HomeMy WebLinkAbout444 ZEPPELIN WAY - SPECIAL INSPECTIONS - 3/1/201744 w
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March 1, 2017
CalAtlantic Group, Inc
6161 South Syracuse Way
Suite 200
Greenwood Village, Colorado 80111
Subject. Soils and Foundation Summary Letter
East Ridge Phase 1
Lot 7/8, Block 4
Fort Collins, Colorado
CTLIT Project No FC06953 004-120 R2
ft HOME BUYER ADVISORY
Expansive sods are present at this subdivision,
this results in a geologic hazard This letter
describes the soil conditions on this lot more
specifically Prospective home buyers are 3
strongly advised to read this letter and the
referenced documents.
.i,
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 geotech-
nical (sods) engineer for advice,
CTL I Thompson, Inc is performing a Soils and Foundation Investigation for res-
idential construction in East Ridge Subdivision (CTLIT Project No FC06953 004-120)
This letter presents a summary of our findings and recommendations for the subject lot
The report referenced above should be reviewed for foundation design
Colorado is a challenging location to practice geotechnical engineering The cli-
mate is relatively dry and the near -surface soils are typically dry and comparatively
stiff These soils and related sedimentary bedrock formations tend to react to changes in
moisture conditions Some of the soils swell as they increase in moisture and are
referred to as expansive soils Other soils can compress significantly upon wetting and
are identified as collapsing soils Most of the land available for development east of the
Front Range is underlain by expansive clay and/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,
home owners must assume responsibility for maintaining the structure and use appro-
priate practices regarding drainage and landscaping
In summary, the strata encountered in the boring on this lot consisted of 7 feet of
fill underlain by interlayered clay and sand to the maximum depth explored of 20 feet
Groundwater was encountered at a depth of 19 feet Use of footings with minimum
deadload is recommended A slab -on -grade concrete basement floor can be used
provided risk of movement and cracking is acceptable Further details are described in
following paragraphs
Expansive soils are present at this site This constitutes a geologic hazard There
is risk that ground heave or settlement will damage slabs -on -grade and foundations The
nsks associated with swelling soils can be mitigated but not eliminated by careful design,
construction and maintenance procedures We believe the recommendations in the
400 North Link Lane I Fort Collins, Colorado 80521
Telephone (970) 206-9455 Fax (970) 206-9441
referenced report will help control risk of foundation and/or 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 swelling soils Home owner
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
Samples from this lot swelled 0 to 1 3 percent when wetted Based upon results of
laboratory tests and other factors, we judge basement slab performance risk for this lot
to be low Exhibit A provides a discussion of slab performance risk evaluation, as well as
slab installation and maintenance recommendations We performed calculations of total
potential ground heave and basement heave as part of our study The calculated
potential heave for this lot is 0 8 inch at the ground surface and less than 0 5 inch at the
basement level It is not certain this movement will occur. If home buyers cannot toler-
ate 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 with minimum deadload Footings
should be designed for a maximum allowable soil pressure of 2,000 psf with a minimum
deadload pressure of 600 psf Footings should be at least 16 inches in width Column
pads should be at least 20 inches square There should be a 4-inch continuous void
between the footings to concentrate the deadload, if interrupted footings are necessary
Exterior footings should be protected from frost action with at least 30 inches of cover
We recommend over -excavation, moisture treatment and recompaction 4 feet below
non -basement footings, or extending all footings to basement level It is sometimes
necessary to alter the foundation design based on conditions exposed during
construction The 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 backfiii 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 55 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 home owner
"A Guide to Swelling Soils for Colorado Homebuyers and Homeowners; Second Edition Revised and Updated by David
C Noe, Colorado Geological Survey, Department of Natural Resources, Denver, Colorado, 2007
CALATLANTIC GROUP
EAST RIDGE PHASE 1
LOT 7/8, BLOCK 4
CTL I T PROJECT NO FC06953 004-120 R2
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, pn-
manly because the methods used to develop geotechnlcal recommendations do not
comprise an exact science We never have complete knowledge of subsurface condi-
tions Our analysis must be tempered with engineering judgment and experience
Therefore, the recommendations presented in any geotechnlcal evaluation should not be
considered risk -free Our recommendations represent our judgment of those measures
that are necessary to Increase the chances that the structure will perform satisfactorily It
is critical that all recommendations in the referenced report are followed Home owners
must assume responsibility for maintaining the structure and use appropriate practices
regarding drainage and landscaping
As this letter is meant only as a summary of our findings and recommendations
for the subject lot, we recommend home buyers review the Soils and Foundation Inves-
tigation from which this summary is taken
CTL I THOM
46"6
f?-0�•!
