1. Introduction
Any open pit and almost any underground mine is a vast sump collecting water. The water naturally tends to accumulate at the bottom of the workings and the flow scours fine material and holds it in suspension. Dewatering a mine encompasses not only the water but also the fines contained in the water. The task is aggravated in some mines because fines can significantly alter the pH of the mine water. Many base metal mines have to contend with acid water (pH as low as 2) while other mines have problems with high alkalinity.
The mine dewatering process includes the following activities.
• Prevention
• Collection and containment
• Removal
• Disposal
Prevention
Rainfall and snow cannot be prevented from falling directly into an open pit; however, ditches diverting the flow away from the workings can prevent the overland flow of water into the excavation. Water flow from the overburden soil at a pit rim can be collected and pumped to the diversion ditches. Water that seeps through the rock walls (ground water) of the pit may be redirected by collection from drill holes or lowering the ground water table in the bedrock by drilling and pumping from deep wells.
Entries to underground mines are prone to collect water. Surface flow is prevented by locating entries on high ground, sloping the terrain away from entries, or placing a reverse slope at the top of a ramp entry. Flow of water through the overburden is prevented by sealing the entry down to (and into) the bedrock or collecting and redirecting the water.
Ground water flowing into an underground entry (shaft, raise, ramp or adit) is most often controlled by injection grouting. In special cases a vertical entry is sealed with an impervious (hydrostatic) lining designed to withstand the pressure of the ground water. Many open pit mines and some underground mines reduce the flow of ground water with deep well in-the-hole pumps. A few underground mines reduce the flow with curtain grouting.
Collection and Containment
Mine water that reaches the workings is typically collected and confined to a central location(s) using ditches, boreholes, and piping arranged to prevent accumulation and limit fines contamination. The containment is required to provide surge capacity in the event of a power outage or pump failure and offers the opportunity for settling fines (slimes) before pumping the decanted water.
Removal
If the terrain permits, collected water may be removed through a drainage tunnel, but usually pumping is required. For most applications, centrifugal motor pumps are used as the prime movers. Water is normally directed to a settling sump(s) and the overflow of clear water to a “clean water” sump for main line pumping. Handling the sediment (slimes) that deposit in the sumps is a significant problem, especially for underground mines.
Centrifugal pumps are available for high volumes that can pump “dirty” mine water (not allowed to settle). If the quantity of dirty water is relatively small, piston diaphragm pumps can deliver in a single stage from great depths. Diaphragm pumps may be used for new mines but are typically installed in existing operations to pump from a new horizon up to an existing clear water system.
Disposal
Treatment and disposal (or recycling) of mine water on surface is discussed in detail in Section 5 – Environmental Engineering. Treatment underground is confined to dosing with a flocculant. Adding lime underground is believed to promote calcium compound deposits (principally CaCO3) inside pipelines and should be avoided.
2. Rules of Thumb
Water Balance
• The average consumption of service water for an underground mine is estimated at 30 US gallons per ton of ore mined per day. The peak consumption (for which the water supply piping is designed) can be estimated at 100 USGPM per ton of ore mined per day. Source: Andy Pitz
• Ore hoisted from an underground hard rock mine has moisture content of approximately 3%. Source: Larry Cooper
• A water fountain left running underground wastes 1,100 USGPD. Source: Jack de la Vergne
• A diesel engine produces 1.2 litres (or gallons) of moisture for each litre (or gallon) of fuel consumed. Source: John Marks
• In the hard rock mines of the Canadian Shield, ground water is seldom encountered by mine development below 450m (1,500 feet). This may be because the increased ground stress at depth tends to close the joints and fractures that normally conduct water. Source: Jim Redpath
Layout
• The main pump station underground must have sufficient excavations beneath it to protect from the longest power failure. The suggested minimum capacity of the excavations is 24 hours and a typical design value is 36 hours. Source: Jack de la Vergne
• The main pumps should be placed close to the sump so that the separation will allow for a minimum straight run of pipe equal to five times (preferably ten times) the diameter of the pipe.
