INTRODUCTION
Landslides in soils like shales, sandstone, and mudstone are common globally, with a notable increase in earthquake-induced landslides in Japan since the Mid Niigata Prefecture Earthquake in 2004 These landslides differ significantly in scale, movement, and slip surface locations compared to rainfall-induced landslides During an earthquake, existing landslides with slickensided ruptures often experience shear displacement along weaker slip surfaces A typical example from the Mid Niigata Prefecture Earthquake illustrates a landslide occurring at the boundary between weathered and un-weathered mudstone Most landslides develop along discontinuous surfaces, such as bedding planes, where the strength of the upper layer contrasts with that of the lower layers However, the strength and deformation properties of the contact surfaces between different soil layers under static and dynamic loading require further investigation.
Naturally cemented clays exhibit significant bonding between particles due to diagenesis, primarily through carbonate precipitation and crystal growth on soil grains This natural cementation enhances soil resistance to deformation, and when disrupted, can lead to substantial failure and rapid deformation Landslide soils demonstrate the mechanical properties of cemented soil, attributed to long-term diagenetic bonding in marine and arid environments, as well as weathering processes The unique behavior of these soils stems from the in situ development of natural cementation shortly after deposition, which can create chemical binding and result in over-consolidation.
2 similar to that of over-consolidated clays, such as strain softening and higher initial stiffness
Researchers have extensively examined the behavior of artificially cemented soils to better understand naturally cemented soils By mixing clay with a small amount of Portland cement, artificially cemented clay can be created to simulate the long-term effects of natural cementation This laboratory method allows for faster cementation bonding compared to the natural diagenetic processes Recent studies have focused on the stress-strain relationships and strength properties of these cemented soils, utilizing various testing methods such as triaxial, direct shear, unconfined compression, and ring-shear tests, primarily on artificially cemented samples Despite the differences in soil types, common mechanical characteristics like yield stress, initial stiffness, peak strength, residual strength, and dilatancy are observed across various cemented soils.
Aging in clays refers to the long-term cementation process, impacting their mechanical behavior through delayed compression, higher consolidation yield stress compared to effective overburden pressure, and stress overshooting in e-log p relations (Bjerrum, 1967) Research by Leroueil and Vaughan (1990) indicates that naturally cemented soils, such as claystone and sandstone, exhibit similar mechanical behaviors, regardless of the cementation cause Consequently, artificially cemented clay samples can replicate many characteristics of natural clays, particularly under stresses below the apparent pre-consolidation stress, where they are sensitive to stress changes and loading duration Studies show that artificially and naturally bonded soils are comparable in yield compression stress and strain-softening behaviors (Cuccovillo and Coop, 1999) Furthermore, the behavior of artificially cemented kaolin is qualitatively similar to sensitive natural clays (Sangrey, 1972; Burland, 1990), while Fischer et al (1978) found that cemented Drammen clay behaves like non-cemented clay with an over-consolidation ratio of approximately 1.7 Kasama et al (2000) noted that the failure envelope of cemented clay parallels that of non-cemented clay.
Recent studies by Horpibulsuk et al (2004, 2005) indicate that the behavior of cemented clays significantly differs from that of over-consolidated soils, as evidenced by three consolidated undrained triaxial compression tests performed on cemented clay.
On the other hand, residual strength of these soils is one of the most important characteristics in evaluating the stability of reactivated landslides (Skempton, 1964;
Following Skempton's groundbreaking research in 1985, ring shear tests and reversal direct box shear tests have become crucial for assessing the residual strength of soil, offering advantages over traditional methods like the triaxial test (Bishop et al., 1971; La Gatta, 1970; Yatabe et al., 1996; Toyota et al., 2009) Key factors influencing residual strength across various soil types include physical and chemical properties, mineral composition, effective normal stresses, re-consolidation, over-consolidation ratio (OCR), and shear displacement rate (Lupini et al., 1981; Skempton, 1985; Lemos et al., 1985; Gibo et al., 1987, 2002; Moore, 1991; Yatabe, 1991; Stark and Eid, 1994; Tika et al., 1996; Suzuki et al., 2000, 2001; Vithana et al., 2012; Kimura et al., 2013) Cementation significantly impacts the residual strength of landslide materials, particularly at bedding planes with low confining pressures, where effective cohesion is vital for slope stability (Mesri and Abdel-Ghaffar, 1993) While Wissa et al (1965) found that residual strength remains unaffected by cementation, Sasanian and Newson (2014) noted that it increases with curing time and cement content, albeit at a slower rate than peak strength However, there is a scarcity of literature regarding the residual strength of varied cemented soil layers evaluated using ring-shear test apparatus (Suzuki et al., 2007).
This study investigates the interface of artificially cemented clays to model the behavior of slip surfaces between two soil layers under rapid earthquake-induced loading The primary aim is to clarify the residual strength characteristics of discontinuous planes that represent realistic slip surfaces with varying degrees of cementation Utilizing a ring-shear test apparatus, the research seeks to enhance the assessment of earthquake-induced landslide risks, as highlighted by Suzuki et al (2007).
The strength of cemented clay is influenced by the loss of cementation and the alignment of platy clay particles during shearing Landslides involving cemented clays are common worldwide, making it crucial to assess the stability of slopes composed of naturally cemented materials, like mudstone Understanding the residual strength characteristics of soil samples with natural cementation is essential for evaluating landslide risks.
