Glucose-stimulation of natural microbial activity changes composition, structure and engineering properties of sandy and loamy soils

High inputs of easily available organic matter to the subsurface may quickly activate the native microbial communities, thereby changing soil engineering properties. We studied the effect of glucose addition, an easily available carbon source, on stress-strain properties, mineralogy, and microstructure of several loamy and sandy soils over 30 days in laboratory experiments. During the period of high microbial activity, direct shear tests revealed a reduction of the friction angle of 15-30 % and a raise of cohesion in sands. Unconfined compression tests showed a 20-30 % decrease in the compressive strength of loamy soils. With the decline of microbial activity, the stress-strain properties recovered partially. The alterations of the stress-strain properties with increasing microbial activity were linked to changes in mineralogy and composition. X-ray diffraction showed that the proportion of smectite layers in illite-smectite mixed layer minerals increased, as well as the overall imperfection of clay minerals. Glucose addition resulted in a temporary increase in the content of microaggregates (0.1-0.05 mm). Newly formed linkages of organic matter between solid particles, biofilm formation and direct interaction of cells with mineral surfaces were observed by scanning electron microscopy. Our data show that microbial-mediated processes may adversely influence stress-strain properties of soils and endanger the safety of buildings.