HomeMy WebLinkAboutSP 144C; Encina Plant; Soil Investigation; 1976-08-09SQll INVESTIGATION FOR THE PROPOSED
SDG&E WATER TREATMENT FACILITY
ENCINA PLANT
CARLSBAD, CALIFORNIA
For
San Diego Gas & Electric Company
Post Office Box 1831
San Diego, California 92112
RECEIVED
^^^^/e or
AUG 9 1976
CITY OF CARLSBAD
y Engineering Department
WOODWARD-CLYDE CONSULTANTS
Consulting Engineers and Geologists
5P
Project Mo. 76-123
SCOPE
This report describes a soil investigation for primary and
secondary waste water containment ponds and a waste water treatment
plant, at the Encina Power Station. The ponds are proposed to have a
seven day capacity and are planned to be constructed with compacted
^ — - --earth embankments and an impervious lining. The basic assumption in
design of the lining is that no losses will be permitted from the ponds
to the underlying groundwater. The scope of the work includes a review
of the existing soil foundation information, a program of exploratory
borings and laboratory testing of soil samples, engineering analysis to
prepare recommendations regarding site development and pond construction
and preparation of guide specifications for pond embankment earthwork
and pond lining. General subsurface conditions at the site have been
determined, including depth to groundwater, depth of existing fills,
suitability of embankment or fill material available on site and of
borrow from the required excavation for Encina Unit 5 and the general,
characteristics of the natural formational soils.
Three previous soil investigations made in the power plant and
fuel tank areas and a project design guide were furnished for our
information. The first is a report dated October 3, 1951, done by Dames
& Moore, entitled "Report of Foundation Investigation, Proposed Encina
Steam Station, San Diego County, California." The second and third
reports dated July 14, 1970 and February 1, 1973 were prepared by Benton
Engineering, Incorporated and were entitled "Soil Investigation, Encina
Power Plant Unit No. 4, Carlsbad, California" and "Soil Investigation,
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Project No. 76-123 , Page 2
Encina Power Plant Unit No. 5, Carlsbad, California," respectively. The
design guide was entitled "Power Plant Waste Water Treatment Facilities"
Project 75024 dated March 26, 1976, prepared by Hugh Carter Engineering
Corporation, La Jolla, California.
FIELD INVESTIGATION
Eleven test borings were made during the period of April 6
through April 9, 1976 at the approximate locations shown on Fig. 1
entitled "Site Plan, San Diego Gas & Electric Waste Water Treatment
Facility, Encina Plant." Test Borings 1, 2, 3, 8, 9 and 10 were made
with 8-in. diameter, hollow stem, continuous flight augers with drive
sampling done through the augers. Test Borings 4, 5, 6 and 7 were made
with a 6-in. diameter, continuous flight auger. Samples from these
borings were taken with the drive sampler in the open test boring or
from auger cuttings. All relatively undisturbed samples were taken with
a 2-1/2 in. outside diameter California Sampler using a 140 pound
hammer falling 30 in. The sampler is described on Fig. 2, Legend of'
Test Borings.
The test borings were made under the supervision of an engineering
geologist from our firm who visually classified the soils encountered
and logged the materials. The logs of test borings shown on Figs. 3
through 13 are based on the field log, visual classification of the
materials encountered, an inspection of the samples recovered and the
results- of laboratory tests. The elevations shown on the logs of test
borings were determined from the Rick Engineering Company preliminary
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topography of the site dated May 11, 1976. The penetration blow counts
are shown at the sample locations on the logs of test borings. The
borings were located in the field with the aid of an aerial photograph
of the plant site at a scale of 1 in. equals 100 ft.
LABORATORY TESTING
The soils at the site were visually classified and evaluated
with respect to strength, swelling, grain size, plasticity characteristics,
I density and moisture content and compressibility characteristics. Soil
iI classifications were made in accordance with the Unified Soil Classification
T;
'I§ System and were verified by the results of laboratory grain size analyses
I
1 and Atterberg Limits tests on representative soil samples. The strength
of the soils was evaluated by consideration of the dry density and
moisture content of the samples, the results of -direct shear tests, the
penetration resistance of the sampler and by consideration of the material
type and the geologic history. Compressibility characteristics of the
clayey soils at the site were evaluated by means of consolidation tests'."
Laboratory compaction tests were performed on samples of the on-site
silty to clayey sand fill, as well as on two samples taken from the
proposed Unit 5 excavation. The results of the lab compaction tests
were then used to fabricate laboratory specimens of these soils for
direct shear testing.
