SEEPAGE MODELING FOR EVALUATION OF DEWATERING

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21st Century Dam Design

Advances and Adaptations

31st Annual USSD Conference

San Diego, California, April 11-15, 2011

On the CoverArtist's rendition of San Vicente Dam after completion of the dam raise project to increase local storage and provide

a more flexible conveyance system for use during emergencies such as earthquakes that could curtail the regions

imported water supplies. The existing 220-foot-high dam, owned by the City of San Diego, will be raised by 117

feet to increase reservoir storage capacity by 152,000 acre-feet. The project will be the tallest dam raise in the

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Evaluation of Dewatering Efforts 831

SEEPAGE MODELING FOR EVALUATION OF DEWATERING EFFORTS FOR CONSTRUCTION OF THE COACHELLA CANAL LINING PROJECT

Geraldo R. Iglesia, Ph.D., P.E./G.E.1

Christopher M. Dull, P.E.2 Kenneth A. Steele, P.E.3

Halla Razak, P.E. 4

ABSTRACT During construction for a new concrete-lined canal to replace a ~35-mile stretch of the existing earthen Coachella Canal, the required dewatering efforts had substantially exceeded initial expectations. Because the new canal had to be constructed without impacting operation of the existing canal, the section between the new and existing canals functioned much like a levee or fill for an elevated canal. With the aid of finite element modeling software, seepage analyses of the site conditions were performed to obtain an understanding of the underlying physical processes that likely contributed to the situation. Based on the seepage modeling results, the presence of a predominantly fine-grained layer overlain by relatively coarser-grained material tended to keep the water seeping from the existing canal into the new canal. These results were mostly corroborated by construction experience in the field, which showed a strong correlation between the location of dewatering drain tile installations and where the predominantly fine-grained soil layer was present.

INTRODUCTION

The Coachella Canal Lining Project (CCLP) was a major water conservation action developed as part of the historic Quantification Settlement Agreement (QSA), a consensus reached in 2003 among multiple parties (from local to state to federal level) to settle longstanding disputes regarding allocations of Colorado River water. The CCLP involved the construction of approximately 35 miles of concrete-lined canal adjacent to the existing Coachella Canal from Siphon 7 to Siphon 32. In addition, construction included 25 inverted siphon undercrossings of stream channels, one railroad crossing, and six check structures, along with various measures adopted to mitigate consequential environmental impacts.

As shown in Figure 1, the Coachella Canal branches off the All-American Canal near the Mexican border in Imperial County, 16 miles west of Pilot Knob at Drop 1, and travels northwest 123 miles to the Coachella Valley in Riverside County. The Coachella Canal is located between the Salton Sea and the Chocolate Mountain range, and serves irrigated

1 Principal, G2D Resources, LLC, 7966 Arjons Drive, Suite 204, San Diego, California 92126-6361. [email protected]. 2 Vice President, R.W. Beck, Inc., 15373 Innovation Drive, Suite 390, San Diego, California 92128. [email protected]. 3 Consultant, 1740 Burnside Way, Stockton, CA 95207. [email protected]. 4 Colorado River Programs Director, San Diego County Water Authority, 4677 Overland Avenue, San Diego, California 92123-1233. [email protected].

21st Century Dam Design Advances and Adaptations 832

agriculture with a total annual crop value exceeding $550 million, as well as serving cities north of the Salton Sea, including Palm Springs, Palm Desert and Indian Wells.

Figure 1. CCLP Location Map

The Imperial and Coachella Valleys are blessed with a 12-month growing season, benefiting local farmers and neighboring residents, and enabling one of the major food production areas of the United States. The Boulder Canyon Project Act authorized, among other infrastructure, the construction of the All-American Canal in June 1929, including the Coachella Canal branch to help bring water to these valleys. The 123-mile long Coachella Canal was first excavated on August 11, 1938, delayed during the onset of World War II in 1941, and was the final feature of the Boulder Canyon Project in 1948.

