A typical compaction grout mix consists of sand, cement, flyash (if available) and water. Silty sand near the finer limits produce the best mixes, as suggested by Brown & Warner (1974)3. The gradation of the sand (figure 8) is extremely important in compaction grouting. A uniform or gap graded sand typically lacks the fines necessary to produce a dense homogenous mix. Additionally, natural rounded edge particles produce the best grout mixes.
Figure 8 Desirable Sand Gradation Curve
Best and Lane 4 have previously undertaken a study to investigate the nature of flow rate versus pumping pressure on stiff displacement type grouts. Sand gradation in grout combined with angularity and surface texture influence the pumpability of grout. Coarse, uniform sands in a grout mix tend to plug or "sand block" the injection line due to the water segregating from the mix. A well graded grout mix design containing a sufficient percentage of fine particles (-200 sieve), minimizes the separation by lowering the porosity of the mix. Another advantage to a well graded mix is that low slumps (1 to 3 inches (2.5-7.5cm)) can be achieved with low water:cement ratios while maintaining pumpability.
Flyash is a common additive used to supplement many design mixes. Flyash is produced by burning coals which have been crushed and ground to a fineness of 70 to 80 percent passing the No. 200 sieve. Specifications for flyash are given in ASTM C 618, which defines two types of flyash - Class "C" and Class "F". Class "C" flyash usually has cemeticous properties in addition to pozzolanic properties, while Class F flyash is rarely cementitious when mixed with water alone5. The small particle size of flyash is helpful in void filling and increasing the density of the mix, while the generally spherical shape of the particle significantly reduces the frictional line losses. Lane and Best's data indicated that the fineness of the flyash has a significant influence on performance in concrete. Concrete strength and abrasion resistance are functions of the proportions of the flyash finer than the No. 325 sieve.
In areas where sufficient amounts of fines are not available, bentonite has been found to be a useful additive6. This is primarily due to the water retaining ability of the montmorillonite clay particle. Testing by Jeffries7 has shown that hydration of the bentonite prior to the addition of cement is necessary to minimize the bleeding and segregation effect. The use of bentonite in compaction grouting may cause excessive mobility and a loss of compressive strength of the grout material.
To verify this theory, Borden & Groome performed a field test using a positive displacement pump to produce a constant flow rate and varying the bentonite content in a sand, flyash, cement grout mix. The bentonite addition to the mixture was varied as a percentage of the flyash content and ranged from 2.5% to 15%. Attempts were made to hold the slump constant at approximately two inches (5cm). It was found that the optimum bentonite percentage was 5% for flow rates of one cu. Ft./min(28.3 l/min), and approximately 10% for flowrates of four cu. Ft./min (113.2 l/min).
