HomeMy WebLinkAbout; ; Design of Squires Dam and Appurtenant Structures for Carlsbad Municipal Water District; 1961-02-16C
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DESIGN OF SQUIRES DAM
AND APPURTEMANT STRUCTURES
FOR CARLSBAD MUNICIPAL WATER DISTRICT
San Diego County, California
FEBRUARY, 1961
BOARD OF DIRECTORS
Allan o. Kelly, President
Mac Carnohan
Wesley F. Greek
Richard R. Coe
Fred W. Maerkle
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TABLE OF
Letter of Transmittal
SECTION I -General Information
A -Introduction
B -Design Conclusions
C -General Site Conditions
D -Hydrology
E -Previous Studies
SECTION II -Dam Design
A -Foundation
1. Foundation Exploration
2. Foundation Conditions
CONTENTS
3, Foundation Stripping and Preparation
B
4. Earthquake Potential
5, Foundation Design Properties
-Embankment Materials
1. Construction Materials Exploration
2. Construction Material Sources
3, Embankment Design Properties
4. Design Criteria
5, Embankment Layout & Design
6, Seepage Analysis
7, Stability Analysis
8, Piezometers and Settlement Markers
(Design)
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C -Freeboard, Settlement and Camber
1. Freeboard Design
2. Settlement Analysis
3. Camber
D -Spillway
1. Location
2. Capacity
3. Design
E -Inlet-Outlet Pipe
1. Location
2. Construction
Appendix
Figure 1 Plan and Location Map
Figure 2 Embankment Layout and Details
Figure 3 Geologic Profile and Drill Hole Logs
Figure 4 Dam Embankment Seepage Analyses
Figure 5 Dam Embankment Stability Analysis
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ARCHITECTS • ENGINEERS
4525 MISSION GORGE PLACE • SAN DIEGO 20, CALIFORNIA • ATWATER 3-5481
February 16, 1961
Board of Directors
Carlsbad Municipal Water District
P. 0. Box 278
Carlsbad, California
Gentlemen:
Presented herewith is our report "Design of Squires Dam and Appur-
tenant Structures, San Diego County, California" outlining the basis
of design for your proposed Squires Dam .
The purpose of this report is to give you in some detail our design
assumptions and considerations in order that you may be informed re-
garding the design characteristics of this structure. This report
will be most useful to you in presenting your plans to various State
of California agencies for preliminary approval of your proposed con-
struction project. Upon receipt of such preliminary approvals, we
will be in a position to proceed immediately with the final construc-
tion plans and specifications for the project.
We have been pleased to perform this portion of the work for you and
look forward to an early completion of planning followed by immediate
construction.
LAC:bhb
Respectfully submitted,
BOYLE ENGINEER1N· =/·· /) ,, ~< (__;&-,;.;,,
L, A. Clayt
Project Design
W, A, WAHLER AND ASSOCIATES
(,,il.7:'j (A.... le' t,_' f., J
W. A. Wahler
Earth Dam Design
ft&Aaoa.c:..1ANA.L ENGINEERING AHO
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A -INTRODUCTION
DESIGN OF SQUIRES DAM
AND APPURTENANT STRUCTURES
SAN DIEGO COUNTY, CALIFORNIA
I. GENERAL
The purpose of this report is to present and substantiate the
design of the Squires Dam and Appurtenant Structures with special
reference to those portions of the design which have a bearing on
the public safety of the project. Those features of the project
which do not have a bearing on the safety of the dam, spillway, or
outlet works are not covered in detail in this report .
The proposed Squires Dam project will serve as a regulating
and short-term emergency supply reservoir for the Carlsbad Municipal
Water District, Carlsbad, California. The water will be supplied by
the Metropolitan Water District of Southern California through the
San Diego Aqueduct. The site was chosen near Mt. Hinton in San Diego
County for its location relative to the District's distribution system
and the San Diego Aqueduct, its elevation, and its very small contrib-
utary watershed. Since the reservoir drainage basin is only about
60 acres, very little water will originate in this area, thereby
minimizing the spillway requirements and water contamination problems.
The principal features of this project are the reservoir formed
by an earthfill dam across Squires Canyon, a small overflow-type spill-
way, an outlet works through the base of the dam, and the feeder pipe-
line with chlorination station. The spillway is located in a small
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saddle in the right abutment and the outlet works are located in
the bottom of the canyon near the right abutment. The feeder pipe-
line is 27 inches in diameter and one mile long, connecting the res-
ervoir with the Carlsbad Municipal Water District distribution system
near Palomar Airport.
The maximum pool water surface is about 14 acres at elevation
505, The capacity of the reservoir at that level is about 450 acre
feet. All except 10 acre feet of storage is located above the outlet
works intake level. The 10 acre feet of storage below the outlet
level are provided for silt storage and can be evacuated as required
by portable pumps. The exact surface area and capacity of the res-
ervoir will depend on the amount of earth material excavation from
that area for construction of the dam.
