HomeMy WebLinkAboutLETTER Geotechnical Evaluation Report 2750 Ypsilon Cir 2021-09-28
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September 28, 2021
Nathan Kinley
Kinley Built
600 S. St Vrain Ave, Estes Park, CO 80517
RE: Geotechnical Observation for Lot 6A, Amended Mountain Meadow Subdivision
To Whom it May Concern:
As requested, Trail Ridge Consulting Engineers, LLC (TRCE) has performed a geotechnical
evaluation at the subject site. Included in this report are the results of the field evaluation and
review of the “Geologic Hazards and Mineral Extraction Evaluation Report Mountain Meadow
Subdivision” by Earth Engineering Consultants (EEC), and recommendations for foundation and
surface & subsurface drainage. The following is a summary of the findings and recommendations.
Project Description
This report presents the results of a geotechnical evaluation for planned single family residence at
the subject site. TRCE understands that the structure will be of a typical size and nature for a
single-family residence with a single story slab on grad or crawlspace construction. If the project
description differs from that depicted herein, TRCE should be notified to re-evaluate these
recommendations presented, if necessary.
9/28/21
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Site Conditions
The site is approximately .34 acres located in the Fall River valley of Estes Park, Colorado. The
property elevation is approximately 7980 feet. Ground cover consisted of natural landscaping,
previous earthwork (including removal of small buildings) and un-improved areas (Figure 1, 2
and 3). The ground surface slopes towards the north, northeast throughout the property, with the
slope flattening in the proposed project area. The property appears to have good surface drainage,
with no drainage channels or rills observed at the time of TRCE’s site visit (September, 2021).
Figure 1- Proposed project area- Looking South from road, North property corners circled
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Figure 2- Center of project area looking north, Toward Fall River Valley.
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Figure 3 Center of project area looking south. Note remnants of previous earthwork
Geotechnical Evaluation
The geotechnical evaluation of the site was completed by Earth Engineering Consultants in January
2016, the following description of soils consists of reported observations of the excavations on the site
in January 2016 (see referenced report) and TRCE’s site visit September, 2021. The United States
Department of Agriculture Soil Conservation Survey (Figure 4) attached map describes the surficial
soils mainly as gravelly to extremely gravelly, sandy to course sandy loams. The gravelly to extremely
gravelly, sandy loam surficial soils of the steep slopes of the Legault Series (21) are generally classified
as exhibiting moderate to severe erosion potentials with no shrink/swell potential. The gravelly to very
gravelly, sandy loam surficial soils of the slight to moderate slopes of the Lumpyridge-Rofork Complex
Series (23) are generally classified as exhibiting slight to moderate erosion potentials with low
shrink/swell potential.
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Utilizing TRCE’s site evaluation, USDA Soils Reports, and the EEC Mountain Meadow Subdivision
Report; TRCE concludes none-to-minimal risk for the following geologic hazards for the property
known as Lot 6A, Mountain Meadow Subdivision (2750 Ypsilon Circle, Estes Park, CO):
Avalanche- None to Minimal (property borders steep slopes on the south)
Landslide/Mud Flow/ Debris Fan- None to minimal (The property is located outside of direct
paths of potential flows, no existing signs of past damage was observed)
Rockfall/ Unstable Slopes- None to minimal (the southern portion of this property is listed as
Zone A, defined as a potential for rockfall if ground conditions change significantly)
Seismic- Class B
The below recommendations are meant to minimize potential impacts for geologic hazards associated
with this site.
Site Preparation
Grading and construction work is to be limited outside of the Zone A Rockfall Susceptibility area
(southern portion of property, Figure 5, “Geologic Hazards and Mineral Extraction Evaluation Report
Mountain Meadow Subdivision” attached).
Prior to site grading, areas to be graded should be stripped of surface vegetation, topsoil, debris and
other deleterious material. Stripped materials should be wasted off site in accordance with applicable
regulations. However, non-hazardous stripped soil with vegetation can be used as shallow fill in non-
structural, landscaped areas of the site. All manmade fill, demolition debris should be removed from
below the planned foundations and floor slabs and replaced with properly compacted structural fill.
Structural Fill Material: Imported structural fill material should be non-expansive at the recommended
placement moisture content and density, classify as a coarse-grained material (sand or gravel) by the
Unified Soil Classification System (USCS), and have a Liquid Limit less than 40% and a Plasticity
Index less than 20. Frozen or organic material, ice, snow, cobbles and boulders larger than 3 inches,
man-made debris and other deleterious material should be excluded from fill materials. TRCE should be
contacted to evaluate whether proposed onsite or imported materials are suitable structural fill. Suitable
imported structural materials would be: CDOT Class 5 or 6 road base, CDOT Class 1 Structure Backfill,
or a manufactured ‘crusher fines’ material. Imported materials should be approved by the Geotechnical
Engineer.
