Hydro-mechanical properties of unsaturated residual soil from a flysch rock mass

Journal Pre-proof Hydro-mechanical properties of unsaturated residual soil from a flysch rock mass

Josip Peranić, Mariagiovanna Moscariello, Sabatino Cuomo, Željko Arbanas PII:

S0013-7952(19)30671-4

DOI:

https://doi.org/10.1016/j.enggeo.2020.105546

Reference:

ENGEO 105546

To appear in:

Engineering Geology

Received date:

10 April 2019

Revised date:

21 January 2020

Accepted date:

20 February 2020

Please cite this article as: J. Peranić, M. Moscariello, S. Cuomo, et al., Hydro-mechanical properties of unsaturated residual soil from a flysch rock mass, Engineering Geology (2020), https://doi.org/10.1016/j.enggeo.2020.105546

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© 2020 Published by Elsevier.

Journal Pre-proof Hydro-mechanical properties of unsaturated residual soil from a flysch rock mass Josip Peranić(1), Mariagiovanna Moscariello(2), Sabatino Cuomo(2), Željko Arbanas(1)* (1)

University of Rijeka, Faculty of Civil Engineering Radmile Matejčić 3, 51000 Rijeka, Croatia

(2)

University of Salerno, Lab. Geotechnics, Department of Civil Engineering Via Giovanni Paolo II, 132, 84084 Fisciano (SA), Italy

(*)

Corresponding author

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HIGHLIGHTS

- Intact soil samples were used in determination of hydro-mechanical properties of the

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unsaturated residual soil from flysch rock mass.

Hydraulic conductivity around two orders of magnitude lower for remolded samples.

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Samples at natural water content present matric suction values ranging from 30 to 400 kPa.

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ABSTRACT

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Rainfall is a major triggering factor of landslides in flysch deposits along Europe. Physical and

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mechanical changes of flysch rock masses caused by the weathering process, result in a complex soil profile with residual soil typically present at the slope surface. The research encompasses residual

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soils from the Rječina River Valley in Croatia. Built in flysch deposits, the valley is known for several deep-seated and shallow historical landslides. This manuscript presents the hydro-mechanical properties of the soil existing in unsaturated zone of the slope that could play an important role in landslide activation on flysch slopes. Presented unsaturated soil property functions are essential for modelling of the transient rainfall infiltration process and how it affects the stability of flysch slopes in time. For the first time, shear strength and hydraulic conductivity determination were performed in saturated and unsaturated soil conditions, using both the intact as well as remolded samples of the residual soil form flysch rock mass. The results indicate that, in order to correctly define hydraulic and mechanical properties of the in-situ soil, measurements have to be performed on intact samples. Also, the presented results highlight the importance of hysteresis effects and hydraulic paths that fine-grained residual soil has been subjected to in the past.

Keywords: residual soil, flysch, landslides, hydraulic conductivity, unsaturated shear strength

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Journal Pre-proof 1. INTRODUCTION Heavy rainfall is the most common triggering factor of shallow and deep-seated landslides in flysch deposits throughout Europe. Previous studies of rainfall-induced landslides in similar formations were mainly focused on the build-up of positive pore-water pressures along the sliding surface as a landslide triggering factor (e.g. Eberhardt et al., 2005; Berisavljević et al., 2015; Arbanas et al., 2017; and, Berti et al., 2017) and on the effects of the weathering process on shear strength, physical properties and/or mineralogical composition (e.g. Vivoda Prodan and Arbanas, 2016; Vivoda Prodan et al., 2016; Vlastelica et al., 2017). For the first group of studies, various researchers employed conventional slope stability analyses based on limit equilibrium or finite-element method

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formulations, where the effect of rainfall on the slope stability is accounted for through simple variations of the groundwater level. This type of analysis assumes saturated, steady-state water flow,

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where for the worst-case scenario the phreatic surface reaches the soil surface (Collins and Znidarčić, 2004). This approach does not have the physics or governing equations required for solving the

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problem of transient rainfall infiltration through the unsaturated part of the slope and is usually based on assumptions that the soil is either completely dry or completely saturated, depending on

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the position of the groundwater level. The influence of the water content or difference between air pressure (ua) and water pressure (uw) (here called the matric suction) on the permeability or shear

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strength of the soil existing above the phreatic line is usually not considered. Since the time is not a variable in governing equations, a coupling between the rainfall infiltration and change of slope

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stability state through time cannot be considered within this approach.

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Factors that primarily control rainfall-induced slope failures are both rainfall characteristics and soil properties (e.g., Rahardjo et al., 2007; 2016). In general, rainfall causes a transient infiltration process in a slope, which affects slope material changing the moisture content and pore water pressure distribution in the soil cross-section, thereby changing the effective stress and shear strength of the soil that can finally lead to the slope failure. To investigate the process of transient rainfall infiltration through the unsaturated part of the slope and its influence on slope stability, the functional relationship between suction and soil-water content, the hydraulic conductivity, i.e. the coefficient of permeability with respect to the water phase (kw), and shear strength have to be known. While both the soil-water retention curve (SWRC) and the hydraulic conductivity function (HCF) govern the seepage process and build-up of pore water pressures during rainfall infiltration, the shear strength is related to the SWRC through the effective stress (e.g. Lu et al., 2010; Oh, et al. 2012). The relationship between the matric suction (ua-uw) and the shear strength of the soil is necessary for analyzing the stability of the slope where a part (commonly present in deep-seated landslides) or the entire sliding surface (typical for shallow landslides) is developed above the 2

