Mining Rule of Thumb

The primary usage of Rules of Thumb should be in the development of conceptual designs and feasibility studies or, when a quick decision is required in the solution of an operating problem. Although an approximated answer, derived from a Rule of Thumb may solve an immediate problem, Rules of Thumb are not a substitute for the application of sound engineering and design methodologies. Although we firmly believe that the presented Rules of Thumb provide great continuing value to our industry, McIntosh Engineering does not guarantee their validity, nor do we (or the referenced individual sources) accept responsibility for application of the Rules of Thumb by others. Where possible, direct quotes have been provided from individual references; however, it is possible that referenced sources may not have directly stated the Rule of Thumb for which they are assigned credit. Although we have endeavored to accurately quote all individual references contained in the Rules of Thumb compilation, we apologize in advance for any misquotes that may be attributed to individual sources. We will provide updates to the Rules of Thumb compilation, as we become aware of corrections that may be necessary.

Field: Collars and Portals

Area: Collars

• The elevation of a shaft collar should be 2 feet above finished grade. Heinz Schober
• The typical thickness of a concrete lining for a production shaft collar is 24 inches in overburden and 18 inches in weathered bedrock. For a ventilation shaft collar, it is 18 inches in overburden and 12 inches in weathered bedrock. Jack de la Vergne
• The finished grade around a shaft collar should be sloped away from it at a gradient of 2%. Dennis Sundborg
• A shaft collar in overburden, completed by any means other than ground freezing (which may take longer), will be completed at an overall rate of 1 foot per calendar day. Jim Redpath
• For a shaft collar in deep overburden, the minimum depth of socket into bedrock is 3m (10 feet) in good ground, more if the rock is badly weathered or oxidized. Jack de la Vergne
• The minimum depth for a timber shaft collar is 48 feet (15m). Jack de la Vergne
• The minimum depth for a concrete shaft collar is 92 feet (28m). If a long round jumbo is to be employed for sinking, it is 120 feet. Jack de la Vergne
• For a ground-freezing project, the lateral flow of subsurface ground water in the formation to be frozen should not exceed 1m per day. Khakinkov and Sliepcevich
• To determine the diameter of a proposed circle of freeze pipes around a shaft collar, 60% should be added to the diameter of the proposed excavation. Sanger and Sayles
• When ground freezing is employed for a shaft collar, the area of the proposed collar excavation (plan view) should not be greater than the area to remain inside the circle of pipes (area that is not to be excavated). B. Hornemann
• The minimum practical thickness for a freeze wall is 4 feet (1.2m). Derek Maishman
• The maximum practical thickness for a freeze wall with a single freeze circle is 16 feet (5m). Concentric circles of freeze pipes should be employed when a thicker freeze wall is required. Derek Maishman
• The radiation (heat transfer) capacity of a freeze pipe containing brine may be assumed to be 165-kilocalories/square meter of pipe surface. However, if the brine velocity is too slow (laminar flow), this capacity will be reduced by 40%. Jack de la Vergne
• The capacity of the freeze plant selected for a ground freezing project should be 2-2½ times the capacity calculated from the radiation capacity of the total length of freeze pipes installed in the ground. Berndt Braun
• Groundwater movements over 3 to 4 feet per day are significant in a ground freezing operation. U.S. National Research Council
• If the drill casing is left in the ground after installing the freeze pipes, it will cost more but the freeze pipes will be protected from blast damage or ground movement and the heat transfer will be increased due to the greater surface area of the steel casing. Jim Tucker
• The heat gain from circulating brine is equal to the sum of the friction losses in the pipes plus the heat generated due to the mechanical efficiency of the brine pump. The value calculated for the heat gain should not exceed 10% of the refrigeration plant capacity. Jack de la Vergne
• The amount of liquid nitrogen (LN) required to freeze overburden at a shaft collar is 1,000 Lbs. of LN/cubic yard of material to be frozen. Weng Jiaje
• Due to the heat of hydration, the long-term strength of concrete poured against frozen ground will not be affected if the thickness exceeds 0.45m (18 inches). Below this thickness, designers will sometimes allow a skin of about 70-mm (2¾ inches). Derek Maishman

Area: Portals

• The minimum brow for a portal in good ground (sound rock) is normally equal to the width of the decline or ramp entry. It may be reduced in steeply sloped terrain or leaving "shoulders" (instead of a vertical face) and/or by proper ground support with resin grouted rebar bolts. Various
• When slurry walls, freeze walls, or sheet piling are employed for portal entries in deep, saturated overburden, they should be placed to a depth 50% greater than the depth of the excavation to avoid uplift on the bottom. Jacobs Engineering
• The maximum practical depth for sheet piling in cohesive soils approximately 60 feet (18m). In granular soils, it is usually little more than 40 feet (12m). Jack de la Vergne
• Standard well point systems are based on suction (vacuum) lift and the practical limit for lowering the groundwater is normally about 5m (16 feet). It is typical to provide a second stage of well points to lower it further. Stang Dewatering Systems
• Well point systems employing jet eductor pumps are capable of lowering the ground water by 12 to 15m (40 to 50 feet) in one lift. Golder