HomeMy WebLinkAbout1229 Peony Way - Special Inspections/Engineering - 02/16/2015b v2.q P'e'q UO cl
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February 16, 2015
Century Communities
8390 E. Crescent Parkway
Suite 650
Greenwood Village, Colorado 80111
Subject: Soils and Foundation Summary Letter
Lakeview Subdivision
Lot 28
Fort Collins, Colorado
FC05622.003-120
CTL I TH•OMPSON
i_ ADVISORY
Expansive soils are present at this
subdivision; this is a geologic hazard.
This letter describes the soil conditions
on this lot more specifically. Prospective
home buyers are strongly advised to read
this letter and the referenced documents.
If you do not understand the risk(s)
associated with the hazard and the
important role you must accept to
manage and mitigate the risk(s), we
recommend you contact a competent
geotechnical (soils) engineer for advice.
CTL I Thompson, Inc. performed a Soils and Foundation Investigation for lots within
the Lakeview Subdivision (CTL Project No. FC05622.003, report dated January 7, 2015) in
Fort Collins, Colorado. This letter presents a summary of our findings and recommendations
for the subject lot.
Colorado is a challenging location to practice geotechnical (soils) engineering. The
climate is relatively and and the near -surface soils are typically dry and relatively stiff. These
soils and related sedimentary bedrock formations tend to react to changes in moisture
conditions. Some of the soils and bedrock swell as they increase in moisture and are called
expansive soils. Other soils can settle significantly upon wetting and are referred to as
collapsing soils. Most of the land available for development east of the Front Range is
underlain by expansive clay or claystone bedrock near the surface. The soils that exhibit
collapse are more likely west of the continental divide; however, both types of soils occur
throughout the state.
Covering the ground with houses, streets, driveways, patios, etc., coupled with
landscape irrigation and changing drainage patterns, leads to an increase in subsurface
moisture conditions. As a result, some soil movement is inevitable. It is critical that all
recommendations in the referenced report are followed to increase the chances that
foundations and concrete slabs -on -grade will perform satisfactorily. After construction,
homeowners must assume responsibility for maintaining the structure and use appropriate
practices regarding drainage and landscaping.
In summary, the subsurface conditions encountered in the boring on this lot
consisted of sandy clay with occasional gravelly sand layers to a depth of 20 feet.
Groundwater was encountered at a depth of 15 feet. The soils are predominantly low
swelling. Use of a footing foundation system with a minimum dead load is recommended.
Slab -on -grade construction can be used for the basement floor. Foundation and floor slab
recommendations are discussed in greater detail in the following paragraphs.
Based on our investigation, low swelling soils are present at depths that will likely
influence the foundation and slab performance at this site. We believe there is low risk of
ground heave or settlement and associated damages of slabs -on -grade and foundations.
The foundations and slabs may settle if loose or soft soils are present underthe footings and
slabs. The risk of foundation and slab movements can be mitigated, but not eliminated by
careful design, construction and maintenance procedures. We believe the recommendations
400 North Link Lane I Suite 140 1 Fort Collins, Colorado 80524
Telephone:970-206-9455 Fax:970-206-9441
in our report will help control risk of foundation and slab damage; they will not eliminate that
risk. The builder and home buyers should understand that slabs -on -grade and, in some
instances, foundations may be affected by the subsoils. Homeowner maintenance will be
required to minimize this risk. We recommend the builder provide a booklet to the home
buyer that describes swelling soils and includes recommendations for care and maintenance
of homes constructed on expansive soils. Colorado Geological Survey Special Publication
43' was designed to provide this information.
Laboratory tests were performed on samples from this lot and nearby lots. Based
upon results of laboratory tests and other factors, we judge basement slab performance risk
for this lotto be low. Exhibit A provides a discussion of slab performance risk evaluation, as
well as slab installation and maintenance recommendations. If home buyers cannot tolerate
movement of a slab -on -grade basement floor, they should select a lot where a structurally
supported floor will be constructed or request that a structurally supported floor be installed.
Considering the subsurface conditions at this lot, we recommend construction of the
proposed residence on a footing foundation system. Footings should be designed for a
maximum allowable soil pressure of 1,500 psf and a minimum deadload of 500 psf. Footings
should be at least 16 inches in width. Column pads should be at least 20 inches square.
Exterior footings should be protected from frost action with at least 30 inches of cover. It is
sometimes necessary to alter the foundation design based on conditions exposed during
construction. The home buyer can discuss the changes, if any, with the builder.
Basement and/or foundation walls and grade beams that extend below grade should
be designed for lateral earth pressures where backfill is not present to about the same
extent on both sides of the wall. Our experience suggests basement walls can deflect or
rotate slightly under normal design loads and that this deflection typically does not affect the
structural integrity of the walls. We recommend design of the basement walls on this lot
using an equivalent fluid density of at least 50 pounds per cubic foot. This value assumes
slight deflection of the wall can occur, generally less than 0.5 to 1 percent of the wall height.
