The main purpose of the paper is to present the usefulness of microwave ovens in the everyday work of geotechnical laboratories. Microwave ovens are commonly used to determine soil moisture and to dry soils that are too moist for other laboratory tests. The activities related to the above processes are not standarized in the majority of countries. The only available guidelines are based on the research projects. For the purpose of this particular research 345 tests have been conducted for moisture content determination for 3 soil types: fine sand (FSa; SP – poorly graded sand), sandy gravel (saGr; GW – well graded gravel-sand mixtures) and silty clay (siCl; CH – inorganic clay of high plasticity), using both – standard and microwave methods. The research cycle consisted of 3 different variants which includes soil moisture content, specimen initial mass, number of specimens in microwave oven and their placing. It was established that different factors have impact on the length of the process and it lasted between 2 to 17 minutes. During the heating different undesirable phenomena were observed, e.g., soil explosions or melting and glowing of clay. Based on the above research results this paper introduces guidelines for the soil drying process in the microwave oven (the optimization of the procedure and handling of the dried samples).

Abstract:The subject of the paper comprises tests of cohesive soil subjected to low-frequency cyclic loading with constant strain amplitude. The main aim of the research is to define a failure criteria for cohesive soils subjected to this type of load. Tests of undrained cyclic shear were carried out in a triaxial apparatus on normally consolidated reworked soil samples made of kaolinite clay from Tułowice. Analysis of the results includes the influence of number of load cycles on the course of effective stress paths, development of excess pore water pressure and stress deviator value. Observed regularities may seem surprising. The effective stress path initially moves away from the boundary surface and only after a certain number of load-unload cycles change of its direction occurs and it starts to move consequently towards the surface. At the same time, it has been observed that pore water pressure value decreases at the beginning and after few hundred cycles increases again. It is a typical behaviour for overconsolidated soil, while test samples are normally consolidated. Additionally, a similar change in deviator stress value has been observed -at first it decreases and later, with subsequent cycles, re-increases.

This paper presents the results of consolidated and undrained (CU) triaxial cyclic tests related to the influence of tire waste addition on the strength characteristics of two different soils from Southern Poland: unswelling kaolin and swelling red clay. The test procedure included the normally consolidated remolded specimens prepared from pure red clay (RC) and kaolin (K) and their mixtures with two different fractions of shredded rubber powder (P) and granulate (G) in 5%, 10%, and 25% mass proportions. All samples were subjected to low-frequency cyclic loading carried out with a constant stress amplitude. Analysis of the results includes consideration of the effect of rubber additive and number of load cycles on the development of excess pore pressure and axial strain during the cyclic load operation and on the maximum stress deviator value. A general decrease in the shear strength due to the cyclic load operation was observed, and various effects of shear strength depended on the mixture content and size of the rubber waste particles. In general, the use of soil–rubber mixtures, especially for expansive soils and powder, should be treated with caution for cyclic loading.

Abstract. The paper deals with comparison of tangent shear moduli G s of kaolin from Tułowice obtained from cyclic triaxial tests on the basis of external and internal reading in the small strains range (10 -5 ÷10 -3 ). The tests were carried out on a modernised test bed, enabling full saturation of specimens using the back pressure method as well as a precise internal measurement of strains by means of contactless microdisplacements sensors. The value of linearity factor L is one of adopted quality criteria for two measuring methods. Maintaining a constant deformation rate the influence of various cyclic process parameters (deviator stress amplitude -constant or variable; high or low; initial level of stress and strain, at which the unloading and reloading cycles were started; overconsolidation ratio OCR as well as cycles' number and arrangement) on the "shear modulus -axial strain" characteristic was studied. The obtained values of G int and G ext (or L int and L ext ) clearly show an underestimation (even 5 times) of G s value within the range 10 -5 ÷10 -3 when using an external measurement. In addition, the differences between G int and G ext , which develop differently depending on specified cyclic process parameters, gradually decrease with increasing axial strains.

This paper presents results of undrained and unconsolidated (UU) triaxial tests related to the influence of tire waste addition on strength characteristics of red clay from Patoka in Southern Poland. Angle of internal friction and cohesion values were estimated for 30 specimens prepared from pure red clay (RC), its mixtures with two different fractions of shredded rubber in 5%, 10%, and 25% mass proportions as well as for pure powder (P) and granulate (G). It has been observed that the addition of granulate contributes more to the increase in the angle of friction than the addition of powder (uu = +1% (G-5) / +16% (G-10) / +31% (G-25), uu = +1% (P-5) / +10% (P-10) / +19% (P-25)). On the other hand, rubber additions reduce cohesion in mixtures, and the effect is enhanced with increases in their grain size and percentage composition (cuu = −31% (G-5) / −63% (G-10) / −87% (G-25), cuu = −67% (P-5) / −58% (P-10) / −58% (P-25)). It has been noticed that a change of parameters uu and cuu causes a decline of shear stresses at increasing granulate content. There is an inverse relationship for powder. At the same time, it has been shown that the failure strain, hence a change in red clay-rubber (RCR) mixtures plasticity, is related to the level of confining stress 3 and the type of rubber waste. Results of tests and their comparison with results of other researchers show that each time it is necessary to experimentally verify a given soil with specific rubber waste.