Lab-Scale Study of Ultrafine Cement Grout Curtains ... - ASCE Library

Geo-Congress 2014 Technical Papers, GSP 234 © ASCE 2014

Lab-Scale Study of Ultrafine Cement Grout Curtains using a Physical Model L. Sebastian Bryson1, Ph.D, P.E., M. ASCE and Ryan Ortiz2, E.I.T

1

Associate Professor, Department of Civil Engineering, 161 Raymond Bldg., University of Kentucky, Lexington, KY 40506 USA, phone: 859.257.3247, email: [email protected] 2

Research Assistant, Department of Civil Engineering, 161 Raymond Bldg., University of Kentucky, Lexington, KY 40506 USA, email: [email protected]

ABSTRACT: Curtain grouting is a construction technique by which a continuous grout wall is formed from a series of cylindrical columns placed adjacent to one another. Grout curtains are typical used in permeable soil or rock masses to decrease seepage. Grout curtains are vital to dam and other earthen impoundment restoration projects. In particular cement grouts, such as ultrafine grout, have proven to be very effective in reducing seepage through these impoundments. While the primary purpose of curtain grouting is for seepage control, mechanical behavior of the grouted mass such as excessive settlement and lateral movement is also of concern. In this study, an apparatus and methodology for a lab-scale physical model of curtain grouting was developed. Various measures of mechanical behavior related to grout curtain field practices were investigated. These measures included cure time and interface strength between the soil and grout. In addition, the grout-soil interface strength was a minimum of 66 percent more than the sand-sand interface strength. This study showed that a lab-scale grout curtain can be adequately implemented to evaluate grout curtain behavior. Finally, this research showed that utilizing a labscale physical model of a grout curtain is a valid approach for investigating fieldscale grout curtain effectiveness. 1

INTRODUCTION

Curtain grouting is defined in this study as the construction technique by which a continuous grout wall is formed from a series of cylindrical grout columns placed adjacent to one another in a permeable soil or rock mass in order to decrease seepage

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(UFC, 2004). Typically, cementitious grouts are used to create the curtain. Current standards specify one linear row or multiple linear rows as appropriate, depending on target seepage decrease. Grout curtains have been fundamental in dam rehabilitation for many years. Several examples are available (Bruce and Dugnani, 1996; Amos et al., 2007) that show the benefits of using grout curtains for seepage reduction applications. Unfortunately, these and many other case histories reported in literature tend to only present the seepage reduction data. While it would appear that only the seepage reduction data is necessary, given the primary purpose of a grout curtain, focusing only on the performance data precludes the development of consistent and rational design methodology. In general, most grout curtain designs are performed by the grouting contractor, using subjective “local experience” or proprietary design techniques (Weaver, 1991). Further research is needed to investigate the factors that most influence grout curtain effectiveness so that reliable design methodologies can be developed. In particular, factors such as the mechanical behavior of the grout material, the index properties of the geologic mass, and the installation characteristics of the grout curtain must be quantified. Further research will optimize the design and effectiveness measures, as it applies to grout curtains. In this study, an apparatus and methodology for a lab-scale physical model of curtain grouting was developed. The physical model allowed for various measures of grout curtain effectiveness to be investigated in a controlled manner. These effectiveness measures included cure time, injection pressure, extent of lateral grout penetration in the soil mass, and interface strength between the soil and grout. 2

LAB-SCALE PHYSICAL MODELS FOR SEEPAGE STUDIES

An essential component to this experiment is the development of a lab-scale physical model. Previous lab-scale studies (Liua et al., 2003) have shown that physical models can be used to evaluate foundation water flow, such as modeling seepage through a concrete dam over bedrock. Other studies have effectively used physical models to investigate the seepage behavior of a dam with a highly permeable sand foundation (Luofenga et al., 2012); and to investigate the seepage behavior and the residual slope stability of a dam after a landslide (Awal et al., 2009). These aforementioned studies noted the difficulty in solely developing numerical models for complex geological insitu field conditions and concluded that physical models were essential to developing an understanding of the behavior associated with insitu construction. While a major portion of this study investigates the lab-scale grouting apparatus development, the properties of the test materials were also analyzed. 3

MATERIALS 2


3.1

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Ultrafine Cement Grout

The grout used in this study was a Type V Standard Grout. It was supplied by Avanti International and manufactured by U.S. Grout, LLC. By weight, the cement grout contains 55 percent pumice, 45 percent Portland cement, and 0.09 to 0.12 percent modified polymer powder. Ninety percent of the particles are distributed below 8 microns and the average particle size is 4 microns. The Blaine Fineness is 1,511 m2/kg, which is the specific area attributed to a gram of the cement (Henn and Soule, 2010). The cement mixing instructions include a 0.8:1 water-to-grout ratio by weight. 3.2

