HomeMy WebLinkAboutSubsoil Study for Foundation Design 02.05.19l(+rt
::
Kurmr & Àsg¡cffin lnc.
ûeotechnical and Malerials Engineers
and Environmental Scientisls
kumarusa.com
5020 County Road 154
Glenwood Springs, CO 81601
phone: (970) 945-7988
fax: (970) 945-8454
email: kaglenwood@kumarusa.com
www.kumarusa.com
ACEC
MEMRER
Off¡ce Locations: Denver (HQ), Parker, Colorado Springs, Fort Collins, Glenwood Spr¡ngs and Summit County, Colorado
RECEIVED
,APR 2 2 2OI9
GARFIELD COUNTY
COMMUNITY DEVELOPMENT
SUBSOIL STUDY
FOR FOT]NDATION DESIGN
PROPOSED RESIDENCE
LOT 3, BLOCK I, OAK MEADOWS 3
TBD OAK CREST DRIVE
GARFIELD COUNTY, COLORADO
PROJECT NO. 19-7-117
FEBRUARY 5,2019
PREPARED FOR:
HEYL CONSTRUCTION
ATTN: DAVE HEYL
6560 COIINTY ROAD 335
NEW CASTLA, COLORADO 81647
d.he yl(ãihevlcivil.co m
TABLE OF CONTENTS
PURPOSE AND SCOPE OF STUDY .....
I
I
PROPOSED CONSTRUCTION
SITE CONDITIONS
FIELD EXPLORATION
SUBSURFACE CONDITIONS
FOUNDATION BEARING CONDITIONS .....
DESIGN RECOMMENDATIONS .......
FOLINDATIONS
FOIINDATION AND RETAINING WALLS
FLOOR SLABS
UNDERDRAIN SYSTEM
SITE GRADING............
SURFACE DRAINAGE
PERCOLATION TESTING
LIMITATIONS
FIGURE 1 - LOCATION OF EXPLORATORY PITS
FIGURE 2 - LOGS OF EXPLORATORY PITS
FIGURES 3 AND 4 - SWELL-CONSOLIDATION TEST RESULTS
FIGURE 5 _ USDA GRADATION TEST RESULTS
TABLE I- SUMMARY OF LABORATORY TEST RESULTS
TABLE 2 _ PERCOLATION TEST RESULTS
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Kumar & Associates, lnc.Project No. 19-7-117
PURPOSE AND SCOPE OF STUDY
This report presents the results ofa subsoil study for a proposed residence to be located on Lot 3,
Block 1, Oak Meadows 3, TBD Oak Crest Drive, Garfield County, Colorado. The project site is
shown on Figure 1. The purpose of the study was to develop recommendations for foundation
design. The study was conducted in accordance with our agreement for geotechnical engineering
services to Heyl Construction, dated January 21,2019.
A field exploration program consisting of exploratory pits was conducted to obtain information
on the subsurface conditions. Samples of the subsoils obtained during the field exploration were
tested in the laboratory to determine their classification, compressibility or swell and other
engineering characteristics. The results of the field exploration and laboratory testing were
analyzedto develop recommendations for foundation types, depths and allowable pressures for
the proposed building foundation. This report summarizes the data obtained during this study
and presents our conclusions, recommendations and other geotechnical engineering
considerations based on the proposed construction and the subsurface conditions encountered.
PROPOSED CONSTRUCTION
At the time of our study, design plans for the residence had not been developed. The building is
proposed in the area roughly between exploratory Pits 1 and 2 shown on Figure l. We assume
excavation for the building will have a maximum cut depth of one level, about 10 feet below the
existing ground surface. For the purpose ofour analysis, foundation loadings for the structure
\Mere assumed to be relatively light and typical of the proposed type of construction.
If building loadings, location or grading plans are significantly different from those described
above, we should be notified to re-evaluate the recommendations contained in this report.