The bulk of the embankment fill materials will be obtained from
within the reservoir area. The drain sand and probably the riprap
will have to be imported from beyond the reservoir area. Supple-
mental core and shell earth fill materials are available at the head
of the canyon just above the proposed reservoir water level if they
are required. Since the stream in Squires Canyon only flows during
and shortly after storms, minimal diversion facilities are all that
will be required. Construction of this project is proposed for
Summer 1961.
The design of the Squires Project was by Boyle Engineering, 4525
Mission Gorge Place, San Diego, California. The earthfill dam design
was by W. A. Wahler and Associates, 3850 Middlefield Road, Palo Alto,
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California. The field exploration and laboratory testing work was
by Benton Engineering, Inc., 4342 Fairmount Avenue, San Diego, California.
The geological investigation was by Professor Baylor Brooks, San Diego
State College, San Diego, California. The project design was prepared
for the Carlsbad Municipal Water District of Carlsbad, California.
B -DESIGN CONCLUSIONS
The basic concept of the design of the Squires Dam, spillway and
outlet works was to provide a structure that is adequate beyond any
doubt with regard to public safety, and to apply sound engineering
and economical Pfincipals throughout the design of the project to
make it economical to build, operate and to maintain.
On the basis of the design studies, investigations, and data pre-
sented herein it is concluded that a safe and economical rolled earth
fill type dam can be built at the proposed site using earth materials
that are available in the vicinity of the site or within economical
haul distances.
C -GENERAL SITE CONDITIONS
The proposed dam site is located in a well defined canyon adJa-
centto and North of Mt. Hinton. The canyon is very short, the drain-
age area being limited to about 60 acres. The stream which eroded
the canyon is intermittent -having very little flow except for
occasional storm periods when the majority of the annual runoff
occurs. The general country-side in the area is rolling, varying
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from sea level to about 600 feet in elevation. The site is located
just east of the coastal plain underlain by marine sediments and
sedimentary rocks, The dam site is in an area underlain by the
crystalline igneous rocks which form the local basement complex.
This rock classifies as a quartz diorite but is locally referred to
as granite. The surface soils are "slope wash" deposits of trans-
ported disintegrated quartz diorite material from higher up on the
slopes, This material is relatively shallow and overlies in-place
weathered and disintegrated quartz diorite which is locally weather-
ed to a considerable depth. The rock is extensively jointed, the
joint frequency being one of the controlling depth and extent of
weathering factors, Major faults are not believed to be present in
the immediate dam site area. No unusual or unexpected geological or
topographical conditions were found in the area of the dam site.
D -HYDROLOGY
Intensity duration curves developed for the years of record 1897
to 1941 from San Diego,California,Weather Bureau Records indicate one
hundred year storms on the order of the following:
Duration in
Minutes
10
30
60
12 hour
24 hour
Rainfall Intensity
Inches Per Hour
3,5
2
1.3
0.3
0,2
The above rainfall intensities would indicate the maximum runoffs in
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this area on the order of 60 cubic feet per second if reservoir
storage is neglected. Taking reservoir storage into account, how-
ever, a storm yielding one-inch runoff in one hour would give a re-
sulting rise in water level of only 4 inches. This probable rise in
water level will be accounted for in the spillway design,
E -PREVIOUS STUDIES
The project design presented herein is based in part on prelim-
inary studies prepared by Boyle Engineering to locate, size, and de-
termine the feasibility of the Squires Reservoir. These studies were
augmented by specialized studies prepared by consultants to Boyle
Engineering and to the Carlsbad Municipal Water District. The most
important of these studies are:
1. BOYLE PRELIMINARY REPORTS
2. "GEOLOGICAL CONDITIONS AT SQUIRES DAM SITE, CARLSBAD,
CALIFORNIA, 11 July 1960, by Mr. Baylor Brooks, San Diego
State College, San Diego, California.
3. "SOIL INVESTIGATION -PROPOSED DAM SITE ADJACENT TO MT.
HINTON, SAN DIEGO COUNTY, CALIFORNIA," August 1960, by
Benton Engineering, Inc., San Diego, California.
4. "PRELIMINARY DAM DESIGN REPORT," September 1960, Wahler
& Associates, Palo Alto, California .
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II. DAM DESIGN
A -FOUNDATION
1. Foundation Exploration. The foundation exploration program
was carried out during the sUllllner of 1960. The surface indications
at the site were studied by engineering geologists and soils and
foundation engineers. The subsurface conditions at the site were
investigated by use of diamond drill and bucket auger holes, and by
bulldozer trenches .
Three deep rotary drill holes were put down somewhat downstream
from the present axis of the dam. Continuous NX size diamond cores
were obtained of all competent rock materials encountered. Hole lA
was extended the last 7,5 feet with a BX size hole, A SUllllnary of
the drill hole depth information is presented below:
Diamond Core Holes
Percent
Total Depth Recovery
Hole No. Location Depth to Rock in Rock
lA Midslope -left abutment* 781 581 83%
2A Midslope -right abutment 80 1 59' 69%
3A Valley bottom 74• 59' 72%
*looking downstream
The rock encountered in these holes was a crystalline igneous quartz
diorite locally referred to as granite. The rock varied from a hard
fresh material in the bottom reaches of the holes to a weathered mate-
rial with soil like properties. The rock is overlain by insitu weathered
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quartz diorite, The degree of weathering of this material varies
only indirectly with depth because the weathering has progressed
faster along old joint and fracture planes than it has through the
blocks of material in between, This results in the formation of in-
situ boulders in a matrix of in-place weathered rock of igneous origin,
The insitu weathered bedrock is overlain on the surface by a few feet
of slope wash debris and a minor amount of stream alluvium in the
valley.