Figure 1.1 Typical case of earthquake-induced landslide in the 2004 Mid Niigata
Earthquake-induced slope failures and landslides in naturally cemented clays pose significant hazards due to their susceptibility to cyclic shear stresses of varying amplitudes and frequencies, which can diminish their stiffness and strength This cyclic degradation contributes to deformation and instability in natural slopes While extensive research has been conducted on the dynamic behavior of sandy soils, there is a notable lack of studies focusing on clayey soils, particularly naturally cemented clays Therefore, further research is essential to assess the extent and degree of soil deformation under different monotonic and cyclic loads During earthquakes, soil elements experiencing initial static shear stresses on potential sliding surfaces are subjected to additional cyclic shear stresses, highlighting the complexity of these dynamic properties (Jurko et al., 2008).
The impact of dynamic loading on the pre-existing shear surfaces of cemented clayey soil remains under-researched, particularly regarding cyclic degradation Previous studies, such as those by Ansal and Erken (1989) and Grachev et al (2006), have explored various soil properties, including clay content and shear strain amplitude However, the effects of dynamic loading on lightly cemented kaolin clay and its behavior under cyclic conditions require further evaluation to enhance our understanding of this largely unexplored area.
Residual strength is assessed in laboratories using various testing devices, with the conventional ring shear apparatus, based on the Imperial College design by Bishop et al (1971), being widely favored for its advantages The Bromhead ring shear apparatus, developed in 1979, is particularly popular for commercial testing due to its simplicity, cost-effectiveness, and minimal sample volume requirements Research indicates that similar residual strength values can be obtained using different laboratory shear devices and testing methods (La Gatta, 1970; Bishop et al., 1971; Townsend and Gilbert, 1973; 1976) The traditional single-stage ring-shear test method consolidates each specimen and shears it in a single process, yielding only a stress-displacement relationship, with peak and residual strengths identified at failure This approach necessitates multiple specimens tested at varying normal stress levels to accurately predict the failure envelope's shape.
The multistage test technique involves a single specimen subjected to varying normal stress levels followed by shearing to determine the complete residual strength envelope This method can also assess the effects of shear rate on residual strength by gradually increasing the shear rate on an individual specimen The multistage ring-shear test offers significant advantages over the single-stage ring-shear test, primarily because it accurately plots the failure envelope due to its comprehensive approach.
The method involves testing six different normal stresses, requiring a smaller sample volume compared to individual testing, and significantly reducing test duration Additionally, it allows for the extraction of numerous strength parameters from a single specimen However, the primary limitation of this approach is the difficulty in accurately measuring peak shear strength.
The multistage technique is widely accepted due to its independence from stress history, as noted in existing literature However, further research is needed to enhance the application of this technique in ring-shear apparatus, which generates multiple residual stress states for each specimen Currently, there is limited literature on the residual strength of cemented soil layers assessed through multistage ring-shear tests, where normal stress and shear rate levels vary at each stage.
This thesis presents a laboratory-based experimental study on the shear strength characteristics of discontinuous planes in ring shearing, utilizing both monotonic and dynamic ring-shear apparatuses The research investigates the strength parameters of non-cemented and cemented kaolin clay through a series of tests on reconstituted and cemented specimens, as well as two-layered samples with varying cementation to simulate bedding planes Laboratory-simulated cementation was achieved by incorporating different amounts of a cementing agent into kaolin clay The findings were compared with existing literature to analyze the impact of various parameters on the strength properties of the contact surfaces between cemented and non-cemented kaolin.
The overall objective of this research is to study the monotonic and dynamic ring shear strength characteristics of discontinuous plane materials
The key objectives and scope of the research work were formulated as follows:
LITERATURE REVIEW
Residual strength behavior of clays
Residual shear strength is the minimum constant shear resistance of soil at large displacements, regarded as a fundamental property largely unaffected by stress history, original structure, or initial moisture content This strength is crucial for assessing long-term stability issues, particularly in regions prone to landslides or with complex geological formations (Skempton, 1985) Key factors influencing the residual shear strength of clays include the type of clay mineral, soil index properties, pore water chemistry, and shear displacement rate (Lupini et al., 1981; Mesri and Cepeda-Diaz, 1986; Stark and Eid, 1994; Tiwari and Marui, 2003; Ramiah et al., 1970; Suzuki et al., 2001).
In recent decades, extensive research has focused on understanding the residual strength of clay soils and enhancing testing techniques for accurate measurement (Skempton, 1985; Lupini et al., 1981; Stark and Eid, 1994, 1997) Despite these advancements, there is a notable lack of literature regarding the residual strength of materials with varying cemented soil layers, particularly those assessed using a ring-shear test apparatus (Suzuki et al., 2007).
Heavily over-consolidated clays demonstrate high peak strengths but experience a significant drop from peak to residual strength, accompanied by an increase in void ratio and water content In contrast, normally consolidated clays have lower peak strengths and show a smaller decrease in strength, which is linked to a reduction in void ratio caused by the alignment of particles in the direction of shearing This behavior of soil types is visually represented in Fig 2.1.
