CEMENT BENTONITE SLURRY WALL STRENGTH

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21st Century Dam Design

Advances and Adaptations

31st Annual USSD Conference

San Diego, California, April 11-15, 2011

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Cement Bentonite Slurry Wall Strength 653

CEMENT BENTONITE SLURRY WALL STRENGTH TUTTLE CREEK DAM SEISMIC REMEDIATION

Amod K. Koirala, Ph.D.1

Glen M. Bellew, P.E.2 John C. Dillon, P.E.3

David L. Mathews, P.E.4

ABSTRACT Cement bentonite (CB) slurry walls were constructed as a seismic remediation to stabilize the downstream slope of Tuttle Creek Dam in Manhattan, Kansas. A full-scale test program was conducted to evaluate various slurry mixes and construction techniques prior to main construction. Grout mixes with cement/water (c/w) ratios of 0.3, 0.4, 0.45, and 0.5 were used. Construction equipment in the test program included long-reach and clamshell excavators. Sampling and testing were performed on wet grab samples and core samples. Wet grab samples were obtained from freshly constructed slurry walls and cured in a laboratory. Core samples were obtained from cured walls. Evaluation of the test section results led to the selection of a c/w ratio of 0.5 and the use of a clamshell excavator for seismic stabilization construction. The required peak unconfined compressive strength (UCS) of the cured wall was 300 psi based on stability and deformation modeling. Observations showed UCS of core samples were less than wet grab samples. UCS generally increased with specific gravity and c/w ratio.

INTRODUCTION The Corps of Engineers-Kansas City District conducted extensive seismic evaluations for Tuttle Creek Dam located in Manhattan, Kansas. These evaluations concluded that an earthquake with a magnitude of 5.7 or greater would cause liquefaction of the foundation sand and result in large deformations of the embankment. The maximum credible earthquake (MCE) from the nearby Humboldt Fault Zone is a magnitude 6.6 earthquake. Fast Langrangian Analysis of Continua (FLAC) modeling demonstrated that downstream slope and toe deformations would exceed acceptable limits during the MCE. Limit equilibrium slope stability analysis confirmed these findings. To reduce the risk of deformation and slope instability during and after the MCE, cement bentonite (CB) slurry walls were constructed in the foundation sands through the downstream portion of the embankment. The first phase of wall construction was a full-scale production test section. The purpose of the Production Test was to refine materials and methods for use in the remainder of construction (Stage One Stabilization and Main Construction Option).

1Civil Engineer, US Army Corps of Engineers, 601 E 12th St. Kansas City, MO 64106, [email protected] 2Geotechnical Engineer, US Army Corps of Engineers, 601 E 12th St. Kansas City, MO 64106 , [email protected] 3Project Manager, US Army Corps of Engineers, 601 E 12th St. Kansas City, MO 64106, [email protected] 4Chief, Geotechnical Branch, US Army Corps of Engineers, 601 E 12th St. Kansas City, MO 64106, [email protected]

654 21st Century Dam Design Advances and Adaptations

Additional work was completed as part of the overall dam remediation, including construction of a buried collector system to fill in a collector ditch at the downstream toe of the dam, upstream riprap overlays, and emergency spillway gate modifications. The Tuttle Creek Dam modification plan with all construction stages is shown in Figure 1.

Figure 1. Plan view of Tuttle Creek Dam Downstream Seismic Stabilization

SUBSURFACE CONDITIONS The soil in the alluvial foundation of Tuttle Creek Dam consists of 8 to 27 ft of silt and low plasticity clay underlain by sand, silty sand, and gravelly sand. The sand deposits vary in thickness from about 25 to 60 ft and can be separated into two distinct zones. The upper zone consists of a 15 to 20-ft-thick layer of loose fine to medium sand (SM, SP and SW) and the lower zone consists of a 25 to 30-ft-thick layer of dense coarse to gravelly sand that increases in grain size with depth (SP, SW, GP and GW). Due to the alluvial nature of the foundation deposits, multiple lenses of cohesive soil exist within the coarse-grained layers. The upper sand zone was determined to be potentially liquefiable during large earthquake motions. The upper silts and clays were also expected to suffer significant strength loss due to large strains caused by liquefaction of the underlying sand. Bedrock consists of alternating layers of shale and limestone. The silt and clay form a natural cohesive soil blanket over the more-permeable sands. This natural cohesive blanket is an important component of underseepage control. Underseepage pressures are controlled by a line of pressure relief wells along the downstream toe.

