Practical Application of the Microwave Oven in the Geotechnical Laboratory

Abstract

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).

The Impact of Microwave Drying on the Structure of Exemplary Soils—Insights Using X-ray Microtomography

“…Certainly, this is influenced by the dominant share of tough quartz and low humidity, and hence, the limited effect of microwave radiation. For fine quartz sands with high water content (above 23%), the phenomenon of microwave drying has a completely different nature and can even lead to the explosion of samples resulting from too much evaporation (tests conducted on samples of 25 g [ 6 ]). For this reason, it is worth expanding the range of samples with higher water content and determining critical saturation in subsequent tests by using XµCT.…”

“…The selection of the specific heating power and the mass of the specimen are still, generally, arbitrary decisions of the researcher. Numerous examples can be found in Jastrzębska’s work [ 6 ].…”

“…The selection of soil mass and drying time strictly correlated with the heating power of the device. Care should be taken to avoid explosion or burning of dried soils (e.g., clay soils and fine sand, gravel particles, and hard brittle rock, especially when they have a high water content [ 6 , 17 , 28 , 37 ]). General recommendations include using 700–800 W microwave power and 2–50 minute drying time intervals.…”

“…General recommendations include using 700–800 W microwave power and 2–50 minute drying time intervals. Details can be found, among others, in studies by [ 6 , 36 , 38 ].…”

“…Many experimental studies have explored rapid microwave drying [ 6 , 7 ] and the influence of multiple wet–dry cycles (such as those that occurred during laboratory drying) on the physical and mechanical properties of soil or its mixtures with various additives (e.g., with cement, liquid modifiers, bentonite, etc. ), which include thermally modified swelling parameters [ 8 , 9 ], hydraulic conductivity/permeability coefficients [ 8 , 10 , 11 ], shear strength [ 12 , 13 ], durability, stiffness, and the void ratio [ 12 ], Atterberg limits [ 8 ], and even intensification of landslides [ 11 , 14 , 15 ].…”