SITE CONDITIONS
The proposed development area is located on a north'trending, broad ridge with slopes between
5 and 15 percent. Steeper slopes up to about 40 percent are located downhill to the north and
northwest of the development area. The proposed building area had been stripped of vegetation
Kumar & Associates, lnc.Project No. '19"7-117
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and topsoil at thc tirne t¡f our site visit. Vegetation in the surrounding area consistctl uf nrostly
oak brush with an understory of grass and weeds. Basalt boulders up to about 5 feet in size were
observed scattered on the natural ground surface. There was about lYz to 2 feet of snow cover at
the time of our site visit.
FIELD EXPLORATION
The field exploration for the project was conducted on January 22,2019. Four exploratory pits
were excavated with a trackhoe at the locations shown on Figure 1 to evaluate the subsurface
conditions. The pits were logged by a representative of Kumar & Associates, Inc.
Samples of the subsoils were taken with relatively undisturbed and disturbed sampling methods.
Depths at which the samples were taken are shown on the Logs of Exploratory Pits, Figure 2.
The samples were retumed to our laboratory for review by the project engineer and testing.
SUBSURFACE CONDITIONS
Graphic logs of the subsurfaoe profiles encountered at the site are shown on Figure 2. In Pits 1
and 2 in the proposed building area, the subsoils consisted of very stiff, silty sandy clay with
scattered gravel and basalt cobbles and boulders down to the maximum depth explored of 8 feet.
The soils encountered in the pits are similar to the soils encountered at other nearby lots. The
clay portions of these soils can possess an expansion potential when wetted.
Laboratory testing performed on samples obtained during the field exploration included natural
moisture content and density. Swell-consolidation testing was performed on relatively
undisturbed liner samples of the clay subsoils. The swell-consolidation test results, presented on
Figures 3 and 4, indicate low compressibility under relatively light surcharge loading and
typically a low to moderate expansion potential when wetted under a constant light surcharge.
The laboratory testing is summarized in Table 1.
No free water was encountered in the pits at time of excavation. The subsoils were slightly moist
to moist.
FOUNDATION BEARING CONDITIONS
The subsoils encountered at the site possess low to moderate expansion potential when wetted.
The expansion potential can probably be partly mitigated by load concentration to reduce or
Kumar & Associates, lnc.Projecr No. 19.7-117
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prevent swelling in the event of wetting below the foundation bearing level. The rock content of
the soils will also be a mitigating factor but the overall clay soils could still be expansive when
wetted. Surface runoff, landscape irrigation, and utility leakage are possible sources of water
which could cause wetting.
DESIGN RECOMMENDATIONS
FOUNDATIONS
Considering the subsurface conditions encountered in the exploratory pits and the nature of the
proposed construction, we recommend the residence be founded with spread footings placed on
undisturbed natural clay soils with a risk of differential foundation movement.
The design and construction criteria presented below should be observed for a spread footing
foundation system.
1) Footings placed on the undisturbed natural soils can be designed for an allowable
bearing pressure of 3,000 psf. The footings should also be designed for a
minimum dead load pressure of 800 psf. In order to satisfu the minimum dead
load pressure under lightly loaded areas, it may be necessary to concentrate loads
by using a grade beam and pad system. Wall-on-grade construction is not
recommended at this site to achieve the minimum dead load.
2)Based on experience, \rye expect initial settlement of footings designed and
constructed as discussed in this section will be up to about I inch. There could be
some additional movement if the bearing soils were to become wet of around
I inch.
The footings should have a minimum width of i6 inches for continuous footings
and 24 inches for isolated pads.
Continuous foundation walls should be reinforced top and bottom to span local
anomalies and limit the risk of differential movement. One method of analysis is
to design the foundation wall to span an unsupported length of at least 14 feet.
Foundation walls acting as retaining structures should also be designed to resist a
lateral earth pressure as discussed in the "Foundation and Retaining Walls"
section of this report.
Exterior footings and footings beneath unheated areas should be provided with
adequate soil cover above their bearing elevation for frost protection. Placement
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of foundations at least 36 inches below the exterior gratle is typically used ilr tltis
area.
Prior to the footing construction, any existing topsoil and loose or disturbed soils
should be removed and the footing bearing level extended down to competent
bearing soils.