In the 1960s, investigations were carried out to develop a program to upgrade the original Coachella Canal system. The rehabilitation program, funding by a loan authorized in 1961 under the Rehabilitation and Betterment Act, provided a supervisory remote control and telemetering system to operate the canal and distribution system; constructed a regulating reservoir, Lake Cahuilla, at the end of the canal; built two flood control dikes; rehabilitated an existing check gate; and added 10 traveling demossing screens and a new check gate.

Historically California had used more of its allocation of Colorado River water, relying on underused allocations of other states. In the 1990s it was recognized that the other states would soon need to use their full allocations. In October 2003, the QSA was signed along with related agreements intended to consensually settle longstanding disputes regarding priority, use, and transfer of Colorado River water and to establish


distribution of the water among the parties for up to 75 years. These agreements also facilitate actions that will enhance the certainty and reliability of Colorado River water supplies and meeting demands within Californias apportionment of Colorado River water by identifying terms, conditions, and incentives for the conservation and distribution of Colorado River water within California. The project also contributed to the settlement of water rights disputes involving the Federal government and several Native American groups on the San Luis Rey River in California.

As a measure of conservation, the Allocation Agreement (collateral to the QSA) was signed, memorializing the allocation of conserved water from the lining of the Coachella Canal, as authorized by Title II of Public Law 100-675. The Allocation Agreement, among the U.S. Bureau of Reclamation, Metropolitan Water District of Southern California, Coachella Valley Water District, Imperial Irrigation District, San Diego County Water Authority, the San Luis Rey Indian parties, the City of Escondido, and the Vista Irrigation District, provides for, among other things, the identification of quantity of water conserved by the CCLP and subsidized funding of the CCLP by the California Department of Water Resources. Additionally, other agreements associated with the project established timelines for achieving conservation targets and using State funds.

TECHNICAL ASPECTS OF CCLP

The CCLP includes construction of approximately 35 miles of concrete-lined canal adjacent to the existing canal including construction of 25 inverted siphon undercrossings of stream channels from Siphon 7 to Siphon 32, six check structures and a variety of consequential environmental mitigation measures. Environmental mitigation included cultural resource surveys, mitigation for aquatic and riparian habitat, desert riparian habitat, tree replacement, large mammal monitoring and mitigation measures, and fishery mitigation. The CCLP results in 26,000 acre-feet of conserved water per year.

It was originally planned to line the old canal in place with pipelined bypasses going around reaches where work would be taking place. This plan would have allowed use of existing siphons and check structures. However, the pumping and pipelines required for such a bypass operation turned out to be vastly more costly than had been anticipated. Additionally such a bypass operation could be very risky in terms of water supply interruptions and damage to work in progress in the event of a failure or washout. Due to the cost and risk, following the 60 percent design review, the design was changed to a parallel canal with new siphons and check structures.

One of the most unique aspects of the CCLP is that the new concrete-lined canal was built adjacent to the old unlined canal in the same right of way, roughly 400 to 500 feet in width. Shoulder to shoulder, the two canals are occasionally only about 20 feet apart. To minimize the potential for seepage flows between the existing and new canal during construction, the existing canal was operated with all check structures open during the entire period of canal construction.

The 25 new (replacement) inverted siphons ranged from 200 feet to 800 feet in length, and directed the canal underneath the railroad crossing and the major washes coming off


the Chocolate Mountains. They needed to be constructed during the dry (hot) season to minimize the associated dewatering and surface water issues.

The CCLP incorporated a traditional construction bid package based on established specifications, conditions, and criteria. Since the CCLP was particularly unique, only three proposals were received. The construction contract was awarded to the lowest bidder, R & L Brosamer.

As depicted in Figure 2, the new concrete-lined canal had a trapezoidal section built with side slopes inclined at 1.5H:1V (horizontal to vertical), while the existing earthen canal had 2H:1V side slopes. More than 1.3 million square yards of 3-inch thick concrete was used for lining 35 miles of the new Coachella Canal from Siphon 7 to Siphon 32. Technology for the lining machine employed for the CCLP was first developed in the 1950s, and has been continually refined with each successive use. Other equipment included a trimmer, a finisher and a cure machine.

