A variety of backfill materials are used in the construction industry. Commonly used backfill materials are described below along with their engineering properties.

1. coarse-grained soil

Coarse-grained soils include gravel and sandy soils and range from clay sands (scheduled caste) through well graded gravel of a gravel-sand mixture (Guinea worm) with little or no penalty. They will exhibit little to no plasticity. All well graded soils that fall in this category have fairly good compaction characteristics and when sufficiently compacted provide good backfill and foundation support.

(a) One difficulty that may arise with soils in this range may be achieving good compaction of poorly graded sand and gravel. These poorly graded materials may require saturation with downstream drainage and compaction with greater compaction effort to achieve sufficiently high densities. Also, where a sufficient amount of silt is present, close control of the amount of water is required. Coarse grain materials compressed to low relative densities are susceptible upon saturation to liquefaction under dynamic loads.

(b) For sand with little or no fines and sand with gravel, good compaction can be achieved in air dry or saturated condition. If saturation is used to aid in compaction, downstream drainage is needed to maintain the leakage forces in a downward direction.

It can be considered an economy of adding cement to stabilize moist clean sand which is difficult to compact especially in narrow confined areas. However, adding cement can generate areas of greater stiffness than the untreated adjacent backfill and create “hard spots” resulting in non-uniform stresses and deformations in the structure.

(C) Noncombustible materials are suitable for placement in confined areas in and around structures where heavy equipment is not permitted and under and around irregularly shaped structures such as tunnels, culverts, utilities and tanks. Clean, granular, well-graded materials for use in these areas with a maximum size of 1 inch with 95 percent No. 4 sieve and 5 percent or less No. 200 sieve pass. However, there exists the danger of creating areas where seepage water can accumulate and saturate adjacent cohesive soils resulting in undesirable consolidation or swelling. In such cases, provisions for draining the granular backfill, sealing the surface, and draining surface water away from the structure are necessary.

2. Fine grained clay of low to medium plasticity

in organic soil (chlorine) low to medium plasticity (gravel, sandy, or silty soils and loamy soils) and inorganic silts and very fine sands (ml) low plasticity (silty or smooth fine sand and clay silts) are included in this category. Inorganic clays are relatively impermeable and can be compacted fairly easily with heavy compaction equipment to provide a good stable backfill.

Soils in the CL group can be compacted to a fairly high level in confined areas with proper water content and lift thickness control. Clay sands of the SC group and silts of soils of the ML group can be compacted to fairly high densities, but close control of water content is necessary and sometimes important, especially on the wet side of the optimum water content. Feather. A few mL of soil, if stored on the dry side of the optimum, can lose considerable strength upon saturation after compaction. There can be quite a bit of compromise.

Therefore caution should be exercised in using such soil as backfill, especially below groundwater level. Furthermore, saturated ML soils are likely to be more susceptible to liquefaction when dynamically loaded. Where such soils are used as backfill in earthquake prone areas, laboratory tests should be carried out to determine their liquefaction capacity.

3. Rock

The suitability of rock as a backfill material is highly dependent on the elevation and hardness of the rock particles. The amount of hard rock excavated at most subsurface formation sites is relatively small, but selective material can be difficult to find or costly. Therefore, the excavated hard rock can be specified for crusher processing and used as a selective cohesive material.

4. shale

Although shale is commonly referred to as rock, the tendency of some shales to crack under heavy compaction equipment and when exposed to air or water after placement warrants slack deserves special attention.

(a) Some soft shells break under heavy compaction equipment, leaving the material after compaction with completely different properties than before compaction. This fact should be recognized before this type of material can be used for backfill. Establishing proper compaction criteria may require that the contractor construct a test fill and vary the water content, lift thickness and number of coverages with equipment proposed for use in the backfill operation. This type of backfill can only be used in unrestricted open areas where heavy tow or self-propelled equipment may operate.

(b) Some shales have a tendency to crack or mold when exposed to air. Other rock-like rocks will become soft or loose during excavation and will deteriorate when wetted after being applied as rock fill. Alternating cycles of wetting and drying accelerate the slaking process. The extent of material breakdown determines the manner in which it is treated as backfill material. If the material is completely eroded into constituent particles or small chips and flakes, it should be treated as a clay-like material with property characteristics similar to ML, Cl, or CH content, depending on the intact structure of the parent material. Complete degradation can be facilitated by alternately wetting, drying and discing the material prior to compaction.

5. marginal material

Marginal materials are materials that, either because of their poor compaction, cohesion, or swelling characteristics, would not ordinarily be used as backfill when a source of suitable material is available. Materials considered marginal include fine-grained clays of high plasticity and wide clays. The decision to use marginal materials should be based on economical and energy conservation considerations, which should include the cost of obtaining suitable materials, whether from distant lending areas or commercial sources, the potential distress due to the use of marginal materials, repair costs , and the additional costs involved in processing, holding and adequately compressing marginal material.

(a) Difficult to handle fine, highly plastic materials, using water-content controls, and condensation create poor backfill. In highly plastic fine-grained soils, the water content is critical for proper compaction and is very difficult to control by aeration or wetting in the field. In addition, such soils are less plastic and much more compact than coarse-grained soils; The shear strength and thus the pressure of the Earth can fluctuate between wide ranges with changes in water content; And in colder climates, frost will occur in fine-grained soil that is not properly drained. The only soil type in this category that can be considered suitable as a backfill is inorganic soil (CH). The use of CH soil in confined areas should be avoided if a high degree of compaction is required to reduce backfill settlement or to provide a high compressive modulus.

(b) Broad soil swelling (and shrinking) characteristics vary with changes in the type of clay mineral present in the soil, the percentage of that soil mineral, and the amount of water. Active clay minerals include montmorillonite, mixed-layer combinations of montmorillonite and other clay minerals, and chlorites and vermiculites under certain conditions.

Elimination of groundwater rise, seepage, seepage, or surface evaporation can cause problems that can increase or decrease the water content of compacted soils and lead to a tendency to expand or shrink. If the developed swelling is greater than the compressive pressure, fever will occur and structural distress may occur. Condensation on the wet side of the optimum moisture content will produce less magnitude of swelling and swell pressure. Extensive soils showing significant growth should not be used as backfill where the potential for structural damage may exist. Suitability should be based on laboratory swell tests.

(C) Additives, such as hydrated lime, quicklime and fly ash, can be mixed with some highly plastic clays to improve their engineering characteristics and allow the use of certain materials that would otherwise be unacceptable. Hydrated lime can also be mixed with some wide clays to reduce their swelling characteristics.

Laboratory tests should be performed to determine the amount of the additive used and the characteristics of the backfill material as a result of the use of the additive. Due to the complexity of soil-additive systems and the almost perfect empirical nature of the current state of the art, test mixes must be verified in the field by test fill.

6. commercial by-products

The use of commercial by-products, such as furnace slag or fly ash as backfill material, may be beneficial where such products are locally available and where suitable natural materials cannot be found. Fly ash has been used as a lightweight backfill behind a 25-foot-high wall and as an additive to highly plastic soils. The suitability of these materials will depend on the desirable characteristics of the backfill and the engineering characteristics of the products.

Er. Mukesh Kumar

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Er. Mukesh Kumar is Editor in Chief and Co-Funder at ProCivilEngineer.com Civil Engineering Website. Mukesh Kumar is a Bachelor in Civil Engineering From MIT. He has work experience in Highway Construction, Bridge Construction, Railway Steel Girder work, Under box culvert construction, Retaining wall construction. He was a lecturer in a Engineering college for more than 6 years.