ǂThe ǂdynamic properties of the snail soil from the Ljubljana marsh

Bojan Žlender (Avtor), Ludvik Trauner (Avtor)

Povzetek

A series of cyclic triaxial tests was performed on snail-soil samples with different porosities. The cyclic loading was performed with a Wykeham Farrance cyclic triaxial system. The investigation was based on a series of tests in which the following conditions were varied: the initial effective pressures (50, 100, and 150 kPa), the void ratio after consolidation (2.0–1.2) and the cyclic loading expressed by the cyclic stress ratio CSR (0.1–1.0). Measurements were made of the stress, the deformation and the pore-water pressure. The results of the tests show that interdependency exists between the geomechanical characteristics and the porosity. These relationships can be expressed as functions of the density, the porosity or the water content. It is evident from the results that the changes in the coefficient of permeability, the coefficient of consolidation, and the coefficient of volume compressibility are non-linear with respect to the changes in the porosity. However, the changes at high porosity are much greater than the changes at low porosity, and the changes of the mechanical parameters, such as the Young’s modulus, Poisson’s ratio, and the friction angle, are indistinct and almost linear at lower changes of porosity, and after that become non-linear. The initial void ratio e is extremely high and the snail soil is liquid after consolidation; a volume strain of εvol > 16 % is needed for the plastic limit state. The chemical and mineral composition, the particle size distribution and the remains of micro-organisms in the snail soil are constants. In addition, the specific surface is independent of the porosity and the density or unit weight, the porosity and the volume strain are in the well-known correlation. The performed cyclic triaxial tests show the dynamic characteristics of the snail soil and the influence of the porosity on the cyclic loading strength. The snail soil was recognized as a highly sensitive material. A large strain appears after the initial cycles. The pore pressure, increases already during the first cycle, to the hydrostatic part of the cyclic loading, or more (depending on CSR). The damping ratio increases exponentially with strain, after some cycles it reach its maximum value, and after that it decreases to the asymptotic value. The reason for such behaviour is the large deformation. The maximum and asymptotic values of the damping ratio are a changed minimum with a void ratio. There is obviously no influence of the porosity on the damping ratio. The shear modulus is described in relation to shear strain. The increasing of the pore pressure is independent of the porosity until it reaches some value of the pore-pressure ratio (>0.7). Similarly, the increasing of the shear strain becomes dependent on the void ratio until it reaches some particular value of the shear strain (>3%). The deformation and failure lines for the different porosities are determined from the relationship between the shear stress and the effective stress at some shear strain, after 10 cycles. The relationships between the shear stress and the effective stress at some value of the pore-pressure ratio are expressed in a similar way. Two kinds of criteria were used to determine the triggering of liquefaction during the cyclic triaxial tests: first, when the pore pressure becomes equal to the effective confining pressure, and, second, when the axial strain reaches 5% of the double amplitude.

Ključne besede

geodinamika;Ljubljansko barje;polžarica;fizikalne lastnosti;

Podatki

Jezik:	Angleški jezik
Leto izida:	2007
Tipologija:	1.01 - Izvirni znanstveni članek
Organizacija:	UM FGPA - Fakulteta za gradbeništvo, prometno inženirstvo in arhitekturo
UDK:	624.131.27
COBISS:	246150144
ISSN:	1854-0171
Matična publikacija:	Acta geotechnica Slovenica
Št. ogledov:	1135
Št. prenosov:	69
Ocena:	0 (0 glasov)
Metapodatki:

Ostali podatki

Sekundarni jezik:	Slovenski jezik
Sekundarni naslov:	Dinamične lastnosti polžarice iz Ljubljanskega barja
Sekundarni povzetek:	Na vzorcih polžarice iz jugozahodne lokacije Ljubljanskega barja je bila izvedena raziskava njenih dinamičnih lastnosti. Izvedeni so bili ciklični triosni preizkusi. V preiskavi so bili spreminjani pogoji: začetna efektivna napetost (50, 100, 150 kPa), količnik por (2.1 do 1.2) in faktor ciklične obremenitve CSR (med 0.1 in 1). Med posameznim preizkusom so bile merjene časovne spremembe napetosti, deformacij in pornega vodnega tlaka. Parametri fizikalnih lastnosti so podani kot funkcije zgoščenosti polžarice, torej volumenske deformacije, gostote, poroznosti ali vlažnosti. Kemični sestav delcev in mineralna sestava ter zrnatost polžarice se zaradi zgoščevanja ne spremenijo. Delež mikroorganizmov v polžarici je zelo majhen in ga lahko smatramo kot konstanto. Specifična površina je neodvisna od zgoščevanja. Nekateri parametri fizikalnih lastnosti polžarice (prostorninska teža, vlažnost, poroznost) se spreminjajo v odvisnosti od zgostitve, kar lahko izrazimo s splošno znanimi odnosi. Spremembe vodoprepustnosti, konsolidacije in stisljivosti so nelinearno odvisne od sprememb poroznosti, pri začetnem zgoščanju so spremembe očitne, z nadaljnim zgoščanjem se njihove spremembe manjšajo. Spremembe mehanskih parametrov kot so Youngov modul, Poissonov količnik in strižni kot, pa so pri pri začetnih spremembah poroznosti neizrazite in skoraj linearne, pri nadaljnem zgoščanju pa se njihove spremembe večajo. Vpliv zgoščanja na Poissonov količnik je skoraj linearen. Polžarica je kljub židkosti in neugodnim fizikalnim lastnostim ter nizkim parametrom trdnosti dokaj odporna na pojav likvifakcije. Se pa že v začetnih ciklih obremenjevanja pojavijo velike specifične deformacije. Tudi porni vodni tlaki že po nekaj ciklih izrazito narastejo, vendar pri manjših vrednostih CSR praviloma ne dosežejo efektivne celične napetosti. Dušenje narašča eksponentno z deformacijo. Zaradi velikih deformacij pa praviloma že po nekaj ciklih doseže maksimalno vrednost in zatem upade na neko asimptotično vrednost. Maksimalna in asimptotična vrednost dušenja se s spremembo poroznosti polžarice minimalno spremenita. Na strižni modul vpliva zgoščevanje posredno, saj je le ta izražen kot funkcija strižne deformacije. Deformacija je manjša pri gostejšem materialu in večja z večanjem CSR. Za različne vrednosti začetne efektivne napetosti in poroznosti polžarice so določene deformacijske in porušne ovojnice tako, da so podane vrednosti CSR pri katerih je po nekem številu ciklov (npr. 10) dosežena določena deformacija. Kot mejna vrednost je izbrana dvojna amplituta osne deformacije velikosti 5%. Podobno so podane ovojnice za različna stanja pornega tlaka in za mejno stanje, ko razmerje naraščajočega pornega tlaka in efektivne celične napetosti doseže vrednost 1.
Sekundarne ključne besede:	geodinamika;Ljubljansko barje;polžarica;fizikalne lastnosti;ciklični triosni preizkus;poroznost;vodoprepustnost;konsolidacija;Youngov modul;strižni modul;koeficient dušenja;Poissonov količnik;strižni kot;
URN:	URN:SI:UM:
Vrsta dela (COBISS):	Znanstveno delo
Strani:	str. 48-61
Letnik:	ǂVol. ǂ4
Zvezek:	ǂ[no.] ǂ2
Čas izdaje:	2007
ID:	10935049