Journal Pre-proof phreatic line. So far, no comprehensive studies have investigated the SWRC, HCF or the influence of the matric suction on shear strength properties of fine -grained residual soils from the flysch rock masses. Systematic investigations have been conducted mainly for materials of higher permeability, such as sandy or silty soils, where the unsaturated soil property functions are easier to obtain. The main reasons could be long time required for the investigation, but also the fact that no single measurement technique or device can be used to obtain the complete SWRC in case of fine-grained soils with low hydraulic conductivity. In addition, most of the measurements reported in the literature were performed using reconstituted samples of sandy or silty soils, assuming that samples produced in the laboratory would exhibit a similar behavior to the soil existing in the field. In this

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study, tests were performed on both intact and remolded samples. Obtained results suggest that the

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compaction and consolidation procedure cannot entirely restore the structure of a clay present insitu, resulting in the need for use of intact samples. This especially pertains to soil materials such as

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the residual soil from a flysch rock mass close to the ground surface, where a specific structure is characterized by the occasional presence of macro voids that originate from the biological activity

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and presence of siltstone particles that vary in size and weathering degree. Generally, studies that

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investigated the effects of sample disturbance or matric suction on hydraulic and strength properties of fine-grained soils are very rare or not existing at all.

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2. STUDY AREA AND BASIC SOIL PROPERTIES

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The Rječina River Valley, located in the outback of the City of Rijeka, Croatia, is well known by numerous historical and recent deep-seated and shallow landslides (Arbanas et al., 2014).

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Cretaceous and Paleogene limestone form the top of the slopes, while Paleogene siliciclastic rocks are present in the lower part of the slopes and the bottom of the valley, where the sliding processes have primarily developed. Intact soil samples used in this study were taken from the sampling pit location of the deep-seated Valići Landslide. According to the preliminary investigation results, the Valići Landslide occurred after several months of mostly uninterrupted rainfall as a reactivation of the part of a larger dormant landslide on 13 February 2014, (Mihalić Arbanas et al., 2017). Residual soil covering slope surfaces in the study area was found to have similar granulometric composition and plasticity limits (Peranić et al., 2018). Basic properties of the soil, classified as a low-plasticity clay, are summarized in Table 1. It was found that the superficial soil layer undergoes desaturation during dry summer months. Determination of the natural water content (wn) (Figure 1) was performed at the sampling pit location in several campaigns using the equipment for undisturbed soil sampling (Eijkelkamp Soil&Water, Inc.). For the first several meters of the profile in dry summer periods, the degree of saturation values (S) lower than 50% were 3

Journal Pre-proof measured. Short-term rainfalls of higher intensity occurring in September were able to saturate only the very superficial layer of the soil. According to the SWRC of the material and considering the significant hysteresis between the drying and wetting paths ( Peranić et al., 2018), ua-uw values corresponding to measured water contents of soil could vary from several kPa up to several hundred kPa. For example, ua-uw > 350 kPa were measured for intact samples collected at the wn during summers of 2016 and 2017 (Peranić and Arbanas, 2019). On the other hand, measurements performed on samples collected after rainy periods at the end of October in 2016 and 2017, indicated ua-uwvalues ranging from 30 to 50 kPa. TABLE 1: Mean values of the basic properties of the residual soil samples used in the study (Peranić et al. 2018).

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Atterberg’s limits PI USCS [%] [%] [/] 44 24 20 CL = plastic limit; PI = plasticity index

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Grain size distribution Clay, C Silt, M Sand, S Gravel, G 3 [kN/m ] [/] [%] [%] [%] [%] 2.7 20.9 30.3 53 10.4 6.3 = specific gravity; = saturated unit weight; = liquid limit;

(b)

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(a)

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FIGURE 1. Measured values of a) gravimetric water content, and b) degree of saturation for the superficial soil during sampling on 14 October 2016, 18 July 2017, 13 September 2017, 2 October 2017 and 28 May 2018.

3. SWRC AND DETERMINATION OF THE HYDRAULIC CONDUCTIVITY FUNCTION Peranić et al. (2018) successfully combined different devices and measurements techniques to obtain a complete SWRC of the residual soil from flysch rock mass. The obtained results suggest that, although using reconstituted samples has clear advantages in terms of the consistency of the test results and homogeneity of the specimens, different retention properties and material response were observed when intact samples were used instead of for remolded and consolidated ones. For example, the measurements results obtained from intact samples undergoing the drying process have indicate air entry value (AEV) of around 200 kPa, while samples exhibited both the over consolidated and normally-consolidated behavior, depending on the applied suction values. On the other hand, the results obtained from remolded samples indicated AEV higher than 400 kPa, with 4

Journal Pre-proof material exhibiting the normally-consolidated behavior only, undergoing a large reduction in the void ratio from the beginning of the drying process. The best-fit SWRC parameters of the van Genuchten (1980) equation, used to derive the HCF of the soil are provided in Table 2. TABLE 2: Best-fit SWRC parameters of the investigated soil (Peranić et al. 2018). van Genuchten (1980) 3

Drying curve

-1

[kPa ] 0.004

[/] 1.186

[/] 0.323

0.011

0.005

0.973

0.348

Wetting curve and

3

[m /m ] 0.028 constants;

residual volumetric water content

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There are different approaches used to obtain the functional relationship between the kw

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and ua-uw. For example, various setups of column test (e.g., McCartney et al.,2007; Cui et al., 2008; Li et al., 2009), rigid and flexible-wall permeameters (e.g., Huang et al., 1998; Samingan et al., 2003;

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Gallage et al. 2013), or centrifuge methods (e.g., Singh and Kuriyan, 2002; Zornberg and McCartney, 2010) can be used to directly obtain points of the HCF. However, depending on the soil type and

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measurement range, various studies have outlined limitations of direct measurement methods,

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where prolonged testing times, expensive equipment, and limited measuring range are especially pronounced in a case of testing fine-grained soils (e.g. Leong et al., 2004; Cui et al., 2008; Marinho et al., 2008).

