Shaft collars consist of the uppermost portion of the shaft and act as a protective barrier to prevent water and soil from entering the shaft. They typically extend from ground surface down to solid bedrock, which the collar is anchored in to. They also provide a rigid support around the shaft to protect it from external loading conditions caused by both the headframe which is constructed on top of the shaft, and horizontal stresses resulting from nearby structures such as hoist housing or mills. Collar linings are constructed in a similar manner and with the same materials that are used to construct the rest of the shaft. The thickness of the lining is larger to accommodate for the additional stresses and loading which may be present.
The collar is constructed in a ‘step down’ geometry, meaning it is thickest closest to ground surface and decreases in distinct steps as it nears bedrock. A collar usually contains a maximum of three ‘steps’. The first lining step typically ranges between 1 to 1.5 m in width, but can be as high as 2m. It is recommended that the first step occur below the limit of frost penetration. The second step ranges between 0.6 – 1m (or approximately 2 times the thickness of the shaft lining) while the third has a thickness somewhere between the second step and the shaft lining thickness. The final step, known as the foot, is usually a double conical shape which acts to transfer the load of the collar to the bedrock and should be placed 3 meters below the overburden to ensure secure anchorage into bedrock. Where weak or badly fractured rock is present, it is recommended the collar extend further into the bedrock. Dimensions of the collar are important to ensure they can handle the loading which they are subject to. Some factors for consideration when determining depth, cross section and thickness include the method used to sink the shaft, overburden soil characteristics, in situ ground stresses, hydrology and additional loading conditions. Calculations of lining thickness are done according to structural needs and stresses which the collar will be subject to.
The stresses are calculated and required Factors of Safety are determined. From these values the necessary collar dimensions and material properties are obtained. As a general rule of thumb a collar must have a minimum length of 28 m for a concrete lined shaft and 15 m for a timber lined shaft.
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