A representative ofthe geotechnical engineer should observe all footing
excavations prior to concrete placement to evaluate bearing conditions.
FOTINDATION AND RETAINING WALLS
Foundation walls and retaining structures which are laterally supported and can be expected to
undergo only a slight amount of deflection should be designed for a lateral earth pressure
computed on the basis of an equivalent fluid unit weight of at least 60 pcf for backfill consisting
of the on-site clay soils and at least 50 pcf for backfill consisting of imported granular materials.
Cantilevered retaining structures which are separate from the residence and can be expected to
deflect sufficiently to mobilize the full active earth pressure condition should be designed for a
lateral earth pressure computed on the basis of an equivalent fluid unit weight of at least 50 pcf
for backfill consisting of the on-site clay soils and at least 40 pcf for backfill consisting of
imported granular material s.
All foundation and retaining structures should be designed for appropriate hydrostatic and
surcharge pressures such as adjacent footings, traffic, construction materials and equipment. The
pressures recommended above assume drained conditions behind the walls and a horizontal
backfill surface. The buildup of water behind a wall or an upward sloping backfill surface will
increase the lateral pressure imposed on a foundation wall or retaining structure. An underdrain
should be provided to prevent hydrostatic pressure buildup behind walls.
Backfill should be placed in uniform lifts and compacted to at least 90o/o of the maximum
standard Proctor density at a moisture content slightly above optimum. Backfill placed in
pavement areas should be compacted to at leastg1o/o of the maximum standard Proctor density.
Care should be taken not to overcompact the backfill or use large equipment near the wall since
this could cause excessive lateral pressure on the wall. Some settlement of deep foundation wall
backfill should be expected even if the material is placed correctly ancl could result in tlistress to
facilities constructed on the backfill.
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'We recommend imported granular soils for backfilling foundation walls and retaining structures
because their use results in lower late,ral earth pressures. Imported granular wall backfill should
contain less than 15% passing the No. 200 sieve and have a maximum size of 6 inches. Granular
materials should be placed within 2 feet of the ground surface and to a minimum of 3 feet
beyond the walls. The granular backfill behind foundation and retaining walls should extend to
an envelope defined as a line sloped up from tlre base of the wall at an angle of at least 30
degrees from the vertical. The upper 2 feet of the wall backfill should be a relatively impervious
on-site soil or a pavement structure should be provided to prevent surface water infiltration into
the backfill.
The lateral resistance of foundation or retaining wall footings will be a combination of the
sliding resistance of the footing on the foundation materials and passive earth pressure against
the side of the footing. Resistance to sliding at the bottoms of the footings can be calculated
based on a coefficient of friction of 0.30. Passive pressure of compacted backfill against the
sides of the footings can be calculated using an equivalent fluid unit weight of 350 pcf. The
coefficient of friction and passive pressure values recommended above assume ultimate soil
strength. Suitable factors of safety should be included in the design to limit the strain which will
occur at the ultimate strength, particularly in the case of passive resistance. Fill placed against
the sides of the footings to resist lateral loads should be a nonexpansive, granular material
compacted to at least 95Yo of the maximum standard Proctor density at a moisture content near
optimum.
FLOOR SLABS
The on-site soils possess an expansion potential and slab heave could occur ifthe subgrade soils
were to become wet. Slab-on-grade construction can be used provided precautions are taken to
limit potential movement and the risk of distress to the building is accepted by the owner. A
positive way to reduce the risk of slab movement, which is commonly used in the area, is to
construct structurally supported floors over crawlspace.
To reduce the effects of some differential movement, nonstructural floor slabs should be
separated from all bearing walls and columns with expansion joints which allow unrestrained
vertical movement. Interior non-bearing partitions resting on floor slabs should be provided with
a slip joint at the bottom of the wall so that, if the slab moves, the movement cannot be
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transmitted to the upper structure. This detail is also important for wallboards, stairways and
door frames. Slip joints which will allow at least 1% inches of vertical movement are
recommended. Floor slab control joints should be used to reduce damage due to shrinkage
cracking. Slab reinforcement and control joints should be established by the designer based on
experience and the intended slab use.