Figure 2. Coachella Canal Typical Section

The concrete lining process was adjusted to nighttime paving in order to avoid the extreme desert heat in the daytime. Too much heat adversely affects the curing of the concrete resulting in excessive cracking and degraded durability. Rescheduling to nighttime paving proved successful in accelerating the schedule for paving and allowed the project to stay on schedule. While extensive lighting made the work possible, it was still riskier, due to considerably more provisions needed for nocturnal operations. Even so, concrete placement rates approached 2,000 feet per day, and up to 200 feet per hour on some days. At peak performance, 150 to 160 crewmembers were working on site.

During construction, it was imperative to maintain continuous flow in the canals so that water deliveries to the Coachella Valley agricultural community would not be interrupted. Unique construction of the parallel canal with only two tie-ins enabled the project to meet this requirement. Canal tie-ins were developed and constructed below Siphon 32 and above Siphon 7. The tie-ins provided transfer of service from the old canal to the new canal, redirecting water at Siphon 7 from the existing canal to the new canal for 35 miles and then directing canal water at Siphon 32 back to the old canal. Plans for the tie-ins, including a plan for the transfer of service, were developed and submitted to the Coachella Valley Water District for review prior to initiating work. This was a unique aspect of the project to ensure continued water delivery at a prescribed flow rate in the canals.


Construction of the siphon undercrossings and check structures included reinforced concrete transition structures, earthwork, roadways, riprap, sheet piling, safety cable and floats, metalwork and chain link fencing. Construction of check structures also included control buildings, stilling wells, commercially designed radial gates complete with appurtenances, and electrical instrumentation.

Construction of the CCLP began in October 2004, lining activities commenced in January 2006, water began flowing in the newly lined canal in December 2006, and the facility was taking full flow within a few weeks. A ribbon-cutting ceremony was held in November 2006, and the project was officially completed in April 2007.

GEOLOGIC SETTING AND SITE CONDITIONS

The geologic conditions prevalent in the vicinity of the CCLP are depicted in Figure 3. The CCLP is situated along the eastern margin of the Salton Trough physiographic province. The Salton Trough, a geologic structural depression resulting from large-scale regional faulting, is bounded on the northeast by the San Andreas Fault and Chocolate Mountains, and on the southwest by the Peninsular Range and faults of the San Jacinto Fault Zone. The Salton Trough represents the northward extension of the Gulf of California, containing both marine and non-marine sediments since the Miocene Epoch. Tectonic activity that formed the trough continues at a high rate, as evidenced by deformed young sedimentary deposits and high levels of seismicity (Southland Geotechnical, 2002).

The geology along the CCLP alignment can be divided into three distinct portions. The southern portion (Siphons 7 to 21) of the project route is characterized by unconsolidated marine and non-marine sediments of the Borrego Formation. These sediments consist of interbedded sand, silt, and clay deposited during incursions of the Gulf of California and/or ancient lakes created by overflows of the Colorado River into the Salton Trough.

Along the central portion (Siphons 21 to 30) of the CCLP alignment, the geology consists of alluvial fan deposits derived from erosion of the Chocolate and Orocopia Mountains. The Chocolate Mountains consist of the pre-Cambrian Chuckwalla Complex (gneiss, migmatite, and anorthosite) intruded by Mesozoic granitic rocks. The Orocopia Mountains are composed of pre-Cambrian Orocopia Schist.

On the northern portion (Siphons 30 to 32) of the canal route, the geology consists of Plio-Pleistocene deposits of the Palm Spring Formation. These sediments consist of sandstones and siltstones derived from granitic rocks and deposited as alluvial fans. The Palm Spring Formation is highly folded and faulted.

