Rheometer use in analysis of soil creep behaviour

Hydrogeology, Engineering Geology and Geotechnics

RHEOMETER USE IN ANALYSIS OF SOIL CREEP BEHAVIOR MSc Lukasz Kaczmarek1,2 1 2

University of Warsaw, Faculty of Geology, Żwirki i Wigury 93, 02-089 Warsaw, Poland Warsaw University of Technology, Faculty of Materials Science and Engineering, Wołoska 141, 02-507 Warsaw, Poland

ABSTRACT Soil creep is an important geodynamic process, which can adversely affect engineering geology at construction sites. This paper presents results of rotational controlled shear stress tests, oscillatory tests and creep tests of remolded clay samples performed with a rheometer. The results were used to characterize the creep behavior of Warsaw MioPliocene clay paste samples. The application of rheometer techniques in soil mechanics is uncommon, but they have key advantages: short test time, small size of samples and unsophisticated sample preparation. An AR 2000 rheometer equipped with a parallel plate was used in the testing process. In soil creep analyses, rheometry is made possible by changes in soil structure at specific constant stress states. This research presents a novel insight into the phenomenon of soil creep. Keywords: clay, constant stress state, soil rheology, Mio-Pliocene, Poland INTRODUCTION Soil behavior depends primarily on three properties: elasticity, viscosity and creep. These properties may be reproduced in material models that constitute the basis for further advanced mathematical models. In the case of clay soils that contain a large amount of fine fraction, viscous and creep properties are revealed, which is due to the soil structure [1]. Additional factors potentiating those properties are elevated humidity and soft-plastic state of soils. In engineering it has a direct influence on human safety and on the probability of a building suffering damage if development takes place in the vicinity of slopes or deep excavations for foundations. This article shows the use of rheometer to characterize creep properties of clay soils taken from the excavations of a metro station in Warsaw at the base of the Warsaw Slope. It also presents the results of rotational controlled shear stress testing and amplitude sweep testing with controlled shear strain as soil rheological properties. MATERIAL AND METHODS Samples Mio-Pliocene clay samples were taken in the form of monolites from an excavation at a metro station in Warsaw. Physical characteristics of the samples were presented in the article written by [2]. The investigated soil - of semisolid consistency - comes from a group of highly cohesive soils and has natural humidity of 28%. Its bulk and grain density are ρo=1.82 g/cm3 and ρs=2.68 g/cm3, respectively, whereas its liquid limit was estimated at wL=76.6%. Analyzed soil composites were from 35% clay with particle

16th International Multidisciplinary Scientific GeoConference SGEM 2016

size <0.002mm, 42% silt with 0.002mm
Fig. 1. Rheological curves of ideal materials: (a) Stress-strain relations of ideal Hookean solid and flow curve of ideal plastic material with distinct yield stress (τy). Spring and slider are mechanical analogs of ideal elastic and plastic solids, respectively. (b) Flow curves of ideal Newtonian fluid and its mechanical analogs, dashpot [9]

The study was performed with a parallel plate rheometer AR 2000 (Fig. 2). Tests were conducted at a temperature of 20°C where the diameter of samples was 2 cm and the thickness of samples was 3 mm.


Fig. 2. A parallel plate rheometer AR 2000

Rotational controlled shear stress test (CSS) In the first stage, unsaturated soil deformation under controlled shear stress state was analyzed. During this analysis the increment of shear stress was controlled by a rotating top cone and a fixed base. The result of this study revealed information about the yield stress point (τy) beyond which soil behavior is similar to the flow of viscous material [10], [11] and if shear stress is equal to this stress value then plastic deformation occurs (Fig. 3). Such behavior is commonly referred to as viscoplasticity [9] and is a reaction to an acting force. A description of this parameter with a case study based on supersoft clay can be found in [12].

Fig. 3. Bingham viscoplastic body and its mechanical analogs, dashpot-slider [9]

Furthermore, CSS tests are used to simulate flow or creep that occurs during landslides and avalanches [8]. Oscillatory test (Amplitude sweep test with controlled shear strain) Basically, the present analysis gives an idealized illustration of shear strain. Due to an applied force F on a defined amount of material and by moving a round plate over a stable surface. According to the results, complex viscosity η*, which represents the flow


resistance of the sample, can be calculated from the formula below [8]. Equations that were used during results analysis based on the relations below, according to the Maxwell model, which is combination of a dashpot and a spring in a raw.

where: τ - shear stress, η*- complex viscosity, η’- real part of the complex viscosity, η’’- imaginary part of the complex viscosity, ω - angular frequency, G*- complex shear modulus, G’- storage modulus, G’’- loss modulus, δ - phase shift angle, γ - shear strain, γA - is the variable strain amplitude, t - time.

