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Drilling Methods & Equipment
Several different methods exist for advancing a steel casing or grout injection pipe to a desired design depth for compaction grouting. The differing techniques are typically dictated by soil conditions, project size, injection depth and access limitations. Caution must be exercised to specify a drilling technique that does not aggravate a problem soil condition by subjecting the subsurface strata to excess vibration, water or uncontrolled air pressure. These can actually induce settlement and can be risky when drilling near existing structures. Some of the more popular methods used to advance injection pipes include:
- Auger Drilling Techniques
- Percussion Drilling Techniques
- OdexTM or TUBEX
- Vibratory Drilling Techniques (Crane & Mandrel)
- Air Rotary or Rotary Wash
- Grout Pumps
- Grout Sources
- Grout Mixes
- Slump Test
Auger Drilling Techniques- This method utilizes continuous flight augers to remove the soil from the borehole as the drill bit advances. Once the augers have reached the desired depth, the drill tools are removed from the hole and a steel injection casing is inserted to the full depth. The drill hole is typically sized slightly larger than the outside diameter of the steel casing. The annular space between the drill hole and the casing can be filled with a fine sand to establish intimate contact if necessary. Additionally, the compressive forces produced during the injection of the grout tend to compress the soils around the grout injection pipe. This technique is the least disruptive to the insitu soils. Auger drilling can typically be used in any soil condition that will maintain an open hole long enough to install the steel casing, however, in soils where boulders exist or where rock must be drilled, other methods should be utilized.
Percussion Drilling Techniques- This technique typically has two variations. The first involves simply driving a steel pipe or casing into the soil, normally with a disposable or lost point. The pipe is driven with either hydraulic or pneumatic percussion equipment (figure 4) to a design depth by displacing the soil directly in front of the pipe point. The soil remains tight around the pipe and provides a good seal. This method is highly utilized for its low cost, effectiveness and the ability to use small equipment in limited access areas. Pipe driving depth is typically limited by the size of the percussion equipment and the ability of the equipment to overcome the skin friction between the steel pipe and the surrounding soils. Pipe driving also subjects the surface and surrounding soils to some vibration but is very effective in advancing injection holes in collapsible soil conditions. This method also does not allow for any visual observation of drill cuttings.
Figure 4. Percussion Drill
Figure 5. ODEX or TUBEX- The second percussion method uses an inner drill string that is simultaneously advanced with an outer casing. The drill bit is set slightly ahead of the end of the drill casing. One system of this method used for difficult drilling conditions is the OdexTM system. The OdexTM system utilizes an eccentric under reamer to cut a drilled hole ahead of the bottom of the casing. The casing is then simultaneously advanced by a percussive force applied to either the top or bottom of the casing. As the hole is advanced, the drill cuttings are normally removed by air and travel up the inside of the casing in the annular space between the casing ID and the drill rod OD. The OdexTM method is highly effective in overburden soils where boulders or gravel layers are present, or when layers of rock must be penetrated. This method also subjects surficial and localized soils to some vibration.
Figure 5. Crane & Mandrel Crane Drill
Vibratory Drilling Techniques (Crane & mandrel)- This technique is typically used on large scale, high production compaction grouting projects. Vibratory drilling (figure 5) utilizes a hydraulic vibratory hammer mounted on pile driving leads or a fixed beam and suspended by crane or other support such as an excavator. A continuous grout injection pipe or mandrel is vibrated into the soil with the vibratory hammer. Disposable points or caps are typically utilized to prevent soil from plugging the injection pipe as it is advanced. As in the other percussion drilling techniques, this method provides intimate soil contact with the pipe since the surrounding soils are displaced by the advancing mandrel. Unlike other methods, the injection pipe is normally extracted using the same equipment used for advancing. When used in easily compressible or loose soils this is an extremely effective method for high production compaction grouting. Due to the large equipment required, sites must normally have unrestricted surface access.
Air Rotary or Rotary Wash- This method uses typical drilling techniques and tooling to advance the hole by removing the cuttings with air, water or bentonite slurry. Drill rods or casing may be washed into place and used to inject the compaction grout. This method is effective where the limestone lies deep beneath the surface or when soil conditions cause the drill hole to collapse or cave prior to installing the steel casing.