The results of moisture content and dry density tests are
- .shown with the penetration resistance of the sampler at the corresponding
sample locations on the logs of test borings. The results of grain size
analysis tests and the Atterberg Limits tests are shown on Figs. 14, 15
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Project No. 76-123^ ^ Pa9e 4
and 16. The results of consolidation tests on relatively undisturbed
samples are shown on Figs 17, 18 and 19. Figure 20 shows the results of
the drained direct shear test on a relatively undisturbed sample of the
silty to clayey sand embankment soil from Test Boring No. 4. The
results of fill suitability tests including laboratory compaction, grain
size and direct shear test data are included on Figs. 21 and 22.
GROUNDWATER CONDITIONS
In the area of the treatment plant, which is located at a
ground surface elevation of approximately 38 ft in an area of moderately
compacted fill over Pleistocene terrace deposits, no qroundwater. was
encountered within the 19 ft depth of the test borings.
In the primary pond area, groundwater levels were encountered^
in test borings at the time of drilling at approximate elevations
between 2 ft (Mean Sea Level) and 8 ft (Mean Sea Level). The running
water in the drainage channel'was'at anapproximate elevation of 11 ft
" "'•'*«*-• ' "***i.**.v.and the water surface elevation in the existfri'g waste water pond at--
approximately 12.5 ft at the time of our field investigation. The
general groundwater levels in test borings at the time of drilling
seemed to correspond -to about high tide level of 2 to 4 ft (Mean Sea
Level). It is probable that infiltration of storm drain water from the
channel has caused some increase in groundwater level noted in Test
Boring No. 3 where the groundwater level was approximately 7 ft (MSL) at
time of drilling. Also, the higher water level in Boring No. 4, which
was at approximately 8 ft (Mean Sea Level) at the time of drilling, may
be attributed to waste water seeping from the existing pond.
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Project No. 76-123 ^ ^ Page 5
It is possible that construction of a stabilizing berm across
the lagoon side of the primary pond area will in effect act as an underground
dam to slow the flow of underground water out toward the lagoon. For
this reason, it may be desirable to provide some lining for the invert
of the storm drain channel as well as provide some subsurface drainage
relief beneath the pond lining.
SITE AND SOIL CONDITIONS
The Encina Power Plant site is located on Carlsbad Boulevard
just south and west of Agua Hedionda Lagoon in Carlsbad, California.
The proposed waste water treatment facilities within the plant site are
generally located as shown on the attached site plan.
The proposed primary containment pond area is situated in a •
filled northwesterly draining tributary slough at the west end of Agua
Hedionda Lagoon. The present ground surface in this area ranges from
approximately mean sea level to 17 ft above mean sea level. A silty
sand fill has been placed in the proposed primary containment area to""
develop the existing elevation. Old topographic maps of the area before
development started indicate that the old level in the slough in this
area is approximately 3 ft above mean sea level.
The proposed secondary containment pond area is situated on a
Quaternary terrace at approximate elevation of 35 to 40 ft (MSL Datum) and
is situated approximately 700 ft southeast of the primary containment
.pond area. Test borings indicate 1 to 4 ft of loose to moderately
compacted fill in the area of the secondary containment pond.
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Project No. 76-123 - Page 7
that the embayment soils in the slough area extend to depths of 60
ft or more below sea level. The moisture content and dry density
of the embayment silts and sands range from 19 to 36 percent and 85
to 108 pcf, respectively. Penetration blow counts for this soil
range from 10 to 120 blows per foot within the depths to 38 ft
below grade, which was the deepest test boring made during this
investigation.
The clay layer encountered at depths of approximately 17 to 20
ft below the ground surface on Test Borings 1, 2, 4, and 5 appear to
be normally consolidated to slightly over consolidated and appear
to have relatively high plasticity characteristics. Moisture
contents range from 41 to 65 percent and dry densities range from
57 to 87 pcf. The results of Atterberg Limits tests give a range
of liquid limits of 51 to 87 and a range of plasticity indices of
31 to 61. The results of an unconfined compression test indicate
an unconfined compressive strength of about 400 psf.
The embayment soils in the area of Test Borings 3, 8 and 10,
the general area of which is delineated on the site plan as approximate
limits of loose and mixed foundation soils, appear to have, been
moved and mixed down to a depth of about 10 ft below sea level. The
cause of this disturbance has not been determined. The materials
in the apparently disturbed zone consists of sand with some silt
and shells mixed with chunks of clay. There was no continuous clay
layer encountered in this area. There are no records of dredging
in the area and the area of about 150 by 250 ft seems to be isolated
from the open lagoon area where dredging has taken place. One of
the possible causes for the zone of mixed or reworked materials
could be that a mudwave was formed during fill inn of the slough and
then bridged over. The sandy soils in this mixed zone are generally
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p,-oject No. 76-12^,,,, - Page 8
loose to medium dense with penetration blow counts ranging from 2
to 15 per foot and the clay chunks appeared to be soft to firm. In
general, the moisture contents and dry densities of the soil in
this mixed zone do not seem significantly different than most of
the other embayment soils.