Test Sand Index Properties

Sand was used as the seepage medium for this study. The test sand was classified as poorly graded sand (SP), as determined by the unified soil classification system (USCS). The particle size distribution for the sand used in this study is shown in Figure 1. The figure indicates that the sand was primarily a clean, medium coarse sand and thus was considered well-draining. The specific gravity of the test sand was 2.65. 100 Grain Size Dia (mm) D60 0.92 D50 0.72 D30 0.47 D10 0.32

90 80

% Finer (%)

70 60 50 40 30 20 10 0 0.01

0.1 1 Particle Diameter (mm)

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Figure 1. Grain size distribution curve. 4 4.1

SEEPAGE MODEL Physical Model

The physical model was constructed in a lexan polycarbonate test box. The box was 595 mm long, 595 mm wide and 610 mm deep. The sand was placed in a slightly moist condition and was placed in 76 mm lifts and tamped with a large weight. 4.2

Grouting Apparatus

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The drilling activity was simulated by pushing a metal rod in the soil mass to a prescribed depth. The metal rod was roughly 12.7 mm in diameter. While the pushed rod does not perfectly simulate drilling, it was assumed that the densification of the surrounding sand associated with the pushing would be minimal. The moist soil exhibited an apparent cohesion (i.e. inter-particle bonding) due to matric suction that prevented the drill hole from collapsing. A Hobart C-100 mixer was used to mix the ultrafine cement into the grout. An appropriate mix time of 3 minutes was used to obtain fully mixed grout. A DurhamGeo 152.4 mm diameter constant/falling head permeameter was used as a holding/grout chamber and was agitated by shaking as the test was performed to ensure a proper mixing throughout the curtain grouting. A 457.2 mm long PVC pipe with a 12.7 mm diameter was used for the pressurized grout injector. This grout injector was inserted inside the drilled hole, pressurized, and pulled out gradually, leaving behind a column of the grout slurry. Once the grout cured it served as a lab-scale representation of a grout column. The finalized grouting apparatus is shown in Figure 2.

Figure 2. Curtain grouting apparatus. 5 5.1

MECHANICAL TESTING Grout Compression Testing

Compressive strength testing was performed on grout cylinder samples. The grout sample cylinders were obtained by mixing the ultrafine cement according to the previously described procedure and injecting the cement slurry into an acrylic mold. The diameter and height of these acrylic molds are roughly 101 and 51 mm, respectively. The samples were air-cured in the laboratory in the acrylic molds for 5, 8, 14, and 33 days. Kaufmann and Winnefeld (2002) found that there was a relatively insignificant increase in compressive strength of ultrafine cement specimens for samples that were water cured (i.e. immersed in a water bath) versus samples that were air cured. Thus, air curing in acrylic molds was assumed to be sufficient for the purposes of this study. Unconfined compressive tests were then performed for the 4


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samples in a manner consistent with ASTM D4219. Figure 3 shows that grout strength as a function of cure time. For comparison purposes, data from other studies (Clark et al., 2001; Henn and Soule, 2010) are also plotted in Figure 3.

Grout Strength (MN/m2)

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y = 10.7919 − 26.7785 x R 2 = 0.9251

10 8 6

This Study

4

Clark et al. (2001) 2

Henn and Soule (2010)

0 1

10 Cure Time (days)

100

Figure 3. Unconfined compressive strength versus cure time of grout samples. A simple trend line is fitted through the data to put the observed behavior in context. The equation of the trend line is given as q max = 10.7919 −

26.7785 tc

(1)

where q max is the unconfined compressive strength of the grout cylinders in units of MN/m2 and t c is the cure time in units of days. It not intended that this trend line represent a definitive relation between ultrafine cement grout compressive strength and cure time. However, the trend line does allow rough estimates to be made for cure times between 3 and 33 days, and for ultrafine cement grouts with water-tocement ratios between 0.8:1 and 1:1. For a more definitive relationship, more test data are required.

5.2

Lateral Penetration Testing

Lateral penetration testing was performed to quantify the expected diameter of the individual grouted-sand columns. These tests measured the lateral penetration of the grout through the soil surrounding the drill hole. This testing was used to help determine the column spacing requirements for the physical model. The grouting activity was simulated using the grouting apparatus and methodology previously described. The initial moisture content and dry unit weight of the soil at the time of grouting were 9.6 percent and 16.3 kN/m3, respectively. A typical grout column produced during testing is shown in Figure 4.