The bedrock outcrops exhibit a well developed joint pattern. The
same general joint pattern is evident in the trench cuts into insitu
disintegrated or weathered bedrock, The nature of the bedrock and
weathered bedrock materials is such that it has a relatively low pri-
mary permeability, Seepage through the joints and cracks in the less
weathered rock, however, was considered to be a distinct possibility,
Water pressure tests were made in the diamond drill holes to evaluate
this problem. The results of these tests are not all-conclusive be-
cause water was observed flowing from the top of the hole in some of
the tests indicating that water was by-passing the hole packers. The
results of the tests which were probably valid along with the observed
nature of the rock in the outcrops and cuts indicate that the weathered
material is relatively impermeable, or will be when wetted, and that
the fresher rocks could exhibit leakage through the joints and cracks.
The results of the valid water pressure tests are presented below .
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WATER PRESSURE TEST DATA
Depth
Hole Interval water* Time In-Coefficient of
No . Tested Loss (cf) terval (min) Permeability (ft/yr)
2A 49,5 1 to 80 1 1.0 4 21
2A 19,5 l to 80 1 4.o 4 52
3A 611 to 74 1 no water loss Impermeable
3A 50,5 1 to 741 no water loss Impermeable
3A 491 to 741 no water loss Impermeable
3A 19,5 l to 74 1 2.6 7 20
*Applied pressure= 50 psi.
The near surface soils in the dam foundation were explored by the
use of bucket auger holes and bulldozer trenches. Four 24-inch diam-
eter bucket auger holes were put down in the valley bottom to depths
ranging from 4-1/2 to 6-1/2 feet. Undisturbed and disturbed samples
were taken of the representative materials encountered in the holes .
These samples were tested in the laboratory to define the state and
the physical and mechanical properties of the materials encountered .
A summary of these test results is presented below:
Bucket Auger Holes -11 C" Series
Coefficient of
Hole No, ¢ Permeability
lC 0.1 ft/yr
2c* 30-1/2° 25 ft/yr
Max.
Density
127 .1 pcf
Opt.
Moisture
9%
Plasticity
Non plastic
Non plastic
*Tests were performed on samples recompacted at optimum moisture con-
tent to 90% of the maximum density reported above .
The test procedures used are described in the section of this report on
embankment design values .
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The bulldozer trenches were 12 to 16 feet wide, and 3 to 12 feet
deep. They afford an excellent opportunity for inspection and appraisal
of the dam foundation conditions in-place. Approximately 500 lineal
feet of trench within the dam contact area (including access road cut
available for this purpose) were excavated in the right abutment, 500
lineal feet in the left abutment, and 700 lineal feet in the valley
bottom.
The location of the drill holes and trenches is presented on Fig-
ure 1 and drill hole logs are presented on Figure 3. An interpreted
longitudinal cross section of the Squires Canyon Valley, along the
axis of the dam is also presented on this figure.
2. Foundation Conditions. The dam site foundation and abutments
consist of in-place weathered and otherwise decomposed quartz diorite.
This is locally covered by a relatively thin layer of slope wash or
debris consisting of redeposited material of the same general type
that it overlays, although it is slightly more weathered. The slope
wash or debris originates only a short distance up-hill from where
it presently lies. Relatively sound but badly fractured bedrock
occurs in spotty outcrops on the right abutment above elevation 420,
There is very little alluvial material in the creek bed due to the
relatively short distance to the creek head. The insitu weathered
quartz diorite will provide a relatively strong and incompressible
foundation for the proposed dam. The primary permeability of the
foundation material is quite low and the only permeability problems
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of any consequence are the secondary or joint pattern seepage planes •
The altitude of the joint planes is nearly vertical so they are not
serious from a structural point of view, only as potential seepage
paths .
3, Foundation Stripping and Preparation. The embankment foundation
preparation will consist of stripping all organic material and all low
density or low strength material from beneath the entire dam contact
area and of excavating all alluvial or relatively permeable material
from beneath the impervious core contact area. The stripping will in-
clude the removal of a minimum of two feet of material from beneath the
entire contact area to insure the removal of most of the brush root
zones and other undesirable material, The impervious contact excava-
tion will be made a minimum of 5 feet into in-place weathered bedrock
to insure that all deep roots and animal boroughs have been inter-
cepted. A cut-off trench having a bottom width of 20 feet will be
excavated beneath the impervious core contact to an average depth of
about 15 feet in order to minimize seepage losses through the founda-
tion and abutments. Where the impervious core cut-off trench contact
is with relatively fresh but fractured rock fragments, they should be
removed when they are loose, When a firm base has been prepared, the
area will be air and water jet cleaned and all joints and cracks will
be concrete packed or slush grouted as required. These areas will be
pressure grouted when deemed necessary by visual inspection of the area
prepared for placement of impervious core material, If required, pressure
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grouting will follow the surface clean up but precede slush grouting .