Terrace Deposits - Underlying the on-site fills and embayment
soils are medium dense to dense silty sands of Quaternary age
terrace deposits and older Tertiary age deposits consisting of
cemented sand and mudstone. The terrace deposit soils were sampled
at shallow depths of 9 to 13 ft in the treatment plant and secondary
pond area and had dry densities in the range of 109 to 114 pcf,
moisture contents of 4 to 10 percent and penetration blow counts in
the range of 20 to 52 blows per foot. No borings penetrated into
the older Tertiary age deposits.
DISCUSSION
Stability of Pond Foundation Soil
The proposed ponds involve construction of dikes above and
some excavation below the existing ground surface level. The" on-site
soils to be excavated and used for dike construction, as well as the
soils to be utilized from the required excavations from Encina 5, are
essentially granular and should be good materials for construction of
the ponds.
There are three soil conditions at this site that if left
without treatment could detrimentally effect the stability of the ponds;
these are the clay layer found at approximately elevation -2 to -3 ft,
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Project No. 76-123 „ ^ Page 9
the loose to medium dense fill and embayment soils from about 5 ft above
to 5 ft below mean sea level and the zone of mixed sandy and clayey
materials in the southeast part of the site.
The clay layer which appears to be continuous over a fairly
large part of the site is a soft, fine sandy to silty clay and presents
a zone of weakness in regard to embankment stability. Static stability
analysis of the proposed 2 to 1 slope bordering the lagoon indicates
that this slope would be marginally stable if this clay layer is left
in place. Application of a 0.2g seismic lateral forces on the pond
slope further reduces the factor of safety.
Loose to medium dense silty to clayey sands between approximate
elevations +5 ft to -5ft, as represented by samples 2, 3 and 4 from Test
Boring No. 2, were analyzed to estimate the possibility of liquefaction
in the event of an earthquake using the methods presented in the Journal
of Soil Mechanics and Foundations Division of ASCE September, 1971
presented by Seed & Idriss. This analysis using penetration blow counts
indicates that there is a possibility of liquefaction; however, the
clay content of the sand is about 20 percent. This factor indicates
that the soil could act as a cohesive material and not liquify. Whether
these soils liquify or not in the event of an earthquake, they most
probably will lose considerable strength.
The reasoning for the recommendation to construct a
stabilizing berm or buttress across the lagoon edge of the primary
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Project No. 76-12- "-' Page 10
containment ponds is that the proposed slope is marginally stable without
reworking, space is limited so that setbacks or flat slopes are not
.desirable and that liquefaction of the shallow subsoils in.the primary
pond area is a possibility. The berm can be constructed by reworking
the soils down through the clay layer to densify them to make a stable
slope and a buried dam that will contain the ponds and the loose to
medium dense subsoils beneath the ponds in the event an earthquake was
to occur and cause liquefaction. - Such liquifaction would probably cause
some distress in pond linings and piping which could be repaired after
pond contents had been treated and released, but there should be no
breach of dikes and loss of water into the lagoon.
The loose and mixed foundation soils in the southeast part of
the primary pond site are expected to be the source of erratic differential
settlements due to their mixed nature. The loose materials are
located between depths of 10 to 25 ft below the existing ground surface
and extend well below lagoon level. It is estimated that settlements
in the range of 2 to 4 in.may occur under the anticipated load of
additional fills placed for pond construction. Settlements of this
magnitude could be harmful to reservoir lining and pipe structures
crossing that area.
Possible methods of stabilizing these loose soils are be surcharging
or by removing the compressible soil and replacing with compacted soil. The
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Project No. 76-123 „ - Page 11
surcharging involves double handing a volume of earth and rerouting of the
water in the existing drainage channel, as well as a time delay while
settlement takes place. Surcharging will also require evaluation of
settlement data to determine a time for removal. Removal of the 10 ft
or so of fill overlying the loose material, dewatering to permit working
and removal and recompaction of the loose materials involves the expense
of dewatering, as well as a considerable amount of earthwork.
An alternate to the lagoon edge bernv construction and
surcharging or removal of the mixed clay and sand soils is the removal
and replacement of the entire primary pond area subgrade soil down to
dense material at about elevation 3 to 5 ft below mean sea level and
replacement with the granular soils removed during excavation or borrowed
from required excavation of Encina Unit 5. This alternate would produce
a fairly uniform dense granular foundation that would incur negligible
settlements during pond construction and operation, should riot liquify
during the design earthquake and would have . s-table slopes along the...
lagoon and the drainage channel.
