Engineering Geology problems in loess deposits

Susann Jehring

TU Bergakademie Freiberg

Abstract. About five percent of earth’s surface are covered with loess. Especially

in Asia many cities are built on loess. A major problem in practice encountered in

loess deposits are structural collapses caused by loading and soaking. This process

is called hydroconsilidation. It’s a transition from open particle packing to closer

particle packing. Among others, the process depends on the kind of particle pack-

ing and the occurrence of components such as quartz, feldspar and mica. Especial-

ly clays and carbonates play an important role for the intensity of the collapse. In

civil engineering loess deposits represent a difficult group of soils and it is neces-

sary to understand the process of loess collapsing to prevent structural damage.

Under natural moisture conditions loess deposits reveal a relatively high soil bea-

ring capacity that, however, will immediately be lost at water saturation. The pro-

cess of sagging is gaining more and more in importance since people developed

new technologies for water supply.

About Loess

Definition

Loess is a fine, silty, windblown type of unconsolidated aggradation that is usually yellowish or brown in color consisting of tiny mineral particles transported by wind. These particles are finer than sand but coarser than dust or clay. The average grain size is a about 0,02 to 0,06mm. Loess deposits are usually a few meters in thickness, geologically unstable and may form fertile topsoil’s in some parts of the world. They are a product of the past glacier activity.

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Structure

Loess mainly consists of quartz, feldspar and mica grains that are angular showing little polishing or rounding. Because the grains are angular, loess often retains the shape of banks for many years without slumping. This soil characteristic called vertical cleavage causes the forming of cave dwellings is possible. However, loess is highly eroded by water, wind and seismic activity.

Distribution

Loess sediments extend over large areas in Asia, Europe, North America and parts of South America, covering about 10% of the world’s landmass, the aeolian com-ponent covering about 5%. For example in China, in the last 2.4Ma, loess has ac-cumulated creating the Loess plateau, an area of about 440.000km². Loess’ thick-nesses are commonly of the order of 50 -100m, although maximum thicknesses exceeding 300m have been recorded in the semiarid western part of the Loess pla-teau.

Locations in Europe: Poland; areas in Ukraine, Lower Austria, Upper Austria, Styria etc.

Locations in Germany: Jülicher Börde, Zülpicher Börde, Kölner Bucht, Ravensburger Hügelland, Leipziger Tieflandsbucht, Oberlausitz, Lommatzscher Pflege, Thüringer Becken etc.

Loess regions are very important for the agrarian economy. Because of the tiny particle size the soil is rich in air and minerals needed by plants are able to immi-grate into the ground. The fertile ground is also used in medicine as healing earth.

Classification

1. Loess staircases: are vertical erosion gullies, which look like staircases. 2. Loess wells: are circular irruptions (Figure 1). 3. Loess bridges: are areas of intact loess between loess wells (Figure 1).

Engineering Geology problems in loess deposits 3

Figure 1: Loess erosion

Formation

Loess was formed during the time after the last ice age when glaciers covered a huge portion of the earth’s surface. When the climate warmed up, the warm tem-peratures melted the glaciers creating tremendous flows of water running down valleys and exposing vast plains of mud. When these plains dried, strong wind moved the exposed sediments and swept the finer materials from the flood plains into the foreland, i.e. bold steep banks. If silt accumulated, higher hills were formed. Often several loess deposits are stacked on top of each other, because each individual glacier produced new loess deposits.

Characteristics of soil mechanics

Particle packing

The structure that collapse is an open packing of blade shaped particles with a middle size of about 30 µm. The ratio 8 : 5 : 2 is the mean axial ratio of the parti-cles but modifications are possible. The open packing is caused by airfall sedi-mentation and that is also why there is only a slow formation of overburden pres-sure. Grain contacts are often fixed before effective overburden pressure develops. Some of these inter-particle contacts are more important than others. Concerning the process of hydroconsolidation, these inter-particle contacts are the most impor-tant feature of the whole loess system.