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Figure 4. Typical grout column. In general, the penetration diameter was found to increase with injection pressure. However, direct comparisons of penetration versus injection pressure, with case history data is problematic because the extent of penetration is actually a function of the pore structure (i.e. size, shape, and orientation) of the geologic formation at the time of grouting, the moisture content, the type of grout, and the size of the injection point. Isolating these specific factors in the few comprehensive case studies that exist in literature was beyond the scope of this study.

5.3

Interface Shear Tests

The grout columns are semi-rigid inclusions and presumably act to increase the strength of the foundation soil beneath a dam or other impoundment. Thus, shear tests were performed to quantify the possible increase in strength to the foundation soil. For the purpose of this study, interface shear was used as a measure of shear strength. For comparative analysis, the interface strength between the grout and sand, and the shear strength of sand samples were investigated. Direct shear testing was used to obtain the interface shear strength as a function of the normal load for the sand-sand interface and the grout-sand interface. For the sandsand tests, the sand was oven dried and placed in the direct shear device at a targeted dry unit weight of 16.2 kN/m3. For the grout-sand tests, the bottom half of the shear box contained a grout disc and the top half of the shear was filled with sand. The entire sample size, including the grout disc and the sand, was 317.5 mm high and 63.5 mm in diameter. The grout sample used was 10 mm in height. The height for the sand was 21.8 mm. The sand was also placed at a target dry unit weight of 16.2 kN/m3. Figure 5 shows the results of the interface shear tests.

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Interface Shear Stength (kN/m2)


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250 Grout-Sand Interface Sand-Sand Interface

200

R² = 0.9994

150 100

R² = 0.999

50 0 0

50

100 150 200 Normal Stress (kN/m2)

250

300

Figure 5. Cement-sand and sand-sand direct shear results. Figure 5 shows that as the normal load increased, the frictional component between the sand and grout increased and subsequently the difference in interface shear strength also increases. The grout-soil interface had a minimum of 66 percent more strength than the sand-sand shear strength. This minimum increase from the grout-sand interface to the sand-sand interface in Figure 5 corresponds to 34.5 kN/m2. To better quantify the increase in shear strength, the friction angles of the grout-sand samples and the sand-sand samples are determined from the relationship

φ ′ = tan −1

dτ dσ

(2)

where φ ′ is the friction angle and dτ dσ is the change in shear strength with respect to the change in normal stress. For the grout-sand samples, φ ′ = 35.2 o and for the sand-sand samples, φ ′ = 21.4 o .

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GROUT CURTAIN SIMULATION Simulation of the grout curtain installation was performed as described earlier. Unfortunately, the drill holes would occasionally collapse prior to grouting. This occurred due to flooding because matric suction impeded collapse only at low moisture contents. Therefore, PVC casing was added to the process. The casing had an outside diameter of 33.5 mm. The PVC casing was installed by inserting a metal rod inside it, then pushing the PVC and rod into the sand. The metal rod was then pulled out of the PVC casing, leaving the casing in place. The grout injector and the PVC casing were pulled out simultaneously so that only a column of grout remained. The primary columns were constructed and allowed to cure for 48 hours, afterwards the secondary columns were installed in a similar manner, in a continuous line between the primary columns. 7


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The degree of seepage reduction was quantified using an area replacement ratio and a seepage reduction factor. The seepage reduction factor is the hydraulic conductivity of the media prior to installation of the grout curtain, k o divided by the hydraulic conductivity of the media as the grout curtain is being installed, k i . The area replacement ratio is the ratio of the grout column area to the total area that will receive grout. The grout column area is simply the average cross-sectional area of the grout column multiplied by the number of installed grout columns. The total area for the grout curtain is defined as the maximum diameter of the grout columns multiplied by the width of the box. The equation for area replacement ratio is

RR =

Ag

(3)

At

where R R is the area replacement ratio; Ag is the area of the grout curtain at a given point in time; At is the total area to be grouted. Figure 6 presents a plot of the seepage reduction factor as a function of the replacement ratio. The reduction factor was determined for measured seepage rates. For comparison purposes, several case history data have also been included in the plot. 1000

Seepage Reudction Factor, ko/ki

This Study Beaver Dam (Bruce and Dugnani, 1996) Arapuni Dam (Amos et al., 2007) 100 y = e6.2901x R² = 0.8907 10