Some minor cracking of the walls may occur.
A subsurface drain is recommended around the entire perimeter of the lowest
excavation area for this residence. The drain should lead to a positive gravity outlet or to a
sump where water can be removed with a pump. The provision of the drain will not eliminate
slab movement or prevent moist conditions in crawl spaces. The pump must be maintained
by the homeowner.
Proper design, construction and maintenance of surface drainage are critical to the
satisfactory performance of foundations, slabs -on -grade, and other improvements.
Landscaping and irrigation practices will also affect performance. Exhibit B contains our
recommendations for surface drainage, irrigation, and maintenance.
The concept of risk is an important aspect with any geotechnical evaluation, primarily
because the methods used to develop geotechnical recommendations do not comprise an
exact science. We never have complete knowledge of subsurface conditions. Our analysis
must be tempered with engineering judgment and experience. Therefore, the
1"A Guide to Swelling Soils for Colorado I-Iomebuyers and Homeowners," Second Edition Revised and Updated by David C. Noe,
Colorado Geological Survey, Department of Natural Resources, Denver, Colorado, 2007.
CENTURY COMMUNITIES 2
LAKEVIEW SUBDIVISION
CTL I T PROJECT NO. FC05622.003-120
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EXHIBIT B
SURFACE DRAINAGE,
IRRIGATION AND MAINTENANCE
Performance of foundations and concrete flatwork is influenced by the moisture
conditions existing within the foundation soils. Surface drainage should be designed to
provide rapid runoff of surface water away from proposed residences. Good surface
drainage and irrigation practices can help control the amount of surface water which
penetrates to foundation levels and contributes to settlement or heave of soils and bedrock
which support foundations and slabs-on-grade. Good drainage away from the foundation
and avoidance of irrigation near foundations also help to avoid excessive wetting of backfill
soils, which can lead to increased backfill settlement and possibly to higher lateral earth
pressures due to increased weight and reduced strength of the backfill. CTL � Thompson,
Inc.recommends the following precautions. Home buyers should maintain surface drainage
and install irrigation systems which substantiallv conform to these recommendations.
1. Wetting or drying of the open foundation excavations should be avoided.
2. The ground surface surrounding the exterior of each residence should be sloped to
drain away from the building in all directions. We recommend a minimum
constructed slope of at least 12 inches in the first 10 feet(10%) in landscaped areas
around each residence, where practical.
We do not view the recommendation to provide a 10% slope away from the
foundation as an absolute: It is desirable to create this slope where practical, �
because we know that backfill will likely settle to some degree. By starting with good
slope, positive drainage can be maintained for most settlement conditions. There
are many situations around a residence where a 10% slope cannot be achieved
, � practically, such as around patios, at inside foundation corners, and between a ,
� house and nearby sidewalk. �In these areas,we believe it is desirable to establish as
much slope as practical, and to avoid irrigation in the area. We believe it is
acceptable to use a slope on the order of 5% perpendicular to the foundations in
, tfiese limited areas.
For lots graded to direct drainage from the rear.yard to the front, it is difficult to
achieve 10%slope at the high point behind the house. We believe it is acceptable to
use a slope of about 6 inches in the first 10 feet (5%) at this location. �,
Between houses which are separated by a distance less than 20 feet, the
constructed slope should generally be at least 10 percent to the swale used to
' convey water out of this area. For lots which.are graded to drain to the front and
� back,we believe it is acceptable to install a slope of 5 to 8 percent at the high point
� (aka"break point") between houses.
� 3. Swales used to convey water across yards and between houses should be sloped so
that water moves quickly and does not pond for extended periods of time. We
recommend a minimum slope of 2 percent in grassed areas,and believe 2.5 percent
' Exhibit B-1
.. �..
is preferable;this cannot always be achieved without using steep driveways or steps
from the house to the garage. The slope should also be at least 2 percent where
landscaping rock or other materials are present. If slopes less than about 2 percent
are necessary, concrete-lined channels or plastic pipe should be used.
4. Backfill around the foundation walls should be moistened and compacted.
5. Roof downspouts and drains should discharge well beyond the limits of all backfill:
Splash blocks and/or extensions should be provided at all downspouts so that water
discharges onto the ground beyond the backfill. We generally recommend against
burial of downspout discharge. Where it is necessary to bury downspout discharge,
solid pipe should be used and it should slope to an open gravity outlet. All
downspout extensions, splash block and buried outlets must be maintained by the
home buyer.
6. The importance of qood home owner irriqation practices cannot be over-
emphasized. Irriqation should be limited to the minimum amount sufficient to
maintain veqetation; application of more water will increase likelihood of slab and
foundation movements. Landscaping should be carefully designed and maintained
to minimize irrigation. Plants used close to foundation walls should be limited to
those with low moisture requirements. Irrigated grass should not be located within 5
� feet of the foundation. Sprinklers should not discharge within 5 feet of foundations.
Exhibit B 2