Various Sources
• Turbulence will be sufficient to ensure good mixing of a flocculating agent if the water velocity is at least 1m/s and maintained for 30 seconds in a feed pipe or channel. Source: NMERI of South Africa
Design
• Piping for long runs should be selected on the basis that the water velocity in the pipe will be near 10 feet/sec (3m/s). The speed may be increased up to 50% in short runs. Various Sources
• In underground mines, static head is the significant factor for pump design if the pipes are sized properly. To obtain the total head, 5 -10% may be added to the static head to account for all the friction losses without sacrificing accuracy. Source: Andy Pitz
• Pump stations for a deep mine served by centrifugal pumps are most economically placed at approximately 2,000-foot (600m) intervals. Source: Andy Pitz
• The outlet velocity of a centrifugal pump should be between 10 and 15 feet per second to be economical. Source: Queen’s University
• Centrifugal pumps should not operate at a speed exceeding 1,800 RPM (except for temporary or small pumps that may operate at 3,600 RPM). This is because impeller wear is proportional to the 2.5 power of the speed. In other words, half the speed means nearly six times the impeller life. Source: Canadian Mine Journal
• The maximum lift of a centrifugal pump is a function of the motor torque, which in turn is a function of the supply voltage. Since it is a squared function, a 10% drop in line voltage can result in a 20% loss in head. Source: Jack de la Vergne
• The velocity of dirty water being pumped should be greater than 2 fps in vertical piping and 5 fps in horizontal piping. These speeds are recommended to inhibit solids from settling. Source: GEHO
• Slime particles less than 5μ in diameter cannot be precipitated without use of a flocculating agent. Source: B. N. Soutar
3. Tricks of the Trade
• The best layout for a main pump station has the pumps fed from the sump with a positive suction head (i.e. the sump outlet is higher than the pump). If the sump is below the pumps, water is not “pulled” up the suction pipe, it is “pushed” by atmospheric pressure. At a mine where the water flow is relatively small, the lift can be as much as 6m (20 feet). At high rates of flow, the lift is less and for mines at high altitude lift can be reduced to near zero. If the suction lift is too high the result is “cavitation,” which makes the pump sound like it is pumping gravel. Cavitation reduces pump capacity and is blamed for pitting the impeller, erratic power consumption, loss of head, bearing failure, and other mechanical damage from vibration. Source: Travis Glover
• Collecting water from rings (“launders”) in a shaft can provide process water for the operations without water pressure reduction and lowers the total volume to be pumped out of the mine. Source: Peele
• Commercial water blasters (or shop made blow pipes that use compressed air to provide a high velocity water stream) use as little as 10% of the water required for washing equipment with an open water hose. Source: Jack de la Vergne
• Employing disc (instead of button) cutters on raisebore heads will significantly reduce the amount of slimes generated in an underground mine. Source: Pete Guthrie
• Small bulldozers and drags are sometimes used to grade trackless headings; however, the only suitable equipment is a motor grader. On surface roads, these machines may make a first pass each way deflecting material from the crown to the sides and a second pass to restore the crown and provide final grade. The first pass should be omitted underground. The resulting hump at the crown can easily be negotiated by trackless equipment and fines are not directed towards the ditch. Various Sources
• Ground water dribbling from the back of a trackless heading can be diverted to fall in the ditch with the placement of a corrugated sheet metal deflector. This procedure prevents potholes and reduces slimes generation. Source: Marshall Hamilton
• The addition of a flocculating agent to hydraulic fill will reduce the amount of slimes in the decant water. Source: Jim Devlin
• Ore and waste passes should not allow entry of collected mine water for several reasons. It is especially important to ensure that decant water from cemented fill and spill or flush water from fill lines do not enter an ore or waste pass because this water contains particles of fresh cement promoting hang-ups in passes and rat holing in bins. Source: Fred Brackebusch
• A neat way to handle decanted slimes is to pump them into the outlet pipe of a clear water pump station. An electrically driven standard duplex grout pump facilitates the necessary interlock so it will stop when the main pumps stop. Source: Bill Shaver
• Mines usually run centrifugal pumps intermittently to accommodate fluctuations in supply. Another practical method is to choke the output. Choking decreases the power draw and slight choking may increase pump efficiency. Choking is also useful to correct an over-heating pump motor. Source: Lindsay Baxter
• Nuisance water at a face or bench can be pumped into a haul truck or shaft bucket to fill the voids in the muck that may amount to 25% of the volume. Source: Jack de la Vergne
• Mine slimes should be assayed and analyzed to help ascertain the sources and determine whether they should be directed to the mill. Source: Rory Mutch
• Water flows up to 25 USGPM (1.6 l/s) are easy to determine accurately by measuring the time it takes to fill a five-gallon pail. Source: Jim Redpath
• Water flow from a horizontal pipe is easily determined by measuring the distance to a drop of 4 inches (100 mm) in the flow (refer to Section 20.6). Source: Pleuger Unterwasserpumpen GMBH
• Water flowing up from a vertical drill hole is easily determined by measuring the height of flow (refer to Section 20.6). Source: Khoa Mai
• Water flow in a ditch is most practically measured by installing a portable weir box and measuring the true head of flow over the crest (refer to Section 20.6). Source: William Staley
4. Source of Slimes
Water collected in a mine contains particles of solid material referred to as fines or slimes. Limiting the generation of slime material is one of the disciplines directly related to mine dewatering. To enact control measures, sources of slime must be identified (listed below).