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Fill Placement: Structural fill should be placed in lifts 8 inches or less in loose thickness, moisture
conditioned and compacted to 95 percent or more of the standard Proctor Maximum Dry Density
(MDD). Fill which classifies as silt, sand or gravel by the USCS should be moisture conditioned to
within 3 percentage points of Optimum Moisture Content (OMC) and fill which classifies as clay by the
USCS should be moisture conditioned to within OMC to 4 percentage points above OMC. Fill in non-
structural areas should be compacted to at least 90 percent of the MDD.
Foundation Recommendations
The native materials encountered in the excavations completed as part of the EEC report consist of silty,
gravelly sand with an expected low consolidation potential. The design of all foundation elements
should be prepared using an allowable maximum soil bearing pressure of 2,500 pounds per square foot
(PSF). The design bearing pressure applies to dead loads plus design live load conditions. Existing fill at
the site should not be used for support of foundations without removal and re-compaction.
Footings: Based on the results of our subsurface observations, it is our opinion that continuous spread
footing foundations may be used for support of the proposed residence. New pad footings should have a
minimum dimension of 24 inches square and continuous footings should have a minimum width of 16
inches. All new footings should have a minimum embedment of 30 inches below final grade for frost
protection and should be placed directly on native, undisturbed soils or engineered fill.
Foundation Movement: The risk of heave due to potentially expansive soils appears to be low for the
site location. As discussed above, generalized mapping for the site location indicates a Low Swell
Potential. Because a test boring was not performed on this site, the potential foundation movement
cannot be quantitatively determined based only on the foundation excavation observations and surficial
soil sampling and testing. Foundation excavations should be observed by TRCE prior to forming
concrete elements.
Slab-on-Grade Recommendations
The following recommendations should be incorporated into the design and construction of slabs on-
grade on the site.
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1. Slabs-on-grade are commonly designed to be free to move in the vertical direction and are not rigidly
attached to structures. Due to a LOW risk of slab heave due to potentially expansive soils, floor slabs on
this site can be attached to foundation walls if desired due to lateral resistance or other reasons, with a
low risk of heaving and cracking.
2. Concrete slab-on-grade cracking can also result from expansion and shrinkage. Slabs should be
provided with shrinkage control joints. ACI recommends that control joints be spaced no more than 3
feet per inch of slab thickness (12 feet for a 4-inch thick slab). Control joints should also be established
3 feet from foundation walls. Control joints should be continuous across slabs sections. Concrete slabs
should be properly cured using curing compound or water.
3. Cracks and movement of slabs-on-grade can be transmitted through rigid floor coverings such as
ceramic tile. Performance expectations should be taken into consideration in the selection of floor slab
coverings.
4. Slab bearing mechanical units should be provided with flexible connections to allow movement of the
slab. This is not required on monolithic mat foundations or where slabs are tied to foundation walls.
Because a test boring was not performed on this site, the potential floor slab movement cannot be
determined. Based on our evaluation the risk of slab heave at the ground surface appears to be low in the
addition area. The past performance of slabs on the site should be considered when evaluating the risk of
slab on grade construction for this site.
Lateral Earth Pressures
For soils above any free water surface, recommended equivalent fluid pressures for unrestrained
foundation elements are:
1. Active: Cohesionless soil backfill (on-site sands) – 35 psf/ft.
Where the design includes retrained elements, the following equivalent fluid
pressures are recommended:
1. At Rest: Cohesionless soil backfill (on-site sands) – 50 psf/ft
The lateral earth pressures do not include safety factors nor hydrostatic loading.
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Surface and Subsurface Drainage Recommendations
The following drainage precautions should be observed and maintained at all times during and after
construction:
1. Excessive wetting of the foundation excavation and sub-floor soils should be avoided.
2. Exterior backfill should be placed as recommended herein.
3. The ground surface should be sloped away from the building in all directions. TRCE recommends a
minimum slope of 6 inches in the first 5 to 10 feet in landscaped areas (10% min) and 3 inches in 10 feet
in paved areas (3% min) is recommended.
