Machine excavation and material extraction
Once the stakeout is done, the machine proceeds with the excavation of as much earth as possible.
For this purpose, the total amount of earth to be excavated will be calculated by means of topographic plans and the projection of the new building. This volumetric computation is performed when there is a topographic survey of the terrain under analysis.
Contract and measurement method
The excavation of virgin earth material and the respective extraction and hauling off the ground consist of a single item within the unit price analysis.
The earth excavation contract with the contractor will then be based on the projected volume. This projected volume must be multiplied by a swelling factor according to the type of terrain to be excavated.
- 10% swelling for loose terrain
- 15% for compact sand
- 20% to 25% for clays and clays with gravel
- 30% to 40% for rocky material
- 20% to 70% for demolition debris
Many times when the projected volume is not available, the earth extraction contract is made by machine on the number of dump trucks that leave the land. However, under this model, there is no amount calculated a priori that allows a budget for the work to be made for the excavation item.
Excavation by hand
Excavation by hand is done after all the earth excavated by machine has been removed. This type of excavation (by hand) is carried out for the footings and retaining walls sector.
Although a certain percentage of footing excavation can be done by machine, care must be taken that the machine does not exceed the footing offset depth, since if this happens, the accidentally excavated soil would have to be filled in and compacted, running the risk. that the compacted soil does not remain the same as the naturally consolidated soil.
Although a certain percentage of footing excavation can be done by machine, this excavation must be fully budgeted as if it were by hand. This is because the volume that the machine will help excavate the pit where the footing will go cannot be estimated and in the end, the profiling of the hole must always be done by hand. Therefore, the worst case is adopted, which is that everything is excavated by hand.
Types of excavation by hand
Soft ground excavation
As its name says, it is the excavation of organic soil, sand, or loosely compacted clay.
Hard ground excavation
It consists of the excavation of overconsolidated clay soils or gravelly and rocky soils. This type of excavation is more expensive than the previous one.
Excavation with exhaustion
In cases where the water table is high, and when excavating water must be pumped to allow the workers to work, this pumping must be included in the excavation costs.
Excavation at high depths
There is another type of excavation at high depths where more than one person is already needed to accomplish the job, as well as equipment such as ladders, shirts or rings to support the ground, and even winch, depending on the amount of material to be extracted.
Filling and compacting
Finally, for metric computations, the filling and compaction of the earth must also be measured, referring to the voids to be filled after the foundations have been emptied.
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