The residual strength of a clay is described in terms of residual friction angle, r, and a residual strength cohesion intercept, cr, as follows:
= total normal stress on the shear plane u = pore water pressure
In natural soils, the residual strength often shows a cohesion intercept that is near or equal to zero, while the residual friction angle typically falls below the peak friction angle This behavior is characterized by the shearing resistance ratio, expressed as τr/σ' = tanφr.
While the assumption of zero cohesion at residual states is commonly used in design, Tiwari et al (2005) found that some soils can exhibit residual cohesion values as high as 9.2 kPa based on the optimal residual strength envelope.
Figure 2.1 Diagrammatic stress-displacement curves at constant normal stress
Early studies into the residual strength behavior of soils were carried out in the 1930’s The application of residual strength to the stability of slope led to the new
Research into the transition from peak to residual strength in soils has led to the development of new testing devices for measuring residual strength In his Fourth Rankin lecture, Skempton (1964) demonstrated that the long-term stability of slopes in over-consolidated fissured clays is determined by residual strength, which contributes to progressive failure He also noted that once drained residual strength is achieved, further shearing does not alter its value Additionally, Bjerrum (1967) emphasized that over-consolidated clay does not require fissures for its long-term stability to be influenced by residual strength.
2.1.2 Relationship between residual strength with soil index properties
In geotechnical engineering, accurately determining residual strength parameters swiftly is crucial Researchers have attempted to link these parameters to soil index properties, such as clay content and Atterberg limits (Lupini et al 1981; Mesri and Diaz 1986; Stark and Eid 1994) The clay fraction, representing the percentage by weight of particles smaller than 0.002 mm, alongside the liquid limit, serves as a reliable indicator of clay mineralogy Skempton (1985) and Lupini et al (1981) found that significant particle reorientation occurs in clays with platy minerals and a clay fraction exceeding 20-25% Conversely, when the clay fraction is below 25%, the clay behaves similarly to sand or silt, exhibiting residual shearing resistance angles typically greater than 20 degrees.
Numerous studies have explored the relationship between residual shear strength and factors such as clay fraction and plasticity Notably, Skempton's correlation, illustrated in Fig 2.2, incorporates findings from various researchers, including Lupini (1981) Additionally, some scholars argue that the residual friction angle exhibits a stronger correlation with the plasticity index than with other parameters Figures 2.3 and 2.4 provide a summary of these correlations as reported by different authors, including Seycek (1978) and Hatipoglu et al.
In their 1982 study, Lupini et al highlighted that while there is a strong correlation between residual strength and certain soil properties, these correlations are not universally applicable They emphasized the importance of considering additional factors, including particle shape and grading, when assessing soil behavior.
13 mineralogy, pore water chemistry, ect., affect remarkably on the residual strength of soils
Figure 2.2 Relationship between the residual friction angle and the clay fraction
Figure 2.3 Relationship between the residual friction angle and plasticity index
Figure 2 4 The variation of residual shear strength angle with liquid limit (Hatipoglu et al., 2013)
Stark and Eid (1994) argued that relying solely on clay-size fraction or clay plasticity can lead to an overestimation of the drained residual friction angle, which in turn affects the accuracy of safety factor calculations in soil stability analysis They highlighted that the nonlinearity of the residual failure envelope was often overlooked, resulting in significant underestimations of safety factors due to small variations in the residual friction angle Furthermore, the authors proposed that for cohesive soils with a clay fraction below 45%, the residual failure envelope could be approximated by a straight line, while different considerations apply to soils with clay fractions exceeding 50% and a liquid limit between 60 and 80%.
The authors highlighted the significant nonlinearity of the drained residual failure envelope, leading them to propose a new correlation for drained residual strength This correlation is influenced by factors such as the liquid limit, clay fraction, and effective normal stress.
Stark et al (2005) revised the correlation in Fig 2.5 based on new experimental data, introducing a new empirical correlation specifically for an effective normal stress of 100 kPa, as shown in Fig 2.6 This updated relationship exhibits a slight upward shift of less than 1°, enhancing the stress dependency of the secant residual friction angle for soils with a clay fraction of 20% or less However, the relationships for effective normal stresses of 400 kPa and 700 kPa remain unchanged from the previous findings of Stark and Eid.
Figure 2 5 Relationship between the secant residual friction angle and liquid limit, clay fraction, and the effective normal stress (Stark and Eid, 1994)
Figure 2 6 Relationship between the secant residual friction angle and liquid limit, clay fraction, and the effective normal stress (Stark et al., 2005)
Residual strength is primarily influenced by the percentage and type of clay particles present In addition to the clay fraction, the mineralogy of the clay significantly impacts residual strength, particularly when the clay content is substantial Understanding the clay mineralogy is essential for assessing residual strength effectively.
16 are common in clay and shales are platey structures, and are therefore subject to alignment when sheared This leads to high residual friction angles, commonly greater than 25 0
Lupini et al (1981) emphasized that the correlations observed in the data exhibit significant scatter, making them unsuitable for generalization This view aligns with Mesri and Ceped-Diaz’s (1986) assertion that such correlations need not be universally applicable Nonetheless, predicting residual strength based on the clay fraction or plasticity index can be beneficial for specific sites where other factors remain constant Kalteziotis (1993) further highlighted that while these correlations should not be generalized, they can hold significant relevance for soils with similar composition and geological history, particularly in geotechnical engineering and the analysis of reactivated landslides.
