General Concepts 1. Why is shaft support necessary in underground excavations? - Shafts are subjected to stresses from pressure readjustment in the surrounding rock, which can lead to weakening, failure, and collapse. Artificial supports hold the sides of the shaft open to ensure safety during mining operations. 2. What are the main types of shaft supports? - Timber, steel, or pre-cast concrete frames or sets. - Concrete rings poured in place at intervals (monolithic pour). - Continuous linings made of materials like brick, stone, cast-iron (C-I) Tubbing, or concrete poured in place. Factors Influencing Support Choice 3. What factors influence the choice of shaft support? - Nature of the ground and water conditions. - Shape of the shaft (e.g., circular or rectangular). - Cost and availability of materials (e.g., timber may be cheaper in timber-abundant regions). 4. Why is timber support not always the best choice? - Timber can rot if not seasoned properly and is a fire hazard. - It is costly and less durable compared to alternatives like steel or concrete. 5. What are the advantages of concrete supports? - They are fire-resistant, durable, and suitable for shafts with long life spans. - Concrete supports can fit any shaft section and are effective for water sealing and rehabilitation. Design and Construction 6. What considerations are essential when designing shaft supports? - Lining must withstand impacts, such as falling cages or skips in case of rope failure. - Allowances for bad ground or excessive water that may increase wall pressure. - Anchorage methods like wedging cribs or bearer sets provide stability. 7. How is concrete lining applied in a shaft? - It can be poured in place as monoliths, continuous lining, or in sections like slabs or blocks. It is often keyed into the shaft walls for added strength. Support Types 8. What preservation methods are used for steel supports? - Painting with a mixture of tar, kerosene, and cement. - Guniting, where the steel is sprayed with concrete after cleaning. - Filling troughs of H- and I-beams with concrete to prevent rusting. 9. What are the advantages of concrete-lined shafts compared to other supports? - Provide the greatest ground strength and best airflow characteristics. - Lowest maintenance requirements and easy adaptability for future purposes like ventilation. 10. What is the purpose of Tubbing in shaft lining? - Used in circular shafts in wet rock formations to exclude water without requiring pumping. - The process involves bolting Tubbing segments and grouting the space between the lining and rock wall with cement mortar. 1. What is the purpose of sinking in unstable ground? - The aim is to excavate and line the shaft while preventing the sides of the excavation from caving in. 2. What are some methods available for sinking in unstable ground? - Wood piling and steel piling (up to 23 m). - Drop shafts or open caisson (up to 50 m). - Pneumatic caisson (more than 30 m and up to 100 m). - Forced drop shafts (up to 60 m). - Freezing methods (stages of 60 to 90 m). - Cementation (grouting methods). Sheet Piling 3. How is wood piling used in shaft sinking? - Wood piles, typically made of planks tapered at the bottom, are driven vertically around the shaft sides and braced horizontally to stabilize soft or sandy ground. They are temporary and suitable for dry ground but cannot withstand water. 4. What are steel sheet piles, and what makes them effective? - Steel sheet piles interlock, forming a shield against wet running ground and quicksand. They are driven to bedrock or impermeable strata before excavation begins, providing both temporary and permanent solutions. 5. How can steel sheet piles be salvaged for reuse? - After the shaft lining, such as concrete, is installed, steel sheet piles can be pulled out and reused. Drop Shafts and Open Caisson 6. What is an open caisson? - It is a prefabricated reinforced concrete shaft with a steel shoe at the bottom to prevent damage while sinking into the ground. 7. What factors affect friction in open caisson sinking? - Friction varies by material: least for silt, then greater for sand/gravel, clay, and boulders/rock. Friction can be minimized by thick walls, adequate weight, or raising the water level in the shaft above the surrounding groundwater. 8. What are the main challenges with open caisson sinking? - Keeping the shaft vertical, applying enough pressure to force the shoe down, and addressing friction. Pneumatic Drop Shaft 9. **When is the pneumatic drop shaft method appropriate? - When there’s danger of soft ground inflow, numerous boulders below the water level, or difficulty sealing the shaft lining to bedrock. 10. What is unique about the pneumatic drop shaft method? - Compressed air is used below an airtight deck to drive out water, allowing workers to excavate the shaft bottom safely. Certainly! Here are some questions along with their answers based on the key concepts mentioned in the text: Pneumatic Drop Shaft 1. Under what conditions is the pneumatic drop shaft method used? - It is used when: - There is a danger of inflow of soft ground. - There are many boulders below the water level. - Sealing between shaft lining and bedrock is difficult. 2. What is the role of the airtight deck in pneumatic drop shafts? - The airtight deck allows the space below it to be filled with compressed air, driving out water and enabling workers to excavate safely at the shaft bottom. 3. How is access provided to the working chamber in pneumatic drop shafts? - A cylindrical steel shaft, bolted to the deck and extending above groundwater level, provides access. It includes ladder rungs for ascent and descent and an airlock mechanism to prevent air loss. 4. How does the airlock mechanism work in pneumatic drop shafts? - It has two doors. Compressed air is admitted or released to equalize pressure before men or equipment move between the airlock and working chamber. Forced Drop Shaft (Honigmann Method) 5. What is the forced drop shaft method, and when is it applied? - It is a variation of the drop shaft method for unstable sand, gravel, and clay where manual or pneumatic methods are inadequate. Hydraulic jacks and steel rings are used to aid sinking. Freezing Methods 6. What is the purpose of freezing methods in shaft sinking? - Freezing solidifies water-bearing ground, allowing shafts to be safely excavated through soft ground, fissured rock, or very deep strata. 7. How is freezing achieved in freezing methods? - Vertical holes are drilled around the shaft perimeter, and pairs of concentric pipes are installed. Chilled brine (e.g., magnesium or calcium chloride) circulates through the pipes, forming an ice wall. 8. Why is magnesium chloride (MgCl₂ ) preferred over calcium chloride (CaCl₂ )? - MgCl₂ has fewer tendencies to crystallize at low temperatures, reducing the risk of clogging pipes. 9. What challenges are associated with freezing methods? - It is difficult to keep drill holes vertical in deep shafts, and any deviation can cause incomplete ice walls, requiring precise drilling and monitoring. 10. What methods are used to thaw the frozen ground after shaft sinking? The ground may be thawed by: - Filling the shaft section with water. Gradually increasing the brine temperature and circulating it. Aeration of the shaft. Cementation and Grouting 11.What is the principle of the cementation and grouting method? - It involves filling fissures and voids in the ground with cement, solidifying them to prevent water inflow. 12. Where is cementation most effective? - It is effective in fissured rock but not in quicksand, where boreholes cannot remain open. 13. How is cementation carried out in deep shafts? - Cementation is done in stages. Holes are drilled at the shaft bottom, filled with grout under high pressure, and allowed to set. The process is repeated until all fissures are sealed. 14. What are the advantages of cementation? - It is the most economical method for excavating in wet ground and is effective in tunnelling and drifting where excessive water is present.