Foundation pressure grouting will be ineffective in those areas over-
lain by deep weathered bedrock and will not be required in those areas,
Slush grouting will also not be required in these areas. The first
three feet of impervious material placed in the impervious core con-
tact area will be the most plastic material available and it will be
placed at or slightly above the optimum moisture content of the mate-
rial. This layer is designed to be extruded by embankment pressure
into any small cracks in the weathered bedrock material that is other-
wise not filled by the foundation preparation and thereby prevents
seepage and subsequent piping along the core contact .
4. Earthquake Potential, The Squires Dam will be constructed in
a geologically stable canyon, and the embankment will contact insitu
weathered bedrock along all of its base. The abutments are sound
and all of the alluvial valley fill will be stripped from beneath
the embankment fill, Spontaneous liquefaction of foundation materials
due to seismic shock is impossible at this site.
The probable maximum earthquake intensity to be expected in the
vicinity of the dam site has been determined on the basis of the seismic
regionalization studies prepared by Professor C. F, Richter of the Cal-
ifornia Institute of Technology, The results of these studies were
published in his paper "Seismic Regionalization, 11 California Institute
of Technology, Division of Geological Sciences, Contribution No. 897,
December 1958, These studies were made to provide the maximum probable
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seismic intensity to be expected for long range, long term projects
such as dam structures and are not short term insurance types of
estimates. Therefore, the seismic recommendations made below are
believed to be conservative.
The Squires Dam site is situated on the Southern California
batholith. Professor Richter classifies this area as having a maxi-
mum probable intensity of VI (MM). Where local alluvium or weak sur-
face soils are encountered he recommends consideration of an intensity
of VII (MM). When expressed in terms of the maximum probable accelera-
tion, these intensities are expected to develop forces equivalent to
0.03g and 0,07g respectively. Although the consequences of a failure
of the Squires Dam are small at present it was considered to be pru-
dent to design the embankment on the basis of a seismic acceleration
factor of O.lOg and to require a minimum factor of safety equal to
1.10 for the arc having the minimum factor of safety computed for
gravity and seepage forces only,
5, Foundation Design Properties. All of the foundation materials
which are less strong, more compressible, or more permeable than the
adjacent fill zone material will be removed as part of the foundation
and abutment preparation, The insitu weathered bedrock at the dam
site is stronger and less compressible than the. same material in a
compacted fill state.
B -EMBANKMENT MATERIALS
1. Construction Materials Exploration, The construction materials
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exploration program was carried out concurrently with the foundation
exploration program during the summer of 1960. Due to the economic
importance of obtaining as much material from the reservoir area as
practical, exploration for impervious and shell zone materials was
confined to the reservoir area and the area immediately adjacent to
the upstream end of the reservoir. The exploration program for con-
struction materials consisted of a careful surface reconnaissance
and two series of bucket auger holes as well as a road cut excavated
through the reservoir area .
The reconnaissance of the area indicated that there was probably
enough coarse grained insitu weathered bedrock economically available
within the reservoir limits to construct the shells of the embankment.
There is also a possibility that a sufficient amount of finer grained
insitu weathered bedrock is economically available within the reservoir
area for the constructi,on of the impervious core zone. In order to in-
sure that there was enough of both of these types of material within
economical haul distance of the dam, the availability of these materials
adjacent to and just upstream from the reservoir was investigated .
The "B" series of 24-inch diameter bucket auger holes were put
down along the dirt road which crosses the head end of the reservoir .
There were five holes in this series ranging from 2 to 10 feet deep .
Bag samples were taken of representative materials that were encountered
in the holes. These samples were tested in the laboratory to define
the physical and mechanical properties of compacted fill made using
these materials. A summary of these test results is presented below:
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Bucket Auger Holes -"B" Series
Coefficient of Max, Opt.
Hole No. ¢ Permeability Density Moisture Plasticity
l/B 26° 20 ft/yr 122.0 12.1 Slight
5B 32° 50 ft/yr 126.0 8.9 Non plastic
These tests were performed on samples that were compacted at optimum
moisture content to 90 percent of maximum density. The test procedures
used are discussed individually in connection with the section of this
report on embankment design values.
The "C" series of bucket auger holes were discussed in the section
of this report on foundation exploration. These two series of holes
bracket the proposed impervious core and shell zone material source
areas.
The location of the 11 B11 and "C" series of bucket auger holes are
presented on Figure 1 and the logs of those holes are presented on
Figure 3.
2. Construction Material Sources. The Squires Dam will be a
rolled earth fill zoned-type embankment utilizing four types of mate-
rials in five zones. The materials required are rock for riprap, sand
for the drain zone, coarse grained insitu weathered bedrock for the
shell zones, and fine grained insitu weathered bedrock for the im-
pervious core zone .