METHODS AND MATERIALS OF CONSTRUCTION In order to install the walls, a working platform was constructed on the downstream slope of the dam. The working platform was constructed of predominantly sand with an


aggregate surface. The equipment used in the production test section to install walls consisted of a Liebherr HS855DH Crane with a clamshell excavator and a Koehring 1466 long-reach excavator. Figure 2 shows a plan view of a clamshell-constructed wall. Walls constructed with the clamshell excavator were 4 feet wide and were constructed in a series of bites. The clamshell excavator was approximately 15 feet long. Each clamshell wall was constructed in 5 bites: three primary bites (PU, PM, PD) and two secondary bites (SU, SD). Bites were nearly vertical, and the final wall cross section was approximately a rectangle. A steel frame (guide wall) was used to guide the clamshell into the wall excavation. Figure 3 shows wall construction with the long-reach excavator. Walls constructed with the long-reach excavator were 3 feet wide and constructed in one continuous excavation. Due to the operational range of the long-reach excavator, walls were not rectangular in cross section. The bottom corners of long-reach excavator constructed walls were not square due to the reach of the excavator. All walls were constructed with the clamshell excavator after the production test section.

Figure 2. Plan View of Liebherr HS855DH Crane with a clamshell excavation. Figure

from TreviIcos South (2007) The grout mix used in wall construction was mixed at an onsite batch plant. The cement used was Lafarge MaxChem consisting of a 50/50 mixture of Portland cement and ground granulated blast furnace slag (slag). Cement water ratios (by weight) of trial mixes in the test program were 0.3, 0.4, 0.45, and 0.5. Additionally, a 25/75 Portland cement to slag cement mix-ratio was used in a small number of walls in the test program. Bentonite was typically added at a rate of 5% by weight of cement and was Wyo-Ben Hydrogel. The additive Lamsperse-HS was used as a retarder and bentonite antiflocculant to maintain workability of the mix for a minimum of 24 hours. Water was obtained from a well screened in the foundation sands at the downstream toe of the dam.


Figure 3. Operational reach of Koehring 1466 long-reach excavator digging CB wall. Figure from TreviIcos South (2007)

Walls were constructed by excavating and simultaneously placing self hardening cement bentonite slurry in the trenches. By continuously placing slurry into the excavation, the trench would remain open during construction of the wall. Walls were oriented transverse to the axis of the dam. Walls were typically 3 or 4 feet wide, 45 feet long, and approximately 65 feet deep. Walls were spaced with 10 feet of clear space between adjacent walls. The walls extended through the upper foundation sand at least 12 feet into the deeper coarse sand. This was done to allow for stress transfer to stronger materials during shaking. The slurry level in the walls was observed to drop during cure. The observed drop was a combination of slurry permeation into the adjacent soil and slurry bleed during curing. The walls were topped off with fresh slurry daily to account for drop during curing. Total slurry drop was typically 10% of wall depth.

SAMPLING AND TESTING Sampling and testing was performed at the on-site batch plant, on the fluid slurry in the excavations via wet-grab samples, and on the hardened slurry via core drilling. Wet grab sampling was conducted on each wall at equally spaced depths. Wet grab samples were cast in 3-in by 6-in cylinders. The samples were originally stored in a 100-percent humidity curing room until being tested for unconfined compressive strength (UCS).


After it became apparent the samples were dessicating in the wet room, all samples were stored submerged under water until UCS testing. Coring was conducted on 92% of walls in the test section, and approximately 40% of walls in the remainder of construction. Initially, coring was conducted with a Geobore system (double-barrel wireline) producing 4-in-diameter samples. Walls were typically cored between 60 and 90 days after construction, but some were cored as early as 28 days and as late as 200 days to observe strength changes with time. Due to significant strength discrepancies between wet-grab and core sample strengths, the coring operation was changed to a triple-barrel coring device in an effort to reduce sample disturbance and micro-fracturing. However, the change in coring operation did not have a significant effect on cores sample UCS. The specific gravity of most wet grab and core samples was obtained was also. Soil content was calculated from the difference in the specific gravity of slurry at the plant and cured wet grab or core samples.

CB WALL PROPERTIES PRODUCTION TEST SECTION The results of laboratory testing from the production test section were analyzed to determine the appropriate slurry mix to be used in the remainder of stabilization. Table 1 shows a summary of wall properties with various c/w ratios, slag content, and construction equipment. Thirty eight walls were constructed during the production test, approximately 4 walls per mix and method shown in Table 2.