• Drilling.
• Raiseboring.
• Hydraulic fill decant.
• Fault gouge.
• Overloading of explosives.
• Crushing and breaking.
• Attrition in the ore/waste handling system.
• Attrition in the road dressing/rock fill handling system.
• Comminution on haulage ways.
• Flushing fill lines.
• Breaking plugged fill lines.
• Oxidation – Pyrite in the ore produces colloidal ferric hydroxide, Fe(OH)3.
5. The Water Balance
The most important activity in analyzing a mine dewatering system is to compile a water balance that identifies sources and defines the pumping rate. In temperate climates, less water is pumped in winter months than spring and summer. In this event, two separate balances must be compiled. The sources are typically surface water, ground water, service water (drilling, dust suppression and washing), decant from hydraulic fill, flush water from fill and slurry lines, and condensation from ventilation air or chillers. Some of this water is removed to surface in the ore and waste rock stream or evaporates into a ventilation circuit. The remainder must be pumped. Table 20-1 is an example of an underground mine water balance.
Table 20-1 Underground Mine Water Balance
6. Estimating and Measuring Water Flows
Predicting anticipated flow in the overburden soils is a highly developed science (hydrology), which has its roots in estimating the capacity of drilled wells. Calculations for a mine application are typically complex and best performed by a specialist. Predicting ground water flows in porous rock may be accomplished by determining the fall of head in a drilled well or by packer tests in a borehole to produce reliable results.
Predicting ground water flow into a hard rock mine is difficult because its source is typically from irregular fissures and joints in the rock; therefore, it is very difficult to predict with any accuracy. Normally, packer tests in drilled holes provide an order of magnitude for the anticipated flow, but in numerous instances the resulting estimates were completely wrong. As a rule, an accurate estimate of the ground water flow into a proposed hard rock mine can only be obtained from driving an exploration or development entry (shaft, adit, or ramp).
The measurement of relatively small quantities of water is most easily accomplished by the time required to fill a pail or bucket of known volume (for example, a 5-gallon pail). The flow measurement from a horizontal open-ended pipe may be determined by measuring the distance to a predetermined drop of the water stream. If the distance to where the drop is 4 inches (100 mm) is measured, the flow can be obtained from Table 20-2 (providing flow rates in USGPM for schedule 40 pipe).
1 m3/min = 16.67 l/s = 35.3 cfm = 264 USGPM
1 l/s = 15.85 USGPM
Table 20-2 Flow Rates (USGPM)
The quantity (Q) of water flowing upwards from a hole drilled vertically can be estimated by measuring the height of flow (H) over the collar of the hole.
Q = 5.1 D2H½
In which,
D is the hole diameter (inches)
H is the height of flow (inches)
Q is the flow in USGPM
The flow rate from a sinker drill (plugger) hole can be quickly determined by measuring the height of flow and comparing it with the data in Table 20-3.
Table 20-3 Flow Rate from a 1.38-inch Diameter Hole
The quantity of water flowing in a typical underground hard rock mine ditch can be measured with a portable weir box made from wood or sheet metal. A three-foot box is long enough for the flows found in typical ditches. If the weir extends across the full width of the box and the box is placed level in the ditch, the quantity (in USGPM) may be determined by using the following formula.
Q = 3.0 bH1.5
b is the length in inches of the weir (box width), H is the head measured in inches well back from the crest of the weir, and Q is USGPM. Table 20-4 provides flow rates for an 8-inch wide weir box (a convenient dimension for a portable unit).
