Triaxial Tests On Weak Cohesive Soils – Some Practical Remarks (Part 2)

S t r e s z c z e n i e Niniejszy artykuł stanowi kontynuację opisu zagadnień poruszanych w części pierwszej artykułu „Triaxial tests on weak cohesive soils – some practical remarks (part 1)”, dotyczących specyfiki badania słabych gruntów spoistych w warunkach trójosiowego obciążenia. W tej części (2/2) omówiono i przedyskutowano na podstawie doświadczeń własnych oraz danych dostępnych w literaturze zagadnienia związane z montowaniem próbki w komorze aparatu trójosiowego oraz przygotowaniem jej do badania. Wytypowano rozwiązania ułatwiające pracę ze słabymi gruntami spoistymi i zwiększające wiarygodność wyników – m.in. stosowanie bezkontaktowych lokalnych czujników przemieszczenia z możliwością ich przestawienia w trakcie badania. Zagadnienia dotyczące pobierania próbek in situ i ich formowania do badań trójosiowych zawarto w części 1/2. Zdaniem Autorek treść obu artykułów (części 1 i 2) może stanowić istotną wskazówkę dla badaczy kompletujących stanowisko do badań trójosiowych słabych gruntów spoistych. K e y w o r d s : Local displacement transducers; Soft clays; Triaxial tests; Weak cohesive soils. 3/2016 A R C H I T E C T U R E C I V I L E N G I N E E R I N G E N V I R O N M E N T 81 A R C H I T E C T U R E C I V I L E N G I N E E R I N G E N V I R O N M E N T The Si les ian Univers i ty of Technology No. 3/2016 M . K o w a l s k a , M . J a s t r z ę b s k a ters of soil constitutive models for normally consolidated Speswhite kaolin [5, 6]. 2. SPECIMEN INSTALLATION Installation a specimen of weak cohesive soil in the triaxial chamber and its preparation for further testing is not simple for several reasons. At this stage the chamber construction itself and also the particular parts of the triaxial apparatus equipment may cause difficulties. The influence of all the elements on testing quality and reliability of results depends on the type and consistency of the soil tested and on the procedure and purpose of the experiment (with or without consolidation, with or without saturation, drained or undrained shearing). Most of the requirements amount to full control of the current specimen’s size and its variations during the experiment – from the prevention of sample disturbance during the installation in the apparatus, to the measurement of soil deformations not only during the main stage of the test, but also during the specimen preparation (e.g. flushing and back pressure application). This forces the use of an appropriate system of displacement measurement, which will be discussed further in this paper. Transportation of the specimen from the place of its preparation to the apparatus and its set up should be done with the use of a special rigid bipartite mould (see Fig. 1(a)). It is usually designed to fit a specific triaxial chamber as they may have very different constructions. The main purpose of such a mould is to centre and orient the specimen by means of pins installed on the pedestal while limiting the necessity of touching the specimen by hand to a minimum. In case of a rigid connection of a top cap with piston the same mould has to be used also to install a membrane and O-rings on the specimen (Fig. 1(a)). Otherwise the membrane and O-rings are mounted by means of simpler tools (Fig. 1(b), Fig. 1(c)). When very soft clays are tested in an apparatus with top cap non-integrated with piston, the mass of the top cap becomes important – it should be made from a lightweight material, e.g. acrylic or aluminium. End friction, disturbing the homogeneity of stress and strain distribution in triaxial tests, is a very significant issue. Its impact is commonly reduced by maintaining an appropriate slenderness of the specimen 82 A R C H I T E C T U R E C I V I L E N G I N E E R I N G E N V I R O N M E N T 3/2016 Figure 1. Tools used to set up a specimen: a) orienting and membrane fitting mould, b) O-ring and c) membrane fitting tool (photograph: M. Kowalska) a b