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To overcome these problems, many researchers have proposed indirect methods for

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obtaining the HCF. Regression-based approaches can be used to estimate hydraulic properties of the soil from observed flow quantities. For example, Kool et al. (1985), Eching and Hopmans (1993), van

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Dam et al. (1992; 1994), Šimunek et al. (1998), Wayllace and Lu (2012) used the parameter estimation method on experimentally obtained data in the laboratory. The same method was applied to in-situ measurements (e.g., Šimunek and van Genuchten, 1996; Gribb et al., 1996; Inoue et al., 1998; Nakhaei and Šimunek, 2014). Despite the possibility of estimating the HCF without the need for direct measurements of permeability in unsaturated conditions, complex calculation procedures and the requirement for specialized engineering analyses have resulted in very limited application of the approach in the engineering practice. Another commonly adopted approach for estimating HCF of the soil is combining some of the quantities that are measured more easily than the HCF, such as the saturated hydraulic conductivity (ks ) with SWRC. The performance of such methods was investigated by many researchers for different types of soil, testing conditions, and specimen preparation techniques (e.g., Agus et al. 2003; Cai et al. 2014; Rahimi et al. 2015; Zhai and Rahardjo 2015; Zhai et al. 2019a). By using twenty sets of data found in the literature, which included measurements of both SWRC and kw(ua-uw) on

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Journal Pre-proof different soil types, ranging from sand to clay, Rahimi et al. (2015) concluded that the best-fit SWRC equation has a more significant effect on the estimation of the HCF than the relative permeability equation used in the calculation procedure. Agus et al. (2003) found that all the statistical models provide reasonable estimations of HCFs, while the best fit for the experimental data was generally obtained from Mualem’s (1976) model combined with the Fredlund and Xing’s (1994) SWRC equation. Due to the low ks values and wide range of matric suction values that an investigated soil material can exhibit, the HCF in this study was estimated using the SWRC data provided in Table 2 and directly measured ks values. The ks was measured both for intact as well as remolded and

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consolidated samples at different confining pressures. Readings obtained from mini-tensiometers in

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the HYPROP device were used to calculate the kw for ua-uw values up to 130 kPa, according to the Darcy-Buckingham law and the Extended Evaporation Method (Schindler et al., 2015) as well.

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3.1. Experimental procedures and results

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Saturated hydraulic conductivity was measured on both intact and remolded samples using the falling head method (ASTM D5084-03) in conventional, front-loading oedometer apparatus 26-

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WF0302 (Controls S.p.A.), and the constant head method (ASTM D5084-03) using the triaxial apparatus 28-WF4050 (Controls S.p.A.). The ks values were obtained for different values of effective

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vertical or isotropic stresses. Remolded specimens were trimmed from the sample prepared from a slurry material mixed at the water content above the wL and consolidated under vertical stress of 50

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kPa. Specimens of 38 mm diameter and a diameter/length ratio of 0.5 were saturated by incremental back-pressurization in a triaxial apparatus, until B value ≥ 0.95 was reached. Figure 2 shows intact

apparatus.

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and remolded specimens used for determination of ks with the falling head method in triaxial

The HYPROP device was used on intact samples taken in a steel cylinder 80 mm in diameter and 50 mm height, starting from the wn (HYPwn), or saturated by immersion into the water with (HYP1) and without application of a vacuum (HYP2). The kw was calculated as a function of ua-uw according to the Darcy-Buckingham law using the following expression (1): (1) where

is the mean matric suction value between the upper and the lower tensiometers,

geometrically averaged over a time interval of

,

with

,

,

representing the sample mass difference in time interval recorded by the scale (g), which is assumed to be equal to the volume of water loss

due to evaporation from specimen;

is the density

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Journal Pre-proof of water, assumed to be 1 (g cm-3),

is the flux factor depending on the soil type being tested,

denotes the cross-section area of the specimen (cm2), and

denotes the hydraulic gradient

averaged over the time interval (Schindler et al., 2015). Table 3 and Figure 3 present the experimentally obtained ks values for intact and remolded samples consolidated at different values of the effective stress. The saturated hydraulic conductivity (ks = 4.60E-08 m/s) was adopted along with the SWRC equation parameters obtained for the drying and wetting process to estimate the HCF of the soil according to the equation proposed by van Genuchten (1980) (2): {

[

]

(2)

]

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[

}

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The obtained results are presented in Figure 4 along with the SWRC (Table 2) presented on the second axis. Continuous and dashed green lines represent the HCF estimated by (2) for the drying

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(HYPwn), black (HYP1) and purple (HYP2) colours.

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and wetting paths, respectively. Results obtained with the HYPROP device are represented with blue

(b)

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FIGURE 2. Samples used for determination of the ks : a) intact, and b) completely remolded and consolidated sample.

TABLE 3: Saturated hydraulic conductivity from the constant head tests performed in conventional triaxial apparatus. Intact samples ' [kPa]

Remolded samples

[/]

ks [m/s]

[/]

ks [m/s]

25

0.95

4.60E-08

0.97

3.50E-10

50

0.90

2.79E-08

0.93

1.64E-10

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Journal Pre-proof 100

0.85

2.71E-09

0.88

4.98E-11

200

0.80

3.02E-10

0.81

2.50E-12

400

0.75

9.39E-11

/

/

void ratio;

sat. hyd. cond.

