Why soil strength increases with depth:

Soil strength is the ability of soil to resist deformation or failure under stress. It is a critical factor in determining the suitability of soil for various engineering applications, such as foundation construction and road building.

There are several reasons why soil strength typically increases with depth. These include:

• Increased consolidation: As soil particles are buried deeper, they are subjected to greater pressure from the overlying layers of soil. This pressure causes the particles to pack more tightly together, resulting in a denser and stronger soil structure.
• Reduced water content: Generally, soil water content decreases with depth. Water can act as a lubricant, reducing the friction between soil particles and making the soil more prone to deformation. Therefore, drier soils tend to be stronger than soils with higher water content.
• Changes in soil composition: Soil composition can vary significantly with depth. For instance, deeper layers may contain more clay minerals or coarser particles, both of which can contribute to increased soil strength.

Role of soil depth in the decomposition of soil organic matter:

Soil organic matter (SOM) is a crucial component of healthy soil, contributing to fertility, water retention, and soil structure. Decomposition is a natural process that breaks down SOM into simpler organic compounds, which are then released into the atmosphere or utilized by plants and microorganisms.

Soil depth plays a significant role in the rate of SOM decomposition. Deeper soils tend to have lower decomposition rates compared to shallow soils. This is due to several factors, including:

• Reduced oxygen availability: Oxygen is essential for the activity of decomposer organisms, such as bacteria and fungi. As soil depth increases, oxygen levels generally decrease, limiting the activity of decomposers and slowing down the decomposition process.
• Lower temperatures: Soil temperatures typically decrease with depth. Decomposition rates are generally faster at warmer temperatures, as enzymes produced by decomposers work more efficiently. Therefore, cooler deeper soils experience slower decomposition rates.
• Increased moisture content: Deeper soils often have higher moisture content than shallow soils. Moisture can hinder the movement of oxygen and limit the activity of decomposers, thereby slowing down decomposition.
• Greater soil density: Denser soils have fewer air pores, which can restrict the diffusion of oxygen and reduce the activity of decomposers. As a result, decomposition rates tend to be slower in denser, deeper soils.

In summary, soil strength generally increases with depth due to increased consolidation, reduced water content, and changes in soil composition. Soil depth also plays a significant role in the decomposition of soil organic matter, with deeper soils typically exhibiting slower decomposition rates due to reduced oxygen availability, lower temperatures, increased moisture content, and greater soil density.

Dr Murtadha Shukur thank you for your contribution to the discussion

The undrained shear strength of naturally deposited, consolidated and saturated clay usually increases with depth due to the overburden pressure and the decreasing water content. Undrained shear strength is not a fundamental soil parameter. It generally varies over depth as a function of insitu confining stress at a given depth. Note that larger confining stress reduces void ratio over the historical time. By the decrease of void ratio, undrained shear strength increases. The undrained cohesion of the clay is considered to be anisotropic and to increase linearly with the depth below ground surface. Bed shear stress values increase for lower flow depths and decrease for higher flow depth cases. They found that as depth of foundation increases, bearing capacity increases. They found that these are increased in dry density and decrease in optimum moisture content. They are certainly correlated but there is no simple relationship. A commonly used rule of thumb, where I live, is c′=0.1cu for over consolidated clays and zero otherwise, but reality is much more complex. The undrained shear strength (su) is defined as one-half of the peak value of (P1 - P3). For CU tests, the slower strain rates of Raymond should be retained to more closely approximate field rates of loading. The UU test provides a measure of Su at the in situ water content of the sample. At sea level, the air that surrounds us presses down on our bodies at 14.7 pounds per square inch . You don't feel it because the fluids in your body are pushing outward with the same force. Dive down into the ocean even a few feet, though, and a noticeable change occurs. Enhancing organic carbon storage below the soil surface (“sub-soil”) is an attractive option because in most soils, organic matter content decreases with increasing depth from the surface. Soil depth can greatly influence the types of plants that can grow in them. Deeper soils generally can provide more water and nutrients to plants than more shallow soils. Furthermore, most plants rely on soil for mechanical support and this is especially true for tall woody plants.Deep soils can hold more plant nutrients and water than can shallow soils with similar textures. Depth of soil and its capacity for nutrients and water frequently determine the yield from a crop, particularly annual crops that are grown with little or no irrigation.