Earth Backfilling and Compaction

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  • Always go for approved earth material for backfilling. After soil testing, the Soil with Liquid limit > 65% and Plasticity index > 35% should not be used.
  • Clay soil is not allowed for backfilling, the reason being that, the characteristic of clay is, it shrinks when dry and expands when wet, results in development of cracks at later stage.
  • Construction debris and soil with organic material like grass, roots, shrubs and vegetation are not to be used for backfilling.
  • Backfill material shall confirm to one of the following types:

Type 1 – Granular Backfill

Sieve Size % Passing Standard Sieve
Granular Limestone
37.5 mm 100 100
25.0 mm 85 – 100 85 – 100
4.75 mm 25 – 80 25 – 80
425 mm 15 – 40 15 – 35
75 micron 8 – 18 0 – 10
Min. Crush Count 15%
Max. Los Angeles Abrasion Loss 40% 32%
Max. Shale Content 20%

Type 2 – Gravel back fill

Sieve Size % Passing Standard Sieve
37.5 mm 100
4.75 mm 25 – 80
425 µm 15 – 40 (mechanically compacted)
425 µm 5 – 10 (water jetted)
75 µm 6 – 18 (mechanically compacted)
75 µm 0 – 8 (water jetted)
  • The process of Compaction is to be done in layers of 150 mm depth. Based on the nature of soil, the compaction equipment will be selected.
Type of Compaction Equipment Cohesive soil Well graded granular soil Uniformly graded soil
Pneumatic tyred and Smooth wheeled roller Suitable Suitable Unsuitable
Vibratory roller Unsuitable Suitable Suitable
Vibratory compactor Unsuitable Suitable Suitable
  • Field Density of Soil : Earth backfilling to be done with Proper consolidation, compacted at optimum moisture content to a minimum of 95% standard proctor density or Field dry density FDD for all i) Footings up to top level, ii) Inside and Outside Plinth and iii) Below GSB level. Sand replacement method or Core cutter method is to be used to ascertain insitu field density of soil.

FDD reading for all column footings location inside and outside the building.

Weak strata of soil :

  • Sometimes, if weak strata of soil is found in excavated trenches, footings and any other foundation areas, this has to be removed completely and replaced by strong compacted earth depending upon the type of structure, loads coming over foundations etc.
  •  Advice: Actually before starting excavation work, it is very much important, that the strength and stability of soil at various depths of foundation with respect to the Natural ground level of soil is to be determined by conducting soil exploration with reputed agency. (We will discuss the same in our future article)

Earth Sliding : 

  • Caution: During excavation  in all types of soil and all weather conditions, there might be chances of earth sliding, sudden earth collapse, percolation of water in foundation trenches, seepage of water at the sides or from the bottom, all these factors not only affect the work but results in safety of men working at the location. (We will discuss this in our future article) It is the responsibility of Project manager and safety manager to anticipate and foresee the problems, take necessary safety measures and to be documented.
  • The volume of backfilling for the designated area to be estimated and plan accordingly before the actual work starts and always keep extra cushion of 25 -30 % quantity to accommodate loose earth volume to compacted earth volume.

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