Beach ridges and other shoreline features of ancient Lake Cahuilla can be seen along the west side of the canal. Ancient Lake Cahuilla reached an elevation of about 45 feet above mean sea level some 300 years ago. In general, the water in the unlined Coachella Canal sat at a slightly higher elevation than the shoreline.


ABBREVIATED GEOLOGIC LEGENDQUATERNARY (Pleistocene - Holocene) Deposits:

Alluvium, lake, playa, and terrace deposits; unconsolidated and semi-consolidated. Mostly non-marine, but includes marine deposits near the coast.

Older alluvium, lake, playa, and terrace deposits

Extensive marine and non-marine sand deposits, generally near the coast or desert playas

TERTIARY Deposit:

Tertiary intrusive rocks; mostly shallow (hypabyssal) plugs and dikes.

Note:Base geologic map is obtained from information provided by the California Geologic Survey (http://www.conservation.ca.gov/cgs/cgs_history/Pages/2010_geologicmap.aspx).

For further information on the other types of deposits shown on the map above, please refer to the website indicated.

CCLPAlignment

S24

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Figure 3. Geologic Map of CCLP Site

Geotechnical investigation was performed for the CCLP (Southland Geotechnical, 2002), which characterized the upper 40 feet of subsurface soil from 36 selected borehole locations. The geotechnical investigation had three primary elements: (1) soil composition, strength, and driving resistance to 25 feet below the bottom of the new canal; (2) groundwater levels in the construction zone; and (3) location of existing earthquake faults, if any, crossing the canal alignment.

Figure 4 shows a plot of the approximate subsurface profile along the length of the CCLP alignment, including the level of groundwater detected during the geotechnical investigation, the estimated invert elevation of the new concrete-lined canal, as well as the locations of the various siphons. The stratigraphy of the soils along the CCLP alignment basically consists of a predominantly coarse-grained material (sand or gravel)


overlying a mostly fine-grained material (clay or silt). The approximate location of this predominantly fine-grained layer along the CCLP alignment is shown in Figure 4. Notably, there are locations where no predominantly coarse-grained layer was encountered in some of the geotechnical borings (e.g., along Stations 2890 through 3150); conversely, in some borings (e.g., along Stations 3700 through 4250), no predominantly fine-grained layer was encountered. Also, in certain locations (e.g., between Stations 2800 and 2900, and between Stations 3190 and 3235), a layer of coarse-grained material (though not visible in Figure 4) was found interbedded within the fine-grained layer.

Figure 4. Approximate Subsurface Profile along Length/Stationing of CCLP

DEWATERING CHALLENGES DURING CONSTRUCTION

Dewatering at some canal sections was essential before excavating to final grade within areas of high groundwater, both in the canal and at canal siphon and check structures. The groundwater level was lowered to at least one foot below the canal invert subgrade or structure subgrade, and maintained at this level until the embankments were constructed, the lining placed, and the canal filled with water above the established groundwater level, or until backfill work was completed to a point two feet above the groundwater level or the canal was filled. In all cases, the contractor was responsible for preventing flotation of the siphons and canal liner. This proved to be much more challenging and significant than originally anticipated. More water than anticipated entered the new canal section during construction. Additional dewatering efforts were substantial, requiring additional pumping, vigilant attention, and added work.

The contractor employed a drain tile and sump system for dewatering the canal. Its dewatering subcontractor used a specially designed trenching machine to insert the drain tile and pea gravel pack in the invert of the canal in the same operation. The work could

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be accomplished even with water in the invert of the canal. In its bid, the contractor reportedly expected to dewater using drain tiles totaling about 12,000 linear feet along the length of the canal. During the course of construction, however, the contractor had to install drain tiles over 85,000 linear feet along the length of the canal, due to significantly higher quantities of groundwater within the construction zone than anticipated. The locations of the drain tile installations for dewatering for CCLP construction can be superimposed with the subsurface profile, as displayed in Figure 5. It can be inferred from Figure 5 that the contractor tended to install dewatering drain tiles where the predominantly fine-grained soil layer was present.