A minimum 4 inch layer of free-draining gravel should be placed immediately beneath basement
level slabs-on-grade. This material should consist of minus 2 inch aggregate with less than 50o/o
passing the No. 4 sieve and less thanTo/o passing the No. 200 sieve. The fi'ee-draining gravel
will aid in drainage below the slabs and should be connected to the perimeter underdrain systern.
Required fill beneath slabs should consist of imported granular material, such as 3/¿ inchroad
base, excluding topsoil and oversized rocks. The fill should be spread in thin horizontal lifts,
adjusted to at or above optimum moisture content, and compacted to at least 95o/a of the
maximum standard Proctor density. All vegetation, topsoil and loose or disturbed soil should be
removed prior to fill placement.
The above recommendations will not prevent slab heave if the expansive soils underlying slabs-
on-grade become wet. However, the recommendations will reduce the effects if slab heave
occurs. Al1 plumbing lines should be pressure tested before backfilling to help reduce the
potential for wetting.
UNDERDRAIN SYSTEM
Although groundwater was not encountered during our exploration, it has been our experience in
this area and where clay soils are present, that local perched groundwater can develop during
times of heavy precipitation or seasonal runoff. Frozen ground during spring runoff can also
create a perched condition. Therefore, we recommend below-grade construction, such as
crawlspace and basement areas, be protected from wetting by an underdrain system. The drain
should also act to prevent buildup of hydrostatic pressures behind foundation walls.
The underdrain system should consist ofa drainpipe surrounded by free-draining granular
material placed at the bottom of the wall backfill. The drain lines should be placed at each level
of excavation and at least I tbot below lowest adjacent finish grade, and sloped at a minimum
lo/o grade to a suitable gfavity outlet. Free-draining granular material used in the drain system
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should consist of minus 2 inch aggregate with less than 50Yo passing the No. 4 sieve and less
than2%o passing the No. 200 sieve. The drain gravel should be at least lYz feet deep. Void form
below the foundation can act as a conduit for water flow. An impervious liner such as 20 mil
PVC should be placed below the drain gravel in a hough shape and attached to the foundation
wall above the void form with mastic to keep drain water from flowing beneath the wall and to
other areas of the building.
SITE GRADING
The risk of construction-induced slope instability at the site appears low provided the building is
located above the steep slope as planned, and cut and fill depths are limited. 'We assume the cut
depth for a basement level will not exceed one level, about 10 to 12 feet. Fills should be limited
to about 8 to l0 feet deep, especially at the downhill side of the residence where the slope
steepens. Embankrnent fills should be compacted to at least95Yo of the maximum standard
Proctor density near optimum moisture content. Prior to fill placement, the subgrade should be
carefully prepared by removing all vegetation and topsoil and compacting to at least 95o/o of the
maximum standard Proctor density. The fill should be benched into the portions of the hillside
exceeding 20o/o gade.
Permanent umetained cut and fill slopes should be graded at2honzontal to 1 vertical or flatter
and protected against erosion by revegetation or other means. The risk of slope instability will
be increased if seepage is e,ncountered in cuts and flatter slopes may be necessary. If seepage is
encountered in permanent cuts, an investigation should be conducted to determine if the see,page
will adversely affect the cut stability. This office should review site grading plans for the project
prior to construction.
SURFACE DRAINAGE
The following drainage precautions should be observed during construction and maintained at all
times after the residence has been completed:
1) Excessive wetting or drying of the foundation excavations and underslab areas
should be avoided during construction. Dryrng could increase the expansion
potential of the clay soils.
2) Exterior backfill should be adjusted to near optimum moisture and compacted to
at least 95Yo of themaximum standard Proctor density in pavement areas and to at
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least 90% of the maximum standard Proctor density in landscape areas. Free-
draining wall backfill should be capped with about 2 to 3 feet of the on-site soils
to reduce surface water infiltration.
The ground surface surrounding the exterior of the building should be sloped to
drain away from the foundation in all directions. rWe recommend aminimum
slope of 12 inches in the first 10 feet in unpaved areas and a minimum slope of 3
inches in the first 10 feet in paved areas.