The results of the field exploratory program and the analysis of
the material source conditions that form the basis for the dam design
are covered in detail below for the various types of material required .
The location and approximate limits of the local material source areas
are presented on Figure 1.
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The impervious core and shell zone materials will be obtained from
the same sauce area, namely from the reservoir area and from the area
just upstream of the reservoir within the project watershed. Careful
material selection will be made on the basis of material grain size
gradation to obtain an average permeability difference factor of at
least 10 between the upstream shell and the impervious core zone. The
permeability of the downstream shell zone will not be important to the
design of the dam .
The geological conditions within the reservoir area are essentially
the same as those at the dam site. The surface material is somewhat
more fine grained than the less weathered material at greater depth
although this variation is by no means uniform with depth. The sur-
face soils in the topographically steeper areas are coarser grained
than in the flatter areas due to the effects of erosion and creep.
The most probable reliable sources of "impervious" core zone material
are therefore the flatter areas. The same principle holds true for the
coarser grained materials; they are most likely to be readily available
in the steeper hillside areas. Many large insitu boulders will also
be encountered within the weathered bedrock zone .
The design intent is for the finest grained and therefore presum-
ably the most impervious materials to be selected for placement in the
core zone; for the coarsest grained and therefore presumably the most
pervious materials to be selected for placement in the upstream shell
zone, The intermediately graded materials and excess materials meeting
either of the other zone requirements are intended for placement in the
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largest zone, the downstream shell zone. It is believed that this zoning
combination will result in the most satisfactory dam embankment; however,
the design analyses assume the zoning to be effective or ineffective de-
pending on the consequences of the zoning assumption for the case being
analyzed. The selection is made in such a manner as to achieve the most
conservative seepage assumption for each case.
Excavation within the reservoir will be more difficult than in
flatter areas because of the steepness of the topography. A short
haul will compensate to some degree for this difficulty. The use of
material from the reservoir will develop storage capacity having a
much greater value than any added cost that may result due to the
topography of the area for excavation purposes.
Excavation of in-place weathered material in this area will be
hampered by the presence of insitu boulders. Considerable ripping
will be required if the use of self loading scrapers is considered;
however, the use of power shovels may be necessary in order to obtain
the maximum possible amount of material from the reservoir area. Pro-
vision has been made in the outer one-half of the upstream shell for
the deposition of excess boulders from the reservoir area if it is
economical to dispose of these rocks in that manner. If this alternate
is selected, riprap will not be necessary in that portion of the zone
actually so constructed.
Two alternative riprap sources have been considered during the
project design although the contractor will be allowed to use material
obtained elsewhere if the quality of the material selected is satisfactory.
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These alternates are to select and use sound rock having a satisfactory
gradation from the boulders encountered in the shell and core material
source areas, or to quarry rock from the Calavaras Quarry about 3 miles
north of the dam site for this purpose. Since the wave problems will be
minimal at this site due to the small size of the reservoir and since
freezing and thawing will not occur due to the southerly location of
the site, the soundness of riprap required will not be severe provided
that the material is well graded, or that a filter zone is placed if
larger sized or poorly graded rock is to be used without secondary
shooting on the fill .
There is no satisfactory drain zone sand at the dam site. This
material will be imported to the site. The allowable gradation limits
of this material along with its relation to the proposed impervious
core and downstream shell zone material gradation limits are shown
on Figure 2.
3. Embankment Design Properties. The probable gradation limits
of the material available in the reservoir area for impervious and
shell zone are shown on Figure 2. The required gradation for the
drain zone sand has been determined so as to meet the filter require-
ments of the impervious core zone material. Shell zone material will
automatically meet the filter requirements of both the impervious core
and drain zone materials. The imp.ervious core and shell zone gradation
limits were selected to make the most advantageous use of the materials
available for the construction of the dam. The design, however, is
17
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based on the most disadvantageous seepage assumptions for each case
considered, therefore these zone gradations are relative, rather than
absolute. The gradation test procedure on which these limits are
based is A.S.T.M. Test Procedure D 422-54T.
The Atterberg Limit tests were performed using A.S.T.M. Test
Procedures D 423-54T and D424-54T. The material at the project site
was found to be essentially non-plastic and the apparent liquid limits
were found to be less than 26 percent .
The compaction tests were made using A.S.T.M. Test Procedure
D-698 modified to use 26 blows of a 10 pound hammer with an 18-inch
drop on each of 3 layers in a I/30th cubic foot mold. This results
in a compactive effort of 35,100 foot pounds. The required minimum
fill density will be 90 percent of the maximum density so obtained.
The compaction moisture will be controlled within the limits of 3
percent dry and 2 percent wet of the optimum moisture content so ob-
tained while attempting to compact at optimum. The stability analyses
have been.made using a dry unit weight of 113 pcf for all material in
the embanl<Jnent and in the foundation. This is a slightly conservative
assumption because the unit weight of the core·zone material will prob-
ably be lower than this amount and the shell zone material may be heavier .
