1.Introduction

Loess is a type of windblown sediment consisting of loosely compacted silt-sized particles, with some sand and clay mixed in. It is typically yellowish-brown in color, and can be several meters thick in some areas. Loess is formed from the accumulation of sediment carried by wind from arid or semi-arid regions, and deposited in areas with little or no vegetation cover. Loess deposits are found in many parts of the world, but are particularly common in the mid-latitudes of Europe and Asia. In these regions, loess is often associated with former glacial periods and the retreat of ice sheets, which created vast areas of exposed, dry land that were susceptible to wind erosion. Loess soils have some distinctive characteristics, including high fertility and good water-holding capacity. They are often used for agriculture and horticulture, and can support a wide range of crops and plant species. However, they can also be prone to erosion and instability, particularly in areas with steep slopes or heavy rainfall. Loess is an important geological formation, and can provide valuable insights into past climate and environmental conditions. Studies of loess deposits have helped to reconstruct past climates, including temperature and precipitation patterns, and to understand how these factors have influenced landscapes and ecosystems over time. Loess deposits also contain important archaeological and paleontological records, providing clues to human and animal activity in past eras.

Collapsible soils are generally wind-blown (aeolian) deposits of silts, dune sands, and volcanic ash. Typically they are loose but stable, with contact points well cemented with a watersoluble bonding agent, so that certain conditions of load + wetting produce a collapse of the soil structure with a resulting large settlement. Loess is a special—but widely distributed—case of a silt deposit characterized by having been deposited by wind. The grain distribution of loess deposits tends to be limited to approximately the range of 0.01 to 0.10 mm and is usually contaminated with clay and sand particles (< 0.05 mm) and later by organic leachates. Nonsaturated, aged deposits are capable of standing on vertical cuts or banks due to interparticle cementation, but saturation can produce slough-offs and/or a large vertical settlement, termed soil collapse. The collapse of the soil structure may occur interior to any bank or vertical cut (i.e., is confined by K0 lateral pressures) as noted by Feda et al (1995). Loess is the predominating collapsible soil that engineers are confronted with. It is very widespread, covering about 17 percent of the United States (see Fig. 7-5)—principally adjacent to the major rivers (Mississippi and Missouri). About 15 to 17 percent of Western Europe, including parts of Belgium and France as well as portions of Germany, and Eastern Europe (Slovakia, Romania, Hungary) are covered with loess deposits. The European deposits are generally found adjacent to the major rivers, such as the Danube, the Rhine, and their larger contributories. About 10 to 15 percent of the Russian Federation (south and southwest of Moscow to the Caucasus mountains) and a part of Ukraine is covered by loessial deposits, as well as large parts of China. Very little loess is found in Canada and none in Australia or Africa (but both the latter have other wind-blown deposits), according to Flint (1971). Loess appears to have been formed from the wind picking up inert pulverized silt-sized rock particles produced by glacial action. These were carried to locales where either glacial outwash or a flowing stream produced sufficient humidity in the air that the wind-borne soil grains precipitated—usually on the eastern, or leeward sides of flowing water (at least in the United States). It follows that nonglacial areas do not have loess deposits. Depths of loess deposits range from less than 1 meter to more than 50 meters. Depths of 2 to 3 meters are