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mean eff. stress;

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FIGURE 3. Saturated hydraulic conductivity vs. effective (vertical or mean) stress for intact and remolded

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samples of residual soil from a flysch rock mass.

FIGURE 4. Measured ks , kw (ua-uw) and the estimated HCF of residual soil from a flysch rock mass along with the SWRC presented on the second axis.

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Journal Pre-proof 4. INVESTIGATION OF THE SHEAR STRENGTH PROPERTIES Shear strength of soil above the phreatic line relies both on the shear strength components associated with the effective cohesion and frictional component, as well as the strength component associated with the suction. The latter can be obtained by performing shear strength tests in devices that are modified to control or measure suction (and/or water content of soil) during the shearing. So far, shear strength properties of the residual soil were investigated using the conventional direct shear and ring shear apparatuses on completely remolded and consolidated samples only (e.g. Oštrić et al., 2012; Vivoda Prodan et al., 2016). A study performed by Vivoda Prodan et al. (2016) indicated that mobilized friction angle

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increased while cohesion decreased with increasing weathering grade of siltstones from a flysch rock mass. Conventional direct shear and ring shear apparatuses were used on samples produced in the

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laboratory, by mechanically crushing siltstone blocks of varying weathering degrees to sand-sized particles. Oštrić et al. (2012) used the undrained ring shear apparatus to obtain shear strength

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parameters of completely remolded and consolidated samples taken from the superficial soil layer in the Rječina River Valley. The influence of the disturbance effects or matric suction on shear strength

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properties of the investigated soil was never investigated and it is completely unknown. In this study, a significant effort was made to obtain intact samples from the residual soil from the flysch rock mass

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that is the superficial soil layer over the Valići Landslide body. Due to the possible presence of roots and siltstone particles of various size and weathering degrees, and general difficulties related to

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collection and installation of intact samples of the material, sampling was performed using the steel

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cutters in the field with the preservation of wn. Sampling depth varied from 0.5 to 1 m. There are two basic approaches that have been used extensively for determination of the shear strength of unsaturated soils: the effective stress approach ( Bishop, 1959) and the independent stress state variables approach (Fredlund and Morgenstern, 1977). A major part of the shear strength equations of unsaturated soil are modifications of Bishop’s effective stress formulation for the partially saturated soil: (3) where pressure

and

are the effective and total stress (kPa), the difference between total stress σ and air

represents the net stress

represents a matric suction

, the difference between the air and water pressure , while the parameter is the coefficient of effective stress

which is a constitutive property of soil related to the water content of soil (Bishop, 1959). Bishop proposed the effective stress parameter to be simply a degree of saturation (

). For completely

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Journal Pre-proof saturated soil (

), the

stress. For the dry soil (

term vanishes and the equation (3) reduces to Terzaghi’s effective ),

Although the

term vanishes and the effective stress equals the net stress.

can be measured or controlled with a certain degree of confidence, to

date it is not possible to directly measure changes of the effective stress in soil due to changes in . Consequently, the effective stress parameter χ cannot be determined directly from the equation (3) (Lu and Griffiths 2004). However, Bishop (1954) proposed an indirect way to obtain the parameter χ from the stress conditions at failure. The following shear strength equation is obtained by substituting the (3) into the conventional Mohr-Coulomb failure criteria (4)

is the shear strength of unsaturated soil,

internal friction, while

is effective cohesion,

is the effective angle of

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where

]

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[

and

are the net normal stress and matric suction at

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failure, respectively. The last two can be obtained at the failure by using, for example, the axis translation technique in modified direct shear or triaxial apparatus.

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Bishop’s proposal of the effective stress formulation has encountered several difficulties to quantify the contribution of the matric suction to the effective stress of unsaturated soil ( Seboong

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and Lu, 2014). The impossibility to explain the collapse phenomena (Jennings and Burland, 1962), the non-unique relationship between χ and S (Coleman, 1962), or the fact that the equation (3) contains

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the parameter which is material property (Fredlund and Morgenstern, 1977) are limitations which are often noted in the literature. To overcome the difficulties associated with quantifying the

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value, Fredlund and Morgenstern (1977) introduced the independent stress state variables approach

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to define stress state in partially saturated soil. Fredlund et al. (1978) extended the Mohr-Coulomb failure criterion to unsaturated soils with the equation:

where

(5)

represent the “effective cohesion” located on the extended M-C failure envelope as the

intercept on the shear stress axis where the net normal stress and the matric suction at failure are equal to zero, while

and

suction on the failure plane at failure, normal stress state variable, while

represent the net normal stress state and matric is the angle of internal friction associated with the net represents the contribution to the shear strength due to

matric suction, and is generally a function of matric suction. The three -dimensional failure envelope can be observed in two 2D planes: net normal stress vs. shear stress plane parallel to the matric suction axis provides contour lines of the failure envelope defining values of the angle of internal friction

and the effective cohesion

cohesion for

for saturated conditions,

0 or apparent

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Journal Pre-proof Although a linear relationship between

and

might exist over a limited range of

matric suction, experimental results obtained for a wide range of suctions on different types of soil (e.g., Escario and Saez, 1986; Gan and Fredlund, 1988; Vanapalli et al., 1996; Huat et al., 2005; Kim et al., 2010; or Marinho et al., 2013) showed a non-linear relationship between shear strength of unsaturated soil with respect to property

and an increase in

. The need for defining the nonlinear soil

over a wide range of suctions implies a large number of tests and possible uncertainties

in obtained results, which resulted in limited application of the independent stress state variable approach in practice (Khalili and Khabbaz, 1998; Nuth and Laloui, 2008; Lu et al., 2010).

