Herein are presented the results of laboratory and field tests which were carried out on loess in Yugoslavia.
Experience gained during recent decades show that the loess soil in some cases undergoes structural collapse and subsidence due to inundation. In order to find the explanation of such behaviour, numerous laboratory and field load tests on loess soil have been performed. Using the obtained results, several correlations have been established.
On the basis of the unconfined test results, a correlation between the initial dry density, initial water content and unconfined compression strength has been established.
A relatively large number of the consolidation subsidence tests (about 550), carried out on undisturbed loess samples, made possible the determination of the degree of subsidence and the corresponding values of the dimensionless coefficients of subsidence. These coefficients have been determined for several values of the initial dry density, for various degrees of saturation and for several stress levels upon wetting. Using these coefficients, the values of the additional settlement caused by flooding of loess under the foundation can easily be calculated. It is of particular interest to note that, in general, water penetrates under one part of the building, producing differential settlements, which are in most cases very dangerous.
By the comparative laboratory investigation of the undisturbed loess samples obtained by thin walled sampler and samples obtained from pits, it has been shown that the mechanical disturbance is an extremely important factor which governs the shear and deformation parameters of loess soils.
The laboratory test results obtained on undisturbed loess samples, cut from blocks in the vertical and horizontal direction, have shown that this soil exhibits anisotropic properties. For this reason a stress deformation problem in an anisotropic medium has been treated by the finite element method.
Static penetration tests and field load tests in loess soils with natural water content and also after saturation have been performed and are described.
The results of the observed settlements for two statically identical multi-storey buildings are also presented. Using the coefficients of subsidence for the undisturbed samples cut from blocks, very good agreement between the calculated and the observed settlements has been obtained. 相似文献
The site preparation for foundations and records of performance of three storage structures founded on loess are presented to illustrate the engineering problems and the range in compressibility of loess associated with different moisture conditions. The sites are located in western Iowa and eastern Nebraska. The structures are a 7500 m3 capacity steel water storage reservoir founded on dry collapsible loess along the bluff line of the Missouri River; a 7200 m3 capacity concrete grain elevator supported by a rigid mat founded on saturated and partially saturated loess; and a 10 m high soil preload for a 54 m diameter ammonia storage tank underlain by saturated and partially saturated loess.
The foundation design, the site preparation and the observed settlements are presented for each structure. The settlement behavior varies markedly with the individual moisture conditions and the apparent preconsolidation of the loess. The apparent preconsolidation is attributed to desiccation and appears to vary systematically. 相似文献
The frequent failure of loess slopes in the western part of the Chinese Loess Plateau is closely related to progressive weathering along zones in these slopes, which causes a dramatic decrease in strength from the peak strength condition. Progressive weathering is common in the loess slopes in the western part of the Chinese Loess Plateau. During the process shear strength reduction along potential slip surfaces may be achieved by leaching of readily soluble salts, destruction of cementation bonds, and redistribution of particles. Localized collapse of the loess fabric causes internal deformation and consequently peak strength conditions are concentrated on a progressively smaller area of the failure plane. Therefore, the mode of failure of loess slopes is generally determined by brittle failure of the undisturbed, and unweathered, central parts of the slopes. It is important that both the weathered and unweathered strength of the loessmaterials in this area be established in order to analyze the stability of existing loessslopes, many of which are steepand lie directly above domestic and industrial urban areas. 相似文献
We present here an experimental work about the engineering behaviour of a calcareous loess from the Parisian Basin, the Rungis loess, in a temperature range of 5°C and 80°C. Mechanical testings on a collapsed and remolded material were realized with a specific apparatus. It allows us to give a stress and temperature history to the material while the process remains simple.
The temperature speeds up primary consolidation which is attained five times quicker at 80°C than at 20°C. The viscous flow of the skeleton and the compressibility are also increased in the same range. The consolidation phenomenon is theoretically analysed using both the classical Terzaghi approach and a rate process theory. The volume variations induced by a modification of the sample temperature is analysed and each phenomenon (thermic dilation, double layer modification, granular rearrangement) is emphasized.
