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Military

Chapter 11

Auxiliary Activities

11-1. Exploratory Drilling.

a. Geological Exploration. Use the same drilling methods for subsurface geological exploration as for standard well drilling. However, you will need special accessory equipment. See Table 11-1 for drilling methods and material samples. Auxiliary equipment for exploration drilling may include solid and hollow-stem augers, sampling tubes, core barrels and bits, crackerjack bits, wash casing, and bailers. All the equipment is effective for sampling but varies for exploratory drilling, depending on conditions. In shallow depths, use augers. However, auger samples are disturbed and are not suitable for strength determinations. The samples can provide data such as soil type, water content, layer thickness, and changes in formation.

Use a rotary or a down-hole drill for depths beyond an auger's range. Use these methods to obtain data for placing bridge abutments and structures and for determining the location, depths, and bearing stratum for other structures such as foundations, piles, dams, and underground facilities. You may have to drill boreholes to reach the materials that have good bearing capacities, to establish soil and bedrock profiles, and to reveal flaws in rock formations. You may perform the same well-drilling procedures for several tasks. When drilling, be aware of water occurrences and where drilling fluid loses circulation. These signs could indicate flaws in the rock. In shallow depths, use the jetting method to obtain the same information.

b. Logging Techniques. In exploratory drilling on military installations or major facilities, a geologist will often be assigned to conduct the logging at the rig. The geologist provides directions on objectives and assistance on methods of sampling. Inexpedient field operations, the driller may have to serve as a technician and make all observations and logs.

The driller should log each borehole regarding the depth reached and material taken. For field logging of unconsolidated materials, use the Unified Soil Classification System in FM 5-410 for consistency and compatibility of terms. The driller may have to rely on initial visual examination of cuttings to describe the formation being penetrated.

c. Drilling Action. The driller should recognize changes in earth formations by the actions and sounds of all well-drilling equipment. For example, a change in pump circulation could indicate an aquifer. The driller should include such information in the log.

11-2. Sampling Soil and Rock. A valuable source of geologic and hydrologic information is cuttings from the penetrated rock. Handle these cuttings carefully. Collect samples from various depths to obtain the complete lithologic character of the formations penetrated. Use the coring method to take samples. With this method, you extract a solid piece of the formation. However, you can achieve the same results by carefully collecting loose cuttings.

a. Rotary Cuttings.

(1) Method. Examine the cuttings deposited in the return ditch or around the collar of the hole to determine the material the bit penetrates. If you are digging a deep well, take a sample every 5 feet. If you are digging a shallow well, take samples at shorter intervals. Hoist the bit about 1 foot off the bottom, and rotate slowly while the mud circulates at full volume. Continue until the hole washes free of drill cuttings. Resume drilling for another 5 feet, raise the drill pipe slightly, and let the mud circulate until it is clean of drill cuttings. If you use this procedure, the mud is screened carefully as it flows in the return ditch. You can obtain samples to identify the different beds.

(2) Contamination. Soft formations have a tendency to cave in and add extra materials to the drilling fluid. When the drilling mud circulates, this material comes to the surface and contaminates the sample. Material from the upper formation can also contaminate the sample when drilling mud is not heavy enough to cushion the impact of the drill pipe against the walls of the well. You can eliminate much of the contamination if you make the mud pits large enough to ensure the settling of all the particles of the cuttings before the mud circulates into the hole. If you want an uncontaminated sample, stop drilling and continue circulation until all cuttings are washed to the surface. Clean the ditch, proceed with drilling for a few inches, and catchall the cuttings that wash out. This sample represents the material drilled, provided the fine sand and silt are not carried into the settling pit in the drilling mud.

b. Depth Determination. Determine the depth of a sample by measuring the lag time, which is the time it takes for material to reach the surface. Lag time depends on--

  • Size of the hole.

  • Condition of the hole.

  • Type of formation penetrated.

  • Type and viscosity of the mud.

  • Actual depth of the hole.

You can measure lag time by placing material such as cut straw into the intake drilling mud pipe and recording the time it takes for the material to math the surface. Or, you can--

  • Stop the drill.

  • Circulate the mud until it is free of cuttings.

  • Drill down a few inches.

  • Stop drilling and measure the time required for the cuttings to reach the surface.

