Cement and chemical grout curtains
(1) Cement or chemical grouts are injected through pipes installed in the soil or rock. Generally, pervious soil or rock formations are grouted from the top of the formation downward. When this procedure is followed, the hole for the grout pipe is first cored or drilled down to the first depth to be grouted, the grout pipe and packer set, and the first zone grouted. After the grout is allowed to set, the hole is redrilled and advanced for the second stage of grouting, and the above procedure repeated. This process is repeated until the entire depth of the formation has been grouted. No drilling mud should be used in drilling holes for grout pipes because the sides of the hole will be plastered with the mud and little, if any, penetration of grout will be achieved.
(2) Mixing tanks and pump equipment for pressure injection of cement or chemical grouts vary depending upon the materials being handled. Ingredients for a grout mix are loaded into a mixing tank equipped with an agitator and, from there, are pumped to a storage tank also equipped with an agitator. Pumps for grouting with cement are generally duplex, positive displacement, reciprocating pumps similar to slush pumps used in oil fields. Cement grouts are highly abrasive, so the cylinder liners and valves should be of case-hardened steel Chemical grouts, because of their low viscosity and nonabrasive nature, can be pumped with any type of pump that produces a satisfactory pressure. Grout pump capacities commonly range from 20 to 100 gallons per minute at pressures ranging from 0 to 500 pounds per square inch. The maximum grout pressure used should not exceed about 1 pound per square inch times the depth at which the grout is being injected.
(3) The distribution system for grouting may be either of two types: a single-line system or a recirculating system. Because of segregation that may de-
velop in the pressure supply line from the pump to the grout injection pipe, the line must occasionally be flushed to ensure that the grout being pumped into the formation is homogeneous and has the correct viscosity. The grout in a single-line system is flushed through a blowoff valve onto the ground surface and wasted. A recirculating system has a return line to the grout storage tank so that the grout is constantly being circulated through the supply line, with a tap off to the injection pipe where desired.
(4) Additional information on grouting is contained in TM 5-818-6.
a. Slurry walls.
(1) Slurry cutoff trenches can be dug with a trenching machine, backhoe, dragline, or a clam bucket, typically 2 to 5 feet wide. The walls of the trench are stabilized with a thick bentonitic slurry until the trench can be backfilled. The bentonitic slurry is best mixed at a central plant and delivered to the trench in trucks or pumped from slurry ponds. The trench’is carried to full depth by excavation through the slurry, with the trench being maintained full of slurry by the addition of slurry as the trench is deepened and extended.
(2) With the trench open over a limited length and to full depth, cleaning of the slurry is commenced in order to remove gravelly or sandy soil particles that have collected in the slurry, especially near the bottom of the trench. Fair cleanup can be obtained using a clamshell bucket; more thorough cleaning can be obtained by airlifting the slurry to the surface for circulation through desanding units. Cleaning of the slurry makes it less viscous and ensures that the slurry will be displaced by the soilbentonite backfill. After cleaning the “in-trench” slurry, the trench is generally backfilled with a well-graded mix of sand-clay-gravel and bentonite slurry with a slump of about 4 to 6 inches. The backfill material and slurry may be mixed either along and adjacent to the trench or in a central mixing plant and delivered to the trench in trucks.
(3) The backfill is introduced at the beginning of the trench so as to displace the slurry toward the advancing end of the trench. In the initial stages of backfill, special precautions should be taken to ensure that the backfill reaches the bottom of the trench and that it assumes a proper slope (generally IV on 5H to IV on 10H). In order to achieve this slope, the first backfill should be placed by clamshell or allowed to flow down an inclined ramp, dug at the beginning end of the trench. As the surface of the backfill is built up to the top of the trench, digging the trench resumes as shown in figure 5-4. As the backfill is bulldozed into the back of the trench, it flows down the sloped face of the already placed backfill, displacing slurry as it advances. Proper control of the properties of the slurry and back-
fill is required to ensure that the slurry is not trapped within the backfill.
(4) The backfill should be placed continuously as the trench is advanced. By so doing, sloughing of the trench walls will be minimized, and the amount ofben – tonitic slurry required kept to a minimum. The level of the slurry in the trench should be maintained at least 5 feet above the groundwater table. Care should be taken to control the density and viscosity of the bentonitic slurry as required by the design. To minimize wastage of bentonitic slurry, it may be necessary to screen out sand and gravel in order to reuse the slurry. (Construction techniques are still being developed,)
(5) The toe of the backfill slope should be kept within 50 to 150 feet of the leading edge of the trench to minimize the open length of the slurry-supported trench. During placement operations, excavation and cleaning operations proceed simultaneously ahead of the advancing backfill. (It should be noted that because of the geometric constraints set by the backfill slope, the amount of open trench length supported by slurry is a function of the depth of the trench. For example, if the trench is 100 feet deep and the backfill slope is IV on 8H, the open length will be about 900 to 950 feet-800 feet along the slope of the backfill face plus 100 to 150 feet from the backfill toe to the leading edge of the trench.)
(6) When the trench is complete and the backfill occupies the entire trench, a compacted clay cap is normally placed over the trench. Key steps in this construction sequence involve the mixing of the bentonite-water slurry, excavation and stabilization of the trench, cleaning of the slurry, mixing of the soil-bentonite backfill, displacement of the slurry by the backfill, and treatment of the top of the trench. Each of these items must be covered in the specifications.
b. Steel sheet piling. Steel sheet pile cutoffs are constructed employing the same general techniques as those used for driving steel sheet piles. However, precautions should be taken in handling and driving sheet piling to ensure that the interlocks are tight for the full depth of the piling and that all of the sheets are driven into the underlying impermeable stratum at all locations along the sheet pile cutoff. Methods and techniques for driving steel sheet piling are described in numerous referencestm this subject.
c. Freezing. Freezing the soil around a shaft or tunnel requires the installation of pipes into the soil and circulating chilled brine through them. These pipes generally consist of a 2-inch inflow pipe placed in a
б-inch closed-end “freezing” pipe installed in the ground by any convenient drilling means. Two headers are requried for a freezing installation: one to carry chilled brine from the refrigeration plant and the other to carry the return flow of refrigerant. The refrigeration plant should be of adequate capacity and should include standby or auxiliary equipment to maintain a continuous operation.