The influence of the grain-size distribution and soil structure on the unsaturated shear strength of loess sediments in Belgrade, Central Serbia

Abstract

There is a negative pore water pressure or matric suction in the zone above the ground water level in silty loess soil, which can be as deep as 5-10 m in the Belgrade area. This primary characteristic of unsaturated soil, i.e., matric suction, should be included in laboratory testing and geotechnical analyses. Direct shear or triaxial testing of unsaturated soil are very expensive and time-consuming and require specially modified equipment. Instead, the prediction of unsaturated shear strength using the soil water characteristic curve, SWCC, and the effective shear strength parameters c' and ?' is a widely accepted practice. In this study, constitutive soil-water characteristic curves were obtained from the results of experimental testing by draining saturated soil samples under different pressures. This testing was performed for the first time in Serbia in a 15 bar pressure plate extractor according to ASTM standards. The laboratory testing included natural samples of loess sediments with the original macroporous structure and loess sediments with a destroyed soil structure. The influence of the grain-size distribution and natural soil structure on the unsaturated shear strength of Belgrade loess sediments above the ground water level was also evaluated. The obtained results are in accordance with the results from other investigations.Nadizdanska zona u prasinastim lesnim sedimentima u podrucju Beograda moze da bude debljine 5-10 m. U ovoj nadizdanskoj zoni postoji matricna sukcija (negativni porni pritisak). Osnovno svojstvo nezasicenog tla - matricnu sukciju bi trebalo ukljuciti u laboratorijska ispitivanja nezasicenog tla, ali i geostaticke analize. Opiti direktnog smicanja ili triaksijalne kompresije nezasicenog tla su dugotrajni, skupi i podrazumevaju izmenu konvencionalne laboratorijske opreme. Umesto toga, sve cesce se koristi postupak odredjivanja nezasicene cvrstoce tla pomocu osnovne konstitutivne zavisnosti nezasicenog tla vlaznost/sukcija i efektivnih parametara cvrstoce zasicenog tla c' i ?'. U ovom radu su, prvi put kod nas, prikazani rezultati odredjivanja konstitutivnih zavisnosti efektivni stepen zasicenja/matricna sukcija, na osnovu laboratorijskih opita dreniranja uzoraka tla pod razlicitim pritiscima. Opiti su vrseni u 15-bar ekstraktoru sa polupropustljivom plocom prema standardima ASTM. Opiti dreniranja su sprovedeni na neporemecenim uzorcima sa ocuvanom i izmenjenom primarnom makroporoznom strukturom. Analiziran je i uticaj granulometrijskog sastava i primarne strukture na konstitutivne zavisnosti lesnih sedimenata nadizdanske zone u Beogradu. Dobijeni rezultati uporedjeni su sa rezultatima inostranih istrazivaca i dobijena su dobra slaganja.