4. Roof down spouts should discharge well beyond foundation wall backfill zones (5 feet minimum).
5. Any settling of foundation or foundation wall backfill should be re-graded to maintain the slopes
noted above. A perimeter foundation drainage system is typically used to help protect below grade,
finished living space in new construction. Our office should be contacted for information regarding
subsurface drainage if desired. Positive surface drainage should be developed and maintained adjacent to
below grade areas as part of the planned construction.
Limitations
This report was prepared for the exclusive use of TRCE’s client for the referenced project on the subject
site. Information or recommendations in this report should not be used for other purposes without the
express written consent of TRCE. TRCE has endeavored to perform the geotechnical engineering
services for this project in a manner consistent with the practices of the geotechnical engineering
profession at the time and in the location the services were performed. TRCE makes no warranties,
expressed or implied. Evaluation, sampling and testing of the surface soils or water or subsurface soils
or groundwater for the presence of hazardous or toxic materials was not included in TRCE’s scope of
services for this project.
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We appreciate the opportunity to provide geotechnical engineering services for this project. If there are
any questions about the information contained herein please don’t hesitate to contact me.
Sincerely,
David A. Bangs, PE
Trail Ridge Consulting Engineers
Ph: 970-308-8221
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Custom Soil Resource Report
Soil Map
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Map projection: Web Mercator Corner coordinates: WGS84 Edge tics: UTM Zone 13N WGS84
0 50 100 200 300
Feet
0 25 50 100 150
Meters
Map Scale: 1:1,690 if printed on A portrait (8.5" x 11") sheet.
Soil Map may not be valid at this scale.
Figure 4- USDA Soils Map
MAP LEGEND MAP INFORMATION
Area of Interest (AOI)
Area of Interest (AOI)
Soils
Soil Map Unit Polygons
Soil Map Unit Lines
Soil Map Unit Points
Special Point Features
Blowout
Borrow Pit
Clay Spot
Closed Depression
Gravel Pit
Gravelly Spot
Landfill
Lava Flow
Marsh or swamp
Mine or Quarry
Miscellaneous Water
Perennial Water
Rock Outcrop
Saline Spot
Sandy Spot
Severely Eroded Spot
Sinkhole
Slide or Slip
Sodic Spot
Spoil Area
Stony Spot
Very Stony Spot
Wet Spot
Other
Special Line Features
Water Features
Streams and Canals
Transportation
Rails
Interstate Highways
US Routes
Major Roads
Local Roads
Background
Aerial Photography
The soil surveys that comprise your AOI were mapped at
1:24,000.
Warning: Soil Map may not be valid at this scale.
Enlargement of maps beyond the scale of mapping can cause
misunderstanding of the detail of mapping and accuracy of soil
line placement. The maps do not show the small areas of
contrasting soils that could have been shown at a more detailed
scale.
Please rely on the bar scale on each map sheet for map
measurements.
Source of Map: Natural Resources Conservation Service
Web Soil Survey URL:
Coordinate System: Web Mercator (EPSG:3857)
Maps from the Web Soil Survey are based on the Web Mercator
projection, which preserves direction and shape but distorts
distance and area. A projection that preserves area, such as the
Albers equal-area conic projection, should be used if more
accurate calculations of distance or area are required.
This product is generated from the USDA-NRCS certified data as
of the version date(s) listed below.
Soil Survey Area: Estes Park Area, Colorado, Parts of Boulder
and Larimer Counties
Survey Area Data: Version 6, Jun 5, 2020
Soil map units are labeled (as space allows) for map scales
1:50,000 or larger.
Date(s) aerial images were photographed: Sep 30, 2010—Oct 7,
2017
The orthophoto or other base map on which the soil lines were
compiled and digitized probably differs from the background
Custom Soil Resource Report
10 Figure 4- Continued
Figure 1: Van Horn Engineering Preliminary Subdivision Plat Map
Mountain Meadow Subdivision
Larimer County, Colorado
EEC Project #: 1162002 Date: February 2016
EARTH ENGINEERING CONSULTANTS, LLC
Area of Geologic
Hazard
Figure 5
Figure 5: Rockfall Hazard Susceptibility in the Estes Valley Development Code Area, Estes Park, Larimer County, CO
by Wait, T.C., Berry, K - CGS 2006
Mountain Meadow Subdivision - Larimer County, Colorado
EEC Project #: 1162002 Date: February 2016
EARTH ENGINEERING CONSULTANTS, LLC
Project Site
Legend
Zone A Rockfall Susceptibility Zone B Rockfall Susceptibility
Figure 5- Continued