For many years now, state and federal government agencies have identified bentonite grouting materials and placement methods and strongly have recommended that positive placement of grout by tremie under pumping pressure be used for all types of wells where bentonite grout is to be placed. It is essential that the bentonite grout always be introduced from the bottom of the space being grouted and pumped upward to avoid separation or bridging of the grouting materials and to remove undesirable cuttings from the bore hole.
The bentonite grout is pumped into the annular space through a tremie pipe that has been inserted to the bottom of the borehole along with the casing. The annular space between the borehole and casing is usually 2 inches or larger. This can vary depending on state regulations, depths and drilling conditions. Grouting is complete when the grout slurry leaving the borehole at the surface is of the same weight and consistency as the grout slurry being introduced into the bottom of the borehole. This method assures that the entire length of the annulus is grouted in a consistent manner and is the recommended grouting technique for most well installations.
Pumping and Placement Considerations to Note
Selection and specification of a bentonite grout that will be placed using conventional pumping and tremie pipe methods must consider the mixing and pumping properties of the grout being used and the pumping equipment when in a slurry form. In some cases, boreholes are relatively deep (300 feet plus). Selection of a grout that sets quickly with a relatively high viscosity will result in very high pumping pressures that will make field applications very difficult if not impossible. Sometimes this problem can be overcome by increasing the tremie pipe size to reduce pressure and improve flowability. Proper mixing of the grout prior to pumping also is an important factor in successful placement of the grout. The quality of the mixing water is very important, and potable water is mandatory for proper hydration of the bentonite grouting materials. High levels of chlorine will act to retard the hydration process. Surface waters or water containing a high dissolved mineral content should not be used since they might adversely affect the performance of the grouting material. Temperature of the mix water also is very important and should be kept as low as possible to prevent rapid hydration of the bentonite grout. The warmer the mix water, the more rapid the rate of hydration and the higher the pumping pressure will be at a given pumping rate.Pumping the Grouting Material
Selection of the appropriate grout pump is the most important factor in developing an efficient and effective grouting procedure. The grout pump should be selected only after fully considering the grouting material that will be most often used and the depths the grout will be placed. Grout pump suction and discharge hoses should be adequately sized to overcome friction losses and decrease chances of plugging. Suction and discharge hose connections to the pump should be made using quick-connect style couplings. This will save time when attempting to locate a blockage in the hose and allow faster clean up. When the grout pump also is used for mixing, the discharge hose should be plumbed in such a manner to allow changeover from mixing to pumping grout without shutting down the pump.
A pressure gauge should be installed on the pump discharge and monitored to assure that the working pressure does not exceed the hose and pipe maximum pressure rating. A sudden increase in pressure may indicate that the tremie pipe has become plugged.
Grout Mixing Equipment
The mixing of high-solids bentonite grout should be done with a mixing procedure that minimizes shearing action of the slurry. Therefore, jet mixing and recirculation mixing are not recommended. A paddle mixer should be used. Paddle mixing consists of a mechanical agitation of the grout slurry by paddles or blades rotating in a barrel-like container in small batches to the manufacturer's specifications. Smaller batches can then be pumped directly down hole or into a holding tank and pumped from there as the next batch of grout is being prepared. Complete portable grouting units that include a paddle mixing tank, a grout pump and a portable power unit are available from many sources.Grout Pump Selection
This is the most important factor when using the tremie pipe bentonite grout placement method. Grouting material is pumped down the tremie pipe and up the annulus to the surface. In doing so the drilling mud or water in the borehole, which is of lower specific weight is displaced from the annulus. Elevation head is not a major consideration since the grout returning to the surface is at or below the grout pump level. The pump must be capable of developing enough pressure to overcome the pressure drop through the tremie pipe. After the first batch of grout is pumped into the borehole, the tremie pipe should be pulled at about the same rate that grout is filling the borehole. This will minimize hydrostatic head on the tremie pipe outlet and reduce any chances of the tremie pipe being stuck in the borehole due to hydration of the grout. NOTE: The outlet end of the tremie pipe should always be kept below the grout level at all times.There are two major classifications of pumps - positive displacement and non-positive displacement. Non-positive displacement pumps, such as the centrifugal pump, utilize an impeller to develop pressure in the fluid stream. A very high rate of shear is applied to the fluid. This is not desirable when pumping any high-solids bentonite grout because hydration of the bentonite will be accelerated and increased pumping pressures cannot be overcome. Therefore, centrifugal pumps are not recommended for pumping high-solids bentonite grouts.