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4.1. Experimental procedures and results obtained using the conventional direct shear apparatus

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Saturated shear strength properties were investigated using the conventional direct shear apparatus 27-WF2160 (Controls S.p.A.) The adopted shearing rate 5E-03 mm/min was around half to

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one order of magnitude lower than the maximum displacement rate calculated from the

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consolidation theory according to equations provided in CEN ISO/TS 1789-10:2004 (E) and ASTM D3080-11 test standards. The time to reach failure conditions was about ten times less than the

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default minimum times to failure according to the USCS results suggested by ASTM D3080-11. After being consolidated for 24 h under 50, 100 and 200 kPa of normal stress, the shearing stage was started until the maximum shear displacement was reached. After the tests were performed on

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intact samples, soil material from each testing device was thoroughly mixed with distilled water and

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left to rest overnight. Following this, the same amount of material was added in the same steel cutter used for in-situ sampling and installed in the same testing device. The identical testing procedure

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regarding the applied vertical stress, consolidation stage and shearing conditions was repeated on the produced specimens. The aim was to identify possible differences in strength or stiffness characteristics of material only due to specimen disturbance effects, avoiding problems of data interpretation and differences due to different shapes and volumes of used samples or differences between the three testing devices that were used simultaneously. Table 4 summarizes peak and final shear stress values measured on intact and remolded specimens, along with corresponding horizontal and vertical displacements. As shown in Figure 5, specimens sheared under vertical stress of 200 kPa were still undergoing compression when maximum horizontal displacement was reached. The final values obtained at the end of test were used for the interpretation of test results. Figure 5 summarizes the results for intact (solid line) and remolded (dashed line) specimens sheared under vertical stresses of 50 (black), 100 (orange) and 200 kPa (green). Mohr-Coulomb shear strength envelopes obtained for intact and remolded samples using a) peak and b) final values are shown in Figure 6. 11

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FIGURE 5. Shearing stage test results for intact (solid) and remolded (dashed line) samples in terms of a)

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horizontal displacement vs. shear stress and b) horizontal displacement vs. vertical displacement.

TABLE 4: Peak and final conditions obtained in conventional DSA on intact and remolded samples.

50 100 200

Intact Remol. Intact Remol. Intact Remol.

τ [kPa] 37.8

Peak values Stress ratio [/]

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Sample

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σv' [kPa]

34.0 64.0 59.0

129.5 115.4

0.90 0.92 0.89

dh [mm] 11.36

dv [mm] 0.53

τf [kPa] 37.0

10.94

0.48

33.1

8.51

0.66

59.5

7.76

0.53

52.8

6.31

0.67

115.4

13.36

0.57

112.8

Final values Stress ratio dhf [/] [mm] 16.90 0.89 16.93

dvf [mm] 0.52

16.87

0.76

16.93

0.63

16.48

0.95

16.75

0.67

0.89 0.98

0.54

σv' = vert. stress; τ = shear stress; dh = hor. displ. for τ; dv = vert. displ. for τ; subscript f denotes final values

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Journal Pre-proof

(a)

(b)

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FIGURE 6. a) Peak and b) final shear strength envelopes for intact (solid line, filled markers) and remolded

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specimens (dashed line, empty markers), obtained in conventional direct shear apparatus.

4.2. Experimental procedures and results obtained us ing the modified direct shear apparatus

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Investigation of shear strength properties of residual soil in unsaturated conditions was performed

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using two modified, axis-translation based, direct shear devices. The Suction Controlled Direct Shear Apparatus (SCDSA) (Megaris s.a.s.) was used at the University of Salerno, Italy to perform three

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standard (suction-controlled) tests (DSUVCJ01, DSUVCJ02, and DSUVCJ03) on intact samples starting from the wn. Initial matric suction was measured with a closed drainage valve and under low confining pressure. Following this, samples were consolidated for 24 h under vertical stresses of 50,

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100 and 200 kPa and equilibrated at ua-uw = 36 kPa afterwards. Shearing was performed at a

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constant shear rate of 5E-03 mm/min under constant ua-uw value of 36 kPa up to a maximum shear displacement of around 20 mm. The unsaturated version of the Back Pressured Shear Box (GDS

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Instruments Ltd.) (UBPS) was used at the University of Rijeka, Croatia to perform suction-controlled tests on intact samples installed at the wn or saturated outside the shear box by submersion into the de-aired water prior to installation. Samples with lower wn (UVSCDS04, UVSCDS06, UVSCDS11, and UVSCDS12) ranging from 14.1% to 17.4%, were collected at the end of July 2017, during the summer dry period. Other samples with higher water content values were collected during March 2017 (UVSCDS01, UVSCDS02, UVSCDS03, and UVSCDS07) and September 2017 (UVSCDS08, UVSCDS09, and UVSCDS10), periods with higher amounts of rainfall. Shearing was performed under different net vertical stress values and a constant shear rate of 5E-03 mm/min, except for the two long-lasting wetting tests that were performed on samples UVSCDS11 and UVSCDS12 and are excluded from the interpretation here. Specimen characteristics and testing conditions for the tests performed using the UBPS are summarized in Table S1 (provided as supplementary material). Conditions at failure for all direct shear tests are summarized in Table S2 (provided as supplementary material). Shearing stage test 13

Journal Pre-proof results obtained on intact samples under the same value of the net vertical stress are summarized in Figures 7 to 9. The volume of water change during the shearing was measured by the volume measuring system in case of the modified direct shear apparatuses. It was assumed that samples were completely saturated (ua-uw = 0 kPa) in the case of the conventional direct shear apparatuses, while the volume of water change during the shearing was assumed to be equal to the total volume change of the specimen calculated from the axial deformation readings. All shear strength results obtained on intact samples under 50, 100 and 200 kPa of the net vertical stress are summarized in Figures 7, 8 and 9, respectively. Results are shown in terms of the shear stress, vertical displacement, degree of saturation, and change of water volume vs. horizontal displacement in a), b), c), and d)

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plots, respectively.