The permeability of the available impervious and shell zone mate-
rials was obtained by use of both constant and falling head tests. The
coefficient of permeability so obtained varied from about 0.1 to 50 feet
per year. Since the tests were run on the more pervious materials at
the site both the shells and the core zone will be relatively impervious .
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The shear strength of the impervious core and shell zone materials
was obtained on samples compacted at the optimum moisture content to
90 percent of the maximum density obtained by the compaction test
discussed above. The samples weresaturated under a head of one-half of
the chamber pressure, The samples were stored for 24 hours. Triaxial
shear tests were then run in approximately 15 minutes without permitting
sample drainage. Failure was assumed at 12-1/2 percent strain and Mohrs
diagrams were plotted to determine the resulting coefficient of internal
friction. Cohesion was ignored for stability analysis purposes because
it is not considered to be a reliable property of the materials at the
project site for long term embankment conditions. The shear strength
obtained was an apparent shear strength under near saturated or satura-
ted conditions. The Coefficient of Internal Friction of the impervious
core zone material is conservatively estimated to be 0.48 and of the
shell zone material to be 0,625.
4. Design Criteria, The design of the Squires Dam is based on
the results of economic, seepage, stability, and settlement analyses
which include consideration of-all critical conditions that were in-
dicated by the site foundation, and construction material source in-
vestigations, and the probable reservoir operation. The basic criteria
for the design of this dam has been to develop a structure that is
adequate beyond any doubt with regard to public safety and to apply
sound engineering and economic principles in the design of those as-
pects of the project which affect the initial cost and maintenance of
the structure .
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5. Embankment Layout and Design. The most practical and economical
development of the subject dam site required a detailed study of the em-
bankment quantity-reservoir capacity relationship for several dam heights
and axis alignments. The dam axis and crest elevation presented herein
was based on the results of this study along with a study of the site
topographical, geological, and engineering conditions presented in
this report. The final layout, embankment design, and material utili-
zation was then again reviewed at the site and the detailed seepage,
stability, and settlement analyses presented on Figures 4 and 5 were per-
formed on the resulting final embankment section.
The embankment section design was based on the economical availa-
bility of large quantities of in-place weathered rock material which
is both fine grained enough to be relatively impervious, and strong
and incompressible enough to support the water tight core with reason-
able side slopes as well as on the geological and foundation engineer-
ing aspects of the site.
The Squires Dam will be a zoned type compacted earth fill embank-
ment with a 20 foot wide, 802 foot long crest at Elevation 510. The
emergency spillway crest will be at Elevation 505 with the maximum de-
sign flood water surface at Elevation 507 resulting in a reservoir sur-
charge of 2 feet and a net freeboard under design spillway discharge
conditions of 3 feet. The embankment will be approximately 125 feet
high above natural stream bed below the crest and 185 feet above the
downstream toe of the embankment. The dam will be approximately 150
feet above the bottom of the cut-off trench. The layout and details
20
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of the embankment are presented on Figure 2 .
The dam embankment section consists of an off-center impervious
core zone which is drained on the downstream side by a vertical sand
drain zone. The impervious core zone is supported on the upstream
side by a semi-pervious shell zone and on the downstream by a random
permeability shell zone. The downstream shell is underlain by a hori-
zontal sand drain zone which is warped up the abutments to prevent
pressure build-up due to abutment and foundation seepage as well as
to serve as a conduit to carry the vertical sand drain seepage water
to the downstream toe of the embankment. The upstream slope of the
embankment will be protected against wave erosion by a riprap zone,
The downstream slope will have a 15 foot wide berm at Elevation 440
and the slope will be planted with a suitable grass cover to minimize
erosion maintenance due to rain runoff. The downstream groins will
be protected with a ten-foot wide, 2-foot deep layer of well graded
riprap to prevent erosion in this vulnerable area where the abutment
runoff will drain down the embankment slope.
The embankment slopes are one vertical on three horizontal up-
stream and one vertical on two and one-half horizontal downstream
with a 15-foot wide berm at Elevation 440. The embankment slopes
were selected on the basis of the strength of the available shell
construction materials and the site foundation conditions. The
stability analyses discussed later in this report present the com-
puted minimum factor of safety for each condition studied, These
studies were made for major failure sections only. Local, shallow
21
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-..
arc stability consideration involving only maintenance problems will
yield slightly lower factors of safety which, however, are also generally
above the required minimums.
6. Seepage Analyses. Steady state and rapid draw down flow nets
have been drawn for the dam using the procedures set forth for graphical
analysis by Arthur Casagrande in his paper, "Seepage Through Dams,"
Journal of the New England Water Works Association, June 1937, and by
Harry R. Cedergren in discussion of a paper by F. H. Kellogg on "In-
vestigation of Drainage Rates Affecting Stability of Earth Dams,"
Transaction ASCE, Vol. 113, 1948.
An anisotropic ratio of 9 to 1 was used in the construction of the
flow nets for the embankment fill because of the nature of the construc-
tion materials and the specified method of embankment construction.
