Transported by Water Almost any flowing water will transport particles of soil. The particles originate as rain hits the earth and are deposited when the velocity of flow is reduced or when particles of soil form flocs that settle.
Boulders are pushed along in mountainous areas where stream beds are steep and the velocity of the water is very high. The power of flowing water can be illustrated by Figure 2.1, which shows the erosion of limestone along a
Figure 2.1 Erosion of limestone along a stream in Texas.
stream in Texas. At the other end of the scale, rivers transport particles of clay into bodies of water, lakes or the sea, where flocculation occurs and the flocs gain enough weight to fall to the lake bed or sea floor.
Thick beds of clay, with some particles of silt size, can be found on the ocean floor at many locations. An example is at the mouth of the Mississippi River. Year after year, thin layers of clay are deposited as floods along the river erode particles of soil. The weight of the soil above a particular depth causes drainage or consolidation of the clay at that depth. If deposition has ceased at a particular location, the clay will reach a state of equilibrium after a period of time with no outward flow of interstitial water, and the stratum of clay will become normally consolidated. The shear strength of the clay will be virtually zero at the surface and will increase almost linearly with depth.
If deposition continues, the underlying clay is underconsolidated, with drainage continuing. Careful sampling and testing of the clay will reveal its nature, and the engineer will consider the detailed character of the stratum of clay in designs.
Particles of sand and gravel are carried along by flowing water and are deposited with a drop in the velocity of the water. Such deposits occur near the beds of streams that exist now or did so in the past. Such deposits are usually quite variable because of the erratic nature of stream flow. The relative density of such deposits also varies significantly, depending on their historical nature. Fine sand and silt are deposited in bodies of water as the velocity of a stream in flood pushes the suspension into a lake or ocean. Deposits of sand and silt can be found in the deltas of rivers, such as near the mouth of the Mississippi, and sometimes contain decayed vegetation.
Nature creates complex patterns, and the near-surface geology rarely can be known with absolute certainty. Stream flow in the past, when the ground surface was uplifted, incised valleys that, after submergence, became filled with soft sediments. Such examples exist in the Gulf of Mexico near the coast of Texas. Offshore borings may reveal a regular pattern of soil, but such a geologic event may result in an unpleasant surprise to builders of pilesupported structures.
Transported by Wind Over geologic history, wind has played an important role in creating soil. In the deserts in some parts of the world, sand is prominent in the form of dunes, and the dunes may be in continuous motion.
Rainfall is small to nonexistent, and construction is limited. If construction is necessary, the engineer faces the problem of stabilizing the wind-blown sand. Other wind-blown soil is more amenable to construction.
Dust storms in the Midwest of the United States in the first half of the 20th century transported huge quantities of fine-grained soil. Over long periods of time, deposits of great thickness were laid down. Such a soil is loess, found in the United States, in Eastern Europe, and in other places. Vegetation grew during deposition, and deposits of considerable thickness of vertically reinforced soils were developed. Loess will stand almost vertically when deep
cuts are made.
If a sample of loess is placed under load in a laboratory device and water is added, the sample will undergo a significant reduction in thickness or will ‘‘collapse.’’ Loess is termed a collapsible soil. Drainage is a primary concern if shallow foundations are employed. Deep foundations will normally penetrate the stratum of loess.