(c)

(d)

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(b)

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(a)

FIGURE 7. Test results obtained on intact samples sheared under net vertical stress of 50 kPa in terms of a) shear stress; b) vertical displacement; c) degree of saturation; and, d) change of volume of water vs. horizontal displacement.

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(c)

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(a)

(d)

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al

Pr

e-

pr

(b)

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FIGURE 8. Test results obtained on intact samples sheared under net vertical stress of 100 kPa in terms of a) shear stress; b) vertical displacement; c) degree of saturation; and, d) change of volume of water vs. horizontal displacement.

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(c)

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f

(a)

(d)

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al

Pr

e-

pr

(b)

FIGURE 9. Test results obtained on intact samples sheared under net vertical stress of 200 kPa in terms of a) shear stress; b) vertical displacement; c) degree of saturation; and, d) change of volume of water vs. horizontal displacement.

Bishop’s effective stress formulation (3), with the effective stress parameter equal to the current degree of saturation (i.e. at failure), adjusted for the change in total and water volume change during the shearing, was used to present all the shear strength tests results obtained on intact samples in Figure 10. A continuous line fits the result obtained using the conventional direct shear device (Conv. DSD), while dashed line fits all of the results obtained by using two modified direct shear apparatuses (SCDSA and UGDSBPS). Both envelopes represent peak shear strength. Two empty squared markers indicate results from the UVSCDS03 and UVSCDS02b tests, which were excluded from the fitting procedure. Test UVSCDS03 was performed as a standard test in the UGDSBPS on an intact sample installed at a wn inside the shear box, and then saturated by adding the water from the top of the porous disc. The specimen was consolidated under the effective vertical 16

Journal Pre-proof stress of 100 kPa and sheared at a constant shear rate of 5.0E-03 mm/min. However, the obtained results suggest that the saturation process inside the shear box was unsuccessful, with the ua-uw≈30 kPa still existing inside the specimen during the shear. In the following tests, all samples were saturated outside the shear box. In the case of the UVSCDS02b test, which was performed as reshearing of the sample used in UVSCDS02 test, shear stress was still increasing when a maximum horizontal displacement of 12.5 mm was reached. The maximum recorded shear stress was probably lower than would be reached if the shear strength was fully mobilized. The reason for this could be the fact that the specimen was re-sheared, which might have affected the behavior of the specimen

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Pr

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pr

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during the following shearing, especially regarding the stiffness, which could be significantly reduced.

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FIGURE 10. Results in terms of the effective stress formulation (Bishop, 1959), obtained on intact samples by using conventional (DSD) and modified (SCDSA and UGDSBPS) direct shear test apparatuses.

Results are interpreted in two planes of the extended Mohr-Coulomb’s failure envelope (5) in Figure 11. Matric suction vs. shear stress plane (Figure 11 a) provides a

vs.

) relationship.

Figure 11 b) shows the plot for net normal vs. shear stress, a plane parallel to matric suction axis, providing

,

, and c values.

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(b)

vs.

); and b)

vs.

) planes of the extended Mohr-

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FIGURE 11. Results projected onto a)

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f

(a)

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Coulomb’s failure envelope.

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5. DISCUSSION

The ks values obtained using intact samples differed greatly from values obtained on remolded samples. Test results indicate that for the same effective stress or void ratio, ks of remolded samples

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is around two orders of magnitude lower than in the case of intact samples (ks =4.6E-08 m/s).

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Different retention properties were measured when intact samples were used instead of remolded ones as well. These findings suggest that, to correctly define hydraulic properties of the residual soil

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from the flysch rock mass in-situ, laboratory measurements have to be performed on intact samples. The formation process of the soil covering flysch slopes in the valley results with a complex soil structure characterized by a general heterogeneity, presence of macro-voids and siltstone grains of various sizes and weathering degrees (Figure 2 and 12), which all significantly affect the hydraulic properties of the material. The use of remolded and consolidated samples in determination of the ks value can result with significant deviation of real soil properties that would be used in the determination of other soil parameters and processes. Hydraulic conductivity of unsaturated soil depends both on the volume of voids and amount of water inside the voids. For fine-grained soil, change of the effective stress due to change of suction can be substantial, inducing large volumetric deformations. Matric suction, as an environmentally controlled variable, is expected to change effective stress in the slope material more frequently than a change of the net stress (e.g. externally applied loads).

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FIGURE 12. Heterogeneity of the intact specimen: siltstone grains of different size and weathering degree,

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along with some roots present at the shear surface of the specimen used in the DSUVCJ01 test.