Tiwari and Marui (2005) explored the relationship between soil mineralogical composition and the estimation of r, proposing a reliable method that significantly reduces deviation from measured r and minimizes estimation error compared to traditional techniques Their research, which analyzed over 35 mixtures of major minerals such as smectite, kaolinite, mica, feldspar, and quartz, demonstrates the broader applicability of their method relative to previously suggested approaches.
Figure 2 7 Variation in r with clay fraction and liquid limit (Tiwari and Marui, 2005)
Figure 2 8 Variation in r with plasticity index and proportion of smectite (Tiwari and
Characteristics of naturally cemented clay
An important feature of all naturally cemented clays is the bonding that takes place between particles as a result of diagenesis This occurs because of carbonate
Precipitation contributes to the growth of carbonate crystals within soil grains, enhancing the natural cementation that increases soil resistance to deformation When this cementation fails, it leads to significant and rapid deformation Various types of cemented soils exhibit common mechanical characteristics, including yield stress, initial stiffness, peak strength, residual strength, and dilatancy The breakdown of the "metastable" structure during straining is typically linked to relatively weak cementation and bonding among the particles.
According to Abramson et al (1996), soils that demonstrate progressive failure, such as clay and shale, have undergone chemical bond disintegration due to weathering Leroueil and Vaughan (1990) found that the mechanical behaviors of naturally cemented soils, including claystone, sandstone, and weak rocks, exhibit similarities regardless of the different causes of cementation This observation aligns with the findings of Sangrey.
In 1972, research indicated that artificially cemented clay samples can effectively mimic the properties of naturally cemented clays The bonding effects in these artificial clays are particularly notable at stress levels below a certain pre-consolidation threshold, making them sensitive to variations in stress and the duration of loading during testing.
Aging in clays refers to the long-term cementation process that influences their mechanical behavior, characterized by delayed compression, a higher consolidation yield stress than the effective overburden pressure, and stress overshooting in the e-log p relationship The concept of residual strength in cemented clay includes the loss of cementation and the reorientation of clay particles during shearing The drained shear and dilatancy behaviors of cemented and non-cemented soils transitioning through critical to residual states are crucial for understanding slope stability Given the frequency of slope failures and landslides in aged clays worldwide, it is essential to assess the residual strength characteristics of undisturbed soil samples with natural cementation, particularly in mudstone formations.
The schematic diagram illustrates the one-dimensional consolidation and drained shear behaviors of cemented and non-cemented soils, highlighting the relationship between void ratio and effective normal stress, as well as the correlation between shear stress and shear strain.
Residual strength characteristics of cemented clay soils
According to Mitchell (1993), the engineering behavior of clay is influenced by its inter-particle force fabric When soft clay is mixed with cementing agents, a specific micro-fabric forms as these agents fill the spaces between clusters and bond the fabric (Miura et al., 2001) In terms of undrained shear response, the resistance generated by cementation and the fabric occurs simultaneously.
Figure 2 17 (a) Microfabric of uncemented clay and (b) Structure of the induced cemented clay (Horpibulsuk et al., 2003)
Research has shown that the residual strength of cemented materials is unaffected by the degree of cementation, as noted by Wissa et al (1965), who described it with a single strength envelope Fischer et al (1978) found that cemented Drammen clay behaves similarly to non-cemented clay, exhibiting an over-consolidation ratio (OCR) of approximately 1.7 Clough et al (1981) concluded that the failure envelopes for both cemented and un-cemented sands are nearly linear and share a similar slope, with the cohesion intercept increasing alongside the amount of cement, while the friction angle remains unchanged by cementation.
Kasama et al (2000) found that the failure envelope of cemented clay closely resembles that of non-cemented clay, based on their consolidated undrained triaxial compression tests on cemented Ariake clay They proposed that the differences in frictional strength arise because cement mixing not only creates bonds between soil particles but also introduces fine particles to the untreated soil Furthermore, they noted that the contribution of the cementation bond to shear resistance persists even after the bond has been compromised during shear.
Recent studies by Horpibulsuk et al (2004, 2005) on Bangkok and Ariake clay demonstrate that the failure envelope of induced cemented clay forms a single straight line in both pre- and post-yield states, contrasting with un-cemented clay behavior The primary function of cement in this context is to enhance the cohesion intercept while minimally affecting the internal friction angle Additionally, Sasanian and Newson (2014) observed that the residual shear strength of cemented soil increases more gradually than peak strength as curing time or cement content rises Despite this, there is a lack of literature addressing the residual shear strength characteristics of cemented clay soils, leaving the mechanisms at play in their residual state largely unexplored.
Figure 2 18 Relationship between friction angle and cohesion intercept versus cement content of induced cemented Bangkok and Ariake clays (Horpibulsuk et al., 2005)
Multistage ring-shear test technique
The multistage test procedure utilizes a ring-shear apparatus to shear a single specimen to its residual state by varying the normal stress, followed by a second shearing to the same residual state This technique has gained acceptance primarily through test results from over-consolidated clays, indicating that the stress history does not influence the measured residual strength.