All along this work, we have evaluated the specific characteristics of the loess: granularity, geochemistry, physico—chemical forces and structure. We emphasized the study of the microstructure of the material using pore size distribution measures by mercury intrusion and scanning electron microscopy. This permits us to relate the mechanical behaviour of the material to its micro-structure and to the pysico—chemical properties of the particles. 相似文献
As part of a comprehensive research program, laboratory tests were conducted on samples of deep-seated loess from different locations of the loessial regions of northern China, including Shaanxi, Gansu, Shanxi and Honan Provinces. The information thus acquired on some aspects of the engineering properties of the samples, such as the unit collapse-pressure relationships, the initial collapse pressure versus the “preconsolidation pressure” and deformation–pressure characteristics, is interesting and may prove useful to geotechnical engineers who work on projects on water-poor plateaux or high river terraces in loessial regions throughout the world. 相似文献
There are modest amounts of loess in Canada. As in most of North America it is probably produced by a combination of glacial grinding and cold weathering. Atterberg limits of Canadian loess are comparable with reference samples from the U.S.A. A deposit at Kamloops in British Columbia suffers a 15% linear collapse when loaded and wetted. Earlier estimates that the deposit was more than 100 m thick are probably in error; only the top part of the sedimentary deposit is loess. 相似文献
The results from the laboratory tests were used to find a relationship between water content, void ratio, swelling pressure and suction. Two equations for swelling pressure represent the outline of the model.
In the first equation, the swelling pressure developed during water uptake is normalised by a pressure corresponding to the swelling pressure at saturation. This is done in order to be independent of void ratio. A relationship between the normalised swelling pressure and the degree of saturation is suggested.
The second equation describes a relationship between the swelling pressure, the water content and the actual suction (or relative humidity). The equation is based on a thermodynamic relationship and includes the retention curve (i.e. water content vs. suction under free swelling conditions).
The model can be used for a state where two of the four variables; water content, void ratio, swelling pressure and suction are known and can thus be useful to evaluate field measurements and model late stages of the wetting process. An example of an application is given. The equations are mainly based on results from tests with increasing degrees of saturation under constant void ratio but are also suggested for use with increasing void ratio. 相似文献
The natural stability of loess soils can be related to fundamental geotechnical properties such as Atterberg limits, water content and void ratio. Field observations of unstable conditions in loess deposits in the upper midwest, U.S.A. show relationships between instability and the in situ moisture content and the liquidity index of the loess. Unstable loess can attain natural moisture contents equal to, or greater than, its liquid limit. Implications of these observations for applied engineering works are described. 相似文献
Loessial soils, wetted above the limit of collapsibility, remain in the category of difficult foundation grounds. Some case histories are presented herein: surpassing soil bearing capacity and non-stabilized settlements of buildings after initially dry loess wetting; slow but nondamped settlements or damped but great and, eventually, non-uniform settlements of structures erected on wet loess; supplementary settlements by water level lowering in loessial ground; great settlements of deep foundations passing through the collapsible loess to the underlaying wet but noncollapsible loess. Research on wetted loess performed in the laboratory (oedometer, triaxial) or in situ (full-scale experiments or real constructions) have emphasized the dependence of the soil structural resistance, deformability (compressibility and deformation under constant volume) and final resistance on both the moisture content or stress-state and on their history; the depth propagation of the active zone of surface loadings on wetted loess is different from that of the linear elastic theory; some suggestions to estimate the depth of the active zone are presented. The usual foundation systems on wetted loessial grounds, or in their vicinity, adopted in Romania include: loessial or gravel cushions; surface compaction (very efficient results of intensive dynamic compaction); foundation “stamping” or loessial ground “reinforcing”. It is recommendable to take into consideration: surveyed wetting under construction until maximum moisture content of the loessial ground; in situ columns of stabilized loess; the efficiency of geotextiles for filter-drainage and antierosional functions in loessial soils. 相似文献
The Archean organic material shows C isotope values between −47 and −15%. vs. PDB. The Proterozoic metasediments show a smaller range, with δ13C between −30 and −17%.,
A few carbonate samples yielded C isotopic compositions between −5.9 and −4.7%.,
Associated iron sulfides have δ34S-values ranging from −6 to +8%., which are consistent with isotopic fractionation effects either by inorganic or bacterial reactions in the sedimentary sulfur cycle.
The great variability of organic C isotopic composition, especially in the late Archean, although affected by extensive postdepositional alteration, might indicate differences in the environmental conditions and/or the organic content of these sedimentary basins. Some extremely 13C-depleted organic carbon values support a global occurrence of environments likely dominated by methanotrophic organisms at the end of the Archean. 相似文献