Table 20-4 Flow Rates for an 8-inch Wide Weir Box
Figure 20-1 Portable Weir Box in Ditch
The velocity of water flowing in a large rectangular ditch of uniform cross-section may be determined by timing an object floating downstream in the middle of the ditch over a fixed distance. The average velocity will be 74% of this value for a rock ditch and 88% for a smooth concrete-lined ditch. The area is the width of the ditch times the wet depth. The quantity is the product of the average velocity and the area measured.
7. Clear Water Pumping
Clear water is mine water for pumping containing less than 250 ppm of suspended solids that do not exceed 35μ in size. A stricter definition is water containing less than 100 ppm of particles not exceeding 25μ. Clear water pumped from mines often exceeds these criteria causing short life of pump components leading to high maintenance and repair costs.
Sumps
Particles as small as 5μ may be decanted but the retention time and the resulting sump size is not practical in a mine. Typical practice underground is to excavate two horizontal settling sumps, one of which continues to operate while the other is being cleaned of slimes. Smaller operations often employ a single cone or fan shaped vertical settling sump from which the slimes can be drawn off while maintaining the mine water flow. If not wisely designed and carefully maintained, neither a horizontal nor a vertical settler will work with any lasting efficiency.
Slimes
Slimes in a horizontal settling sump are about 15% solids by weight. When the sump is drained, slimes will increase to approximately 30%. This material is difficult and messy to handle, even when left for a week or more to consolidate further. At least one mine doubled the solids content when drained by using a flocculating agent.
Centrifugal Pumps
Mines invariably select centrifugal pumps as the prime mover for dewatering. Centrifugal pumps are reliable, relatively compact, and the multi-stages required for high heads can direct drive with a single motor. Disadvantages include that they are efficient in a relatively narrow operating range making variable speed drives not practical. Additionally, if a centrifugal pump runs dry, or even when the outlet pipe is broken near the pump, they can draw enough amperage to burn out the motor Centrifugal pumps have the following (approximate) characteristics.
• Capacity varies directly with the speed of the impeller (RPM)
• Capacity varies directly with the diameter of the impeller, D
• The head varies with (RPM)2
• The head varies with D2
• The power drawn varies with (RPM)3
• The power drawn varies with D3
8. Dirty Water Pumping
Providing sufficient settling sump capacity in underground mines that experience huge inflows of ground water is not practical. For this application, specially designed centrifugal pumps are employed – dirty water pumps. Characteristics of dirty water pumps are significantly different from the low-head centrifugal slurry pumps employed in mine concentrators where the particles pumped are finely ground. Linings made of natural rubber, neoprene, polyurethane, etc. are often used for these low-head pumps. Dirty water centrifugal pumps for mine service require wear surfaces of hard, tough, abrasion resistant metal. In general, the hardness required (measured on a Brinell scale) is related to the hardness of the particles of sediment (measured on Moh’s scale). Typical rocks in a metal mine have the hardness of feldspar. These particles are approximately as hard as workhardened manganese steel; however, high silica content often exists in hard rock. Silica has the hardness of quartz on the Moh’s Scale. Pyrite is almost as hard as silica. These can only be matched on the Brinell Scale with special alloys, such as one containing 28% chromium.
Hardness is not the only wear factor, dense particles cause more wear since the kinetic energy is higher. Angular particles cause twice as much wear as rounded ones. Up to 10%, the wear rate is almost directly proportional to the concentration of fines. This is one reason why centrifugal dirty water pumps have better application when there is a high volume of water to be pumped from the mine.
Since wear increases exponentially with the velocity (approximately V2.6 ) of the particles in the water, it is prudent to sacrifice some efficiency and use a slightly larger sized pump than would be employed for the same service with clear water.
The minimum clearance between rotating parts in a clear water centrifugal pump is approximately 75 μ, which is why clear water is defined as containing nothing larger than 37 μ (to avoid bridging). This clearance may be altered for a dirty water centrifugal pump in mine service, which can slightly lower its efficiency.
Unlike clear water, dirty water centrifugal pumps are often V belt driven which may add 5% to the power losses, but provides a practical means to obtain an efficient pump design using a standard impeller diameter and a standard motor speed.