Figure 5. CCLP Subsurface Profile with Dewatering Drain Tile Installation Locations

The CCLP Construction Manager also reported (MWH/Bookman Edmonston, 2006) that the water surface in certain reaches of the existing canal was much higher than it should have been due to aquatic grasses and sediment.

SEEPAGE MODELING AND ANALYSES

To validate the apparent correlation of the presence of the predominantly fine-grained material in the vicinity of the canal prism with the dewatering requirements, seepage analyses were conducted for various cross-sections. Computer-aided two-dimensional (plane strain) analyses of steady-state seepage through various cross-sections were performed using GeoStudio SEEP/W (Krahn, 2004) finite element software capable of modeling both saturated and unsaturated soil behavior.

Figure 6 shows a typical finite element mesh used in modeling the seepage flow from the existing canal toward the new canal prism under steady state conditions. The filled circular dots on the boundary of the analysis/model domain (primarily in the wetted perimeter of the existing canal) represent conditions of known hydraulic head. The filled

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Note: Canal stationing is shown below, in linear feet.


triangles indicate conditions where the flux is zero (or near zero), such as on the dry embankment surface, along the vertical plane at the centerline of the existing canal, and at the bottom horizontal boundary of the model. The open (non-filled) triangles are boundary node locations, mostly toward the downstream end, where effluent seepage may be possible, to be determined iteratively by the software algorithm.

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Figure 6. Finite Element Model Mesh Used for Seepage Analysis at Station 3430

Saturated material permeability values input into the seepage modeling were based on the geotechnical investigation report (Southland Geotechnical, 2002) and on published correlations. In addition, the model employed a normalized hydraulic conductivity function that resembled an inverted s-curve of permeability vs. matric suction, as plotted in Figure 7, to simulate saturated-unsaturated material behavior. This hydraulic conductivity function was derived on the basis of published data for various soil types and has been used satisfactorily in other applications (e.g., Iglesia et al., 1998; Iglesia et al., 2009).

Seepage Modeling Results

The graphical output from each modeling run will show the flow patterns simulated for a particular scenario. Besides the contours of total hydraulic head within the system analyzed, of special interest in each Figure 7. Normalized Hydraulic

Conductivity Function

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run is the location of the phreatic surface, which is the boundary between the saturated and unsaturated zones. For example, the graphical output corresponding to the model depicted in Figure 6 is presented in Figure 8. In this case, the phreatic surface tends to stay at a relatively high level due to the presence of the predominantly fine-grained layer not too far below the bottom of the new canal. The rather prominent flow vectors just above the predominantly fine-grained layer also indicate the zone(s) where the seepage is relatively more intense. The result exemplified in Figure 8 is quite typical at the cross-sections where the upper boundary of a fine-grained material lies within a few feet below the bottom of the canal and is overlain by coarser-grained soil.

Figure 8. Seepage Analysis at Station 3430

When a modeling run is performed corresponding to a case where the fine-grained layer is not present, such as at Station 3865 shown in Figure 9, the graphical output is dramatically different than before. For a cross-section with fairly homogeneous material, the phreatic surface drops more steeply in the absence of a less permeable fine-grained layer that tends to impede the flow of water in the downward direction. The homogeneity of the soil also causes the flow to spread out more evenly, as indicated by the flow vectors in Figure 9. These results would help explain why dewatering drain tiles did not have to be installed between approximate Stations 3660 and 4260 (Siphons 22 to 29), where alluvial fan deposits predominate in accordance with the geologic setting at the project site.

The seepage analyses also enable the calculation of the flux or discharge quantity over an area of concern, such as at the inner downstream toe of the new canal. Based on modeling runs at various cross-sections along the CCLP alignment, the calculated flux quantities can be summarized and plotted as in Figure 10. This summary plot suggests that dewatering drain tiles had to be installed where the calculated seepage flux at the inner downstream toe of the new canal would turn out to be relatively high. Looking

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back at the subsurface profile alongside the drain tile installation locations as in Figure 5, the overall trends support the hypothesis that the dewatering requirements at a particular station correlate strongly with the presence of predominantly fine-grained material closely subjacent to the canal and overlain by coarser-grained soil. Perhaps more significantly, such analytical results would make it quite difficult to support or to sustain a contention that the conditions at the site had materially changed since the early stages of the project.

