Typical values of subgrade modulus range between 100-400 pci for coarse grained soils and 25-150 pci for fine grained soils. A subbase—the layer of granular material placed on top of prepared subgrade—is not mandatory for floors on ground.The subgrade reaction coefficient (ks) is the soil stress required to make unit settlement on a soil column in a unit cross- section area. When the settlement amount is d, the vertical stress acting on the soil column in the unit cross-section area is given in (8) (Tezcan et al., 2010).Subgrade strength is mostly expressed in terms of CBR, the California Bearing Ratio. Hence, in all, the pavement and the subgrade together must sustain the traffic volume. The subgrade strength owing to its inconsistency or variable nature poses a challenge for the engineer to come up with a perfect design of pavement.
What is the subgrade strength : strength of the subgrade is a basic factor in determining the thickness of pavement and is evaluated by means of CBR tests. Strength of subgrade corresponding to its CBR value specified by LGED is presented in Table 1.
How is subgrade modulus calculated
The modulus of subgrade reaction is a conceptual relationship between applied pressure and deflection for a plate resting on an elastic support system. The defining equation is k=q/δ where q=pressure, δ=deflection and k is known as the modulus of subgrade reaction (units F/L3) –sometimes called Winkler spring constant.
What is the modulus of subgrade soil : The subgrade modulus is a function of the soil stiffness and compressible layer thickness, as well as the foundation dimensions and stiffness. The subgrade modulus is not constant underneath a foundation.
The defining equation is k=q/δ where q=pressure, δ=deflection and k is known as the modulus of subgrade reaction (units F/L3) –sometimes called Winkler spring constant. In the field, the k-value is determined using data obtained from a plate loading test (typically 1ft by 1ft square).
The minimum CBR value of a sub-base soil should be 20% and 30% for traffic up to 2 msa and traffic exceeding 2 msa respectively. Also, the minimum CBR value for a subgrade soil is usually 6%, although subgrade characterization recommends 10% [4] .
What is the compaction value for subgrade
Generally, adequate density is specified as a relative density for the top 150 mm (6 inches) of subgrade of not less than 95 percent of maximum density determined in the laboratory. In fill areas, subgrade below the top 150 mm (6 inches) is often considered adequate if it is compacted to 90 percent relative density.It can be determined through plate bearing capacity test or standard penetration tests. Even for the same type of soil, the coefficient of subgrade reaction can vary depending on the size, shape, depth of embedment, and direction of the applied load on the foundation.Modulus of Subgrade Reaction (Ks) This term is measured and expressed as load intensity per unit of displacement. For the English unit system, it is often expressed in kip/in2/in; in the SI system it is expressed as kN/m2/m.
The Subgrade Modulus, also known as the Modulus of Subgrade Reaction, is a stiffness parameter typically used in defining the support conditions of footings and mat foundations, such as that shown on Figure 1. The parameter is expressed in units of [Force]/[Length]3.
How do you calculate subgrade modulus of soil : k = 40 x SF x qa, where SF is factor of safety considered in bearing pressure calculation and qa is allowable soil bearing capacity. So, you could employ the above equation in determining the Soil Subgrade Modulus.
How do you test subgrade modulus : This test involves the application of pressure on a steel bearing plate into the ground surface measured with a hydraulic jack and the corresponding surface settlement recorded from dial micrometres on the plate surface, and the modulus of subgrade reaction determined using equation 1.
How do you calculate subgrade CBR
C.B.R. = (PT/PS) X 100
The C.B.R. values are usually calculated for penetration of 2.5 mm and 5 mm. Generally the C.B.R. value at 2.5 mm will be greater than at 5 mm and in such a case/the former shall be taken as C.B.R. for design purpose.
tilled farmland
E.g. CBR = 3 → tilled farmland CBR = 4.75 → turf or moist clay CBR = 10 → moist sand CBR = >80 → hard quality crushed rock Page 3 What is CBR • CBR is a measure of resistance of material to penetration of a plunger under controlled density and moisture conditions.The laboratory CBR test consists essentially of preparing a sample of soil in a cylindrical steel mould and then forcing a cylindrical steel plunger, of nominal diameter 50 mm, into the sample at a controlled rate, whilst measuring the force required to penetrate the sample.
Why is 95% compaction required : It's important to incorporate moisture because the right amount provides particles the ability to 'stick' to each other under pressure. A result of 95% is what is considered best practice; which means the soil density must be within 5% of its maximum achievable compaction.