Earthwork in excavation upto required depth is required for construction of foundation. It is a very important part of construction process, and care must be taken while excavation in safety perspective. Different soil layers may be encountered while excavation, dewatering may be needed sometimes. These points must be kept in mind to take necessary action during excavation and backgilling. Correct measurement of excavation and backfilling is required because excavation cost is major part of the foundation construction.
Following are the sequence of works for excavation and backfilling of soil:
Materials and Tools Used for Excavation:
The following are the equipments used for the earthwork for foundation.
Hydraulic excavator, tractor / trucks, Spade, Kassi, Pick Axe, Crow Bar, Rammer, Wedge, Boning Rod, Sledge Hammer, Basket, Iron Pan, Line and Pins, hydraulic compactor.
Drawings required for excavation:
1. Centerline Drawing
2. Layout Plan
Scope of the work:
- Setting out of corner benchmarks.
- Survey for ground levels.
- Survey for top levels
- Excavation to approved depth.
- Dressing of loose soil.
- Making up to cut off level
- Constructing dewatering wells and interconnecting trenches.
- Marking boundaries of the building.
- Constructing protection bunds and drains
Working Procedure for soil excavation:
The extent of soil and rock strata is found by making trial pits in the construction site. The excavation and depth is decided according to the following guidelines in the site.
- For Isolated footing the depth to be one and half times the width of the foundation.
- For adjacent footings with clear spacing less than twice the width (i.e.) one and half times the length.
- 1.5m in general and 3.5 m in black cotton soils.
Fig: Earthwork in Excavation by using hydraulic Excavator
Setting out or ground tracing is the process of laying down the excavation lines and center lines etc. on the ground before the excavation is started. The center line of the longest outer wall of the building is marked on the ground by stretching a string between wooden or mild steel pegs. Each peg may be projected about 25 to 50 mm form the ground level and 2m from the edge of the excavation. The boundary is marked with the lime powder. The center lines of other walls are marked perpendicular to the longer walls. A right angle can be formed by forming 3, 4 and 5 triangles. Similarly, outer lines of the foundation trench of each cross walls and are set out.
Backfilling and Removal of Excess Soil
Estimate the excavated stuff to be re-utilized in filling, gardening, preparing roads, etc. As far as possible try to carry excavation and filling simultaneously to avoid double handling. Select and stack the required material in such a place that it should not obstruct other construction activities. The excess or unwanted material should immediately be carried away and disposed off by employing any of the following methods.
Quality Checks for Excavation
- Recording initial ground level and check size of bottom.
- Disposal of unsuitable material for filling.
- Stacking suitable material for backfilling to avoid double handling.
- Strata classification approval by competent authority.
- Dressing bottom and sides of pits as per drawing with respect to centerline.
- Necessary safety measures observed.
Quality Checks for Backfilling
- Recording initial ground level
- Sample is approved for back filling.
- Necessary marking/ reference points are established for final level of backfilling.
- Back filling is being carried out in layers (15cm to 20cm).
- Required watering, compaction is done.
- Required density is achieved.
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