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What does hydroconsolidation mean? In literature the definition is still a matter of debate because hydroconsolidation is defined in two languages by Russian and English scientists. In general, the “(…) hydroconsolidation event is essentially a change of packing from an open packing to much closer packing (…)” (Rogers et al., 1994). What causes the process of hydroconsolidation? It can postulate an ideal loess deposit consisting exclusively of a random open packing of blade-shaped quartz and feldspar particles, without clay, carbonate or other mineral components. An ideal loess like that would be rigid and dilatant and it would not collapse when loaded and soaked because the quartz-quartz point contacts are not particularly weakened by water. The typical collapse in loess occurs when con-fined contacts are modified. The ideal quartz-quartz bounding surface tends to be absent in collapsing loess. There are two important minerals responsible for modi-fications: clay and carbonate. These components co-exist in most loess deposits.

Four kinds of modification which are shown in Fig. 2 can be considered: (1) small masses of clays - a small amount of clay minerals in the system, con-

centrated at the bounding surfaces of the main structure. (2) small of amounts of carbonates - the bounding points are amplified by small

amounts of calcareous cement to produce a rigid structure, the bond strength is in-creased but it retains its short range structure.

(3) large contents of clays - the clay begins to fill the large spaces in the basic particle structure and forms a continuous clay phase.

(4) large amounts of carbonates - the carbonate fills interstices and produces large scale cementation (Rogers et al., 1994).

In Fig. 3 the relationships between these four ideal stress shearing systems are shown. The strength is plotted against the deformation. Curve 3b shows a classic short range response in which the strength of the original structure is high, but it is rigid and when it fails the strength drops rapidly. Curve 3c shows a typical long-range bond response in which clay minerals have a powerful influence on the sys-tem and deformation does not significantly reduce the strength. The small clay re-sponse (curve 3a) is perhaps the most interesting. If the amount of clay is small there is still a considerable short-range nature in the system. The clay at the con-tacts modifies less fundamental in the short range and rising deformation causes a moderate fall, but not as in the small carbonate case. The carbonate responses (curve 3b and 3d) on the diagram are not significantly different. The larger amount gives a higher initial strength and rigidity and can cope with a little deformation, but once failure occurs strength drops very rapidly.

In 1973, Handy found out that collapsing loess had low clay mineral content, and when the clay mineral content was greater the loess did not collapse. The small clay system is rigid when it is dry but when wetted bond mobility occurs. The major bond points have to move in such a way that structural collapse occurs. In the large clay system no structural collapse does occur but plastic deformation might be. The system overall has to behave initially as a short range bounded open structure, in which the initial bonds have to be fixed and rigid but lateral local range behavior is required as the clay modified bond points move under the influ-ence of water and stress. The initial short-range nature may be emphasized by some small carbonates in the system causing additional rigidity (Rogers, 1994).

Engineering Geology problems in loess deposits 5

Result: there is a large range of lateral and vertical variations of hydroconsoli-dation in a loess area.

Figure 2: Four ideal bond systems: basic structure of blade shaped primary

mineral particles, plus (a) a small amount of clay; (b) a small amount of carbonate;

(c) a large amount of clay and (c) a large amount of carbonate (Roger, 1994).

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Figure 3: Strength-deformation relationships for four ideal bond systems. Carbonate bonded systems are dominated by short-range bonds, strength drops rapidly after minor de-formation. Large clay system has long-range bonds that allow deformation (Roger, 1994).

Mechanism

The hydroconsolidation based on the transformation of an unstable to a stable par-ticle packing. The variations are triggered by interparticulare bond that will be de-stroyed if the water content increases. This transformation can be divided into four phases (Figure 4). Phase I and II represent water saturation below 15%. In the first phase the hydrosphere around the particles gets bigger, first of all the clay miner-als’ diffuse hydrosphere expands but without accumulation of free particle water. The distance between clay minerals enlarges with the result of decreasing of bound forces of clay minerals. Previously the loose of water forms meniscus within pores’ gussets. Now adhesion forces develop. The second phase includes the solution of easily dissoluble cements. Structural defects form capillary adhe-sion forces reach their maximum strength. Water saturation of 15 to 80% is typical for phase III. With increasing water content the hydration of clay minerals mi-grates into the aggregate. Now the cohesion of particles decreases and structural

Engineering Geology problems in loess deposits 7

defects amplify. The adhesion forces decrease caused by increasing saturation. But the adhesion forces are large enough to ensure the cohesion of particles. If water saturation exceeds 80%, the adhesion forces dwindle abruptly. This event causes the destabilization of the described particle fabric and the collapse starts. The process of collapsing depends on the chemical cementation and the value of sag-ging is dependent on petrographical attributes, structure and the load of buildings. (Feeser, 2001).

