case study: feather river west levee program cutoff walls · cutoff wall construction methods to...

CASE STUDY: FEATHER RIVER WEST LEVEE PROGRAM CUTOFF WALLS Jared P. Williams, GLEI, Rocklin, CA USA 916-462-6400 [email protected] Kyle R. Lackner, GLEI, Rocklin, CA USA 916-462-6400 [email protected] ABSTRACT The Sutter Butte Flood Control Agency initiated the Feather River West Levee (FRWL) Program in 2010 to bring 35 miles of levee along the Feather River into compliance with applicable Federal and State standards for levees protecting urban areas. A combination of soil-bentonite cutoff walls, soil-cement-bentonite cutoff walls, and seepage berms will reduce flood risk by correcting under and through seepage deficiencies and remove more than 34,000 properties from the FEMA Special Flood Hazard Areas. The FRWL program includes a combined total of 7,744,803 square feet (SF) of cutoff wall planned to be constructed between 2013 and 2015. Cutoff wall construction methods to meet the design depths included conventional excavation to a depth of 85 feet and deep soil mixing for depths up to 120 feet. A key component of the FRWL program was the development of site specific mix designs using the design boring logs to evaluate the expected backfill gradation and permeability results. Issues that required a partnership between Contractors, Owner and Engineers included substantial Archaeological and Cultural Resource delays, significant depth changes due to a change in competent key, right of way acquisitions for abutting land, and the magnitude of the project in a resource limited industry. INTRODUCTION The Feather River watershed basin is located 50 miles north of Sacramento, CA and incorporates the agriculture producing communities of Yuba City, Live Oak, Gridley and Biggs. These communities are protected from flooding by levees that contain under-seepage and through-seepage deficiencies and were the cause of major failures in 1955, 1986 and 1997. The Feather River West Levee (FRWL) program was initiated by the Sutter Butte Flood Control Agency (SBFCA) in 2010. The long term objective of SBFCA is to correct the levee deficiencies and provide 200 year flood protection to more than 34,000 at risk properties located within the FEMA Special Flood Hazard Areas. Although FRWL contained a variety of construction components such as earthwork and pipe replacement, the slurry cutoff wall is the primary construction element for addressing under and through seepage deficiencies. From 2013 to 2015 approximately 6,649,210 SF of cutoff wall were installed using conventional excavation and deep soil mixing (DSM) techniques leaving 1,095,593 SF to be completed in 2016. The work was designed, solicited and constructed under two separate contracts, titled Project Area C and Project Area B&D. A summary of the contracts and cutoff wall quantities is presented in Table 1. Table 1. Slurry Cutoff Wall Quantities Contract / Year Conventional Excavation Deep Soil Mixing Project Area C

• 2013 151,906 SF 544,364 SF • 2014 2,091,589 SF 225,513 SF

Project Area B&D • 2014 2,247,470 SF NA • 2015 1,388,368 SF NA • 2016 1,095,593 SF NA

Totals 6,974,926 SF 769,877 SF

© 2016 Deep Foundations Institute 191

FRWL Project Location FRWL’s first contract was Project Area C which is located within the Yuba City and Live Oak city limits with a total length of over ten (10) miles. Project Area’s B&D were the second with Area B located just south of Yuba City extending for just over six (6) miles and Area D which spans from Live Oak to the Thermalito Afterbay encompassing a length of eleven and a half (11.5) miles. Figure 1 shows the geographic locations of the different reaches as well as the designed remedy.

Figure 1. FRWL Project Areas – Credit Sutter Butte Flood Control Agency


CUTOFF WALL METHODOLOGY AND DESIGN Although multiple cutoff wall techniques were allowed per the Contract, the two chosen for FRWL were conventional excavation and deep soil mixing (DSM). DSM was utilized for depths exceeding 85 ft. and provides the opportunity of reaching depths greater than 200 ft. Conventional Excavation Equipment The specialized equipment utilized for conventional excavation cutoff wall construction includes large stock excavators with custom built booms and sticks. Historically, the boom and stick are designed and built by the cutoff wall contractor and an independent equipment manufacturer, and not by the manufacturer of the hydraulic excavator. Consequently, careful planning and design considerations have to be given to details including overall weight, balance, geometry, connections with the parent machine, and hydraulics to achieve a functional boom and stick capable of achieving the maximum depth possible. Figure 2 illustrates the range of motion with regard to maximum reach and excavation depth of a custom built PC1250 long boom and stick.