For the zoned dam steady state seepage analyses, the upstream
shell wasassumed to be at least 10 times more permeable than the core
which for all practical flow net purposes is equivalent to making the
ratio infinite. In the zoned dam drawdown analysis, the core was
assumed to be relatively impervious and the upstream shell was assumed
to be pervious.
Where the embankment was assumed to be homogeneous, the permeabil-
ity of the upstream shell and impervious core zones are considered to
be equal. In all of the seepage studies, the downstream sand drainage
zone was assumed to be effective .
The foundation was assumed to be impervious because it is believed
22
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that it will actually be relatively impervious and because it is more
conservative for the drawdown and downstream drainage considerations
to do so .
The seepage losses through the embankment under full reservoir
seepage conditions is expected to be less than 2 gpm. Abutment and
foundation seepage are believed to be minimal. Any seepage which
occurs through these areas will be localized to open joints or cracks
which can not be appraised prior to construction.
The flow nets are presented on Figure 4. An examination of these
flow nets indicates that for the steady seepage case, the zoned dam
assumptions are the most conservative because no head loss is assumed
in the upstream shell and that for the drawdown case, the vertical
equipotential assumption for the homogeneous dam is the most critical
case, Since all of the other cases will give much higher factors of
safety, these cases were selected for stability analysis.
7, Stability Analysis. Stability analyses were performed to
check the factor of safety which the embankment section will have for
the most critical conditions that it will undergo as a result of the
proposed reservoir operations. These analyses were made on the basis
of saturated shear strength tests. The apparent shear strength of
the soil was obtained by a quick undrained triaxial shear test. This
shear strength was used in the stability analyses as the effective
stress strength of the soil which is a conservative assumption. Co-
hesion, as developed by the laboratory test procedures used for the
project, is not a reliable property of the soil so it has been ignored
in the stability analyses.
23
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Three conditions were analyzed for the upstream slope of the
dam and two conditions were analyzed for the downstream slope, The
upstream conditions were the full reservoir steady seepage case,
with and without seismic forces, and the rapid drawdown case. The
downstream slope was analyzed for full reservoir seepage conditions
with and without seismic forces, The results of these analyses are
presented on Figure 5.
The minimum factor of safety required for each of the stability
cases studied for Squires Dam was selected with consideration given
to the method of analysis, the basis for the adopted soil properties,
the severity of the seepage assumptions, the nature and consequence
of the associated type of failure, and the probability of occurrence
of the conditions assumed in each case.
The minimum factor of safety required for the full reservoir
steady seepage analyses for both slopes is 1.50, The minimum factor
of safety for the drawdown case using vertical equipotentials for
the upstream slope is 1.00, The minimum factor of safety required
for the seismic cases was 1.10,
The trial arc method of analysis was used to analyze the upstream
slope of the dam. This procedure was applied, using graphical pro-
cedures, to determine the arc having the least factor of safety
against sliding. This factor of safety is reported in tabulation on
Figure 5 as the factor of safety for the case studied, This method
considers the resistance of a major segment of the embankment as a
whole against failure by shearing along the circular arc upon which
24
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111111
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...
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the accumulated stresses are the most critical, In this analysis,
the centers for each trial arc were selected by judgment and the
factor of safety was computed. This process was continued until
the arc with the least factor of safety was found.
The downstream slope of the dam was analyzed using infinite slope
procedures. This is conservative for this dam design because the im-
pervious core is too far within the embankment to produce a critical
effect on the slope stability, the internal drain prevents seepage
forces from developing in the vicinity of the downstream slope, and
the foundation strength is greater than that of the embankment fill.
The seepage conditions used in the computation of the "seepage
forces II for the full reservoir steady seepage case were determined
by the use of a flow net seepage analysis. The drawdown "seepage
forces" are based on an assumption that the pressures are represented
by "vertical equipotentials.11 Both of these seepage force determina-
tions were made'with the most critical zoning assumptions. The draw-
down seepage assumptions are believed to be very conservative .
Seismic forces were applied to the arcs found to be the most
critical when acted upon by gravity and seepage forces only. Al-
though the addition of seismic forces would shift the location of
the critical arc slightly, this simplified method of application of
these complex forces is considered to be adequate .
The results of the stability analyses for the dam slopes are pre-
sented on Figure 5, The seepage assumptions, and all of the pertinent
data for each case are shown on this figure. The results of the
25
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l
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stability analyses on the dam section indicate that the slopes for
the structure will be stable for the conditions analyzed which in-
clude all critical conditions that can now be anticipated,
A review of the results of the analyses indicates that the de-
sign of the upstream slope of the embankment is controlled by the
conditions obtained by two-thirds drawdown (with the assumption of
vertical equipotentials) with both gravity and seepage forces act-
ing. The computed factor of safety for this case is 1,08 as com-
pared with the minimum required design value of 1.00.
The most critical conditions for the downstream slope exist dur-
ing an earthquake with seismic forces acting on the dam slope. The
computed factor of safety for this condition is 1.16 as compared with
the required minimum design value of 1.10 .
