PWP Tester - A simplified cyclic shear test for pore water pressure evolution in granular soils

Bozana Bacic

AutorIn

Bozana Bacic

Titel

PWP Tester - A simplified cyclic shear test for pore water pressure evolution in granular soils

Zitierfähige Url:

https://nbn-resolving.org/urn:nbn:de:bsz:14-qucosa2-945638

Übersetzter Titel (DE)

PWD–Tester - Vereinfachter zyklischer Scherversuch für den Porenwasserdruckaufbau in granularen Böden

Schriftenreihe

Mitteilungen / Institut für Geotechnik

Bandnummer

Erstveröffentlichung

2024

Datum der Einreichung

10.06.2024

Datum der Verteidigung

30.09.2024

ISSN

1434-3053

Abstract (EN)

The build-up of excess pore water pressure (PWP) usually takes place in loose, saturated, coarse-grained soils when subjected to external shear loading (monotonic or cyclic) under prevented water drainage. When the amount of the excess PWP reaches the value of the total stress, the grain structure collapses and the shear strength and stiffness of the soil disappear - the soil behaves as a liquid. Such soil phenomenon is commonly referred to as soil liquefaction. Experimental laboratory testing of the excess PWP build-up in coarse-grained soils is almost restricted to sophisticated undrained cyclic triaxial and simple shear tests. A novel experimental method, the PWP Tester, enabling a simple and fast testing of the excess PWP accumulation in coarse-grained soils has been designed and presented in this thesis. The evolution of the excess PWP during cycling shearing of a cylindrical sand specimen in undrained conditions is measured during the test, and parameters characterising this evolution are determined. Besides an extensive repeatability study, the influence of several soil state variables, e.g. soil density, consolidation pressure, or loading amplitude on the accumulation of the excess PWP was tested. It was shown that the generation of the excess PWP decreased with rising density and consolidation stress, while increasing loading amplitude resulted in faster excess PWP build-up, which confirmed the plausibility of the experimental method. A comparison with the results of undrained cyclic triaxial tests in the case of eight different sands demonstrated a good agreement between both experimental methods, which further corroborated the suitability of the proposed test for a fast investigation of the excess PWP build-up in coarse-grained soils. Specimens prepared according to the proposed procedure in the PWP Tester are pluviated with a funnel under water. The extraction of the sand fabric from the µCT images acquired after the underwater deposition of different sands demonstrated a similar geometrical fabric for different grain size distibutions. Despite having a very different relative density after underwater pluviation, which ranged from a loose to a dense state, a similar resistance to the accumulation of the excess PWP was obtained for different sands. These results have indicated that the same soil fabric prescribed by the same specimen preparation method has at least the same or even stronger impact on the excess PWP accumulation than (different) relative densities. The influence of different sand fabrics on the excess PWP evolution was further tested in the case of a coarse and a fine sand that were prepared via several different procedures. A large impact of the sand different fabrics on the excess PWP build-up was identified for the coarse sand, while the tests on fine sand resulted in a negligible influence of different grain compositions on the evolution of the excess PWP. In total, 42 different materials, including natural sands as well as mixtures of glass beads and crushed sand were tested with the PWP Tester. The results on these materials have shown no clear trend of the excess PWP evolution with the main soil physical properties, e.g. uniformity coefficient, mean grain diameter, and fines content. However, observing one soil property and keeping all other similar has shown that the resistance to the excess PWP accumulation rises with the mean grain diameter but reduces with an increase in the uniformity coefficient and fines content. The influence of different grain shapes and grain angularities was evaluated by performing tests on three sands with similar grain size distribution curves. Here, a sand consisting of elongated and angular grains with a rather rough surface, exhibited lower relative density when deposited under water and showed a larger resistance to the excess PWP generation with growing density than sand with a more round grains having smoother grain surface. Different grain descriptors of these sands were determined using the dynamic image analysis and digital microscopy. The same analysis was performed for natural sand and mixtures of glass beads and crushed sand with the same grain size distribution curves. Glass beads and crushed sand were both more resistant to the excess PWP build-up than natural sand. This was attributed to a stronger fabric in the case of glass beads and a high angularity (and thus grain interlocking) of the crushed sand grains. Furthermore, an idealized application of the PWP Tester results for in situ conditions was presented, where the calculation of the excess PWP evolution in a sand layer under speci c density, stress and loading conditions was shown.

Freie Schlagwörter (EN)

Pore water pressure build-up, Index test, Cyclic shear test, Porenwassedruckaufbau, Indexversuch, Zyklischer Scherversuch

Klassifikation (DDC)

690

Klassifikation (RVK)

ZI 6130

GutachterIn