Figure 2. PC1250 Range of Motion


Equipment limitations for the custom boom and stick, require that the constructed trench width is 36 inches. This measurement is determined and monitored in the field by measuring the width of the excavator bucket. The following are the advantages to utilizing this method for cutoff wall construction:

• The cutoff wall can be excavated to the full required depth in a single pass • The cutoff wall backfill is mixed ex-situ thus allowing for visual inspections • Keyed-in cutoff wall samples can be verified through visual inspection of removed samples

In addition to the specialized excavation equipment, there is also standard size equipment used for mixing and placing backfill. This includes at a minimum, one excavator and one dozer (i.e. PC 200 excavator and CAT D6 LGP dozer) for mixing the backfill materials on the surface of the work pad. Soils can be pre-mixed by the excavator to break down large clumps of clay or soil, prior to tracking through the material with the dozer. When sections of levee experience work pad width limitations (reduced mixing platform) or geotechnical challenges, as was prevalent during FRWL, a standard long reach excavator as well as haul trucks (i.e. Hyundai 290 excavator and HM350 haul trucks) can be used to move suitable material to and unsuitable material from the backfill operation. Also, import material was evenly distributed based on design borings along the cutoff alignment, ahead of the excavation using a front end wheel loader (i.e. Komatsu WA270). Slurry mixing equipment included a jet shear mixer, multiple six (6) inch trash pumps for circulation, four (4) inch HDPE pipe for conveyance, and an all-terrain forklift for handling 3,000 pound bentonite super sacks. The slurry pond consists of constructing a minimum 50,000 gallon capacity cell, usually by excavating a location within a staging area or on the workpad, into the ground surface by 2-3 feet, and building 3-5 foot berms around the perimeter of the pond. The size of the pond is generally dependent on the project demands, including constructing one slurry trench heading versus multiple slurry trench headings, anticipated daily production rates, and other slurry demands. The length (linear feet) of the project would also dictate the need for intermediate slurry ponds and transfer pumps for slurry conveyance. Contract Acceptance Criteria The contract acceptance criterion for conventional excavation includes the following:

• Wall Depth • Minimum Wall Thickness of 36 inches • Maximum Bulk Sample Permeability of 5x10-7 cm/s • Continuity and Homogeneity: Evidenced by the preponderance of Quality Control and Quality

Assurance test results and inspection of the soil-bentonite backfill placement by the Agency. The contract acceptance criterion for the DSM method includes the following:

• Wall Depth – Depth’s based on the verification drilling results which were performed before construction and at the Agency’s direction.

• Wall Thickness – Minimum 27 inches • Wall Continuity – The maximum incremental deviation between two adjacent panels at any depth

shall not be greater than twenty (20) percent of the specified minimum wall thickness. The accumulated total deviation of any single panel from vertical shall not exceed one (1) percent of the panel depth.

• Continuity and Homogeneity: Evidenced by the preponderance of Quality Control and Quality Assurance test results and inspects.

• Maximum Bulk Sample Permeability of 5x10-7 cm/s • Bulk Sample Strength of minimum twenty (20) psi at 28 days, and maximum 100 psi at 28 days • Minimum Cement Content of 0.5% of the cutoff wall mix design


The wall depth is a critical criterion established for correcting the under seepage deficiencies. Although typical cutoff walls are keyed into an underlying confining layer, the analyzed head pressures and groundwater hydrology concerns could be addressed by the design of a hanging, or non-keyed, cutoff wall. In these cases, the design depth of the cutoff wall was independent of identifying an underlying confining layer. However, other locations required a deeper cutoff wall, and confirmation of keying into a previously identified confining layer, such as verifiable and competent clay or silt. The minimum wall thickness for conventional excavation was specified at 36 inches on all contracts for FRWL. This was achieved during the conventional excavation method by making a single unobstructed pass with the excavator through the slurry trench and dictated by the width of the excavator bucket. For DSM, although the auger is 36 inches wide, the wall thickness requirement was only 27 inches to accommodate the width of the radical line between the overlapping augers. The maximum bulk sample permeability requirement for all contracts was 5x10-7 cm/s. Testing was performed in a flexible wall permeameter in accordance with ASTM D5084, using the following parameters:

• Average effective confining stress of one (1) to five (5) psi • Maximum hydraulic gradient of fifteen (15) psi

Unconfined compressive strength (UCS) applied only to the DSM method in which soil-cement-bentonite (SCB) was used. The specified parameters were between 20 and 100 psi at 28 days. Continuity and homogeneity acceptance criteria were met based on results of the following tests:

• Slump testing in accordance with ASTM C143/143M to be between four (4) and seven (7) inches • Slurry testing per applicable American Petroleum Institute (API) procedures • Backfill density testing per ASTM C117 and C136 to be at least fifteen (15) pcf greater than the

maximum in place slurry density • Visual inspection performed by both experience quality control and quality assurance personnel • Soil gradations testing in accordance with ASTM D1140 and D422 within the limits of Table 2

Table 2. Specified SB Cutoff Wall Backfill Gradation Sieve Size or Number Percent (%) Passing by Dry Weight 2-inch 100 No. 4 40 to 100 No. 40 25 to 90 No. 200 20 to 60

The lower end of the No. 200 sieve gradation is based on having sufficient material to meet the specified maximum bulk sample permeability requirement. The upper end of this requirement is based on minimizing long term consolidation of the backfill material due to an overabundant presence of fine clays or silts. Bentonite Slurry Quality Control Implementing proper quality control procedures has become a critical element of slurry cutoff wall construction. Today’s procedures and specification requirements, versus those as few as ten (10) years ago, are more stringent and refined in relation to defining the overall performance of the cutoff wall. Although the API procedures are relatively similar with the past, the specifications may require additional testing or parameters that are unique to individual site conditions rather than simply following the standard API requirements. The advancement in testing requires that the Contractor maintain a technically knowledgeable staff to both perform and analyze the results.


FRWL required field testing of the initial bentonite slurry for viscosity, density and filtrate loss to be within the following specified parameters: Table 3. Initial Bentonite Testing Methods and Parameters Test Method Parameters Viscosity – Marsh Funnel Minimum of 40 seconds Density – Mud Balance Minimum of 64 pounds per cubic foot Filtrate Loss – Filter Press Maximum 1.75 cubic inches in 30 minutes at 100 psi

For FRWL, the trench slurry was tested at multiple depths and locations within the trench. This included a top, middle and bottom sample at a distance no less than 5 feet from the toe of the soil-bentonite backfill slope. Testing procedures were the same as the initial bentonite slurry with differing result parameters for both density and filtrate loss.

Table 4. Trench Bentonite Slurry Testing Methods and Parameters Test Method Parameters Viscosity – Marsh Funnel Minimum of 40 seconds Density – Mud Balance Minimum of 64 pounds per cubic foot, maximum of 85 pounds per cubic foot Filtrate Loss – Filter Press No parameter just submit result of the filtrate loss in 30 minutes under 100 psi

As seen with most slurry cutoff walls, the quality control testing for FRWL revealed a direct correlation between the depth of the trench and the density of the slurry, with denser slurry being found at the bottom of the trench likely due to settlement of the suspended silts and sands. However, the bentonite slurry viscosity and gel strength allow for suspension of the silts and sands, generated during the excavation process, which if managed correctly, do not settle rapidly enough to cause quality control concerns. A major focus of the quality control program was managing the density of the trench slurry. At times, particularly when performing deep excavations, densities would rise above 85 pounds per cubic foot. To mitigate this becoming an evolving issue, we would install a desander unit that would be moved along with the excavation. Additional measures also included maximizing the distance between the backfill toe and the excavation to introduce fresh slurry or in extreme cases, pumping out / disposing the trench slurry while also adding fresh slurry. Soil Gradations and Settlement Monitoring Soil gradations were performed in the past generally to verify if the percentage of silt and clay was acceptable to produce a low permeability mixture. However, design concerns in levee restoration involve the potential for long term secondary consolidation of the cutoff wall which could potentially create a void between the top of the cutoff wall and levee fill material. Long term secondary consolidation concerns are minimized by limiting the allowable amount of silt and clay in the backfill material to less than 60% passing the No. 200 sieve. Because the soil-bentonite backfill is placed in a loose and saturated state, some initial settlement or consolidation is anticipated immediately after backfilling the cutoff wall. Therefore, backfilling over the top of the slurry cutoff wall is delayed until the initial settlement is complete. Locations of settlement plates include the beginning of each cutoff wall heading and at significant changes in depth, over 20 ft. Settlement plates were installed every 200 ft. at 1000 linear foot intervals and surveyed using convention surveying equipment and closed level loops over a 21 day period. The consolidation rate outcome was nearly undetectable within the settlement period signaling that the initial settlement was complete.


Permeability Testing Hydraulic conductivity, commonly referred to as permeability, is the primary acceptance criteria element and key function of the slurry cutoff wall. Permeability testing of the soil-bentonite backfill was performed on wet bulk samples collected at frequency of once every 50 LF of top out. Permeability testing was performed in accordance with ASTM D 5084 with results typically ranging from 5x10-8 to 2x10-7 cm/sec. To date, all samples have achieved passing results for permeability and are shown below in Figure 3.