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SWRC measurements showed that the soil material presents normally- and overconsolidated behavior depending on the applied suction value and previous hydraulic paths to which samples were exposed. In this study, we use important observation about soil behavior during the

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wetting process, which reveals that the soil presents suction-overconsolidated behavior when

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exposed to conditions of increasing water content and reduction of the matric suction, undergoing a negligible increase in volume (i.e., swelling). This especially may be expected considering the range of

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matric suctions which is of interest for studies of rainfall-induced landslides in flysch slopes of the Rječina River Valley: ua-uw measurements with the axis-translation technique performed on intact samples collected at the wn showed that, depending on the weather conditions in a period preceding to sampling, matric suction ranged from 30 to 400 kPa. We utilize this observation to estimate the HCF relevant for analyzing changes in flysch slopes exposed to a rainfall event using the ks value and SWRC obtained for the wetting process (Figure 4). Although very few measurements were performed with the HYPROP device on intact samples, the calculated kw values are consistent with the estimated HCF. In all tests, an increase of ua-uw resulted with a decrease of the kw. The lowest kw was obtained for specimen installed at the wn (HYPWn) due to the lowest S, while the highest kw was obtained for the specimen saturated with the application of small vacuum (HYP1) (Figure 4). Shear tests performed on intact as well as remolded samples using the conventional direct shear apparatuses showed that, although the intact samples exhibit greater compression during the shearing stage, disturbance effects seem to have negligible effects on the shear strength properties 19

Journal Pre-proof of the investigated soil material. This is especially true for final (residual) conditions. According to the results summarized in Table 4, maximum shear strength was mobilized at horizontal displacement values of 11.4, 8.5 and 6.3 mm for intact specimens sheared at normal stresses of 50, 100 and 200 kPa, respectively (Figure 5a). Resulted Mohr-Coulomb peak (and final) shear strength parameters, the effective cohesion ( ), and the effective angle of internal friction (

), were 5 (9) kPa and 31.7

(27.8)° for intact samples, and 6 (3) kPa and 28.6 (28.4)° in a case of remolded samples (Figure 6). As emphasized by Sheng et al. (2011), available shear strength results of partially-saturated soils were mostly obtained on specimens prepared from slurry soil consolidated to a specified pressure, or with a compaction method with desired water content. Studies performed on intact

f

samples are rare and there is a need for a greater number of this type of laboratory studies.

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Schnellmann et al. (2013) concluded that the effect of net normal stress on SWRC should be considered in the shear strength equations to obtain a reasonable prediction of the unsaturated

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strength behavior. Using the moist-tamping method to produce low-plasticity clay samples, Chiu et al. (2014) found that the unsaturated strength depended on the hydraulic history of samples and

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that the main wetting surface should be used when predicting the shear strength of unsaturated soil

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subjected to different hydraulic histories. Findings obtained for intact samples in this study are on the same trace.

Insights from the first suction-controlled direct shear tests revealed that the multistage tests

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are inapplicable in the case of the investigated soil. Multistage direct shear tests have proven to be

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useful, especially for testing intact samples of low hydraulic conductivity in unsaturated conditions, where equilibration and testing times are considerably shorter than in case of triaxial tests. At the

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same time, it is possible to obtain more data using a single specimen, which is generally more expensive and challenging in the case of intact samples (e.g., Gan and Fredlund, 1988; Nam et al., 2011). However, one of the requirements is that the material exhibits flat shear stress vs. shear displacements curves for very low values of shear displacements ( i.e., bellow 1 or 2 mm), which was not the case for the investigated soil. Instead, some of the tests were performed as re-shearing under increased values of σ-ua and ua-uw. The interpretation of results showed that these can be successfully used, and that disturbance and re-shearing effects did not affect the results. Several observations could be outlined from the shear strength results obtained on intact samples (Figures 5, 7, 8 and 9). In any case, the increase of ua-uw resulted in an increase of the shear strength (Figures 5a, 7a, 8a, and 9a). Compressive behavior during shearing in saturated or nearlysaturated conditions changed to dilative as the ua-uw was increased during the shearing (Figures 5b, 7b, 8b, and 9b). On the other hand, as the ua-uw increases, so does the rate at which water drains from the specimen. Consequently, the S rapidly decreases during the shearing at higher ua-uw values (Figures 7c to 9c), due to dilation of specimens and the lowering of the water content (Figures 7d to 20

Journal Pre-proof 9d). Compression and water flow inside the specimen during shearing at low ua-uw values increase the S during the shearing stage. The threshold ua-uw value, separating two behavior patterns during the shearing (between 36 and 72 kPa for σ-ua = 50 kPa), was found to increase with the increase of the net vertical stress. Dilative behavior and desaturation of samples sheared under σ-ua = 100 kPa was observed only ua-uw was increased to 72 kPa. On the other hand, for ua-uw < 72 kPa, the tested specimens presented contractive behavior with the increase of a S during the shearing. The dilation and significant desaturation of samples shear under σ-ua = 200 kPa was observed only when ua-uw was increased to 138 kPa. A strong relationship exists between the confining stress (Ng and Pang, 2000) or void ratio (Tarantino, 2009) and retention properties of the soil. Many of the shear strength

f

models that predict the unsaturated shear strength of soil use the intrinsic relationship between

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shear strength and SWRC. The results obtained in this study agree with suggestions from Schnellmann et al. (2013) regarding the consideration of the effect of σ-ua on SWRC in the equations

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for prediction of the unsaturated soil strength. However, Schnellmann et al. (2013) drew conclusions based on test results obtained on coarse sand with AEV of 1 kPa and ks = 1.10E-04 m/s.