The last procedure has the advantage of furnishing an accurate sample of the formation at the depth of the bit. Use the procedure when you want very accurate formation samples.

c. Undisturbed Soil Samples. To collect these samples, advance the hole using a rotary or auger rig to a position just above the sampling interval. Clean the hole in the dry (when using an auger) or by flushing (when using a rotary rig). Recover the soil sample using either of the following methods:

(1) Push-Tube Method. With this method, you can use thin-wall, fixed-piston samplers in very soft to stiff clays, silts, and sands that do not contain appreciable amounts of gravel. Use 5-inch-diameter sample tubes for all clays and silts that you can remove from the tube and keep in an undisturbed state. Some very soft clays and silts will not support their own weight, so you will not be able to remove them in an undisturbed state. You can take samples of these soil soils with either 3- or 5-inch-diameter sampler tubes. Seal the sample in the tube with expanding packers. Generally, a 3-inch-diameter sample of clean sand is sufficient.

Before sampling, chuck the drill rods in the drill rig with the drive mechanism raised to its maximum height. Doing so prevents the weight of the drill rods and sampler from bearing on and possibly disturbing the sampled material. Rotate the piston-rod extensions to the right until the piston-rod clamp or locking mechanism in the sampler assembly is released. Clamp the piston-rod extensions to the drill-rig mast. Place a mark on the piston-rod extension where it emerges from the drill rod. Marking the rod will help you determine the exact length of drive. The sampler is ready to push. The capability of the drilling rig to perform a continuous, smooth push limits the length of the drive.

In noncohesive soils below the water table, the vacuum caused by the piston can cause piping of the sample if the drive length is excessive. Samplers 24 to 30 inches in length are reasonable. The most satisfactory method of pushing an undisturbed sampler is with the drive mechanism (preferably hydraulic) on the drill rig. The rig must be firmly anchored to prevent the reaction of the drive from raising the rig and the sampler piston. Screw-type earth anchors are sometimes used. You must advance the sampler in one continuous push at a uniform rate. Do not rotate the sampler during the drive. If the drive is interrupted, do not restart it. Adhesion and friction develop between the sample and the tubing wall during an interruption. Restarting the drive will result in increased penetration resistance and disturbance of the sample. Withdrew the sampler, advance and clean out the hole, and make a new sample drive.

(2) Core Method. If the sample material contains gravel or is too hard for a thin-wall sampler, use a double-tube core barrel or a Denison sampler. These barrels are commonly used in obtaining samples of very stiff, brittle, dense, or partially cemented soils and of soft, broken, fissured, or friable rocks. Use the following procedures for this method:

    • Step 1. Lower the core barrel to within a few feet of the bottom of the hole.

    • Step 2. Circulate the drilling fluid to remove any excess cuttings that may have settled to the bottom.

    • Step 3. Lower the core barrel to the bottom. Rotate and force the barrel downward at a uniform rate.

If the soil erodes easily and the inner-barrel shoe will penetrate the soil under the pressure exerted on the core barrel, the inner-band shoe should extend below the cutting teeth of the outer bard. If this does not occur, use a shorter shoe that has a cutting edge even with or slightly above the cutting teeth of the bit. Adjust the rotation speed and the advance rate to ensure that the bit penetrates continuously. If the bit advances too rapidly, it will become plugged and grind away the core. If the bit advances too slowly or intermittently, the core will be exposed to excessive erosion and torsional stresses. Use a rotational speed of 50 to 150 RPM for most soil coring.

You must control the drilling-fluid pump pressure and flow when removing cuttings. Too much drilling-fluid pressure and flow will erode the core. Too little drilling-fluid pressure and flow will allow the cuttings to enter the core barrel, along with the core, and plug the bit. Try to use experienced personnel to regulate the bit pressure, speed of rotation, and drilling-fluid pressure. However, you might have to experiment due to the variety of core barrels and drilling fluids and variations in soils.

d. Rock Core Samples. You may recover undisturbed core samples from hard rock formations with the rotary-drilling rig. As with other sampling methods, core drilling requires special auxiliary equipment and procedures. The diamond core bit and double-tube core bard are basic equipment (Figure 11-1). The assembly consists of an outer rotating tube or barrel attached to a core barrel head at the top end and the diamond drilling bit at the bottom. An inner tube and core lifter receive and hold the sample stationary within the rotating assembly during drilling. Drilling fluid circulated downward between the inner and outer tubes and the sample remain protected. To take samples:

  • Lower the core barrel and bit into the borehole through the hollow drill rod.