An amplitude sweep test is an oscillatory test with variable amplitude and constant frequency values of shear strain or shear stress respectively. In this study the amplitude of the shear strain is controlled and increases with the logarithm of time (from 0.1 to 20%) as shown in figure below. During tests, angular frequency is constant (ω=10 1/s). Furthermore, Fig. 3 shows the basics and principles of the oscillatory test as well as shear stress (τ) in relation to the shear strain (γ) with delay represented by the phase shift angle (δ).

Fig. 4. A – Principle of oscillatory tests within the linear viscoelasticity deformation range according to Maxwell [8] and schematic stress response to oscillatory strain deformation for various material types [13]; B - Illustration of the amplitude sweep test with controlled shear strain – variable strain amplitude [8]

It is noteworthy that storage modulus G’ represents elastic behavior of a sample and the loss modulus G’’ the viscous component. In the case G’>G’’ the elastic behavior of soil is dominant, and in the case of G’

the oscillatory test as a cyclic test can be used to analyze the influence of frequency changes [9]. Creep test Finally, several creep tests on Mio-Pliocene clay were performed with an AR2000 rheometer. During this test, shear stress (τ) is kept constant and the shear strain (γ) is registered. The example of creep procedure used with Norwegian Glava clay was presented in [15]. In this study, firstly, three single-stage creep tests were done (with three hour stages τI=100Pa, τII=700Pa, τIII=1300Pa). Afterwards, one multi-stage creep test were performed with 45 minute stages (τ1=100Pa, τ2=500Pa, τ3=900Pa), where shear stress (τ) was incrementally increased and kept constant during the time duration of each stage. RESULTS Rotational controlled shear stress test (CSS) The results of the CSS test are presented in the stress-strain relationship curve in log-log scale (Fig. 5). This curve reveals yield point stress (approx.1100 Pa), which establishes the range of elastic and viscosity strains of soil. The failure will occur after exceeding 1300 Pa of shear stress.

Fig. 5. Results of CSS test with yield stress point and soil behavior regime

The results are similar to CSS test results of clay from USA [9] and Norwegian Glava clay [15]. Oscillatory test The character of the presented curves (Fig. 6) corresponds to paste type materials [8]. The structure shows a certain rigidity and elastic behavior that dominated over the viscous one. Additionally, the phase shift angle (δ) was calculated to demonstrate differences between elastic and viscous ideal materials, and how it changes with shear strains. The results obtained by the oscillatory test define the yield point, which is the


border value for the linear viscoelasticity range. The inflexion point of storage modulus (G’) is equal to the yield point. Compared to G’’, the first inflexion represents the imaginary part of lost elasticity. At this point the substance starts to creep and the loss of elasticity reaches its maximum. The state of plasticity is obtained. In the study γL was equal to 0.19%

Fig. 6. Results of amplitude sweep tests with controlled shear deformation of Mio-Pliocene clay paste from Warsaw

Creep tests According to [1], depending on how the strain rate changes, which results from the present shear state, creep may be divided into three types that can run consecutively. All three types could be observed in test results. The first type is suppressive creep (Figs. 7 and 8; shear stress 100 Pa), the second type is secondary creep, i.e., creep with a constant strain rate (Fig. 8; shear stress 500 Pa) and the third type is accelerating creep in which the strain rate increases and, consequently, destruction occurs (Fig. 7; shear stress 700, 1300 Pa and Fig. 8; shear stress 900 Pa).

Fig. 7. Single-stage creep tests of Mio-Pliocene clay paste from Warsaw

Figure 8 shows a multi-stage creep analysis, where in the last stage (τ3=900Pa) the soil structure yields after initial stabilization. It is noteworthy that during the single-stage


creep test the sample yields in the beginning of the test at 700 Pa. Furthermore, in the classical CSS test the yield stress was established at 1100 Pa.

Fig. 8. Multi-stage creep tests of Mio-Pliocene clay paste from Warsaw

CONCLUSION A rheometer can be used to characterize soil creep resulting from structure changes. In the first stage, yield stress of Mio-Pliocene clay paste in unsaturated conditions was determined by a rotational controlled shear stress test. Based on the result from the previous stage, a cyclic oscillatory test (amplitude sweep test with controlled shear strain) and creep tests were defined. The oscillatory test defines the strain regime and yield strain. Creep tests were performed in two phases, the first as single-stage creep tests and the second as a multi-stage test. Soil strength under steady state stress is higher than during the creep process. Incremental load generates multi-stage creep, and therefore, soil structure is more resistant to shear stress. This can be caused by shortening the distance between clay particles and bond formation. One of the main disadvantages of this technique is the lack of pore pressure control. The presented results provide new data, which can be used in the future to compare different soils. Future studies will focus on the influence of changes in water content and temperature and SEM analysis will be used to investigate internal structure.


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Rheometer use in analysis of soil creep behaviour

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