Zmienność właściwości geotechnicznych lessów w świetle współczesnych metod badań

Marian Borowczyk, Zbigniew Frankowski

Abstract


Przedstawiono przydatność różnych metod polowych do badań właściwości geologiczno-inżynierskich lessów. Stosowano pomiary presjometryczne oraz sondowania sondami: obrotową, wciskaną i wkręcaną. Wskazano również na potencjalne możliwości wykorzystania techniki jądrowej do określenia zawartości żelaza i wapnia. Podano nowe kryteria oceny struktury nietrwałej lessów, opracowane na podstawie wyników badań presjometrycznych i radiometrycznych.

VARIABILITY IN GEOTECHNIC PROPERTIES OF LOESSES IN THE LIGHT OF MODERN STUDIES

SpecifIc physico-mechanical properties of loesses have for may years been the subject of various studies in Poland. At present, much attention is being paid over the world to in situ investigations of soil parameters.

The applicability of various in situ investigation methods to study geological-engineering properties of loesses are analysed. The studies covered loesses of south-eastern Poland. There were used pressuremeter, radiometric techniques and static probes: penetrometer, weight penetrometer and vane test. The in situ investigations covered loess profile to the depth of 10 m below terrain surface at the average. In estimations of physico-mechanical loess properties the most important are appropriate stratigraphic and lithological classifications of these soils. The stratigraphic subdivision was accepted after H. Maruszczak (1976). According to that subdivision, the interval down to 10 m below terrain surface comprises the Upper Younger Loess, often very thick, and Middle, Lower and Lowermost Younger Loess. From the point of view of lithogenesis, three basic loess types were differentiated on the basis of differences in grain size distribution, porosity, texture, content of calcium and colour.

Pressuremeter studies were carried out with the use of L. Menard pressuremeter of the type G with probe 60 mm in diameter. There were measured pressuremeter modulus, Ep’ pressuremeter limit pressure, PL’ and creep pressure, Pf. The values of bulk density and moisture by volume were obtained by radiometric techniques with the use of universal probe pattern. The content of calcium and iron in loesses was established by X-ray radioisotopic fluorescence analysis. Undrained shear strength was estimated with the use of vane test. Probing with the use of statistic probes (Borros penetrometer and weight penetrometer) supplied data on cone resistance of loesses to penetrating probe. The basic properties of loesses were also studied in the laboratory on disturbed and undisturbed (thin-walled cyliners) soil samples.

The complex studies showed a marked applicability of in situ investigations in estimating geotechnical properties of loesses. The presence of loesses with unstable structure may be unequivocally evidenced by pressuremeter tests on the basis of the criterion PL-Pf ≤ 0.21 MN/m2. Porosity index en > 0.72 and degree of saturation Sr < 0.60 (i.e. parameters measurable using radiometric methods) may be accepted as orientational criterion. These features are mostly found intypical loesses - silts (Upper Younger Loess according to H. Maruszczak stratigraphic subdivision).

In the areas studied, loesses with unstable structure usually occur at the depth to 3 - 4 m below terrain surface, mostly to 2 m.

Other methods of in situ investigations: penetrometer, vane test and weight penetrometer, give data useable for qualitative interpretation of loess parameters.


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