1 0

0.2

0.4 0.6 0.8 Replacement Ratio, Agrout/Atotal

1

Figure 6. Normalized hydraulic conductivity versus replacement ratio. Figure 6 shows that seepage reduction increases as the area of replacement ratio increases. For example, it can be seen that RR = 0.5 (i.e. coverage of roughly half the grout area) results in a seepage reduction factor of 23.22, which means the resulting hydraulic conductivity is 4.3 percent of the initial hydraulic conductivity. Conversely, to reduce the hydraulic conductivity to 1 percent of the initial hydraulic conductivity, a replacement ratio of 0.73 would be required. An empirical relationship was developed from the study data and the case data that provides an estimate of the

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hydraulic conductivity during curtain installation, given the area replacement ratio and the initial hydraulic conductivity. This expression is given as

ki =

ko exp(βRR )

(4)

where β = 6.2901 and k i will be in the same units as k o .

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CONCLUSION Factors influencing the efficacy of grout curtains such as cure time, injection pressure, extent of lateral grout penetration in the soil mass, and interface strength between the soil and grout were evaluated using a lab-scale physical model of a grout curtain. The following conclusions have been made based on this testing: • •

• •

The strength of the grout material appears to be a simple function cure time and appears to reach a maximum sometime shortly after 33 days. The lateral penetration diameter of the grout through the soil mass to be a function of the injection pressure, but the actual function is dependent on variables related to the geologic formation, grout type, and grouting methods. The interface shear strength of the grouted sand was a minimum of 66 percent greater than the shear strength of the sand alone. The lab-scale grouting apparatus and process proved effective in evaluating the efficiency a grout curtain. This study suggests that future grouting research can be successfully performed in this manner.

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ACKNOWLEDGEMENTS This research was funded in part by the University of Kentucky Appalachian and Minority Science, Technology, Engineering, and Mathematics Majors (AMSTEMM) program under NSF Award # 0431552. This support was greatly appreciated. Grout materials for this research were provided by Mr. Jim Gentry of Avanti International and Mr. Joe Thomas of U.S. Grout, LLC. 9

REFERENCES Amos, P.D., Bruce, D.A., Lucchi, M., Newson, T.S., and Wharmby, N. (2007). “Design and Construction of Seepage Cut-Off Walls Under a Concrete Dam in New Zealand with a Full Reservoir,” Dam Safety 2007, Association of State Dam Safety Officials (ASDSO) Annual Conference, 9-13 September 2007, Austin, TX. Awal, R., Nakagawa, H., Kawaike, K., and Baba Y. (2009) “Three Dimensional Transient Seepage and Slope Stability Analysis of Landslide Dam,” Annuals of Disaster Prevention Research Institute, Kyoto University, Kyoto, Japan, 52(B), 989696. 9


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Bruce, D. and Dugnani, G. (1996). “Rehabilitation of Beaver Dam: A Major Seepage Cut-off Wall,” Ground Engineering, 29(5), 40-45. Clark, W. J., Boyd, M.D, and Helal, M. (1992). “Ultrafine Cement Tests and Dam Test Grouting,” Proceedings of the 1992 ASCE Specialty Conference on Grouting, Soil Improvement, and Geosynthetics, GSP No. 30, New Orleans, Louisiana, 25-28 February 1992, 626-638 Henn, R.W. and Soule, N.C. (2010). Ultrafine Cement in Pressure Grouting, ASCE Press, Reston, VA. Kaufmann, J. and Winnefeld, F. (2002). “Influence of Addition of Ultrafine Cement

on the Rheological Properties and Strength of Portland Cement Paste,” Proceedings of the International Conference on Innovations and Developments in Concrete Materials and Construction, Dundee, UK, 827-836. Liua, J., Fenga, X., Ding, X., Zhang, J., and Yueb, D. (2003). “Stability Assessment of the Three-Gorges Dam Foundation, China, using Physical and Numerical Modeling—Part I: Physical Model Tests,” International Journal of Rock Mechanics and Mining Sciences, 40(2003), 609–631. Luofenga, X., Xiangbaoa, D.,Yangchao. (2012). “Study on Physical Model Experiment of Dam’s Seepage Stability base on Coastal Sand,” Proceedings of the 2012 International Conference on Modern Hydraulic Engineering, Nanjing, Jiangsu Province, China. UFC (2004). “Grouting Methods and Equipment,” UFC 3-220-06 (TM 5-818-6), Unified Facilities Criteria, Department of Defense, Washington, D. C. Weaver, K. (1991). Dam Foundation Grouting, American Society of Civil Engineers, New York, NY.

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