Earthwork Construction Difficulties
A system for dewatering portions of the site to facilitate the
removal and replacement of unsuitable soils may take additional field
and engineering studies to design the system. We have made a preliminary
estimate that the areas could be excavated down to near the groundwater
table, then dewatered with well points on 6 ft centers placed at a depth
of 15 to 17 ft. The well points would need to be placed with a sand
wick to facilitate drainage. The clay layer is about the lower limit of
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Project No. 76-123-, * Page 12
this proposed excavation, as typically soils below the clay layer are
dense. The soils above the clay layer are silty to clayey sand and will
probably be of low to moderate permeability. The soils below the clay
layer are fairly clean and should be quite permeable, at least one order
of magnitude more permeable. However, the clay will impede flow downward
therefore groundwater gradients in the silty sand will be gradual and
additional sumps or other methods may be required inside the perimeter
well points.
Handling wet clay soils may also be a problem during construction.
The soils will dry slowly and may be difficult to mix with other soils
to compact. The clay may become well mixed with the sand during excavation
as the quantity of this material is fortunately not too large. However,
offsite disposal may become desirable if the clay layer becomes a problem
handling, mixing and compacting.
CONCLUSIONS
(1) Geologic and soil conditions at the site consist primarily
of recent fills placed over three major formational units consisting of
Quaternary age estuarine embayment deposits loose to medium dense composed
of silty to clayey sands and a thin clay lense, Quaternary age terrace
deposits consisting of dense silty sand and Tertiary age very dense
cemented sands and mudstones.
(2) Our analysis indicates that in the primary pond area the
proposed 27 ft high combined fill embankment and natural ground slope
bordering the lagoon would be unstable due to the presence of fairly
continuous layer of soft to firm clay at an elevation of approximately 3
ft below mean sea level, loose to medium dense silty to clayey sand soils
above the clay layer, and tidal fluctuation in the lagoon.
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Project No. 76-123 Pa9e 13
(3) Our investigation and analysis revealed a zone of loose
and mixed sand and clay soils in the southeast portion of the site at
depths of 10 to 25 ft below the existing ground surface, which could
consolidate excessively under new fill loads.
(4) The subsoils are primarily granular with some fills and
clays. It can be anticipated that settlement will take place as loads
are applied or within a few weeks after. The only clay layers at depths
within which changes in stress would be significant are not thick
enough to be the source of significant long term settlement. Therefore,
it is not anticipated that there will be long term consolidation settlement
from the construction and operation of the proposed ponds.
(5) Dewatering of subsoils will probably be necessary in
order to remove and recompact a stabilizing berm,along the edge of the
lagoon.
(6) The stabilizing berm may act as a groundwater barrier and
cause water levels to rise in the primary pond area.
(7) Present groundwater levels may range from approximately 2
to 8 ft (MSL Datum) in the primary pond area as measured in test borings.
(8) Settlement due to the dike construction is estimated to
be approximately 1 in. The pond bottom, due to release of load, may
rebound slightly during construction; however, it is more likely that
the action of the construction equipment and compaction of the subgrade
will obscure any rebound.
(9) During operation of the ponds, there will be loading and
unloading of the subsurface soils as pond water levels are raised and
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project No. 76-123^ „ •-* Pa9e 14
lowered. It is estimated that this movement will be in the range of 0.1
to 1.0 in. for a full cycle of the operating range. The 0.1 in. movement
is estimated for the dike crest and the 1.0 in. is estimated for the pond
bottom.
RECOMMENDATIONS '
Site Development - Primary Pond Area
(1) It is recommended that existing fill and embayment soils
be removed down to approximate elevation -5 ft mean sea level and replaced
with compacted soil to form a stabilizing berm or buttress beneath the
primary pond dikes along the lagoon edge of the ponds. This area is
outlined on the Site Plan, Fig. 1.
(2) It is recommended that the existing pocket of loose and
mixed soils in the southeast area of pond foundation area approximately
as delineated on the site plan be surcharged and stabilized before
construction of pond linings or piping system. The minimum surcharge
should be 15 ft of soil. The duration of surcharging should be determined
by placing and making readings of two settlement monuments placed on the
prepared subgrade before surcharging is started. The settlement monument
reading should be evaluated by the soil engineer. It is recommended
that settlement readings be made daily during the time the surcharge is
placed and for 3 days thereafter and then read weekly. It is estimated
that the surcharge will require approximately 30 days.