Figure 4: The mechanism of loess sagging.

Possibilities of experimental determination of the sagging number are oedeme-ter, triaxial test or Monte Carlo simulation.

Engineering geology protections of loess deposits

Increasing clay amount by injection

For Loess there can be used the pores injection. The method contains the packing of high permeably sediments, the advance of tonnage and strain hardening of bro-ken material. Therefore is often used bentonit suspension to increase the clay amount.

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Figure 5: Model of an injection experiment.

Different methods of ground stabilization

Mechanical compression: after compaction with a roller for a better stability sometimes it can use a dynamical intensive compaction, too.

Limestone stabilization: it is the transformation of soil structure by the reaction

between limestone and SiO2- groups, and limestone and Al2O3

- groups. Water re-

sistant limestone-, aluminates- and carbonate silicates were built. This frame will be stable by moisture penetration and even by dryness.

Thermical operations: unusually used for loess. It means heat supply until the soil dried out or burned.

But it is necessary to recognize that there is no chance to regain the loess struc-ture after collapsing. Measures of protection have to done before.

Engineering Geology problems in loess deposits 9

Presentation of the case study Lommatzsch

In the historic town of Lommatzsch located close to Meißen, Saxony, Germany, there had been brewed lager-beer since the 16th century. Storerooms, holes, cellar were built in a depth of 3,5 - 4,0m beneath the buildings. The stability against col-lapse above hollows depends on the rock and the usage. The other problem is that nobody knows were and how many of this caves exist. Today scientists believe that 85 deep cellars exist with a total length of 2km.

Background information

Lommatzsch is part of the Middle-Saxony Loess Plateau whose morphology is flat to hilly sometimes with plateau character. During the Pleistocene up to 15m strong aeolian sediments were deposited. The loess around Lommatzsch is very pure. Its color is bright yellow to yellow brownish. The loess is unlayered and composes of up to 85% silt. Clay and sand portions are rare. The limestone content amounts to 10-15%.

The town of Lommatzsch is characterized by a small ridge. The center of Lommatzsch is located at the northeast flank of the ridge where there is also a lit-tle spring trough. The town hall and the market place are located in this area and especially at this place there are lots of underground cellars. Partially there are multi-storey cellars with a depth of about 3,5 to 8m. The corridors of the cellars are often built without sustaining walls. Old or destroyed cellars often have been filled in, but usually hollows remained.

The first collapse was recognized in 1666. But since the end of the 19th century the collapses have been risen. The reason for the increasing of collapses was the construction of a central drinking water supply. Catastrophically damage at build-ings was the consequence (Figure 6). The principal cause of cellar breakdowns are defect water-pipes, as well as defect wastewater-pipes and roof drainages. The supply of waters had been managed by wells until 1894 when a new central high pressure-pipe was built. The main nutrient was cast iron. But this material is dam-ageable by deformation and stress.

Typical damages are the occurrence of hollows at the land surface, fractures at streets and buildings, and fissures and tilting of buildings demonstrating the first stage of damage.

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Figure 6: Collapses of cellars forced that struts had been necessary (Lommatzsch 1926).

Reconstruction methods

On 17th February of 1996 at three o’clock the guest of the restaurant “Goldene Sonne” heard a loud crash. Near the door they found large centimeters thick fis-sures. In the restaurant’s cellar stood yellow muddy water as well as in the neighborhood’s cellars. Outside the building was a hole with a depth of 1,5m and a diameter of 1m.

The result of the engineering exploration was that the foundation of the build-ing have been exposed to an undercutting. For reconstruction there had been in-stalled ferroconcrete-balks. For filling in the undesired cellars with insulating ma-terial there have been used 65m³ of material with a great fluxion and a high stability.

Engineering Geology problems in loess deposits 11

Figure 7: Expansion of damages caused by sacking of the underground.

Figure 8: Center of deformation at the building.

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References

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Dijkstra, T. A. (1994). The loess of north-central China: Geotechnical properties and their relations to slope stability. Engineering Geology, 36: 153-171

Dijkstra, T. A. (1995). Particle packing in loess deposits and the problem of structure col-lapse and hydroconsolidation. Engineering Geology, 40: 49-64

Feeser, V. (2001). Ingenieurgeologische Probleme bei Gründungen im Löß. Geotechnik, 24: 107-114

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