  • Start circulating the wash water before the core barrel reaches the bottom of the hole to prevent cuttings or sludge from entering the core barrel at the start of coring.

The optimum rotation speed of drilling varies with the type of bit used, the diameter of the core barrel, and the kind of material to be cored. Excessive rotation speed will result in chattering and rapid wear of the bit and the core will break. Low speed results in less wear and tear on the bit and better cores but lower rates of progress. In medium to hard rock, use 300 to 1,500 RPM for diamond bits and 100 to 500 RPM for metal carbide bits.

The advance rate of the coring bit depends on the material's firmness, the downward pressure applied on the bit, and the rotation speed. The driller must carefully adjust the pressure. Too much pressure causes the bit to plug and to shear the core from its base. Bit pressure is controlled by a hydraulic or screw feed on the drilling machine. The weight of the drill-rod column seldom exceeds that of the optimum bit pressure for coring medium and hard rock. You may have to apply additional downward pressure.

11-3. Installing Monitoring Wells. These are small water wells used for measuring water level, estimating well yield, and taking samples for quality analysis. These wells are drilled next to permanent wells at specified intervals.

a. Safety. Before installing a well, the safety of the drill crew and impact of drilling on the environment must be considered, especially in areas containing hazardous wastes. Potentially hazardous categories are chemical wastes and residues, flammable wastes, explosives and explosives wastes, biological materials, toxins, radioactive materials, and harmful secondary vapors. Drilling crews may need protective clothing and respirators while working in such areas.

b. Requirements. Well-drilling equipment is suited for installing monitoring wells. Use the hollow-stem auger for boring clean holes while simultaneously supporting the borehole wall during installation. Some piezometers use stock-porous well points. However, the installation methods are quite similar. Be careful when taking samples for water pollution testing so that contamination is not caused by the installation process.

c. Installation.

(1) Materials and Construction. Casings and screens can be mild steel, stainless steel, PVC, or chemical-resistant plastics such as teflon. Casing ends are threaded for tight couplings. Monitoring wells have a casing, screen, filter pack, and grouted intervals similar to water wells except smaller. A 2-inch casing is the most common. Filter-pack sand should be just coarse enough to stay outside the slotted semen. You can use bentonite pellets above the filter-pack and screened interval to seal the annulus and prevent water from leaking downward into the sampled stratum and contaminating well intake. Make sure that all materials used in the well are sterilized, including the drill rig.

(2) Procedure. To install a monitoring well--

  • Drill the hole about 1 foot below the position of the screen bottom.

  • Place 1 foot of filter-pack sand in the bottom. Insert casing and semen (single-string) and center in the hole.

  • Slowly add filter-pack sand while tapping and twisting the string to settle sand firmly. Continue filter-pack addition upward to at least 1 foot above the screen.

  • Add bentonite pellets to the annulus above the filter pack for at least 2 feet. Seal the remainder of the annulus to the ground surface and grout in a protective collar.

11-4. Supporting Construction and Demolition. Support for drilling operations for a construction or demolition unit is an expedient mission. The supported unit determines drilling requirements. A support mission is used as an alternative to mobilizing and fielding a separate drilling operation.

For construction support, you can use a water-well rig with supplemental equipment for rock excavation. You will need bits designed specifically for drilling rock rapidly without sampling. Rock bits include button bits used in down-hole drilling and tricone and other hard roller bits for rotary rigs. Rock-drilling applications might include foundation preparation, road cuts, and weapons pits.

For demolition support, a demolition specialist will provide specifications and guidance for preparing area for emplacing demolitions or munitions. For information regarding safety when using explosives and demolitions, see FM 5-250. You should use the rotary rig and special bits for expedient drilling and for placing demolitions. In soil, auger bits are ideal for shallow depths. The auger has large-diameter capabilities that may be necessary for placing large charges. The demolition specialist will describe techniques for handling equipment and accessories during emplacement. You can provide information about cavities and joints in formations that could adversely affect demolition and about the position of the water table, which could be important in dry versus wet conditions.

WARNING

Handle and use explosives
and demolitions properly.

 



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