In the case of high heads and relatively small volumes, the modern piston diaphragm pump provides reliable service at a high capital cost. These pumps run continuously but capacity can only be adjusted in a relatively narrow range, unless they are fitted with variable speed motors. The feed water is normally collected in a small sump to facilitate agitation and provide a constant solids content. The piston diaphragm pump has a head capacity high enough to pump to surface in one lift from any depth of mine operations. The pump system cost increases at great depths because pipe beyond schedule 80 normally costs twice as much per pound (or kg) – besides being thicker walled. Maintenance cost for these pumps is low.
9. Drainage Tunnels
In mountainous or hilly terrain, drainage tunnels may be an economical alternative to pumping. Drainage tunnels are driven at an elevation beneath the mine workings. The gradient is normally higher than normal for a rail heading – 1% is typical and 1.5% considered a maximum. The heading incorporates a large ditch on one side that is advanced with the face. The ditch dimension is usually square, 3 feet by 3 feet or 1m by 1m. Driving a heading with this huge ditch is difficult to mechanize, often resulting in slow progress. (In unusual circumstances, the heading may be driven with a conventional ditch and designed to flow nearly full when completed.) Checking by Froude’s Number confirms that these ditches will not experience hydraulic jump. The ditch capacity, Q, may then be simply determined by modifying Manning’s formula.
Q = 25,000A5/3S1/2/P2/3 (metric units)
A is area, S is slope (gradient of the tunnel), P is the wetted perimeter, and the formula assumes a roughness coefficient, n = 0.04, in this case.
For a fixed cross section and depth of flow, the equation is simplified to Q = k S1/2 (k is a constant). The rate of flow is directly proportional to the square root of the gradient. The following tabulation (Table 20-5) may be used to determine the capacity of a standard (1m)2 drainage tunnel ditch flowing 95% full at differing gradients.
Table 20-5 Standard Drainage Tunnel Capacity
The capacity of a drainage ditch cut in rock may be more than doubled if it is smooth lined with concrete (roughness coefficient, n =0.014, in this case). In the case of a square concrete-lined ditch, the following formula may be employed to provide satisfactory answers.
Q = 70,000A5/3S1/2/P2/3 (metric units)
10. Centrifugal Pump Selection
A centrifugal pump is primarily described by its outlet size. The size of a pump is determined by its outlet velocity, which can be determined by the following equation.
Q = V.A
In which, Q = Rate of flow in CFS (m3/s)
V = Average velocity in FPS (m/s)
A = True area of pump outlet ft3 (m3)
Table 20-6 True Area of Outlet
• If the outlet velocity is greater than 15 fps (4.6m/s), the pump is too small.
• If the outlet velocity is less than 10 fps (3.0m/s), the pump is over-designed and oversized.
• Pump efficiency depends on the specific speed (Ns) of its impeller.
Ns =N Q1/2H3/4
In which,
Q = Rate of flow
H = Total friction head
N = RPM of the impeller
The formula is valid for metric or imperial units. The flow rate, Q is expressed in USGPM in many pump catalogues; however, using CFS (cubic feet per second) instead produces values that are simpler to use and cause no confusion between US and Imperial gallons.
Ns (USGPM) = 17.66 Ns (CFS)
Most centrifugal pumps are direct-driven by an induction motor, so N must be a reasonable standard motor speed such as 3,450 RPM (usually small or temporary pumps only), 1,750-RPM, or 1,160-RPM. For typical underground mine service, calculations reveal that higher speeds are more efficient (and higher speed motors are less expensive).
• For most mine service, a pump will be most efficient if the Ns (CFS units for Q) is between 100 and 200.
• If the calculated Ns (CFS) is less than 50 – select as many smaller centrifugal pumps as required so that the specific speed of each exceeds 50 (multi-stage pumps in series).
• If the specific speed (CFS) is between 50 and 200, select a single centrifugal pump.
• If the specific speed (CFS) is over 200 and less than 400, two centrifugal pumps in parallel may be employed.
The following figure plots the range of efficiencies for typical mine application. For a given specific speed, the higher values for efficiency refer to new high volume pumps of efficient design. Centrifugal pumps of relatively small capacity and subjected to wear will have lower efficiency.
Figure 20-2 Range of Efficiencies
• The following formulas may be used to calculate the required motor horsepower with the efficiency determined from the specific speed chart above or from charts in pump manufacturers’ literature.
In which,
Q = Flow Rate (cfs)
W = 62.4 Lbs. per cubic foot (for clear water)
H = Total Head (feet)
E = Pump Efficiency
In which,
Q = Flow Rate (USGPM)
H = Total Head (feet)
E = Pump Efficiency
• With the pump horsepower determined, select the next highest standard sized motor (refer to Table 20-7).
