What Is Subgrade Layer In Road?
What Is Subgrade Layer In Road?
A subgrade layer in a road is the layer of material (usually soil, gravel or asphalt) that lies underneath the pavement or road surface. It is the foundation upon which the entire pavement structure is built.
It provides support to the pavement and helps to distribute the load evenly across its surface. The subgrade also helps to reduce weathering and deterioration of the pavement structure.
The wearing course, or top layer, of a road may be the most visible part of a pavement, but the material underneath it, known as the subgrade, plays a crucial role in the overall success or failure of the pavement. Different subgrade materials can be used, and some are more effective than others.
What Is Subbase And Subgrade?
A subbase is a layer of material that is placed on top of the subgrade, which is the native soil or compacted improved soil that supports a structure.
A subbase is important for providing a stable foundation for the structure being built, such as a slab or pavement.
It should be uniform in nature and have solid edges and joints to prevent cracking or concrete spalling. The subgrade is the layer of native soil that is compacted to support the loads of the structure being built.
It may need special drainage structures if it is made of impermeable soil, and it should be graded to within a certain elevation range.
The terms “subbase” and “subgrade” may be used interchangeably, but generally the subgrade refers to the native soil while the subbase is the layer of soil or aggregate on top of it.
What Is A Subgrade CBR Value?
The California Bearing Ratio (CBR) test is a method used to evaluate the strength and suitability of soil and other materials for use as a base or subgrade in flexible pavement construction.
It involves measuring the resistance of a material to penetration by a standard plunger with a 3 in$^2$ surface area (or 50 mm diameter) at a constant rate of 1.25 mm/minute.
The resulting pressure at a penetration depth of 12.5 mm is compared to the bearing value of a reference material, typically crushed rock, and expressed as a ratio.
The CBR is typically determined at a depth of 2.5 mm, but in some cases, the value at 5 mm may be used instead if it is greater.
The CBR test can be conducted on either remolded or undisturbed specimens in a laboratory setting and is widely used in pavement design due to its simplicity and ability to provide reproducible results.
However, it should be noted that the CBR test is an empirical measure of strength and does not directly determine the resilient modulus of a material.
In order to determine the CBR value of a sample, a laboratory CBR apparatus is used. The apparatus consists of a mould with a diameter of 150 mm, a base plate and collar, a loading frame, and dial gauges.
The sample is soaked in water for four days and the swelling and water absorption values are recorded.
A surcharge weight is placed on top of the sample in the mould, and the assembly is placed under the plunger of the loading frame.
Load is applied to the sample at a rate of 1.25 mm/min using a standard plunger with a diameter of 50 mm.
The resulting load penetration curve is used to calculate the CBR value, which is expressed as a percentage of the actual load that caused the penetrations of 2.5 mm or 5.0 mm, compared to the standard loads of 1370 kg and 2055 kg at those respective penetrations.
If the CBR value at 2.5 mm is greater than that at 5.0 mm, the former value is adopted. If the CBR value at 5.0 mm is higher than that at 2.5 mm, the test is repeated and, if the results are similar, the higher value at 5.0 mm is reported as the CBR value.
The average CBR value from three test specimens is reported as the final CBR value for the sample.
Which Soil Is Best For Subgrade?
Highway engineers are required to identify and classify the different types of soil that can be used as materials for construction.
In India, a survey found a variety of soil types including laterite soil, moorum, desert sands, alluvial soil, and clay, including black cotton soil.
Gravel is a type of coarse material with particles smaller than 2.36 mm and little or no fines that contribute to the cohesion of the material.
Moorum is similar to gravel but has a higher content of fines and is produced from the decomposition and weathering of pavement rock.
Silts are finer than sand, brighter in color than clay, and have little cohesion. When mixed with water, they can exhibit dilatancy, a specific property where a shiny surface appears when the soil is alternately squeezed and tapped.
Clays are finer than silts and have properties such as stickiness, high strength when dry, and no dilatancy.
Black cotton soil and other expansive clays can swell and shrink. When mixed with water to form a paste and rubbed between fingers, clay will leave a stain, unlike silts.
