hydrogeologic Properties........................................................................................ 3
2.3 Registered Wells..................................................................................................... 4
2.4 Permitting................................................................................................................5
3.0 GROUND WATER FLOW ANALYSIS........................................................................... 6
3.1 Method of Analysis................................................................................................. 6
3.2 Underdrain Design Flow......................................................................................... 7
3.3 Outfall Design......................................................................................................... 7
4.0 UNDERDRAIN DESIGN RECOMMENDATIONS....................................................... 10
5.0 REFERENCES................................................................................................................. 14
FIGURES
Figure1 Vicinity Map............................................................................................................2
Figure 2 Underdrain Schematic..............................................................................................9
Figure 3 Cleanout Detail and Bedding/Filter Fabric Detail.................................................11
Figure 4 Conceptual Details of Perimeter Foundation Drains.............................................12
Figure 5 Underdrain Service Wye Detail.............................................................................13
APPENDICES
Appendix A: Site Characterization Data
Appendix A.1: EEC Report, November 2004
Appendix A.2: EEC Report, February 2005
Appendix B: Permeability Testing
i
Appendix B.1: Field Notes and Computer Output
Appendix B.2: RTi Report, September 1995
Appendix B.3: Ayres Report, September 1995
Appendix C: MODFLOW Model Input Parameters and Output
bella vira underdrain report.doc ( ANd ERSON CONSUITING ENGINEERS, INC.
ANALYSIS OF GROUND WATER UNDERDRAIN
FOR THE BELLA VIRA
DEVELOPMENT
PREPARED FOR:
OFP Development Company
203 7 Lexington Court
Fort Collins, CO 80526
PREPARED BY. -
Anderson Consulting Engineers, Inc.
772 Whalers Way, Suite 200
Fort Collins, CO 80525
(A.CE Project No. COFPD01)
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September 26, 2006
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ANdERSON CONSU[TING ENGINEERS, INC.
Civil • Water Resources • Environmental
November 16, 2006
Mr. John Minatta
OFP Development Company
2037 Lexington Court
Fort Collins, CO 80526
RE: Bella Vira Underdrain — Inundation Levels from Minatta Detention Pond
(ACE Project No. COOFPDO 1)
Dear John:
The following is a synopsis of the inundation levels in the Bella Vira underdrain system from the water
levels in the adjacent Minatta Detention Pond:
• Proposed invert of underdrain outfall pipe in the Minatta Pond = 5113.0 feet
• 10-year water surface elevation in the Minatta Pond = 5111.80 feet
• 25-year water surface elevation in the Minatta Pond = 5113.10 feet
• 50-year water surface elevation in the Minatta Pond = 5114.50 feet
• 100-year water surface elevation in the Minatta Pond = 5115.50 feet
Based on the preceding information, the underdrain system will start to be inundated by the water levels
in the detention pond in the 25-year event. It should be noted that this 25-year event will pond into the
solid outfall portion of the underdrain in private property, and will not affect the underdrain system within
the City of Fort Collins right-of-way. As mentioned in the report "Analysis of a Ground Water
Underdrain for the Bella Vira Development', September 26, 2006, the 100-year event in the Minatta Pond
will inundate the underdrain system to a point upstream of West Elizabeth Street and Banyan Drive to an
elevation of 5115.50 feet, which is 2.2 feet below the lowest basement elevation within the development
(5117.70 feet on Lot 1 at West Elizabeth Street).
If you have any questions or comments regarding this information, please do not hesitate to contact me.
Sincerely,
AN ERSON CONS TIN ENGINEERS, INC.
& l 014 -
Mark K. Kempton, P.E.
Project Engineer II
772 Whalers Way, Suite 200 • Fort Collins, CO. 80525
Phone: (970) 226-0120 • Fax: (970) 226-0121 • www.acowater.com