Subgrade is the natural material beneath a manmade road, pavement, or railway track in transportation engineering (US: railroad track). It's also known as the forming level. The phrase can also apply to imported material used to construct an embankment. The word comes from the French term sous-grède, which means "underground."
The main purpose of a subgrade is to provide stability for the base of any structure such as a building or tanking vehicle. The subgrade should be firm and stable before it is used as the base for other structures. If necessary, the subgrade can be reinforced with wire cables, metal rods, or concrete.
The quality of the subgrade has a major impact on the cost and durability of any structure placed on it. For example, if a road is made from gravel, instead of asphalt or concrete, it will last longer but it won't be as smooth or even as wide. However, if a road is made from dirt, instead of gravel, it will be more stable but it won't last as long. The choice of subgrade affects the overall cost and quality of the finished product.
There are two types of subgrades: temporary and permanent.
Temporary subgrades are those that are constructed for a specific project.
The subgrade is the dirt on which the pavement is created in concrete paving language. It is usually addressed by compacting it enough to keep the pavement from settling. The layer(s) of material between the subgrade and the concrete pavement are referred to as the subbase. As with any other base material, the subgrade must be fine grained and stable before it can support the weight of the overlying pavement layer(s). The subgrade should be consistent throughout the project area so that once the concrete is placed, it will not be necessary to remove or fill it in.
The subgrade affects the cost of a project significantly. If it has to be excavated and replaced then this will add to the expense. It also affects the time required for construction. If the subgrade is poor quality or contains excessive amounts of moisture or some other problem then this will add to the length of time needed for completion.
The main purpose of the subgrade is to provide a stable base upon which to build the finished road surface. It does this by absorbing shock waves caused by vehicles driving over it and by preventing water from flowing under the roadway where it can cause erosion or create traffic problems such as potholes. The subgrade should be fine-grained, stable soil that can support the weight of the asphalt pavement that is to come. If the subgrade is too soft or unstable, it will affect the way the pavement responds after it is laid down.
The distinction between embankment and subgrade as nouns is that an embankment is a lengthy artificial hill of earth and stone created to keep back water for protection or to support a road, whereas a subgrade is the layer of natural soil on which a road's foundation is put. The term "subgrade" can also refer to the entire roadway, including its surface and undercarriage.
An embankment or a ramp is used to provide a level crossing over a railway or other means of transport. These structures should be built so that they do not damage the railings or other safety features at the crossing. They may be made from concrete, asphalt, rock, or any other suitable material. An embankment is usually longer than it is high while a ramp is generally sloped towards its top. When constructing an embankment or ramp, care should be taken not to cause erosion near homes or roads.
An earthen bank is a short embankment, usually no more than 3 meters (10 feet) high and 10 meters (33 feet) wide. Earthen banks are used to protect buildings and roads from floods, but they can also be used to create ponds or other decorative features. Earthen banks are often painted white or colored for aesthetic purposes.
A stone wall is a series of stones set in mortar or cement.
Subgrade stabilization is the process of digging for and installing a more appropriate material in place of the old unsuitable material that was removed, typically in subgrade sites found beneath pavement. The most common example of this is the removal of soil from around building foundations to expose clean stone or concrete, which is then used as paving or flooring. Less commonly, old wiring, piping, and other utilities may be removed from site and replaced with metal objects where feasible or otherwise disposed of.
Soil consists of different types of particles held together by glue-like substances called ligaments. Subgrade stabilization involves removing the top layer of soil (the surficial zone) at a depth of about 1 foot. Below this layer is the subsurface zone, which includes any unstable material such as sand or gravel. Underneath the subsurface zone is stable material such as bedrock or clay. Removing soil from around buildings promotes new plant growth and helps prevent future flooding. The new surface can be made of asphalt or concrete, while the space underneath is filled with stable material such as dirt or rock. This prevents water from accumulating in low-lying areas away from houses and allows it to drain away when it does rain, thereby preventing flooding.
Installing subgrade materials is called undercutting.