Spencer Schi
Geotechnlcal
3
CALATLANTIC GROUP
EAST RIDGE PHASE 1
LOT 7/8, BLOCK 4
CTL I T PROJECT NO FC06953 004-120 R2
EXHIBIT A
SLAB PERFORMANCE RISK EVALUATION,
INSTALLATION AND MAINTENANCE
As part of our evaluation of the subsurface soils and bedrock, samples
were tested in the laboratory using a swell test In the test procedure, a relatively
undisturbed sample obtained during drilling is first loaded and then flooded with
water and allowed to swell The pressure applied prior to wetting can
approximate the weight of sod above the sample depth or be some standard
load The measured percent swell is not the sole criteria in assessing potential
movement of slabs -on -grade and the risk of poor slab performance The results
of a swell test on an individual lot are tempered with data from surrounding lots,
depth of tests, depth of excavation, sod profile, and other tests This judgment,
has been described by the Colorado Association of Geotechnical Engineers'
(CAGE, 1996) as it relates to basement slab -on -grade floors It can also be used
to help judge performance risk for other slabs -on -grade such as garage floors,
driveways, and sidewalks The risk evaluation is considered when we evaluate
appropriate foundation systems for a given site In general, more conservative
foundation designs are used for higher risk sites to control the likelihood of
excessive foundation movement.
As a result of the Slab -Performance Risk Evaluation, sites are categorized
as low, moderate, high, or very high risk. This is a judgment of the swelling
characteristics of the soils and bedrock likely to influence slab performance.
REPRESENTATIVE MEASURED SWELL
AND CORRESPONDING SLAB
PERFORMANCE RISK CATEGORIES
Slab
Performance
Risk Category.
Representative Percent
Swell*
(500 psf Surcharge)
Representative Percent
Swell*
(1000:psf Surcharge)
Low
0 to <3
0 to <2
Moderate
3 to <5
2 to <4
High
5 to <8
4 to <6
Very High
> 8
> 6
*Note The representative percent swell values presented are not necessarily
measured values, rather, they are a judgment of the swelling
characteristics of the soil and bedrock likely to influence slab
performance
m
3'Gwdeline for Slab Performance Rusk Evaluation and Residential Basement Floor System Recommendations', Colorado
Association of Geotechnical Engineers, December 1996
EXHIBIT A-1
The rating of slab performance risk on a site as low or high is not absolute
Rather, this rating represents a judgment Movement of slabs may occur with
time in low, moderate, high, and very high risk areas as the expansive sods
respond to increases in moisture content Overall, the seventy and frequency of
slab damage usually is greater In high and very high rated areas Heave of slabs -
on -grade of 3 to 5 Inches Is not uncommon in areas rated as high or very high
risk On low and moderate risk sites, slab heave of 1 to 2 Inches Is considered
normal and we believe In the majority of Instances, movements of this magnitude
constitute reasonable slab performance, more heave can occur. Slabs can be
affected on all sites. On lots rated as high or very high risk, there is more
likelihood of need to repair, maintain or replace basement and garage floors and
exterior flatwork.
CTL I Thompson, Inc recommends use of structurally supported
basement floors, known as "structural floors," for lots rated as high and very high
risk We also recommend use of structural basement floors on walkout and
garden level lots rated as moderate, high or very high risk 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
The home buyer should be advised the floor slab In the basement may
move and crack due to heave or settlement and that there may be maintenance
costs associated during and after the builder warranty period A buyer who
chooses to finish a basement area must accept the risk of slab heave, cracking
and consequential damages. Heave or settlement may require maintenance of
finish details to control damage Our experience suggests that sod moisture
Increases below residence sites due to covering the ground with the house and
exterior flatwork, coupled with the introduction of landscape irrigation In most
cases, slab movements (If any) resulting from this change occur within three to
five years We suggest delaying finish in basements with slab -on -grade floors
until at least three years after start of Irrigation It is possible basement floor slab
and finish work performance will be satisfactory If a basement Is finished earlier,
particularly on low risk sites
For portions of the houses where conventional slabs -on -grade are used,
we recommend the following precautions These measures will not keep slabs -
on -grade from heaving, they tend to mitigate damages due to slab heave
Slab -on -grade floor construction should be limited to areas such as
garages and basements where slab movement and cracking are
acceptable to the builder and home buyer
2 Slabs should be placed directly on the exposed subsoils or properly
moisture conditioned, compacted fill The 2009 International
Building Code (IBC) or 2009 International Residential Code (IRC)
requires a vapor retarder be placed between base course or
EXHIBIT A-2
IF
subgrade soils and the concrete slab -on -grade floor The merits of
Installation of a vapor retarder below floor slabs depend on the
sensitivity of floor coverings and building use to moisture A
properly Installed vapor retarder (10 mil minimum) is more
beneficial below concrete slab -on -grade floors where floor
coverings, painted floor surfaces, or products stored on the floor will
be sensitive to moisture The vapor retarder is most effective when
concrete is placed directly on top of it, rather than placing a sand or
gravel leveling course between the vapor retarder and the floor
slab Placement of concrete on the vapor retarder. may increase
the risk of shrinkage cracking and curling Use of concrete with
reduced shrinkage characteristics Including minimized water
content, maximized coarse aggregate content, and reasonably low
slump will reduce the risk of shrinkage cracking and curling
Considerations and recommendations for the installation of vapor
retarders below concrete slabs are outlined in Section 3.2 3 of the
2006 Amencan Concrete Institute (ACI) Committee 302, "Guide for
Concrete Floor and Slab Construction (ACI 302 R-96)".