very common. Loess and other collapsible deposits are characterized by a complete absence of gravel or pebbles, with most of the material passing the No. 200 sieve (0.075 mm). The specific gravity ranges from 2.60 to 2.80, but most values lie between 2.65 and 2.72. In situ dry densities range from about 10 to 16.5 kN/m3. Atterberg limits depend on the clay and/or organic contamination, and commonly wl ranges from 25 to 55 and wp from 15 to 30 percent. Standard compaction tests (ASTM D 698) produce γ_dry on the order of 15.5 to 17.5 kN/m3 at optimum moisture contents from 12 to 20 percent according to Sheeler (1968). Loess has a high in situ porosity, often more than 50 percent (or void ratio > 1.0), and is thus highly susceptible to collapse upon saturation. Most in situ void ratios eo are in the range from 0.67 to 1.50, according to Drannikov (1967). The density of loess (and other collapsible soils) is one of the most significant parameters in estimating collapse. In the field a major problem is associated with how deep is the saturation zone that participates in the collapse. Problem recognition is considered in some detail by Clemence and Finbarr (1981). One procedure they gave suggests using a consolidation (or oedometer) test where the sample is placed in the confining ring at the in situ water content and consolidated in increments to about 200 kPa; then the ring is flooded with water and the load maintained for 24 hr. If there is a large displacement, this is an indication of a structure collapse within the sample. In general, after recognition of the collapse potential, one may use the site by doing one of the following:

1. Compact (excavate and replace) the soil to γ_dry ≥ 15.5 kN/m3.
2. Use an admixture during compaction. Admixtures may be lime, lime/fly ash, or Portland cement.

3. Use some means to ensure that the collapsible soil does not get wet (often not practical).
4. Use piles through the collapsing soils to a more competent underlying stratum.

2.Engineering Properties

Loess owes its engineering characteristics largely to the way in which it was deposited since this commonly has given it a metastable structure, in that initially the particles were loosely packed. The porosity of the structure is enhanced by the presence of fossil root-holes. The latter are lined with carbonate cement, which helps bind the grains together. This means that the initial, loosely packed structure is preserved and the carbonate cement provides some of the bonding strength of loess. However, the chief binder is usually the clay matrix. On wetting, the clay bond in many loess soils becomes soft, which can lead to the collapse of the metastable structure. The breakdown of the soil structure can occur under its own weight. Loess deposits generally consist of 50–90% particles of silt size. In fact sandy, silty and clayey loess can be distinguished. The undisturbed densities of loess may range from around 1.2–1.36 t/m3. If wetted (or reworked), the density of .collapsible loess increases, sometimes to as high as 1.6 t/m3. The liquid limit of loess averages about 30% (exceptionally liquid limits as high as 45% have been recorded), and their plasticity index ranges from about 4 to 9%, but averages 6%. Normally loess possesses a high shearing resistance and can carry high. loadings without significant settlement when natural moisture contents are low.For instance, natural moisture contents of undisturbed loess are generally around 10% and the supporting capacity of loess at this moisture content is high. However, the density of loess is the most important factor controlling its shear strength and settlement. On wetting, large settlements and low shearing resistance are encountered when the density of loess is below 1.30 t/m3, whereas if the density exceeds 1.45 t/m3 settlement is small and shearing resistance is fairly high. Loess deposits are better drained (their permeability ranges from 10–5 to 10–7 m/s) than are true silts because of the fossil root-holes. Their permeability is appreciably higher in the vertical than in the horizontal direction.

3.Identification

Loess can be identified in the field based on its physical characteristics, location, and geological context. Some common features of loess that can help with identification include:

Color: Loess is typically yellowish-brown in color, although it can also be gray, tan, or reddish in some areas.

Texture: Loess has a distinctive texture that is soft and crumbly, with no visible layers or bedding planes. It can be easily molded or shaped by hand, and often feels like flour or powdered sugar.

Location: Loess deposits are typically found in areas with little or no vegetation cover, such as on hillsides, river valleys, or coastal plains. They may be associated with former glacial periods, desert regions, or other arid or semi-arid environments.

Geological context: Loess deposits are often found in association with other types of sedimentary rocks, such as sandstone, shale, or limestone. They may also contain fossils or other evidence of past environmental conditions, such as ancient soils or buried plant material.

To confirm the identification of loess, additional tests may be conducted in a laboratory setting, such as grain size analysis, mineralogical analysis, or magnetic susceptibility measurements. However, in many cases, the physical characteristics and geological context of loess are sufficient to identify it in the field.