Figure 9. Seepage Analysis at Station 3865

SUMMARY AND CONCLUSIONS Prudent application of modernly available computational tools for analyzing seepage through porous media has provided useful insight into additional dewatering efforts implemented on the CCLP. Graphical displays of simulated subsurface flow patterns, as well as calculation of flux quantities, tend to comport with field observations at various stations along the project alignment. Based on the seepage modeling runs, increased dewatering efforts for construction of the CCLP between Siphons 7 and 22, and between Siphons 30 and 32, can be attributed to the presence of fine-grained subsoil, closely subjacent to the canal and overlain by predominantly coarse-grained material. The relatively lower permeability of the fine-grained soil would tend to impede subsurface flow of water from the coarse-grained material downward, causing the zone of saturation to stay at a shallow level. Higher water levels than anticipated in the existing earthen canal, which was kept in operation during construction of the new concrete-lined canal, likely would have exacerbated the situation. Nevertheless, a better understanding of the mechanism underlying complex field conditions, as afforded by robust numerical modeling of seepage on the CCLP, has led to a satisfactory resolution of several construction-related issues among the project participants.

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Figure 10. Calculated Seepage Fluxes at Inner Downstream Toe of New Canal

ACKNOWLEDGMENT The authors wish to acknowledge the contributions and meaningful participation on the CCLP by the following: U.S. Bureau of Reclamation, State of California Department of Water Resources, Metropolitan Water District of Southern California, Coachella Valley Water District, San Diego County Water Authority, San Luis Rey Indian parties, Imperial Irrigation District, MWH/GEI-Bookman Edmonston, R & L Brosamer, and R.W. Beck, Inc. The assistance provided by Dr. James L. Stiady of G2D Resources, LLC in performing the analyses presented in this manuscript is hereby acknowledged as well. The authors also appreciate the review comments on the draft version of this paper, as provided by Prof. Herbert H. Einstein of the Massachusetts Institute of Technology and by Mr. Paul Shiers of PB Americas, Inc.

REFERENCES

Iglesia, G. R., Keller, T. O., and Meda, J. M. (1998), Seepage Analysis of Miramar Dam, Engineering Mechanics: A Force for the 21st Century. Proceedings of the 12th Engineering Mechanics Conference, La Jolla, CA, pp. 566-569.

Iglesia, G. R., Stiady, J. L., and Zhou J. (2009), Re-Analysis of the Failure of Teton Dam, Managing Our Water Retention Systems. 29th Annual United States Society on Dams Conference, Nashville, TN, pp. 669-684.

Krahn, J. (2004), Seepage Modeling with SEEP/W, Geo-Slope International Ltd., Calgary, Alberta, Canada.

7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 28 29 30 31 32

0.000001

0.00001

0.0001

0.001

0.01

0.1

1

10

2500+00 2700+00 2900+00 3100+00 3300+00 3500+00 3700+00 3900+00 4100+00 4300+00 4500+00

Cal

cula

ted

Seep

age

Flux

(gal

/min

per

100

0 fe

et o

f can

al)

Siphon No.

Based on High-Water Levels

Based on Design Depth

Drain Tile Installation


MWH/Bookman Edmonston (2006), Letter dated May 8, 2006 from the Coachella Canal Construction Management Team to the CCLP Coordinating Committee in regard to canal dewatering for the CCLP.

Southland Geotechnical, Inc. (2002), Preliminary Report Geotechnical Investigation, Coachella Canal Lining Project, Siphon 7 to Siphon 32, Imperial and Riverside Counties, California, Report submitted to MWH Americas, Inc.

Documents

SEEPAGE MODELING FOR EVALUATION OF DEWATERING