Roof downspouts and drains should discharge well beyond the limits of all
backfill.
Landscaping which requires regular heavy irrigation should be located at least 10
feet from foundation walls. Consideration should be given to use of xeriscape to
reduce the potential for wetting of soils below the building caused by irrigation.
PERCOLATION TESTING
Percolation tests were conducted on January 22,2019 to evaluate the feasibility of an infiltration
septic disposal system at the site. Two profile pits and two adjacelrt percolation holes were dug
at the locations shown on Fig. l. The test holes (nominal 12 inch diameter by 12 inch deep)
were hand dug at the bottom of shallow backhoe pits and were soaked with water prior to testing.
Note that the percolation tests were not performed in stict accordance with state regulations and
were for our information only. The results of a USDA gradation analysis performed on a sample
taken from Profile Pit 1 are shown on Figure 5. The soils exposed in the percolation holes are
similar to those exposed in the Profile Pits shown on Fig. 2 and consist of very gravelly sandy
loam to clayey loam with a USDA soil type of 3 to 34.
The percolation test results are presented in Table 2. Based on the subsurface conditions
encountered and the percolation test results, the test area appears suitable for a conventional
infiltration septic disposal system. We recommend the infiltration area be oversized due to the
relatively slow percolation rate. A civil engineer should design the infiltration septic disposal
system including additional soil testing as needed.
LIMITATIONS
This study has been conducted in accordance with generally accepted geotechnical engineering
principles and practices in this area at this time. We make no warranty either express or implied.
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Kumar & Associates, lnc.Project No. 19-7.117
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The conclusions and recommendations submitted in this report are based upon the data obtained
from the exploratory pits excavated at the locations indicated on Figure 1, the proposed type of
construction and our experience in the area. Our services do not include determining the
presence' prevention or possibility of mold or other biological contaminants (MOBC) developing
in the future. If the client is concerned about MOBC, then a professional in this special field of
practice should be consulted. Our findings include interpolation and extrapolation of the
subsurface conditions identified at the exploratory pits and variations in the subsurface
conditions may not become evident until excavation is performed. If conditions encountered
during construction appeff to be different from those described in this report, we should be
notified at once so re-evaluation of the recommendations may be made.
This report has been prepared for the exclusive use by our client for design purposes. We are not
responsible for technical interpretations by others of our information. As the project evolves, we
should provide continued consultation and field services during construction to review and
monitor the implementation of our recommendations, and to verify that the recommendations
have been appropriately interpreted. Significant design changes may require additional analysis
or modifications of the recommendations presented herein. We recommend on-site observation
of excavations and foundation bearing strata and testing of structural fill by a representative of
the geotechnical engineer.
Respectfully Submitted,
Kumar & Associates,
d.
Daniel E. Hardin, P.E.
Reviewed by:
Steven L. Pawlak, P.E.
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AMENDED LOT 3
220,97E sq. ft'
5.073 ôcres
Fig 1LOCATION OF EXPLORATORY PITSKumar & Associates19-7 -117
ÂPPROXIMÀTE SCALE-FEET
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PIT 1
EL. 7549'
Plf 2
EL. 7546'
PROFILE PIT 1 PROFILT PIT 2
EL. 7325' EL. 7323',
0 0
WC= 1 2.3
DD=1 1 6 -l cR¡vEt=¿z-r SAND=29
SILT= 14
CLAY= 1 5
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-LEGEND
ñN TOPSOIL, ORGANIC SILTY SANDY CLAY, FIRM, MOIST, DARK BROWN
CLAY
VERY
(cr-):
SÏIFF
SILTY, SANDY, WITH SCATTERED GRAVEL AND SASALT COBBLES ANO BOULDERS,, BLOCKY, SLIGHTLY MOIST, BROWN.
CLAY AND SAND (CL-SC): SILTY W|TH BASALT COBBLES AND BOULDERS, VERY STtFr,
SLIGHTLY MOIST, TAN.
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HAND DRIVEN 2*INCH DIAMETER LINER SAMPLE.