Figure 3. FRWL SB Backfill Permeability Results

Proportioning and thorough blending of material in the field can often be a difficult task due to the operation being performed in an open and unconfined setting. Because no measuring devices were used, the consistent and successful sample results for soil-bentonite backfill permeability is a monument to the overall quality control program and the performance of each slurry wall contractor. PROJECT OBSTACLES Obstacles are inevitable on any construction project, particularly one of this scope complexity and sheer magnitude. It is important that an ever evolving industry learns from these oversights and mistakes and come up with ways to improve the design, contracting, and construction for future projects. Archaeological and Cultural Resource Delays The original levees, built as early as the 1850’s along the FRWL, were constructed primarily by local farmers with the intention of protecting their homes and crops from flooding. As they were constructed, the levees incorporated several mounds that are actually now known to have been Native American archaeological sites containing buried remains and artifacts. In 2014, construction commenced on Project Area C without knowledge of the extent of Cultural sites that would be uncovered. During levee degrade it became apparent that the amount of these sites were severely underestimated and an action plan between the regulatory agencies, tribes, owner and contractor had to be developed. Because construction was already in progress, the best way forward was to have construction occur around the site investigations leaving gaps that would need to be completed after the artifact recoveries were complete. This substantially increased the cost of construction due to the amount of moves

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and inherent inefficiencies, however this approach allowed for substantial completion of the Project Area C contract without extending into another construction season. Project Area B, which occurred during the same season, only had two such sites and the impact was minimized by pre-planning heading start and finish locations. In 2015, SBFCA began working with the regulatory agencies, namely the US Army Corps of Engineers and California State Historic Preservation Office, as well as the local tribes to come up with pre-construction site specific treatment plans. This process was held up due to the handlings of the artifacts recovered from Project Area C and differing policies between Federal regulatory agencies that conflicted with the tribal requests. Federal policies stated that they must study the artifacts while the State agreed with immediately transferring all findings to the rightful tribe. SBFCA, wanting to comply with Federal permits while also having a good neighbor policy with the local tribes, were put in a very challenging position. Ultimately, negotiations and the development of agreeable site specific action plans resulted in a two and a half month delay that extended the completion of Project Area D into the 2016 construction season. The outcome of the site specific treatment plans allowed tribal monitors to be present during all work activities however, artifact investigations and recoveries would occur after construction was complete and in a manner that would not impact the newly constructed levee prism. Competent Key Changes Design investigations included subsurface drilling and discovery using various methods including core drilling, cone penetration testing (CPT), and sonic drilling every 1,000 ft. along the cutoff wall alignment. Because of the total length of the Project, this interval was viewed as a comprehensive analysis that would accurately identify the subsurface conditions and allow the cutoff wall tip elevation to be determined by the Geotechnical Engineer. For the sections of levee that were determined to be at a depth greater than 85 ft., DSM was the chosen method. For DSM, the contractor was required to perform verification drilling at intervals of 100 ft. along the levee to determine the actual depth of each element. During this investigation process, it was determined by the Geotechnical Engineer that a confining layer was found much higher in elevation than was shown in the design borings. This significant change in elevation altered the methodology from DSM to conventional excavation which created many challenging contractual implications. The Contract was specific to the method and payment based on depth intervals. For example, there were pay items for depths ranging from 50 to 78 ft., 78 to 95 ft., and 95 to 120 ft. This required the Contractor to price the methodology and any work challenges within those depth increments. The Contract also states that the Engineer is limited to decreasing or increasing the depth by maximum of ten (10) ft. This caused two questions to arise: 1) was the Geotechnical Engineer allowed to change depth by such a drastic amount? And 2) if allowed to make such a change which bid item would the work be paid under? Both items required a significant amount of time and effort to reach an agreement. After all issues were closed the one outstanding question was if this scenario could have been prevented. Resource Limitations The cutoff wall industry is a specialized market with limited resources that typically places a cap on how much cutoff wall can be included under a single contract. The Natomas Levee Improvement Program (NLIP), which was a large program with a similar scope, required a maximum per year of one and a half million square feet of cutoff wall under two separate contracts. FRWL, in comparison, had a maximum per year of four and a half million square feet of cutoff wall completed under two separate contracts. This magnitude made the FRWL Project Area B&D Contract, the single largest levee project in California.