3. Conventional slabs should be separated from exterior•walls and
interior bearing members with a slip joint that allows free vertical
movement of the slabs. These joints must be maintained by the
home buyer to avoid transfer of movement
4., Underslab plumbing should be thoroughly pressure tested during
construction for leaks and be provided with flexible couplings Gas
and waterlines leading to slab -supported appliances should be
constructed with flexibility. The homebuyer must maintain these
connections
5. Use of slab bearing partitions should be minimized. Where such
partitions are necessary, a slip joint (or float) allowing at least
2 inches of free vertical slab movement should be used Doorways
should also be designed to allow vertical movement of slabs To
limit damage in the event of movement, sheetrock should not
extend to the floor. The home buyer should monitor partition voids
and other connections and re-establish the voids before they close
to less than 1/2-inch
6 Plumbing and utilities that pass through slabs should be isolated
from the slabs Heating and air conditioning systems constructed
on slabs should be provided with flexible connections capable of at
least 2 inches of vertical movement so slab movement is not
transmitted to the ductwork These connections must be maintained
by the home buyer.
EXHIBIT A-3
7: Roofs that overhang a patio or porch should be constructed on the
same foundation as the residence Isolated piers or pads may be
installed beneath a roof overhang provided the slab is Independent
of the foundation elements Patio or porch roof columns may be
positioned on the slab, directly above the foundation system,
provided the slab is structural and supported by the foundation
system Structural porch or patio slabs should be constructed to
reduce the likelihood that settlement or heave will affect the slab by
placing loose backfill under the structurally supported slab or
constructing the slab over void -forming materials
8. Patio and porch slabs without roofs and other exterior flatwork
should be isolated from the foundation Movements of slabs should
not be transmitted to the foundation Decks are more flexible and
more easily adjusted in the event of movement
9 Frequent control joints should be provided in conventional slabs -on -
grade to reduce problems associated with shrinkage cracking and
curling Panels that are approximately square generally perform
better than rectangular areas. We suggest an additional joint about
3 feet away from and parallel to foundation walls
EXHIBIT A4
EXHIBIT B
SURFACE DRAINAGE,
IRRIGATION AND MAINTENANCE
IMA
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 Proper surface drainage and irrigation practices can help control the
amount of surface water that penetrates to foundation levels and contributes to
settlement or heave of sods and bedrock that support foundations and slabs -on -
grade. Positive drainage away from the foundation and avoidance of irrigation
near the foundation also help to avoid excessive wetting of backfill sods, 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 I Thompson, Inc recommends the following precautions The homebuyer
should maintain surface drainage and Install an irri at�n system that
substantially conforms 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 percent) In landscaped areas around each
residence, where practical
We do not view the recommendation to provide a 10 percent 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 sufficient slope, positive
drainage can be maintained for most settlement conditions. There
are many situations around a residence where a 10 percent 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 We believe It is acceptable to
use a slope on the order of 5 percent perpendicular to the
foundation In these limited areas.
For lots graded to direct drainage from the rear yard to the front, it
is difficult to achieve 10 percent 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 percent) at this location
EXHIBIT B-1
Between houses that are separated by a distance of 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 that 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 advocate a
minimum slope of about 2 5 percent in grassed areas and 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 drams 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 Downspout extensions, splash blocks
and buried outlets must be maintained by the homebuyer
6. The Importance of proper homeowner irrigation practices cannot be
over -emphasized Irrigation should be limited to the minimum
amount sufficient to maintain vegetation; application of more water
will Increase likelihood of slab and foundation movements
Landscaping should be carefully designed and maintained to
minimize irrigation Plants placed 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 Plastic sheeting
should not be placed beneath landscaped areas adjacent to
foundation walls or grade beams. Geotextile fabric will inhibit weed
growth yet still allow natural evaporation to occur
EXHIBIT B-2