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Geotechnics is the application of scientific methods and engineering principles to the acquisition, interpretation, and use of knowledge of materials of the Earth's crust and earth materials for the solution of engineering problems. It is the applied science of predicting the behavior of the Earth and its various materials towards making the Earth more habitable to human activities.

Geotechnics embraces the fields of soil mechanics and rock mechanics, and many of the engineering aspects of geology, geophysics, hydrology, and related sciences. Geotechnics is practiced by both engineering geologists and geotechnical engineers.

Examples of the application of geotechnics include: the prediction, prevention or mitigation of damage caused by natural hazards such as avalanches, mud flows, landslides, rockslides, sinkholes, and volcanic eruptions; the application of soil, rock and groundwater mechanics to the design and predicted performance of earthen structures such as dams; the design and performance prediction of the foundations of bridges, buildings, and other man-made structures in terms of the underlying soil and/or rock; and flood control and prediction.

Foundations
Geotechnics helps to determine subsurface conditions of a building site. The first step is to explore the different strata of soil and rocks underneath the site. Laboratory testing is used to determine the engineering properties of soil and rock samples. The presence of groundwater is important. It will alter the strength properties of soil. All of this information can be found using a test boring.

The presence of rocks will determine the type of equipment used during a test boring. The distinction between residual soils and transported soils is important. Residual soils will gradually phase into unweathered rocks. Boulders can be encountered in the middle of the chosen site. Transported soils, however, will vary widely. They can be fine-grained, loess-type soils deposited by the wind. They may be alluvial if found near a river or lake. If the soil was transported by water, the distribution will be similar to that of residual soils. If the soil was transported by a glacier, however, the distribution will vary. Large boulders can be found. When rock is encountered, it has to be cored to a minimum depth of 3 meters. The sample is used to examine the character of the rock. It will also ensure that the drill hole has penetrated bedrock and not boulder.

Geotechnical engineers use the subsurface information collected ensure a detailed design of the foundation structure.

Landslides
Planning for a landslide requires knowledge of earth movement on slopes. The weight of a segment of soil tends to move downward on a slope (a hillside or an excavation) due to gravity. However, naturally it does not move because of the soil underneath it. This resisting force is known as shear strength of soil. The slope will be stable if the resistant force is greater than the soil’s tendency to move downward. If this is not the case, a landslide will occur. A special case occurs when the resisting force is the same as the downward force. The slope will be in a state of unstable equilibrium. Any slight disturbance will cause a mass of soil to start moving.

Sinkholes
A sinkhole occurs in regions of the world where limestone or dolomite is the local bedrock. Natural sinkholes occur irregularly. The presence of sinkholes is therefore difficult to ensure. Preliminary-site studies can help to identify them. Another solution is to sink a test hole under every important column or load-bearing member.

Floods
Floods can occur for different reasons. One cause is heavy rainfall on paved streets and impermeable roofs. A second cause can be a river or lake that overflows. In the case of coastal areas, floods will occur due to tidal and wind conditions. One way to control floods is to construct strategically located dams. These will hold water in time of flood. Dams will release water into rivers during periods of low water. Geotechnics will be important for the design of the foundation and the impermeability of soil.

An important aspect of flooding within urban areas is floodplains. In times of flood, rivers will overflow into floodplains. Using floodplains for buildings should be avoided. The subsurface of floodplains consists of low-strength soil. In addition, they have high groundwater levels that make the construction of foundations difficult.

Building on Peat and Organic Soils
Peat is an extremely soft, organic soil that is unsuitable as a building site. Peat is considered unstable because localized sinking, slip failures, and widespread settling occur even under moderate loads. In addition, the properties of peat are unreliable. They can vary widely with time. For example, the mechanical properties, such as compressibility, shear strength, and hydraulic conductivity, will change as peat decomposes. Engineers try to avoid building on peat but an alternative does not always exist. In such cases, engineers will use different methods to reduce the challenges of construction. Complementary flood-control measures include better agricultural practices such as contour plowing on sloping land. Soil conservation measures are also used. They can delay runoff of heavy rains and allow it to filter into the ground.

Excavation
One method is to replace peat by excavating peat or by placing imported fill materials on the excavated soil. The excavated soil has to be discarded in an environmentally acceptable place. The excavation distance must be at an economically acceptable distance from the excavation site. This method is only effective up to 5-6 meters.

Surface Reinforcement
Ground can be reinforced by improving engineering properties of soil, such as shear strength, compressibility, stiffness or sustainability. Geotextiles and geogrids can be placed above peat to separate it from granular embankment fill. This prevents the fill material to mix with the peat. As a result, the load-bearing capacities of the fill will be preserved. Geotextiles can also be used to increase tensile reinforcement. Geotextiles will be needed when the layer of fill becomes thicker. Tensile reinforcement will prevent the soil from failing by lateral spreading.

Preloading
Preloading works by placing a thick layer of temporary fill over the final design fill. When loaded, the temporary layer will settle as much as the final design layer will over its lifetime. Then the thickness of the preloaded fill will be the same as the final design thickness. Calculation for rate of settlement in peat and organic material is difficult. Peat and organic soils typically have large secondary compressions. Therefore, the rate of settlement of the final fill layer has to be monitored after construction.