Cement Bentonite Grouts Compatible with Compliant TDR Cables

by David M. Will
Evanston, Illinois
October 1996

Summary

The primary goal of this laboratory investigation was to determine the lowest strength cement-bentonite grout that can shear the most compliant commercially available coaxial cable. A range of cement-bentonite grout strengths exists that can transmit a soil shear band to any cable, with varying degrees of TDR response. As cable stiffness decreases, the grout strength required for cable deformation decreases. Grouts with shear strengths up to 103-psi (unconfined compressive strengths of 206-psi) were sheared with embedded cables with an average peak shear stiffness of 850 lbs./in. to measure TDR response. A minimum shear strength of 48-psi was required to achieve any TDR response with the most compliant cable, which generally increased as strength increased.

Literature provides mix designs to achieve cement-bentonite grout compressive strengths up to about 25-psi at 28 days. TDR application requires higher strengths, therefore new mix designs had to be developed for grouts with compressive strengths between 50 and 200-psi. To accelerate testing and analysis, grouts were designed to achieve these strengths in 3 days. Unconfined compression tests were performed on samples prepared with a range of w/c ratios to determine how grouts with compressive strengths between 50 and 200-psi could be obtained.

The secondary goal was to provide a method to design grouts to reach any strength necessary for compatibility with a given cable. To achieve compatibility, the grout must meet certain material requirements, which can be assessed by measuring strength, modulus of elasticity, bleed, and viscosity. Fluid state properties must meet certain criteria regardless of the cable selected for installation. Hardened state properties, on the other hand, are a function of the soil and cable properties.

The fluid state properties critical to cement-bentonite slurry design are Marsh viscosity and bleeding. A Marsh viscosity less than 50 seconds, which was achieved for most mixes, is in the acceptable range to ensure pumpability with a drill rig water pump. Excessive bleeding, over 3% by volume, typically was not exhibited.

Compression tests show that the unconfined compressive strength is directly related to the cement content and w/c ratio when the bentonite content remains relatively fixed (between 1.8 and 6.9%). A best-fit relationship for 28-day strength as a function of w/c ratio for the combined data of this and Aymard (1996)'s study follows a power law, which is consistent with findings in the literature. To achieve 3-day compressive strengths in the range of 50 to 200-psi with bentonite content between 1.8 and 6.9%, the w/c ratio should lie between 1 and 2. Strength gains from 3 to 28 days show an average strength increase factor of 3.2, ranging from 2.1 to 4.6. To reach strengths of 50 to 200-psi at 28 days, instead of 3 days, the w/c ratio would have to be decreased by a factor of 1 to 2.

Values for Young's tangent modulus ranged from 0.2 to 69-ksi. All values fell into the range expected for clays, which shows that addition of bentonite to cement does result in a clay-like substance. This soil-like consistency allows cement-bentonite grouts to be directly compared to other earthen materials. The average E/Su ratio, calculated with tangent modulus, was 785 and ranged from 67 to 826. The expected E/Su ratio falls between l00 and 500 for clays, and 200 and 500 for sedimentary rock.

Field mixing was simulated to determine the effects of bentonite prehydration and slurry mixing time on grout properties. The closest match to the 3-day strengths obtained by lab mixing occurred when the bentonite was fully prehydrated and the slurry was mixed for 30 minutes. With this procedure, unconfined compressive strengths reached 56-psi compared to an average compressive strength of 53-psi determined in the lab.

Composites of cable and grout were sheared to measure cable-grout interactive response, specifically, cable deformation and grout indentation. Cement-bentonite grouts with compressive strengths of 48-psi or greater exhibited detectable TDR response, while those of 30-psi or less yielded no TDR response. The grout producing the highest TDR signal response had a 4-day shear strength of 103-psi. The highest amplitude TDR responses correlated with the least amount of cable indentation.

Conclusions

Based in the findings of this study, the following conclusions can be made:

1. Cement-bentonite grouts with unconfined compressive strengths between 50 and 200-psi follow the same trends in strength development for grouts of lower strength. For relatively constant bentonite content, the w/c ratio controls strength, even when extending the w/c ratio below 3, which is less than previously observed in practice for low-strength cement-bentonite grouts. Best-fit relations, defined by power equations that relate compressive strength to w/c ratios below 3, are similar to those found for w/c ratios greater than 3.
2. The w/b ratio controls bleeding. It is suggested to use a w/b ratio of 30 or less to avoid excessive bleeding. With such low levels of bentonite (around 2%) it is critical and essential that the bentonite be fully prehydrated and the slurry mixed until homogeneity is obtained in the grout.
3. When designing cement-bentonite grouts, strength gain factors may be implemented as a quality control measure to ensure that a desired strength is obtainable. Early strengths, such as 3 and 7-day, may be used to predict final strengths. These laboratory results show that 28-day strength is approximately 3 times larger than the 3-day strength.
4. Grout for TDR installation may be prepared manually in the field if done properly. In the simulation of field mixing performed in this study, fully prehydrated bentonite and a slurry mix time of 30 minutes were required to achieve a homogeneous slurry with fluid and hardened properties consistent with properties of laboratory-mixed grouts.
5. A minimum shear strength of 48-psi was needed to produce TDR response of the weakest cable during localized shearing of a grout-cable composite. Currently, cement paste, with a shear strength of around 400-psi, is used for grouting TDR cables in rock. Thus, the grout's shear strength can be reduced approximately 10 times. This strength reduction suggests that a cement-bentonite grout may be aufficient for TDR monitoring in soils.

5.3 Recommendations

The following recommendations can be made by virtue of this investigation:

1. Cement-bentonite slurries have traditionally been implemented for cutoff wall applications, where high-strength is not required. Further studies should be performed to gain more information cn high-strength cement-bentonite slurries to confirm the recommended design procedures.
2. A cement-bentonite slurry of aufficient strength to shear the most compliant commercially available coaxial cable has been determined in this investigation. The strength required, however, is relatively higher than that of the soil in which TDR monitoring is expected and a more compliant coaxial cable would allow weaker grouts to perform more adequately. Therefore, a coaxial cable with a lower stiffness than that analyzed in this study should be developed.