Excavation work is the first activity on any construction site. It starts from digging the pit for the structure for either shallow or for deep foundations. It is completed by filling excavated soil or soil brought from outside in the same pit. Construction excavation depends on the size and depth of the foundation, types of soil layers, water table and surrounding structures, etc.
The depth of soil layers may vary from location to location as different types of soil are there.
The safe bearing capacity of soil strata is the important factor for foundation design consideration. It is directly related to structure stability. We need to excavate soil pit up to designed foundation depth so where SBC is available for safety and stability of the structure.
Many accidents happen daily on a/c of sliding of earth or caving in of earth. Such accidents are almost fatal, as the worker generally gets buried and will lose the life on a/c of suffocation.
Further, when you excavate adjacent to existing building, the safety of the existing structure is most important. This is more severe when we have to excavate deeper than foundation and foundation of existing building.
While carrying out the excavation, it is therefore necessary to take care of quality and safety of the earthwork. Always remember that precaution is better than cure. Hence let’s discuss precautions while carrying out the foundation excavation.
01. Things to Keep in Mind Before Excavation Work:
- Clear the area of the site by removing obstructed trees, vegetation and rubbish for building layout. Get permission from government authorities to cut trees before starting the works.
- To overcome these difficulties, shift the layout of the building if possible.
- Get plan approval from the authorities as it is a time-consuming job and may delay the work.
- Check and if needed relocate underground drainage, electrical, and telephone cables crossing over the proposed building and its foundations.
- Make provision for the stacking of excavated soil.
02. Precautions while Excavation:
- Arrange all the materials near the site for successful completion of work. Stack lime powder, shoring materials, blasting powder, blasting equipment and excavator, etc. nearby the site.
- Arrange for necessary equipment like theodolite/total station, level machine and measure tap, etc. Use superior equipment for the layout of the planned building.
Arrange for excavator, dumper, dozer, grader and roller, etc. for handling, transporting and compacting the excavated soil.
- Do the layout of the building with respect to drawing. Make centre line pegs on all around the periphery, and check all centre line before you commence excavation.
- The level of the ground should be taken by theodolite or dumpy level machine before staring and after finishing the work. It should also be recorded for billing purpose.
- Mark the benchmark level till completion of the site. Always follow all level including depth of excavation with respect to reference RL provided with some temporary or permanent bench mark.
- Estimate the excavated stuff to be re-utilized in filling. Try to carry away excavation and filling simultaneously, to avoid double handling.
- Stack separately the excavated earth, which you are going to use for refilling and the one which you are going to dispose of outside.
- Provide ramps or steps for lifting the excavated materials during excavation.
- The size of the pit for excavation should be at least of 300 cm more than the size of foundation PCC, on all sides or as per contract. Increase the pit size with increase in depth of foundation accordingly. Otherwise, top layers would collapse/cave in.
- Keep the excavated materials at least 1 meter away from the edges of the pit.
- Never create high heaps of excavated earth just on the face of pits. This may collapse. Don’t allow the black cotton soil to remain at the site for the longer time as it will start collapsing.
- In black cotton soils or other expansive soil, as the depth increase, the sides will collapse. To prevent this, do shoring or excavate such a large pit in slopes or steps with back filling method.
- It is necessary to go below the level where cracks cease appear particularly in the cease of the expansive black cotton soil.
- Never use black cotton soil in re-filling. Avoid excavation of black cotton soil in rainy days.
- Wet the pit of soft/ hard murum with water a day before the excavation. The strata absorb water and become relatively soft, making the excavation easy.
- The pit size may be exactly of the size to suit the form of work of PCC.
- Carry out excavation in soft/hard rock with skilled labour. Adopt chiselling and hammering for soft/hard rock excavation.
- Adopt wedging by crow-bar and pneumatic breakers to excavate the soft/hard rock.
- Do the blasting for hard rock excavation. It breaks the rock quickly and at a reasonable cost. One may use pneumatic breaker installed on excavator.
- Select and stack the required materials, so that other activities are not obstructed. Immediately remove excess or unwanted excavated materials from the site.
- Excavation should be planned in such way that the movement of vehicle is never obstructed for bringing materials and transporting earth.
- If the excavation goes below the ground water table, Make adequate arrangement for dewatering.
- Keep the dewatering pump on a firm foundation and shift carefully, whenever required.
- Use the dewatering pump of a higher capacity with high suction and long delivery hose pipe.
- Put the dewatering pump in corner sump which is 150 mm down at the level of excavated surface to pumped water easily.
- Diverting the water is better than excavation in flowing water.
- Create artificial trench to divert the flowing water. Construct the coffer-dam using sand bags to divert the flowing water.
- Check the depth of excavation periodically to avoid over excavation. It is necessary to obtain top 150 mm excavated level by manual leveling and dressing with light compaction.
- Excavated levels should be free from excessive moisture content and carry out compaction test if necessary.
- In the case of the deeper excavation than required level, fill the extra depth by using approved filling materials (sand) with proper compaction or 1:5:10 lean concrete to save the cost of column or brick wall.
- Compact the bottom of the required foundation level surface to achieve required safe bearing capacity. Carry out anti-termite treatment before start building foundation work. Do not excavate more than prescribed in the drawing, you might not get payment for it.
- Never excavate below the level of the foundation of existing building. If must, do it under close supervision and guidance of your experienced contractor/engineer, by adequately supporting and propping adjacent building.
- Consult your structural designer or geotechnical consultant if the stratum of soil on the site is different than the soil test report/ soil bore log.
03. Safety Precaution while Excavation Work:
- Prepare firm and broad approach road. Avoid work in rainy season.
- The nail should not be lying around, after the completion of layout work.
- Barricade the site to restrict the entry of animal and the unauthorised person on site.
- Ensure that workers are wearing personal protective equipment. Safety belt, helmet, rubber hand gloves, goggles, facemask and rubber shoes are of ISI mark.
- Provide ladder to Excavation workers to climb in and out of the pit.
- First-Aid kits should be immediately available on site.
- Provide proper ventilation and electric lights at a time of work. Provide barricading to excavated pit. Check the shoring for adequate support.
- Check the material transporting machine for safe operation. The movement of the trucks should be away from the pit. Machine operator must have recognised agency’s license.
- Do the sufficient pit size by providing extra working space on all sides. Cleaning of area and housekeeping of material is necessary.