# Chapter 24: Borehole Dewatering Systems

Dewatering systems are used to remove water from boreholes. After water is removed, some water may remain in the borehole. A small amount may be left as the device is removed, and the borehole walls may remain damp. If water remains in or accumulates, bottom primers may be affected. Plastic borehole liners are used if low water resistant explosives are to be used. Liners prevent water from returning after the borehole is pumped dry.

Inserting and retracting these systems can also damage fragile areas of the borehole such as broken material around the collar or loose rocks as borehole walls. Care must be taken to minimize any damage and to prevent the dewatering device from being caught in the borehole.

Dewatering lowers explosive cost by allowing use of less expensive explosives like ANFO and heavy ANFO products. There are additional advantages from dewatering beyond reducing direct explosives costs. The benefits of dewatering are summarized in table 24.1 and are considered in the blaster-in-charge's decisions.

## Benefits Of Dewatering Boreholes

| Benefit |
|---------|
| Lowers direct explosives cost. |
| Lower indirect explosives cost. |
| Makes borehole loading faster. |
| Minimizes the chance of column separation or initiation system damage. This is true when cartridge or packaged products are used, where each unit needs to be pushed or pulled into and through the water. |
| Improves blasting effectiveness. Stemming that is dry or damp confines better than saturated stemming material. This leads to better use of the energy and results in improved fragmentation, more movement and less back break. |
| Removes high density water containing mud, soil and rock particles. This maintains borehole priming, packaged explosive location and bulk explosive pumpability concerns. |
| Permits use of similar explosive products across the entire blast pattern. This results in a uniform energy package for the blast. |

*Table 24.1 – Benefits of dewatering boreholes.*

Therefore, when wet boreholes are encountered, a well-designed dewatering program can result in lower costs and more efficient blasting.

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## DEWATERING EQUIPMENT

Four types of dewatering equipment are described in this section. Borehole liners (sleeves) are also discussed as a means to keep the borehole dry and prevent degradation of products that would be exposed to the moisture remaining on borehole walls. When working on a bench, it is advisable to discharge water over the face when possible to prevent pumped water from re-entering the boreholes.

### Blowpipes

The blowpipe is one of the earliest borehole dewatering devices. Originally it simply consisted of a rigid metal pipe with an end cut on a bevel and was usually fitted with a quick, quarter turn valve to compress air at one end. This simple setup is still common in some locations and is commonly used underground where compressed air and rigid drill steels are part of the normal environment.

Today the metal pipe is commonly replaced with a semi-rigid black plastic hose. An additional improvement to the blowpipe is a nozzle attached to the end of the hose. It has a ring of holes facing up that directs a swirl of air at the surface of the water in the borehole. Its weight pulls the hose into the borehole.

The main advantage of a blowpipe is that it is almost universally available at any site where there is a compressor and a length of hose. The blowpipe's effectiveness is limited in pressures using multi-diameter boreholes of limited depth. These would generally range from 50 millimeters to 100 millimeters, 2 inches to 4 inches in diameter, and less than 15 meters (49 feet) in depth. Blowpipes are still an accepted means of clearing small diameter dewatered blasting boreholes in underground operations.

Although blowpipes are easy to use they pose several concerns. They tend to accelerate the deterioration of crumbly hole collars. The operator and anyone else in the immediate area can get wet and hit by water and the mud or rock particles ejected with the water. The velocity of flying material can be high so prudent precautions and protection should be taken. Blowpipes are also generally inefficient when there are significant amount of overburden exists.

### Hydraulically Driven Submersible Pumps

Hydraulically driven submersible pumps are suited best for large diameter boreholes and removal of large volumes of water. They were developed to service the mines using large diameter boreholes so lower priced blasting agents could be used. Early dewatering machinery on the Mesabi Iron Range in Minnesota simply consisted of an electric submersible pump rigged with hoses to make it portable from borehole-to-borehole.

The obvious safety objection to the use of electrical devices in the ground next to loaded boreholes led to the development of hydraulically powered units. These devices evolved into an entire family of sophisticated pumping machinery that can dewater holes in the largest diameters and depths currently drilled. These units are offered with single- or multi-stage pumps, and with powered hose reels.

![Figure 24.1 – (left)Hydraulically driven submersible pump. (Courtesy: D. Rensari) (Below)Pump head. (Courtesy D. Rensari)](images/457.png)

Modern units usually are self-contained, and are carried on a vehicle (See figure 24.1) dedicated to dewatering operations. They are operated from a position next to the hose reel or from the vehicle's cab. The pump unit, which is lowered to the end of a hose to the bottom of the borehole, consists of a hydraulic motor driving an impeller. The unit drives the water through a screen at the bottom of the unit and forces it up the hose and out to the surface. The screen filters particulates and prevents damage to the pump itself. The hydraulic lines that supply power to the pump are located inside of the pump's discharge hose. Several companies offer this equipment in a variety of configurations. The submersible hydraulic dewatering pump offers the benefits listed in table 24.2.