Table 1. Summary of cement bentonite wall properties - production test section

Equipment Cement/Water Ratio+ Sample Type

Age (days)

Average UCS (psi)

Ave. Specific Gravity

Average Soil

Content (%)Long-Reach 0.50 Wet Grab 62 568 1.65 24Long-Reach 0.50 Core 134 387 1.50 12Long-Reach 0.45 Wet Grab 47 414 1.65 24Long-Reach 0.45 Core 161 341 1.64 26Long-Reach 0.40 Wet Grab 54 285 1.62 25Long-Reach 0.40 Core 150 284 1.63 26Long-Reach 0.30 Wet Grab 49 130 1.58 25Long-Reach 0.30 Core 131 129 1.49 20Clamshell 0.50 Wet Grab 49 626 1.57 18Clamshell 0.50 Core 100 310 1.59 20Clamshell 0.45 Wet Grab 49 436 1.54 17Clamshell 0.45 Core 125 286 1.57 19Clamshell 0.40 Wet Grab 49 273 1.51 16Clamshell 0.40 Core 174 246 1.52 17Clamshell 0.30 Wet Grab 49 115 1.43 14Clamshell 0.30 Core 191 78 1.49 18Clamshell .40 (75% slag) Wet Grab 45 713 - -Clamshell .40 (75% slag) Core 45 356 - -Clamshell .30 (75% slag) Wet Grab 45 300 - -Clamshell .30 (75% slag) Core 45 239 - -

+cement content was 50/50 Portland cement to slag ratio unless otherwise stated In general, wall strength and specific gravity increased with increasing cement water ratio. Walls constructed with 75% slag content exhibited significantly higher strength


than walls of the same c/w ratio constructed with 50% slag content. Wet grab UCS was higher than core UCS. On average, core UCS was 78% of wet grab UCS for all walls constructed in the production test. The difference in wet grab and core UCS is attributed to sample disturbance and differences in curing environment. Due to the discrepancy between core and wet grab UCS, core samples were used to ensure in situ wall strength requirements were met. Figure 4 shows core UCS for various c/w ratios and construction equipment. Walls constructed utilizing the long-reach excavator had slightly higher core UCS than walls constructed with the clamshell. This is thought to be due to construction duration and method. The long-reach excavator was slightly faster than the clam shell at constructing walls. Walls constructed with the clamshell generally extended into a second day which required slurry agitation over a longer duration. The additional agitation time may have caused some loss of early cement bonding resulting in higher core UCS of long-reach constructed walls. Higher wet grab UCS was not observed in long-reach constructed walls. The specific gravity of long-reach walls was higher than clamshell walls. This is thought to be caused by more soil being mixed in to the slurry during construction with the long-reach excavator. Specific gravity and strength were found to be generally directly proportional, so the higher specific gravity (soil content) in long-reach walls is at least partially responsible for their higher strength.

Figure 4. Core sample UCS variation with cement/water ratio and construction technique,

50% slag walls (Figure from Axtell, Stark, Dillon (2009)) All of the c/w mixes appeared to achieve the majority of their strength between ages of 30 and 45 days. Figure 5 shows the variation in UCS with time for various c/w ratio


mixes. Some samples indicated a decreasing UCS with time after the peak. This is likely caused by testing errors and minor desiccation of samples not stored under water prior to testing.

Figure 5. Wet grab and core sample UCS variation with cement/water ratio and age In the production test walls, cores from the bottom of the wall generally had higher strengths from cores in the middle or top of the wall. The UCS increase with depth is likely the result of increased specific gravity, or soil content, in the lower portion of the wall due to soil particle settlement and slurry consolidation. Materials and equipment for the remainder of construction were selected upon completion of the production test section. A minimum 0.5 cement/water ratio with a 50/50 Portland cement to slag was required in the specifications, and the Contractor elected to use the clamshell excavator on the remainder of construction. The selected materials and method were proven in the production test section as being able to provide a core peak UCS of 300 psi, which was required in the specifications.