8. Piezometers and Settlement Markers. Piezometers and surface
settlement markers will be installed within and on the embankment
respectively as shown on Figure 2. The piezometers gages will be
read periodically during and following construction of the embank-
ment. The settlement markers will be measured periodically follow-
ing construction.
The purpose of the piezometers and settlement markers is to pro-
vide a basis for the systematic evaluation of the stability and gen-
eral condition of the embankment during and after its construction
on a routine basis .
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C -FREEBOARD, SETTLEMENT, AND ·CAMBER ANALYSIS
The height of the Squires Dam is controlled by the spillway
crest elevation to which is added the maximum spillway surcharge and
the freeboard. In order to guarantee that the full design height is
maintained, the embankment is constructed somewhat higher than re-
quired in order to allow for post construction settlement of the
dam crest due to consolidation of the fill. In addition, since a
perfectly horizontal embankment crest gives the impression of sagging
when viewed from the valley floor downstream of the dam, it is custo-
mary to increase the height of the center of the embankment in order
to counteract the optical illusion which occµrs because of the con-
vergence of the valley abutments.
1. Freeboard Design. The maximum spillway surcharge will be
about 2 feet, for which an allowance of 3. feet of free board is made.
Due to the location and nature of the Squires Canyon reservoir, fre-
quency of spill is very low, Due to its size, the crest width, and
the provision for upstream slope riprap, the need for freeboard is
minimal. A total freeboard of at least five feet will be available
most of the time.
2. Settlement Analysis. Consolidation and settlement studies
made on materials similar to those in the Squires Dam indicate that
the post construction settlement of the dam will be about one percent
of its height above the foundation at any point. The post construction
settlement of the dam embankment will be roughly proportional to the
27
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height of the dam at that point. This means that the center of the
embankment will settle more than the portions of the embankment ad-
jacent to the abutments since the abutments and the foundation mate-
rials are relatively incompressible. Since the settlement allowance
varies across the profile of the dam, it will be provided for along
with the camber.
3, Camber. Since public confidence is a function of appearance,
camber will be provided to counteract the apparent sag which the shape
of the valley will give a perfectly horizontal embankment crest, The
camber allowance varies across the profile of the dam, The camber
and settlement allowance is presented on Figure 2.
D -SPILLWAY
1. Location, The spillway is located at the extremity of the
right abutment with the crest at elevation 505,0 in such a position
as to discharge down a natural draw at this location, The downstream
face of the dam is protected from the natural spillway drainage by a
ridge.
2. Capacity, The spillway as designed is an open channel trape-
zoidal in cross section, with a 16 foot bottom width, and 1-1/2 to 1
side slopes. Based on a critical depth occurrence at the entrance to
the channel, super critical flow in the channel, and a 16 foot width,
the computed flow for a water surface elevation of 507 feet is 136
cubic feet per second. Inasmuch as the total drainage area con-
tributing to this spillway is only 60 acres, this capacity appears
28
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to be more than ample.
3, Design. The spillway is proposed to be an open channel lined
with concrete with cutoff walls and side slope protection at the en-
trance, At the outlet a cutoff wall is also proposed with necessary
natural channel alignment being fixed to protect the downstream face
of the dam from any possible erosion.
E -INLET-OUTLET PIPE
l, Location. The 27-inch diameter inlet-outlet pipe is located
to the left of the Squires Canyon drainage channel as indicated on
Figure 2, An inlet box at approximate elevation 425 is provided to
provide screening if necessary and to prevent silted water from enter-
ing the outlet pipe,
2. Construction, The inlet-outlet pipe is 27-inch diameter steel
pipe with cement mortar lining and concrete encasement, Cutoff walls
have been provided on the outlet conduit where it passes through the
impervious core zone of the dam. These walls will serve to lengthen
the seepage path, and to change direction of the seepage flow at reg-
ular intervals, This may decrease the seepage velocities along the
pipe-compacted fill interface. Filter zone cutoffs have been provided
at either end of the outlet conduit where it emerges from the core zone
into the shell zone in order to prevent piping in the event that seepage
does occur along the pipe-soil contact.
The outlet valve has been provided at the downstream face of the
dam, This has been done for reasons of convenience and economy. The
29
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design of the outlet conduit minimizes the probability of catestropic
pipe failure, and the very small size and regulatory nature of the
reservoir makes it possible to assure regular maintenance of the pipe.
These factors have indicated practicability of limiting the outlet con-
trol to a downstream valve system.
30
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STATE OF CALIFORNIA
DEPARTMENT OF WATER RESOURCES
APPLICATION NO, ____ _
APPAOVEO AS TO SAFETY
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DATE REMARKS
CARLSBAD
UNICIPAL WATER
IMPROVEMENT DISTRICT
SQUIRES 0.._
DAM EMBANKMENT
SEEPAGE ANALYSES
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STATE OF CALIFORNIA
DEPARTMENT OF WATER RESOURCES
....,..CATION NO .. ___ _
A~ AS TO SAFETY
CARLS8AD .I WA"l'rD I
DATE
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