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Bishop’s effective stress envelope can be used for both saturated and suction-controlled

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tests according to the results presented in Figure 10. If the residual instead of peak shear strength is used for the conventional DSD tests performed under vertical stress of 200 kPa, a unique shear strength envelope is obtained for all of the results, with the angle of internal friction value kPa. Another interesting observation is made at Figure 11 a)

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and the effective cohesion

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that describes the functional relationship

vs .

). Results indicate that

holds for

ua-uw values up to approximately 75 kPa. Then there is a narrow range of ua-uw for which

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) relationship becomes non-linear, while sharp reduction of ua-uw is increased above 100 kPa,

vs.

occurs afterwards. As the

becomes equal to constant value of around 9°, with very little

variation for the considered range of ua-uw. By analyzing the SWRC results in semi-log vs form, Peranić et al. (2018) obtained that

kPa and air expulsion value

) kPa.

Considering that specimens were saturated prior to installation in the shear box and then dried by equilibrating at a desired ua-uw value, linear

relationship would be expected to extend along

a much wider range of matric suction (up to the AEV) according to existing models (e.g., Vanapalli et al., 1996; Fredlund et al., 1996). Results obtained in this study indicate that a relevant hydraulic path is much closer to the main wetting than the main drying path. This is an important observation because a large part of the equipment used for determination of the SWRC is intended for the drying path only. These findings are important in the case when these and similar fine-grained soils are used in small-scale physical landslide models. Except for the small-scale natural slopes, the build-up of the

21

Journal Pre-proof physical model implies disturbance of the original soil structure and use of the disturbed and compacted soil. An additional fact is that the shear strength associated with the matric suction can have an important role in the small-scale landslide models. Considering a wide range of possible matric suction, this particularly refers to models built of fine-grained soils that research landslide triggering mechanism in static conditions caused by an artificial rainfall, which basically presents a wetting process. 6. CONCLUSIONS Unsaturated shear strength and hydraulic conductivity function (HCF) are the soil parameters

oo

f

required for performing a transient rainfall infiltration analysis and determining its influence on a slope’s stability. Till now, these were completely unknown for the residual soil from a flysch rock

pr

masses. The results suggest that laboratory measurements for the determination of hydraulic properties should be performed using intact samples. Both the SWRC and the hydraulic conductivity

e-

features significantly differ when measured on intact rather than remolded samples. The HCF relevant for investigation of rainfall-induced landslides in flysch slopes was estimated from the ks and

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SWRC obtained for the wetting path. Measurements of the kw performed under a very limited range of ua-uw closely matched the estimated values, indicating that increasing of the matric suction

al

resulted with a decrease of kw. According to the result obtained by Peranić et al. (2018), the kw of the investigated soil should increase when undergoing the wetting process mainly due to an increase of

rn

the water content, while an increase due to swelling should have negligible effects. The results obtained in conventional direct shear apparatus indicated that disturbance

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effects have very little effect on the soil shear strength envelope. Under the same confining pressure, maximum shearing resistance values were around 10% higher for intact samples. Tests performed in unsaturated conditions suggest that multistage direct shear test with control of ua-uw are inapplicable in case of the investigated soil, due to the large shear displacements required for full mobilization of the soil shear strength. However, the effect of the re-shearing of the samples under increasing values of the σ-ua and ua-uw seems not to affect results significantly. Axis translation based direct shear apparatuses were successfully used to measure matric suction values for samples installed at the wn. Depending on the weather conditions existing in the field prior to the sampling, matric suction values ranged from 30 up to 400 kPa. The soil exhibited exclusively contractive behavior when tested in saturated conditions. Contractive behavior in saturated or nearly-saturated conditions changed to dilative as the ua-uw was increased under the same value of the σ-ua. The threshold ua-uw value separating contractive and dilative behavior was found to increase with the increase of the σ-ua applied during the shear, outlining the connection between the SWRC and the 22

Journal Pre-proof behavior of fine-grained soil during the shearing process. The interpretation of the results in terms of the Bishop’s effective stress formulation indicated that a unique envelope exists (

,

kPa) for all tests performed. However, when interpretation was made in terms of the independent stress state variable approach, it was found that the obtained results depart to a certain extent from the behavior patterns described in the literature, obtained mostly on slurry samples prepared and consolidated in the laboratory. The unsaturated shear strength tests indicate that the ua-uw at which the relationship between

vs.

becomes nonlinear is much lower than the AEV of

the soil and is closer to the AExV. The results provide a motivation for further research on hysteresis

f

effects on shear strength properties of intact soil samples.

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ACKNOWLEDGMENTS

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The research presented in this paper was supported by Croatian Science Foundation under the Project IP-2018-01-1503 “Physical modelling of landslide remediation constructions behaviour under

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static and seismic actions (ModLandRemSS)”. This support is gratefully acknowledged. The part of laboratory equipment used for laboratory testing was provided in the frame of Project “Research

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Infrastructure for Campus based Laboratories at the University of Rijeka”, co-funded in part by the Ministry of Science, Education and Sports of the Republic of Croatia and the European Fund for

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Journal Pre-proof Author statement Josip Peranić: Conceptualization, Investigation, Formal analysis, Validation, Writing - Original Draft, Mariagiovanna Moscariello: Investigation, Formal analysis Sabatino Cuomo: Methodology, Resources, Validation Željko Arbanas: Methodology, Resources, Supervision, Writing - Review & Editing, Project

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administration, Funding acquisition

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Journal Pre-proof HIGHLIGHTS

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Intact soil samples were used in determination of hydro-mechanical properties of the unsaturated residual soil from flysch rock mass. Hydraulic conductivity around two orders of magnitude lower for remolded samples. Samples at natural water content present matric suction values ranging from 30 to 400 kPa.

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