WALL PROPERTIES STAGE ONE AND MAIN CONSTRUCTION OPTION

The remainder of wall construction provided a large data set of walls constructed with the same slurry mix and construction method, as well as a smaller data set of higher c/w ratio (0.55 and 0.60) walls constructed at the Contractors option. The higher c/w ratio walls were constructed so the Contractor could gain information regarding stronger mixes and to ensure the last portion of construction would meet strength requirements and equipment could be demobilized. The results of laboratory testing from the remainder of


wall construction, the Stage One and Main Construction Option phases, were analyzed to determine trends in wall properties. Table 2 shows a summary of wall properties from the remainder of wall construction. Typically the Main Construction Option (MCO) exhibited slightly higher strengths than the Stage One Stabilization (S1S). This is likely due to differences in the subsurface conditions between the two reaches. The thickness of the silty clay blanket is significantly greater in the S1S area than in the MCO area. The aggregate qualities of the fine-grained particles are not as good as those of coarse-grained sand particles and likely produce lower strengths. The walls with higher c/w ratios generally had higher wet grab and core UCS. The 0.55 c/w ratio mix core samples are the exception, but this may be due to small sample size or early age of core testing. As was observed in the production test section, wet grab samples exhibited higher strengths than core samples. Core samples exhibited between 38% and 66% of the UCS of wet grab samples for mixes used in the S1S and MCO phases of construction.

Table 2. Summary of remainder of 0.5 c/w ratio wall construction, Main Construction Option

Phase of Construction

Number of Walls

Constructed

c/w Ratio

Sample Type

Age (Days)

Avg. UCS (PSI)

Avg. Specific Gravity

Stage One Stabilization Wet Grab 34 506 1.59

62 0.5

Core 120 333 1.59 Wet Grab 63 658 1.52 235 0.5

Core 95 356 1.65 Wet Grab 63 851 1.57 12 0.55 Core 50 319 1.60 Wet Grab 63 1057 1.70

Main Construction Option

4 0.60 Core 67 604 1.71

Wall UCS and specific gravity with normalized depth for all 0.5 c/w mix walls constructed during the MCO are shown in Figure 6. The middle portion of the walls exhibited a relatively constant UCS and a slightly increasing specific gravity with depth. Research has shown that overburden stresses are transferred to the trench sides and the slurry does not cure under an increasing confining stress with depth (Evans and Ryan, 2005). This explains why there is generally no increase in strength with depth in the middle of the wall. In the upper portion of the wall, there is an inversely proportional relationship between strength and specific gravity - the upper portion of the walls exhibited high strength and low specific gravity. This is likely due to slurry drop and subsequent top off that occurred while the wall was curing. Adding slurry during curing increased the confining stress in this zone leading to higher UCS. The specific gravity was lower because soil particles in the upper portion of the wall settled toward the bottom during cure. The lower portion of the walls exhibited higher strength and specific gravity with depth. This is likely due to the accumulation of settling sand particles at the bottom


of the wall and consolidation of the slurry mix during curing. The higher density and soil content of the lower wall causes higher strengths than less-dense-lower-soil-content-slurry in the upper and middle portions of the wall.

Figure 6. UCS and specific gravity with normalized sample depth for 0.5 c/w mix walls

constructed during the Main Construction Option.

CONCLUSIONS A full-scale production test program at Tuttle Creek Dam was conducted to evaluate various slurry mixes and construction techniques for construction of cement bentonite walls to improve seismic stability. The production test indicated slurry with a c/w of 0.5 would produce the required peak core UCS of 300 psi at Tuttle Creek Dam. Walls constructed with a long-reach excavator produced walls with a higher strength than walls constructed with a clamshell. Walls constructed with higher c/w ratios had higher specific gravities and strengths. Wet grab samples obtained from freshly constructed walls and cured in a laboratory exhibited higher strengths than core samples from cured walls. Because of the discrepancy between wet grab and core sample strength for high strength walls, core sample strength was used to ensure in situ wall strengths were achieved. Strength generally increased with increasing specific gravity, with a trend deviation in the upper portion of the wall likely due to slurry drop and subsequent slurry


top off. Construction of high strength cement bentonite walls at Tuttle Creek Dam proved to be an effective and constructable method to provide seismic stabilization.

ACKNOWLEDGEMENT

The authors acknowledge the support provided by the U.S Army Corps of Engineers-Kansas City District, specifically Joe Topi and Geoff Henggeler, and the expertise of the contractor, Treviicos South.

REFERENCES Axtell P.J, Stark, T. D. and Dillon J.C. (2009). Strength Difference Between Clamshell and Long reach Excavator Constructed Cement-Bentonite Self Hardening Slurry Walls.ASCE International Foundation Congress and Equipment Expo. pp 297-304 Evans, J. and Ryan, C. (2005) Time-Dependent Strength Behavior of Soil-Bentonite Slurry Wall Backfill, GSP-142 Waste Contamination and Remediation. TreviIcos South (2007). Downstream Production Test Modification Final Report.

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CEMENT BENTONITE SLURRY WALL STRENGTH