When the grout slurry is moved as a confined volume of fluid through the pump by some combination of moving parts, the pump is classified as a positive displacement pump. Positive displacement pumps have very closely machined tolerances between the moving parts, which are needed to prevent the fluid from leaking out of the moving confined volumes that are established in the pump housing.
Positive displacement pumps move a specific volume of fluid through the pump for each revolution of the driving shaft, making displacement of the pump a measure of its size. Positive displacement pumps also provide a constant flow rate at a constant speed, regardless of the discharge pressure that is present at the outlet. Since positive displacement pumps can provide uniform flow rates under low or high head conditions, they are well suited for pumping viscous fluids and are considered good selections for grout pumps. Shearing action is minimized, which keeps rate of hydration low and allows longer working time before the grout slurry sets up.
At least five basic types of positive displacement pumps exist that may have applications when grouting boreholes, which include:
- Gear pumps
- Piston pumps
- Progressive cavity pumps
- Air-diaphragm pumps
- Peristaltic pumps
Gear pumps are rotary pumps in which two or more gears mesh to provide the pumping action. The mechanical contacts between the gears form a part of the moving fluid seal between the inlet and outlet ports and the outer radial tips of the gears and the gear body form the remainder of the moving fluid seal between the inlet and outlet ports. The gear pump draws the fluid in and moves it between the gear teeth and the pump body wall.
Piston pumps are constant-speed, constant-torque and constant-capacity reciprocation pumps whose pistons are driven through a crankshaft from an external power source. Double-acting piston pumps will provide flow regardless of the direction the pistons travel. Flow will have a pulsating effect every time the piston reverses direction. Piston pumps are quite flexible in the types of fluids that can be pumped, and fluids with high concentrations of suspended solids can be pumped with minimal wear. Piston pumps can generally produce very high pressures (over 500 psi) which may be required when pumping high-solids bentonite grouts.
Progressive cavity pumps are a special type of rotary positive displacement pump in which the flow through the pumping elements is truly axial. The rotor is eccentric to the axis of rotation and meshes with internal threads on the stator (housing body). The rotating rotor forces the liquid flow through the stator. Progressive cavity pumps can handle liquids with a wide range of viscosity, although performance is sensitive to the viscosity. Also, they are self-priming and have a delivery flow characteristic that is essentially independent of pressure.
Diaphragm pumps utilize mechanical or hydraulic power to impart a reciprocation action on a diaphragm. Mechanically actuated pumps commonly are used for low-pressure service where leakage cannot be tolerated. These pumps are relatively low cost, require minimum maintenance and are capable of pumping slurries and corrosive chemicals. Their disadvantages include low-pressure capabilities and limited pumping capacities.
Peristaltic pumps recently have been used by the mining industry to transfer sludge and slurries. The basic peristaltic principle consists of a smooth-wall flexible tube or hose element to which a rolling or squeezing motion is imparted along a predetermined length. The liquid or powder contained in the tube is displaced positively displaced by each rolling cycle. The pumped material passes straight through the tube from suction to discharge without valves, vanes, gears or other obstructions. This eliminates blockage within the pump and enables viscous, high-solids slurries to be transferred with minimal moisture. Pumps are capable of pumping more than 20 gpm at 200 psi.
Proper grouting and sealing of wells or boreholes is becoming a routine part of our drilling responsibilities and needs to be done with the utmost professionalism to assure the safety of our water resources. I hope this information helps you in selecting the proper equipment for your grouting applications.