(3) It is recommended that the upper 2 ft of the existing
fill soils be processed and compacted to 90 percent relative compaction
before placing additional fills.
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project No. 76-123 Page 15
(4) It Is recommended that the pond dikes and subgrade be
constructed of compacted nonexpansive granular soil.
(5) It is recommended that the upper 2 ft below pond subgrade
in the cut areas be processed and compacted to minimum relative compaction
of 90 percent.
(6) It is recommended that site grading be done in accordance
with the attached "Guide Specifications for Controlled Fill."
(7) It is recommended that all earthwork and lining construction
be done under the observation of the soil engineer.
(8) It is recommended that 4 pneumatic settlement devices be
set in the subgrade in the.primary pond area at locations selected by
the soil engineer and settlement readings taken weekly during the earthwork
and lining construction.
Pond and Slope Protection Construction
(1) It is recommended that ponds be constructed with maximum
interior slopes of 2-1/2 to 1 (2-1/2 horizontal' to 1 vertical);" and --
maximum exterior slopes of 2 to 1.
(2) It is recommended that the ponds be lined with a three
layer lining containing top and bottom layers of hydraulic asphalt
concrete and an internal drainage layer of porous asphalt concrete and
piping to carry any leakage that penetrates the first layer of lining
out of the lining to a suitable sump. This will permit early detection
.of any leakage before the waste waters have time to penetrate into the
subsoil and into groundwater in the area.
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Project No. 76-123 -— ,.,^ Page 16
(3) It is recommended that the ponds have a minimum freeboard
of 1 ft more than that needed to handle the worst possible condition
caused by malfunction of piping, valving, etc.
(4) It is recommended that the dike crests be paved with 3
in. thick asphalt concrete and sloped to drain into the ponds.
(5) It is recommended that interlining subdrains be constructed
of 6 in. perforated pipe bedded in and covered with a minimum thickness
of 4 in. of 1/4 to 3/4 in. aggregate.
(6) It is recommended that the exterior slopes of the pond
area along the drainage channel be protected with paving, sackcrete or
riprap. It is also recommended that the lagoon edge of the dikes be
protected with a 24 in. thick layer of riprap over a 12 in. thick filter
gravel zone to protect against wave erosion from a depth of 4 ft below
mean sea level to a height of approximately 6.ft above mean sea level. The
riprap and filter shall be in accordance with the attached'"Riprap
Specifications." • ' ~
(7) It is recommended that a system of subdrain
pipe with suitable filter gravel backfill be placed beneath the
bottoms of the lower layer of pond lining in the primary containment
pond area to prevent any uplift pressures due to groundwater buildup
under the ponds. It is recommended that such pipe be minimum 6 in.
diameter Class 150 ACP and that the gravel bedding and cover consist of
a minimum thickness of 4 in. of Class 2 permeable material in accordance
with State of California Department of Transportation Standard Specifications.
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project No. 76-123 ' - Page 17
'%»»x "««••**
(8) It is recommended that the hydraulic asphalt lining be
constructed in accordance with the attached "Specifications for Hydraulic
Asphalt Concrete Lining."
(9) It is recommended that the ponds be filled and drained in
one-third total depth increments with a 24 hour pause after each increment.
It is recommended that the subgrade settlement devices be read just
before and just after each increment of filling and draining and 24
hours after the pond is completely filled and then 24 hours after completely
emptied. These readings should be given to the engineer for evaluation
daily as they are taken.
Site Development - Secondary Pond Area, Treatment Plant
(1) It is recommended that the existing fills not removed by
grading, be removed and/or scarified, watered and .compacted before
placement of additional fill or foundations for structures.
(2) It is recommended that the secondary pond dike slope
along the top of the existing natural slope along the west side of the
secondary pond area be designed at an inclination of 2 horizontal to 1
vertical and behind a 2 to 1 slope up from the toe of the existing cut
slope.
(3) It is recommended that the pond, dikes and subgrade be
constructed of nonexpansive granular soil.
(4) It is recommended that site grading be done in accordance
(
with the attached "Guide Specifications for Controlled Fill."
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Project No. 76-123 , , Page 19
Evaluation and utilization of soil materials for support of
structures, includes investigation of the subsurface conditions, analysis,
formulation of recommendations, and inspection during grading. The soil
investigation is not completed until the soil engineer has been able to
examine the soil in excavations or cut slopes so that he can make necessary
modifications, if needed. We emphasize the importance of the soil
engineer continuing his services through the inspection of grading,
including construction of fills and foundation excavations.
WOODWARD-CLYDE CONSULTANTS