Excavation and Backfilling
Excavation
Excavation will be understood as the process of removing volumes of earth or other materials for the conformation of spaces where foundations, water tanks, concrete, masonry and sections corresponding to hydraulic or sanitary systems will be housed according to project plans.
Method of Excavation.
1. Manual
2. Mechanized excavation by excavators.
Before excavation you should check the-
1. Type of soil which is to be excavated.
2. Ground stability.
3. Chance of seepage of water.
4. Adjacent building or structure.
5. Underground utilities.
6. Toxic gases.
Study Of STRATA -TRIAL PIT
The stability and design of the building depends on the strata on which it rests. So, a trial pit per 400 sq.m area of the plot should be made, to know the strata pattern. Trial pit should be of the size 1.2m x 1.2m . The depth depends upon the strata that will be unearthed. Generally, during excavation the different layers of soil are found. The depth of the respective layers may vary from location to location. It's approximate maxim
Generally, during excavation the different layers of soil are found. The depth of the respective layers may vary from location to location. It's approximate maximum safe bearing capacity of various Soils.
1. Very soft. Wet,pasty clay 0 to 5.
2. Black cotton soil (dry) - 4 to 10.
3. Red soil - 10 to 30.
4. Shadu soil - 20 to 30.
5. Loose gravel 15 to 25.
6. Murum - 25 to 45.
7. Soft rock - 40 to 90.
8. Hard rock >90.
Note :- The bearing capacities given above are approximate and given on the basis of average strata.
Actual bearing capacities of the strata should be derived by
1. Static Penetration Test (SPT) - For Soil & clay
2. Plate bearing test For murum,rock etc.
General Rules for foundation excavation
For greater depths, the excavation profiles shall be widened by allowing steps of 50 cm's on either side after every 2 metres from the bottom. Alternatively, the excavation can be done so as to give slope of 1:4 (1 horizontal :4 vertical).
Trench
Any man made cut and cavity whose depth is more than the width called trench. Excavation in which depth is more than 1.5 meters and width no wider than 4.5 meter known as trench.
Important points.
1. Barricade the excavated area at 1m height and provide caution boards.
2. Don't leave open pit.
3. Provide two entry exit for excavation beyond 1m depth, distance between ladder and any point of excavation should not be more than 10m.
4. Provide safe angle access of 45°
5. Excavated soil should me 1m far from edge.
6. Vehicles movement should be far from 2m.
Excavation related Hazards.
1. CAVE-INS or collapse of the sides due to inadequate support.
2. Fall of people and vehicle into the excavation due to inadequate barriers, warning signs and illumination.
3. Water seepage.
4. Moving machinery.
5. Adjacent buildings or any structure.
6. Underground utilities such as electrical cables and pipelines.
7. Lack of oxygen and exposure to toxic fumes and gases.
8. Suffocation due to CO2 gas or other harmful gases.
Bonus 💡
Rate analysis Thumb rule for Excavation
Excavation in hard rock - 800/- per cum
Excavation in soft rock - 280/- per cum
Excavation in soil 0-3 m depth - 160/- per cum
Backfilling
Backfilling is the process of putting the soil back into a trench or foundation once excavation, and the related work has been completed.
method statement for backfilling :
1.compact each layer of earth not exceeding 200mm.
2.Earth / soil should be free of vegetation/twigs if any.
3.Rolling should be done with 5 ton mechanical roller after sprinkling enough water and core cutter samples taken for every 150sq.m
4. Proceed to next layer after achieving minimum of 95% of MDD(maximum dry density). Note: above mentioned values are only to give you rough idea how backfilling usually carried out onsite.
Bonus 💡
How to calculate No trucks required for backfilling
1. volume of soil *Compaction factor(1.30)
2.Total volume of soil including compaction factor÷Volume of a single Truck
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