Table 20-7 Standard Electric Motors
The standard motor sizes produced overseas are the same sizes but expressed in kilowatts (soft conversion). A 15 kW motor is the same as a 20 HP motor. Some additional standard sizes are manufactured that do not correspond to those tabulated above (80 kW, 120 kW).
11. Friction Head Loss in Steel Pipe
The standard (Hazen-Williams) formula is expressed as follows.
In which,
hf = Head loss due to friction in feet of liquid
d = Inside diameter of circular pipe in inches
C = Friction factor (Hazen-Williams)
L = Length of pipe including equivalent length for loss through fittings in feet
Q = Flow of liquid in USGPM
The “C” Factor for steel pipe used in mine de-watering design is typically 120, therefore, the equation can be simplified to the following.
This equation is valid for either clean or dirty water pumping; however, it is not valid for slurry pumping when the solids content exceeds 40% by weight.
Table 20-8 Friction Head Loss (Feet) per 100 Feet of Schedule 40 Steel Pipe (Mine Service)
12. Minimum Wall Thickness of Piping
In calculating the wall thickness of piping for the transmission of clear water, the following formula can be used.
In which,
t = Wall thickness (in.)
D = Outside diameter of pipe (in.)
P = Maximum internal pressure (psi)
y = Temperature coefficient, typically equal to 0.4
s = Allowable stress in pipe (psi) - equal to 50% of yield strength (typically,
s=17,500 psi)
c = allowance for corrosion, equal to 0.062 in. (typical)
+ allowance for depth of thread (screw fitting), or
+ depth of groove (mechanical coupling)
Notes
• The factor of 1.15 is an allowance of 15% for variation in manufactured pipe wall thickness.
• The internal pressure is made up of static head due to the difference in elevation and friction losses. An additional allowance of 250 psi can be considered for surge pressure (water hammer), where applicable.
• The yield strength of common steel pipe (A53-Grade B) is 35,000 psi.
Table 20-9 Dimensions of Commercial Steel Pipe (d = inside diameter, t = wall thickness)
13. Settling Velocity
For particles of diameter less than 1 mm (1,000μ), Stokes’ Law applies to calculate the settling velocity in still water. For particles of diameter greater than 1 cm, Newton’s Law should be used; however, this requires the calculation of Reynolds number and the determination of drag coefficients. In the transition zone, the actual settling velocity is somewhere between the two laws. For typical mine dewatering applications, Stokes’ law is used and may be reduced to the following formula when dealing with water.
vs = 1962 (ρs – 1) d2
vs = particle settling velocity in meters per hour, ρs = specific gravity of the particle*,
d = diameter of the particle in millimeters.
The formula can be used to determine the minimum required plan area for a vertical settler.
Example
Determine the size of vertical settler required to settle 25 μ particles at a given inflow rate of 500 USGPM.
Listed below are the required steps.
1. Determine the settling speed. Assume the S.G. of the particle is 3.0
vs = 1962 x (3.0 – 1) x (0.025)2 = 2.45m/hour.
2. Convert flow rate and determine the settling speed.
1 m3/hour = 4.40 USGPM; therefore 500 USGPM = 113.6 m3/hour.
3. Determine minimum plan area of settler.
A = (113.6 m3/hour) / (2.45m/hour) = 46.4m2 [500 ft2]
Note
1.0 ft2/USGPM
4. Adjust the theoretical result by 20% to account for unintentional agitation in the settler and other inefficiencies.
A = 1.2 x 46.4 m2 = 55.7 m2 [600 ft2]
Solution:
A cone settler 8.4m (28 feet) in diameter
*Note
Specific gravity of a fine particle is usually more than the rock from which it came, due to porosity. Particles derived from typical feldspathic hard rocks (SG =2.65) may have SG =3.
14. Underground Dam Design
The Ministry of Labour (Ontario) has developed a set of standards for bulkhead and dam design for underground mines. They produced tables of pre-designed criteria for use in easily choosing a design without having to perform all the calculations. This section outlines these design criteria for dams only. (The tables for bulkheads may not always be correct.)
Figure 20-3 Underground Dam Design
Table 20-10 Overflow Dam Design (Metric)
Table 20-11 Overflow Dam Design (Imperial)