In very soft soils, continuously increasing normal stress during multistage testing can lead to soil loss through gaps in the confining rings, which poses a significant drawback to the multistage ring-shear technique According to Bromhead (1979), it is crucial to allow sufficient time for soft samples to consolidate under progressively increasing loads to minimize soil loss Therefore, the multistage ring-shear test should be limited to a specific number of stages and corresponding normal stresses to achieve more reliable results.
The multistage ring-shear testing (MRST) technique has been widely utilized by researchers, including Lupini et al (1981), Anderson and Hammoud (1988), Tika et al (1996), and Harris and Watson (1997), to determine the shear strength parameters of various soil types Notably, Bishop et al (1971) demonstrated that residual strength is independent of stress history, indicating that the loading sequence in multistage tests does not affect the results, as a unique curve exists for τ/σN values that solely depends on the magnitude of normal stress.
Anderson and Hammoud (1988) noted that the technique is effective for clays with less than 50% clay particles, as these soils display turbulent or transitional shearing modes In contrast, soils with over 50% clay particles tend to exhibit a sliding mode of shearing, leading to potentially different outcomes in multiage and single-stage tests This discrepancy arises not only from the inaccurate residual strength values obtained from multistage tests but also due to the brittleness of the clay, which significantly affects engineering behavior Additionally, Tiwari and Marui (2004) applied the MRST technique to five different types of natural soils, all of which demonstrated a consistent effective residual internal friction angle.
The multistage technique for shear testing offers a quick and objective method for measuring shear strength across various displacement ranges, as concluded by researchers Harris and Watson (1997) advocated for the multistage test as an optimal approach for the ring-shear test, especially with the Bromhead ring-shear apparatus, which can yield drained residual shear strength values within three working days Their findings indicated that the residual shear strength values obtained closely align with those derived from back analyses of slope failures.
Cyclic degradation in clay in dynamic ring-shear test device
Numerous studies have examined the cyclic behavior of soils through cyclic triaxial and cyclic simple shear tests, as well as dynamic ring-shear apparatus tests, yielding valuable insights into the dynamic response of clayey soils For instance, Osipov et al (1984) utilized scanning electron microscopy (SEM) to analyze the microfabric of clayey soil throughout the cyclic loading process, finding that the microstructure remained intact despite some bond disruptions Research by Mortezaie et al (2013) and Soralump et al (2016) indicated that clay soils with higher over-consolidation ratios and plasticity indices showed minimal degradation in cyclic strength, while Matasovic and Vucetic (1995) suggested that cyclic pore water pressure buildup is not a major factor in the cyclic degradation of normally consolidated clays Proctor and Khaffaf (1985) reported no significant changes in frequency effects relative to the number of cycles, and Yasuhara et al (1982) emphasized the importance of frequency as an influencing factor on clay's mechanical properties during dynamic shear tests Additionally, the interaction between different soil layers under static and dynamic loading requires further investigation (Sassa et al., 1995; Onoue et al., 2006; Wakai et al., 2010).
Cemented clays exhibit low permeability, which can lead to the generation of excess positive pore pressure during cyclic loading, ultimately reducing shear resistance Additionally, the presence of cementing agents may further influence these properties.
The behavior of clays is influenced by their unique structure, which consists of fabric and bonding formed over extended periods through processes like diagenesis and cementation in various environments Discontinuity bedding planes in naturally cemented clay soils are common in areas affected by landslides or tectonic activity Despite this, there is a lack of literature on the impact of dynamic loading on existing shear surfaces, particularly in cemented clay soils Furthermore, the cyclic degradation of these soils, assessed through dynamic ring-shear tests, has not been documented This research aims to evaluate the cyclic degradation of lightly cemented kaolin clay to better understand the behavior of both new and pre-existing slip surfaces in soil, addressing a largely unexplored topic in geotechnical engineering.
Figure 2 19 The effect of vertical consolidation on cyclic degradation for two samples of kaolin (Mortezaie et al 2013)
EXPERIMENTAL TESTING PROGRAMME
Introduction
Residual strength parameters are typically assessed using a ring-shear apparatus Previous studies have indicated that similar values of residual strength can be obtained through different laboratory shear devices and testing procedures, as demonstrated by researchers such as La Gatta (1970), Bishop et al (1971), Townsend and Gilbert (1973, 1976), and Sassa et al (2003).
The Bromhead ring shear apparatus, developed by Bromhead in 1979, is favored for commercial testing due to its simplicity, convenience, low cost, and minimal sample volume requirements It produces consistent results for both remolded and undisturbed samples, comparable to those from more advanced equipment However, the primary factor influencing the measured residual strength is the wall friction generated along the inner and outer circumferences of the confined specimen.
The reversal direct shear test is commonly employed to assess the drained residual strength of clays and clay shales, despite its limitations A key drawback is that the soil is sheared both forward and backward, leading to a maximum displacement of less than 0.5 cm, which prevents a complete alignment of clay particles in the shear direction Additionally, the changing cross-sectional area during shear creates high local strain concentrations, often resulting in soil extrusion from the shear plane, which can yield residual strength values higher than those measured with ring-shear devices However, the test benefits from readily available equipment, ease of operation, and shorter drainage paths, allowing for quicker testing To more accurately estimate field residual strength, Stark et al (1992) recommended using a ring shear apparatus with remolded specimens.