Figure 6. Compaction Grout Pump & truck mixer delivery
Currently, twin cylinder positive displacement piston pumps are the first choice for compaction grouting. This pump ensures a constant delivery volume under varying pressure conditions. The pump should be specifically designed to provide high pressure (1,000 psi, typical (6.9 MPa)) with a constant flow rate which may be varied throughout the construction operation. The grout pump design is significantly different from the typical concrete delivery pump. A concrete pump swing tube or discharge valving typically permits minor leakage of water through the valves. However, where sustained slow delivery volumes and high pressures are required, the water will tend to squeeze out and cause a "sand block" within the pump mechanism. Therefore it is necessary to utilize a specifically designed compaction grout pump which utilizes a "leak free" technology. A compaction grout pump typically has a small cylinder bore (2 to 4 inches (5-10cm)) and a long stroke (40 to 50 inches (100-125 cm)). Most pumps are trailer or skid mounted and powered by a diesel engine. The engine operates a hydraulic motor which powers the grout cylinders. This type of pump is most popular because of the wide range of pumping pressures and placement volumes that can be achieved with a single pump unit.
Figure 6. Compaction Grout Pump & truck mixer delivery
Another key element in developing and implementing a successful grouting program is the supply source for the grout. For many urban compaction grouting programs, truck mixers from a local concrete suppliers may provide a suitable grout supply source. There are several distinct advantages to using a truck mixer supply source. These include delivery of discrete batches, timing of deliveries of grout materials, containment of grout and transportability.
For large scale compaction grouting projects, an on-site mixing plant (figure 7) is often used. The on-site plant is capable of producing compaction grout materials on-demand. One distinct advantage of using on-site grout mixing is the increased control of the grout mix with respect to mix proportions and slump. The gates of the plant are initially calibrated to the design mix, and can be altered slightly throughout the project, should conditions warrant. Typically, two or three aggregate storage bins supply volumetrically metered amounts of sand, flyash or other materials to the mixing auger. Cement is supplied to the mixing auger concurrently from a separate bin. The dry materials are then blended together and volumetrically combined with water and homogeneously blended in the mixing auger. The blended grout is then conveyed to the compaction grout pump.
Figure 7. Mobile Grout Plant & mixing auger to pump, bulk cement tanker in background
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 flowrate 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).
Figure 9 Slump Test
Extensive tests have been performed by Warner to evaluate the effect of bentonite and slump on mobility. The tests concluded that the composition of the grout, the shape and gradation of the sand material, the amount and nature of the fines content and the inclusion of a lubricating agent such as bentonite or other clay materials tend to greatly increase grout mobility and improve the pumpability. It was also found that the grout slump was not directly related to mobility of the grout. Additionally, Warner indicates that the selection of an appropriate grout pump is important for constructing a successful project.
Figure 9 Slump Test
It has been found by these authors, that most compaction grout mixes generally are a combination of a well graded sand, type I Portland cement, class F flyash or other soil fines and water. Occasionally, bentonite has been added for pumpability or by design modification of the project. Other additives are occasionally used such as Calcium Chloride or type III Portland cement to produce an early set. Shrinkage control additives are also available but seldom used. The quantities of the constituents of a typical compaction grout mix per cubic yard are as follows:
|Sand||1,800 - 2,200 lb.||ASTM C-33||Well graded, rounded edge, min. 15% passing #200 Sieve|
|Cement||250 - 500 lb.||ASTM C-150||Control strength of mix, increase density of mix|
|Flyash||400 - 750 lb.||Increase pumpability and provide a well graded grout mix.|
(1) Depending on the fines available from the sand.
Although documented testing is not available at this time, another successful additive to the grout mixture is small diameter aggregate, or pea gravel. This material is typically minus 3/8 inch in diameter and has normally been added at rates between 100 - 200 pounds per cubic yard. The addition of course aggregate tends to knit the grout mix together and decrease the mobility of the grout by increasing the internal friction developed at the soil interface. Even though this has been successfully used in many applications, analytical testing should be performed and evaluated to better define optimum quantities and effects.