DISTURBED BULK SAMPLE.
NOTES.!. THE EXPLORATORY PITS WERE EXCAVATED WITH A CAT 320 BACKI{OE ON JANUARY 22, 2019.
2. THE EXPLORATORY PITS WTRE LOCATED BY THE CLIENI
3. THE ELEVATIONS OT THE EXPLORATORY PITS WERE OBTAINED BY INTERPOLATION BETWEEN
CONÏOURS ON THE SITE PLAN PROVIÐED.
4. TItE TXPTORATORY PIT LOCATIONS AND ELEVAÏIONS SHOULD BE CONS'DERED ACCURATE ONLY
TO TI.{E DEGREE IMPLIED BY THE METHOD USED.
5. THE LINES BETWEEN MATERIALS SHOWN ON THE TXPLORATORY PIT LOGS RTPRESENT THE
APPROXIMAÎE BOUNDARIES BETWEEN MATERIAL TYPES AND THE TRANSITIONS MAY BE GRADUAL.
6. GROUND WATER WAS NOT ENCOUNTERED IN THE PITS AT THE TIME OF DIGGING. PITS WERE
BACKFILLËD SUBSEQUENT TO SAMPLING.
7. LABORATORY TEST RESULTS:
WC = WATER CONTENT (%) (ASTM D 2216);
DD = DRY ÐtNStrY (pcr) (aSrU D 2216);
GRAVEL = PERCENT RETAINED ON NO. 10 SIEVE
SAND = PERCENT PASSING NO. 10 SIEVE AND RETAINED ON NO. 325 SIEVE
SILT = PERCENT PASSING NO. 525 SIEVE TO PARTICLE SIZE .0o2mm
CLAY = PERCENT SMALLER THAN PARTICLE SIZE .002mm
19 -7 *117 Kumar & Associates LOGS OF TXPLORATORY PITS Fig. 2
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SAMPLE OF: Sondy Cloy
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WC = 12.3 %, DD = 1 16 pcf
EXPANSION UNDER CONSTANT
PRESSURE UPON WETTING
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SAMPLE OF: Sondy Sllty Cloy
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ADÐITIONAL COMPRESSION
UNDER CONSTANT PRESSURE
DUE TO WETTING
SAMPLE OF: Sondy Silty Cloy
FROM:Píl 2 @ 5'
WC = 10.8 %, ÐD = 'l 15 pcf
h ol
EXPANSION UNÐER CONSTANT
PRESSURE UPON WETTING
19-7 -117 Kumar & Associates SWELL_CONSOLIDATION TEST RESULTS Fig. 4
HYDROMFTER ANALYSIS
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DIAMETER OF PARTICLES IN MILLIME"TERS
CLÀY COBBL.ES
GRAVEL 42 %SAND 29 "/"stLT 14 %CLAY 15 Yø
USDA SOIL TYPE: Very Gravelly Sandy Loam FROM: Profile Pit 1 @ 3'-4'
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19-7 -117 Kumar & Associates USDA GRADATION TEST RESULTS Fig. 5
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TABLE 2
PERCOLATION TEST RESULTS
PROJECT NO.19.7.il7
Note: Percolation test holes wore hand dug in the bottom of shallow backhoe pits adjacent to the Profile
Pits shown on Figure 1. Percolation tests were conducted on January ?:¿,2019. The average
percolation rates were based on the last two readlngs of each test.
HOLENO.HOLE DEPTH
(¡NCHES)
LENGTH OF
IilTERVAL
(MD{)
UIATER DEPTH
AT START OF
INTERVAL
(rNcHES)
WATER DEPTTI
AT END OF
INTERVAL
(rNcHES)
DROP IN
WATER LEVEL
{rNCHES)
AVERAGE
PERCOIATION
RATE
{nrN.,rNcH)
P-,1 38 t5 414 3Y2 T,
60
gY.31/t ra
3Y.3 Y.
3 2tA Y.
P-2 26 l5 3 2t/.Y.
60
2t/t I/z v,
2Yz 2%Y.
2Y.2 Y.