44 contrast, a reversal direct shear was not recommended to measure the field residual strength on remolded, not precut specimens
The ring-shear apparatus was specifically designed to measure residual strength, with a review of earlier devices conducted by Bishop et al (1971) highlighting their advantages and disadvantages Key benefits of the ring shear test include its ability to provide accurate assessments of soil strength under various conditions.
The continuous shearing of the sample in a single direction, regardless of the magnitude of displacement, facilitates the alignment of clay particles in both intact and remoulded specimens This process promotes a parallel orientation to the shear direction, ultimately leading to the establishment of a residual strength condition.
- The cross sectional area of the shear plane is constant during shear Uniform stress and strain conditions along the shear plane are desirable for reliable interpretation of results
- Because of thinner sample, process of shearing could be conducted at faster shear rate
- Less laboratory supervision is required
The ring shear apparatus, while useful for testing, presents several challenges and limitations, including non-uniform stress and strain distributions, soil extrusion, and a complex undrained testing procedure Additionally, issues such as wall friction and the varying stress effects around the sample edges during shearing complicate the assessment of results.
Monotonic ring-shear apparatus
The test apparatus depicted in Figs 3.1, 3.2, and 3.3 operates on the same principle as the Bishop-type apparatus (Bishop et al., 1971) The ring-shaped specimen, with an inner diameter of 6 cm, an outer diameter of 10 cm, and a wall thickness of 2 cm (illustrated in Fig 3.4), features a ratio of outer to inner ring diameters of 1.7 Table 3.1 compares the specifications of various ring-shear apparatus from different universities and institutes with the device utilized in this study The specimen was subjected to shear forces positioned 1 cm above the base plate during testing.
The ring-shear apparatus accurately measures shear stress, normal stress, frictional force, and vertical displacement through an automated recording system A load cell linked to the upper ring captures the frictional force generated by the vertical displacement of the specimen, which is crucial for determining the total normal stress on the soil sample The load-receiving plate remains stationary, supported by a ball bearing to maintain the integrity of the normal force measurement Consequently, the net normal stress acting on the shear surface is calculated from the recorded frictional force.
Figure 3.1 Essential features of ring-shear test apparatus
Load cell for measuring skin frictional force
Load cell for measuring shear force
Load cell for measuring normal load
Dial gauge for setting clearance
Dial gauges for vertical displacement
46 Figure 3.2 Front view of ring shear-test apparatus
Figure 3 3 Shear box containing test sample
Figure 3 4 A typical testing specimen for monotonic ring-shear test
Table 3 1 Features of ring-shear apparatus from Universities and Institutes, compared with the device used in this study (summarized by Prof Suzuki)
Suzuki et al Shinshu Univ 10 6 0.6 2 0.2 2
Umezaki et al Shinshu Univ 7 4.2 0.6 1.4 0.2 2
Ogawa et al Nagaoka Univ 15 10 0.67 2.5 0.17 2
Yatabe et al Ehime Univ of Technology 16
1 Nakamura et al Tokyo Univ of Agriculture and Technology
Kamai et al Nihon Univ 12 8 0.67 2 0.17 2
Sassa et al Kyoto Univ 35 25 0.71 5 0.14 -
Yamashita et al Aratani Civil Engineering
Gibo et al Ryukyu Univ 10 6 0.6 2 0.2 3
Yoshimi et al Tokyo Institute of Technology 26.6 21.4 0.8 2.6 0.1 2.7
Yasuda et al Tokyo Denki Univ 15 10 0.67 2.5 0.17 2.5
Bishop et al London Univ 15.24 10.16 0.67 2.54 0.17 1.91
Scheffler et al Leipzig Institute of
Bucher Swiss Federal Institute of
Stark et al Illinois Univ 10 7 0.7 1.5 0.15 1
Suzuki et al Yamaguchi Univ 10 6 2 2 0.2 2
Dynamic ring shear apparatus
The study utilized a consolidation-constant volume cyclic loading ring-shear test apparatus, specifically employing a shear-torque-controlled (STC) method for undrained cyclic loading tests The testing device comprises a cyclic ring-shear apparatus, pneumatic servo controller, bellofram cylinders, a constant volume-control device, a dynamic strain data logger, and a personal computer for data recording The ring-shaped specimens, with an inner diameter of 4.2 cm, outer diameter of 7 cm, and wall thickness of 2 cm, feature a shear surface area of approximately 24.63 cm² The apparatus allows for a maximum shear speed of 10 cm/s, a cyclic loading frequency of up to 5 Hz, and a data recording rate of 200 readings/s Additionally, Table 3.2 compares the features of this device with previous dynamic ring-shear apparatuses.
The specimen is subjected to shear by rotating the lower half of the shear box in both directions, while the upper half remains restrained by two resistance transducers that measure shear resistance During the testing process, key parameters such as cyclic shear resistance (τcN), vertical consolidation stress (σN), frictional force, horizontal displacement (δ), and vertical displacement (v) are measured and recorded automatically It is important to note that pore water pressure is not measured in this setup The normal load on the failure plane is determined by subtracting the side friction on the upper confining rings from the total applied normal stress Shear forces developed across the failure plane are transferred through the upper confining rings and loading platen to the moment-transfer arms, where a pair of force transducers measure the moment carried by these arms.