To complete the size, scope and complexity of the FRWL program, a JV partnership was formed between two competitors in Great Lakes Environmental & Infrastructure (formerly Magnus Pacific) and Nordic Industries. Even with two cutoff wall Contractors, there was too much work in 2014 that the JV hired key cutoff wall subcontractors, who are also industry competitors, in Geosolutions, Inquip and Raito. This type of collaboration and partnership was required to fulfill the seasonality project constraints, which only allowed work to proceed between April and November, as well as the completion milestones mandated by the contract. Restricted Cutoff Wall Alignment The alignment of the levee in Project Area D contained a variety of tight corners that could not be constructed per the geometry shown in the plans. The JV came up with a cross configuration that would allow for continuity of the cutoff wall as show in Figure 4.

Figure 4. Cross Excavation for Tight Corners

This type of excavation requires talented operators and precise GPS layout to ensure the as built cutoff wall stays within the allowable distance from the alignment. Corners were monitored visually in the event of substantial settlement or any other unforeseen abnormalities such as slurry loss and trench instability. LESSONS LEARNED AND CONCLUSIONS Every project comes with key lessons learned that can help to improve the efficiency and outcome of future contracts. FRWL is no different with improvements that, if implemented, could have saved the project both time and construction costs. Archaeological and Cultural Resource Delays The delays associated with Archaeological and Cultural Resource findings were due largely in part to a permitting process that indicated there were only a few known sites on the entire project. Knowing now that there were clear deficiencies in the process, there is a lessoned learned on how to improve investigations during the design. This would include additional investigations such as potholing along the levee that could expose midden soil, a common indicator of a cultural site. This work could be accomplished at large enough intervals to be both informative and cost effective.


Competent Key Changes Changes to competent key were expected and anticipated for ranges up to ten (10) ft. However, the drastic changes that occurred on FRWL Project Area D were so substantial that the as-planned methodology had to change from DSM to conventional. With this significant change came an array of contractual discussions that could have been avoided. During the design phase, the Agency performed subsurface investigations at approximately 1,000 ft. increments along the levee that determined the design depths of the cutoff wall. For the areas that had a design depth requiring DSM, verification drilling was to be performed by the Contractor at 100 ft. intervals that provides the Geotechnical Engineer with additional information necessary to then determine the depth of each DSM element. While the investigation intervals are reasonable, the verification drilling scope of work under future contracts should strongly be considered a design responsibility. By performing this work ahead of construction the design would be much more accurate and drastic depth changes, at least in the areas of DSM, would be eliminated. Conclusion Final completion of FRWL will bring 35 miles of levee along the Feather River into compliance with State and Federal standards for levees protecting urban areas. The slurry cutoff wall was the key design component that addressed the under and through seepage deficiencies of the existing levee system. The selected techniques for FRWL included 6,974,926 SF of convention excavation cutoff wall and 769,877 SF of DSM. Although the project contained issues, FRWL was a historic undertaking of magnitude in an industry that is limited by its resources. The partnership formed between the Agency, Engineer, Construction Manager, Prime Contractor, and key subcontractors was paramount to completing the work meeting stringent quality control demands, safely, and on an aggressive schedule. ACKNOWLEDGMENTS The authors wish to acknowledge the following individuals for their contributions on the FRWL program: Louay Owaidat, James Beebe, Sean Rhodes, Matthew Marks, Ezekiel Wilson, and Jon White. REFERENCES Recommended Practice for Field Testing Water based Drilling Fluids. API Recommended Practice 13B-1, Fourth Edition, 2009, 104 p. Standard Test Method for Determining the Amount of Material Finer than 75-µm (No. 200) Sieve in Soils by Washing. Designation D1140 – 14, ASTM International, 2014, 6 p. Standard Test Method for Materials Finer than 75-µm (No. 200) Sieve in Mineral Aggregates by Washing. Designation C117 – 04, ASTM International, 2008, 3 p. Standard Test Methods for Measurement of Hydraulic Conductivity of Saturated Porous Materials Using a Flexible Wall Permeameter. Designation D5084 – 10, ASTM International, 2010, 23 p. Standard Test Method for Particle-Size Analysis of Soils. Designation D422 – 63, ASTM International, 2007, 8 p.


Standard Test Method for Sieve Analysis of Fine and Coarse Aggregates. Designation C136 – 06, ASTM International, 2008, 5 p. Sutter Butte Flood Control Agency | Reducing flood risk and protecting lives and livelihoods. (2010). Sutterbutteflood.org.


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case study: feather river west levee program cutoff walls · cutoff wall construction methods to...

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