### Benefits Of Hydraulically Driven Submersible Pumps

| Benefit |
|---------|
| Self contained construction allows independent operation on the site |
| Versatile |
| Large pumping rate |
| Large volume capability from both large and medium diameter deep lift holes. |

*Table 24.2 – Benefits of hydraulically driven submersible pumps.*

These pumping systems have the following limitations and operating concerns. There is the risk of losing the relatively expensive pump in the borehole if the pump gets stuck in a caved or tight borehole. Pumps can be damaged if used to pump water with abrasive cuttings. In freezing weather, the water in the pumping system needs to be blown out with air or needs to be diluted with an antifreeze solution between runs in order to avoid damage. Finally, most of these units are not appropriate for construction and quarrying operations where close hole spacings are drilled, because it is usually not possible to get this type of dewatering vehicle within arms reach of blast-shot areas.

### Pneumatic Displacement Pumps

The wet blasting environment along the United States northwest coast led to the development of this dewatering device. It was originally designed to meet the needs of the blasters who drilled small diameter, air track size holes. This system (See figure 24.2) uses compressed air to expand a rubber sleeve and seal it against the inside wall of the borehole at a point just above the water level. Air pressure is then diverted into the borehole below the expanded sleeve, displacing the water and forcing it into a discharge line, up through the center of the sleeve assembly, and out to the surface.

![Figure 24.2 – (left) Pneumatic displacement pump components. (Courtesy: RDI Inc.) (below) Pneumatic displacement pump. (Courtesy: D. Rensari)](images/459.png)

These pumps were initially designed to dewater small diameter boreholes. However, they currently are available for boreholes up to about 180 millimeters (7 inches) in diameter in excess of 30 meters (100 feet) deep. One person can place the pump, dewater the hole, and remove the entire unit by hand. Several advantages to this pump are listed in table 24.3.

### Benefits Of Pneumatic Displacement Pumps

| Benefit |
|---------|
| Single person operation. |
| Portable. |
| Limited housekeeping at replacement. |
| Blows hose dry after dewatering each borehole (no antifreeze required). |
| Low hose replacement cost. |
| Low maintenance cost. |
| Undamaged when pumping water containing mud or abrasive cuttings. |

*Table 24.3 – Benefits of pneumatic displacement pumps.*

Its disadvantage is that it requires a reasonably round, competent borehole to form a good seal. Therefore, during operation in very loose or broken ground it can lose pressure through the cracks and reduce its pumping rate.

### Double Diaphragm Air Assisted Pumps

This dewatering pump was also developed for smaller diameter boreholes. The system utilizes a modified, air-powered double diaphragm pump (See Figure 24.3). In operation, a stream of compressed air is directed through a small line located inside of the intake hose to a venturi nozzle positioned near the intake point of the suction hose. That air injection feature causes water to be drawn from holes beyond the depth of the normal suction limits of standard double-diaphragm pumps.

![Figure 24.3 – (left) Double diaphragm pump. (Courtesy: Drylet Pumps) (right) Double diaphragm pump components. (Courtesy: Drylet Pumps)](images/460.png)

The system is capable of pumping significant volumes of water from holes of medium depth and it can lift from nearly 30 meters (100 feet) deep. It is available in various sizes, using 25 millimeters to 38 millimeters (1.25 inches to 1.50 inches) suction hose and may also be used in larger diameter boreholes. It is offered as a portable hand moveable unit, and also in large capacity self contained multiple units that require a separate vehicle to carry them. Table 24.4 shows the advantages of these pumps.

### Benefits Of Double Diaphragm Pumps

| Benefit |
|---------|
| Portable construction. |
| Single person operation. |
| Only hose lowered into the borehole eliminating the potential loss of being expensive parts if the hole caves. |
| Pumps water containing mud and small cuttings without damage. |
| Pumps the discharge the steady dry eliminating the need antifreeze in freezing conditions. |

*Table 24.4 – Benefits of double diaphragm pumps.*

Its limitations include the fact that its pumping volume decreases with borehole depth, and to operate requires a relatively high minimum volume of auxiliary compressed air of 26 liters/second (55 feet³/minute) at a pressure of 83 kilopascals (12 pounds/inch²).

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## BOREHOLE LINERS

Polyethylene borehole liners have been used effectively for many years. They keep ANFO and low water resistant products dry and prevent bulk explosives from migrating into cracks in the rock around the borehole. Poor blasts and NO₂ production are sometimes the result of this migration because when that small cracks are below the explosive's critical diameter the explosive will deflagrate rather than detonate.