The primary limitation of this device is the inability to measure pore water pressure, hindering the assessment of cyclic strength characteristics in cemented clay soil Additionally, the specimen size is smaller than that used in the monotonic ring-shear test, resulting in inadequate comparisons between monotonic and dynamic ring-shear strength, which are crucial for accurate analysis.
49 in this study Furthermore, it is difficult to measure the peak cyclic strength in early few cycles of shear stage due to bellofram cyclinder and pneumatic servo controller
Consequently, only main cyclic degradation parameter is evaluated in relevant subsequent chapter
Figure 3 5 Essential features of consolidation-constant volume cyclic loading ring- shear apparatus
Load cell for measuring skin frictional force
Load cell for measuring shear force
Load cell for measuring normal load
Dial gauge for setting clearance
Dial gauges for vertical displacement
Figure 3 6 Front view of consolidation-constant volume cyclic loading ring-shear apparatus
Figure 3 7 Shear box containing test sample
Pneumatic servo controller Bellofram cyclinder
Figure 3 8 A typical testing specimen for dynamic ring-shear test
Table 3 2 Features of previous dynamic ring-shear apparatus, compared with the device used in this study
Ratio of max height/width
No No No No 0.5 Hz 5 Hz 5 Hz 5 Hz 5 Hz 5 Hz
Undrained testing and pore pressure monitoring
No No No No Yes Yes Yes Yes Yes No
Material and Specimen
This study utilized commercially available kaolin from Japan, obtained in powdered form to ensure its purity and uniformity Ordinary Portland cement (OPC) served as the cementation agent for creating artificially cemented samples, with the cement content defined as a percentage of the dry weight of OPC relative to the clay sample Nine sample types were prepared for monotonic ring-shear tests, including kaolin only (0% cement), 2% cemented kaolin, 4% cemented kaolin (the normal specimen), and various combinations of these percentages The combinations included: 0% and 0% cement, 0% and 2% cement, 0% and 4% cement, 2% and 2% cement, 2% and 4% cement, and 4% and 4% cement, collectively referred to as “0% cement,” “2% cement,” “4% cement,” and “0% + 0% cement.”
“0% + 2% cement”, “0% + 4% cement”, “2% + 2% cement”, “2% + 4% cement” and
Table 3 3 Physical properties of kaolin and cemented kaolin clay
21.8 35.3 Soil particle density (g/cm 3 ) Kaolin
In this study, four sample types were selected for multistage ring-shear tests: 0%, 4%, 4%+2%, and 4%+0%, while another set included 0%, 2%, 2%+2%, and 0%+2% for dynamic ring-shear tests The physical properties of these samples are detailed in Table 3.3, with their combinations presented in Table 3.4 and illustrated in Fig 3.9 The cement content added to kaolin was primarily based on soil hardness measurements taken from a landslide slope site affected by the Mid Niigata Prefecture earthquake Notably, the strength of the cemented soil closely resembles that of the in-situ soil, which is significant as earthquake-induced landslides typically occur along bedding planes as slip surfaces.
53 of the soil has a discontinuity such as that of the bedding plane, the influence of the discontinuity on the stability of the landslide slope can be assessed quantitatively
Figure 3 9Basic features of normal and combined specimens
Table 3 4 Types of combination for combined-cement kaolin samples.
Preparation of normal and combined specimens
Reproducibility is crucial for accurate test results in laboratory tests involving artificially cemented samples As illustrated in Fig 3.10, the testing procedure begins with mixing kaolin clay with distilled water at roughly twice its liquid limit This optimal water content is essential for creating a workable, homogeneous mixture of lightweight cemented clay, which is commonly utilized in laboratory settings.
Lower half of specimen (b % cement)
Upper half of specimen (a % cement) r = 3cm inner r = 5cm outer
Sample Sample types Types of combination
Upper half Lower half Combined specimen 0% + 0% cement
0% + 2% cement 0% + 4% cement 2% + 2% cement 2% + 4% cement 4% + 4% cement
0% cement 0% cement 0% cement 2% cement 2% cement 4% cement
0% cement 2% cement 4% cement 2% cement 4% cement 4% cement
An electric mixer with a paddle-type mixing blade was used to achieve a homogeneous mixture for approximately 5 minutes Afterward, Ordinary Portland Cement (OPC) was incorporated at 2% or 4% of the dry clay mass, followed by further mixing to ensure uniformity The resulting paste, exhibiting suitable workability, was then transferred into a polished double-draining consolidation tank, measuring 150 mm in diameter and 300 mm in height, which was coated with vacuum grease to reduce friction Soaked circular filter papers were placed at both the top and bottom of the sample Before consolidation, a vacuum of about 98 kPa was applied to eliminate trapped air bubbles in the slurry, preventing air entrapment.
Figure 3 10 Flow chart shows test procedure
-Mixing cement with clay slurry.
-Adjustment of water content to 2 times water content of liquid limit
-Pour of cement slurry into mould
-De-air for a half of an hour
98, 196, 294, 392 kPa. -Consolidation period decided by 3t method
-Monotonic and dynamic ring shear test performed on trimmed specimen
-Sample taken from mould after consolidation,wrapped in plastic bag and cured for 3 days.
