For example, excavation of foundation trenches using a backhoe on a wheeled tractor unit will not give accurate alignment of the trench, and the teeth on the bucket will disturb the bearing surface of the trench. Further, it is not possible to dig to a precise level and depth with sufficient accuracy. The backhoe will therefore take out the bulk of the excavated material - say 90 per cent and the trench will be trimmed to level for the foundation by hand. Hand trimming to width may also be required where there is extensive timbering.
Hand excavation may also be necessary for small isolated pits for pads and manholes, excavations for foundations to entrance steps, and so on, either because it is uneconomical to bring a machine to the work or because access is restricted. In the latter instance one could also consider work inside an existing building or in a back garden or enclosed yard or area.
On the subject of trimming excavations by hand, it should be noted that no matter how the excavation is done, the smaller and more confined types of excavation have a higher proportion of trimming to volume of excavated material. Generally in the past this has been taken into account by using lower outputs per hour when calculating the overall rate, which is based purely on volume. However, SMM7 and previous editions have included provision for the preparation of excavations to receive concrete foundations.
Depth and throw
Excavating by hand - even only the trimming of machine-excavated trenches, basements or whatever - brings with it the problem of reconciling the depth stages in SMM7, which are set to suit machines, with the lift and throw achievable by a man with a shovel. The norm for hole depth for man and shovel is 5 ft, which is translated loosely into 1.5 m. This is generally held to be shoulder height. Figure and the following paragraphs explain the reduction in outputs per hour with increasing depth.
Refer first to situation A in Figure The man in position 1 has dug out the earth and has piled it at the side of the excavation. As he has gone down, the pile of spoil at the side has risen, thus increasing further the required throw. It is for this latter reason as much as depth that a limit of 5 ft was set for depth.
Then refer to situation B. The man in position 1 continues to dig down but can no longer throw the spoil clear of the excavation and maintain a reasonable output. Reasonable output is the important point at this transition between depth stages. Of course he could throw clear if the excavation were to stop at say 1.65 m, but the effort required to clear the spoil from the excavation over the growing pile at the surface would slow him down, and that would be unacceptable were the digging to continue for even greater depths. So to keep output up, the man erects a platform or staging 1.5 m down (position 2) and puts the spoil on the platform as he continues to dig down to 3 m. Either he or a second man works off the platform shoveling the now loosened spoil up on to the side of the excavation. We have already stated that the pile of spoil at the side of the trench was making it difficult for the man excavating up to 1.5 m down, so it must also be increasingly difficult for the man shoveling from the platform. What we require to have is the loose spoil cleared back from the side of the excavation (position 3). Again this can be the one man or it could be a second man alternating between staging and starting level.
Finally, situation C shows the saga continuing down to 4.5 m with a second staging, and the need to man both stagings and the surface to clear away.
The man at position 3 can of course do a number of operations. He can merely shovel the spoil back to make room for more and make ready for back filling in the case of pipe
trenches. He might also segregate the spoil according to the different strata being encountered, some being suitable for back fill round pipes; vegetable soil might be kept separate, and the remainder used as general back fill after a certain level. He might load everything into wheelbarrows, dumper trucks, skips or hutches.
Assuming the man at position 3 is clearing back ready for back filling, then typical outputs are as shown in Figure The man getting material out from the bottom takes almost twice as long to handle one cubic meter as those shoveling off the stagings or clearing back at the surface. No matter how many men are required, what the squad size is or what the disposition of the workers can be, the times taken to get out one cubic meter of spoil can be calculated as shown in Table The outputs used in Table are averaged from Table and assume ordinary soil conditions as defined in Table
The hard part about all of this is to reconcile these ergonomically selected depth stages with those given for machine excavation in SMM7. Figure 5.2 has these depths shown as dashed lines across the illustration.
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