### Selection Criteria

Always purchase borehole liners from a supplier that has quality equipment and uses only virgin polyethylene. Recycled polyethylene is produced by grinding and reheating used polyethylene. Repeated grinding and heating can cause degradation and decrease the liner's effectiveness. A good antistatic additive is needed to assist in discharging any static electrical charges that might be present. The lay flat (LF) dimension must be determined from the borehole diameter at which it is to be inserted.

Liners are sold by their lay flat dimension. When the liner is flattened on a flat surface the LF dimension is measured in inches across the flattened tubing. The LF is actually ½ of the circumference of the tubing circle when opened. Users must also specify a thickness measured in mils. This dimension is the thickness of the tube wall measured in 1000ths of an inch. The actual circumference of the liner must be greater than that of the borehole diameter to ensure a smooth fit.

Liners can be purchased in either bulk rolls (See figure 24.4) or as custom lengths with a rock pocket (See figure 24.5) in the bottom. The popular can be filled with stone or made of heavy poly to lower the liner to the bottom of the borehole. When ordering bulk rolls ensure they are not too heavy. To determine the weight of the length wanted in a roll ordered from a supplier, use equation 24.1.

![Figure 24.4 – Bulk liner roll. (Courtesy: Virginia Plastics, Inc.)](images/461.png)

$$Weight = LF \times T \times 0.8$$ <!-- VERIFIED -->

**Equation 24.1**

Where:
- $W$ = weight (pounds/foot)
- $LF$ = Lay flat width (inches)
- $T$ = liner thickness (inches)

![Figure 24.5 – Custom liner with pocket. (Courtesy: Stemlock, Inc.)](images/462.png)

Custom liners should be ordered as lengths equal to the borehole depth plus extra to allow it to be held or secured during loading. It is also possible to buy double liners that have one liner inside the other. In really rough, deep holes this can be an added advantage to preserve the liner's integrity to prevent water intrusion or damage.

### Use Techniques

Boreholes must be dewatered before using liners to prevent the liner from closing at the waterline. Closure prevents the explosives from reaching the borehole's bottom. Charging the boreholes should be done as soon as the dewatering equipment is removed and the liner installed. Stands (See figure 24.6) are designed to hold the liner open so the explosives will load easily and not fall into the borehole with the liner.

When a bulk liner is used, first tie a couple of knots in the liner and place a small weight of explosives into the bottom end to help lower the liner to the borehole bottom. Custom liners are supplied as a weighted rock pocket. Once the rock pocket has been filled, hoist pressure can control the lowering rate to smoothly lower it into the borehole.

When the liner reaches the borehole bottom, secure the top of the liner in a stand as shown in figure 24.6. Once the liner is secured, then prime and load the boreholes.

![Figure 24.6 – Liner stand. (Courtesy: Stemlock, Inc.)](images/463.png)

After stemming the borehole stem along side the liner. Stemming along the side of the liner forces the air to escape the liner and also keeps the downline oriented by applying pressure to the downline against the borehole wall.

> **Caution**
>
> A liner that drops down the borehole can put pressure on the downline and create an air pocket in the stemming. This can cause ejection due to decreased stemming material.

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## ADDITIONAL RESOURCES

Hill, R.J. 1984. Dewatering small diameter bore holes. International Society of Explosives Engineers (ISEE) Proceedings of the 12th Annual Conference on Explosives and Blasting Technique, February 9 - 14, Vol. 1, p 14, Atlanta, GA. ISEE, Cleveland, OH.

Lane, William C. 1986. Using dry ANFO in the Pacific Northwest rainforest. International Society of Explosives Engineers (ISEE) Proceedings of the 12th Annual Conference on Explosives and Blasting Technique, February 9 - 14, Vol. 2, pp. 59 – 69, Atlanta, GA. ISEE, Cleveland, OH.

Polzinc, S. R. "Bob". 1988. Dewatering the blast area — A means of achieving productivity. International Society of Explosives Engineers (ISEE) Proceedings of the 24th Annual Conference on Explosives and Blasting Technique, February 2 – 5, pp. 27 – 35. New Orleans, LA. ISEE, Cleveland, OH.

Satchler, L. and S. Buchanan. 1996. Effect of water on ANFO/Emulsion blends in surface mine blasting. International Society of Explosives Engineers (ISEE) Proceedings of the 22nd Annual Conference on Explosives and Blasting Technique, Vol. 1, p. 52, February 4 – 8, Orlando, FL. ISEE, Cleveland, OH.

Stevens Harold H. 1987. The Wethole Story. Presentation at Puget Sound International Society of Explosives Engineers Chapter Meeting, Post Falls, Idaho.

Stern, M.S. and R. Aquila. 1997. A practical method of field gassing plastic borehole liners for water penetration. International Society of Explosives Engineers (ISEE) Proceedings for the 23rd Annual Conference on Explosives and Blasting Technique, Vol. 1, p. 201, Las Vegas, NV. ISEE, Cleveland, OH.