Each sample underwent normal consolidation under a consistent pressure until primary consolidation was verified using the 3t method After consolidation, the cylindrical samples were unloaded, sealed in plastic bags to prevent moisture fluctuations, and stored in a humidity chamber with a stable temperature.
To effectively simulate the cementation process in natural cemented clay soils, all samples were cured at a temperature of 20 ± 2 °C for a duration of 3 days Furthermore, to assess the impact of extended curing time on the mechanical properties, kaolin samples with 2% and 4% cement content were subjected to additional curing periods of 7, 14, and 28 days.
Figure 3 11 Soil specimen in process of pre-consolidation
Figure 3 12 A soil specimen in process of trimming
Figure 3 13 Removing the inside soil core to make an annual specimen
Figure 3 14 Completing specimen before transferring to the shear box
Figure 3 15 Transferring a soil specimen to the shear box
Figure 3 16 Final setting before ring shear test
Two types of specimens, normal and combined specimens, were tested in this study
A ring-shaped specimen was created from pre-consolidated and cured samples using a cutting tool, with two cutter ring types featuring heights of 1 cm and 2 cm for combined and normal specimens, respectively To reduce side friction during sectioning, the cutting tool's sides were coated with silicon grease Due to the fragility of the cemented samples, careful setup was essential The specimens were then placed into the ring shear apparatus The production of combined specimens involved two stages: first, a 1 cm high cutting tool was utilized to obtain ring-shaped specimens with an outer diameter of 10 cm, an inner diameter of 6 cm, and a height of 1 cm; second, these specimens were transferred to the lower part of the ring shear box.
A complete combined specimen is created by bonding a second specimen of identical dimensions and composition to the first This procedure mirrors the method used for combined samples in dynamic ring-shear tests, with the primary distinction being the size of the samples involved.
Preparation of over-consolidation samples
This study conducted dynamic ring-shear tests on over-consolidated non-cemented and cemented kaolin samples To establish consolidation conditions, the samples were first reconsolidated at a specific pressure until their volume remained unchanged for over an hour Subsequently, the vertical load was reduced to a lower normal stress to attain the desired over-consolidation state The dynamic ring-shear tests were performed only after the vertical settlement of the soil samples stabilized under the new vertical load.
To investigate the effects of stress history on cemented clay soil, two series of tests were conducted on kaolin samples with 0% and 2% cement content, focusing on various over-consolidation ratios (OCRs) Pre-consolidation pressures of 196 kPa and 294 kPa were established as effective normal stresses Following this, the stress was reduced to approximately 98 kPa for each pre-consolidation level, resulting in OCRs of 2, 3, and 4, while maintaining an initial effective normal stress close to the set values.
98 kPa during this testing series.
Test procedure for monotonic ring shear test
Each specimen was normally reconsolidated under a constant consolidation stress,
In the ring shear test apparatus, the consolidation stress applied exceeded the preliminary consolidation pressure during the curing process, which could potentially harm the specimen's cementation Suzuki et al (2005) noted that after a specific curing duration, the unconfined compressive strength of specimens cured under higher overburden pressure was comparable to, if not greater than, those cured under previous conditions.
The additional overburden pressure does not affect the unconfined compressive strength of cemented soil under applied stress, allowing for the neglect of any damage to the cemented matrix when assessing shear behavior To prevent specimen swelling from submergence, pure water was added to a water bath immediately after consolidation stress was applied The specimen was then sheared under constant total normal stress until the shear displacement reached a residual state, defined as an intermediate circular arc between inner and outer rings A fixed gap of 0.10 mm between the rings minimized friction and sample outflow The frictional force at the specimen's perimeter varied with dilatancy, influencing the net normal stress on the shear surface, which was calculated from the frictional force recorded by the load cell Adjustments to the normal stress were made based on changes in frictional force measured before the shear test.
Test procedure for dynamic ring-shear test
The dynamic ring-shear test procedure features distinct differences from the monotonic ring-shear test Initially, the specimen is fully adapted into the shear box, where it is subjected to reversal shear stress at a constant amplitude During the dynamic tests, normal stress remains constant while a sine wave shear stress is applied at a frequency of 0.5 Hz This frequency is specifically chosen as it aligns with the common earthquake frequency range of 0.5 Hz to 15 Hz, making it relevant for various soil dynamic behaviors To reduce friction between the upper and lower rings and prevent sample outflow from the shear surface, the gap between the rings is maintained at 0.10 mm.
The testing program consisted of three test series, each designed to investigate a particular effect of the cyclic behavior of cemented kaolin clay Series 1 included the
In a series of 60 cyclic tests, researchers examined the effects of three distinct normal stress levels, varying cyclic shear stress amplitudes, and different over-consolidation ratios Each test measured the normalized cyclic stress ratio, defined as the ratio of maximum cyclic shear resistance to initial normal stress (τcN/σN0), allowing for effective comparison across different sample types Here, τcN represents the average cyclic shear resistance during the Nth cycle, differentiating it from τc, which refers to cyclic shear discussed earlier Throughout the cyclic shearing process, vertical displacement was meticulously controlled to maintain a constant volume of the specimen using a volume-controlled device.