Discussion on “Experimental Deformation of Opalinus Clay at Elevated Temperature and Pressure Conditions: Mechanical Properties and the Influence of Rock Fabric” of Schuster, V., Rybacki, E., Bonnelye, A., Herrmann, J., Schleicher, A.M., Dresen, G.

The testing procedure and results on saturated samples of Opalinus Clay in the work of Schuster et al. (Rock Mech Rock Eng https://doi.org/10.1007/s00603-021-02474-3, 2021) were conducted and presented using strain rates two to four orders of magnitudes higher than the rates needed to allow pore pressure equilibrium in the material, both in drained and undrained conditions. This leads to an erroneous estimation of the mechanical properties in saturated conditions. We discuss this aspect in the context of shale testing. We also discuss the effect of drying-induced fissuring on the mechanical properties of geomaterials tested in dry conditions.

Three-dimensional cyclic behavior of saturated clays: comparison between undrained and partly drained conditions

The cyclic response of subgrade clays under traffic loadings is likely to be partly drained rather than undrained, and the traffic-induced dynamic stress field is three-dimensional rather than axisymmetric. To compare the three-dimensional deformation behaviors of saturated clays between partly drained and undrained conditions, a large number of cyclic true triaxial tests were conducted. Experimental results show that partly drained condition leads to a remarkable increase of permanent major principal strain ( ) compared to undrained condition, and the differences of between the two drainage conditions are affected greatly by the factors of cyclic stress ratio (CSR), overconsolidation ratio (OCR), and coefficient of cyclic intermediate principal stress (bcyc). The increase of bcyc induces a linear reduction of in undrained condition, while it causes a first increase and then a decrease of in partly drained condition. The clays undergo stiffness softening and hardening in partly drained and undrained conditions, respectively, and the effects of bcyc and CSR on the stiffness evolution are very different between the two drainage conditions. The mechanism of the complex and resilient modulus behaviors in three-dimensional state and partly drained condition is further discussed. In addition, two different empirical models are employed to predict in partly drained and undrained conditions, respectively.

Modelling the monotonic and cyclic behaviour of sands using Artificial Neural Networks

In this study artificial neural networks (ANN) are used to simulate the monotonic and cyclic behaviour of sands observed in laboratory tests on Karlsruhe sand under drained and undrained conditions. A genetic algorithm (GA) is used to obtain an optimal framework for the ANN. The results show that the proposed genetic adaptive neural network (GANN) can effectively simulate drained and undrained monotonic triaxial behaviour of saturated sand under isotropic or anisotropic consolidation. The GANN is also able to predict satisfactorily the cyclic behaviour of sands under undrained triaxial test with strain and stress cycles. In addition, GANN is able to distinguish between monotonic drained and undrained conditions by delivering a good prediction when trained with the combined database.

Behavior of a Large Diameter Bored Pile in Drained and Undrained Conditions: Comparative Analysis

Despite the difficulties in obtaining the ultimate capacity of the large diameter bored piles (LDBP) using the in situ loading test, this method is the most recommended by several codes and design standards. However, several settlement-based approaches, alongside the conventional capacity-based design approach for LDBP, are proposed in the event of the impossibility of performing a pile-loading test during the design phase. With that in mind, natural clays usually involve some degree of over consolidation; there is considerable debate among the various approaches on how to represent the behavior of the overconsolidated (OC)stiff clay and its design parameters, whether drained or undrained, in the pile-load test problems. In this paper, field measurements of axial loaded to failure LDBP load test installed in OC stiff clay (Alzey Bridge Case Study, Germany) have been used to assess the quality of two numerical models established to simulate the pile behavior in both drained and undrained conditions. After calibration, the load transfer mechanism of the LDBP in both drained and undrained conditions has been explored. Results of the numerical analyses showed the main differences between the soil pile interaction in both drained and undrained conditions. Also, field measurements have been used to assess the ultimate pile capacity estimated using different methods.

Effect of the Surface Roughness on the Shear Strength of Granular Materials in Ring Shear Tests

Surface roughness plays an important role in estimating the shear strength of granular materials. A series of ring shear tests with different surface roughnesses (i.e., smooth and rough surfaces) were performed. A large-sized ring shear device, which is applicable for fine- and coarse-grained sediments, was developed to examine the shear strength of large particle sizes (i.e., commercial gravels with a mean grain size of 6 mm). In terms of surface roughness, the drainage- and shear-velocity-dependent shear strengths of the granular materials were examined. In this study, different shear velocities of 0.1, 0.5, and 1 mm/s were applied under drained and undrained conditions. The test results clearly show that shear stress is affected by drainage, shear velocity, and surface roughness. In particular, a typical strain-hardening behavior is exhibited regardless of the drainage and shear velocity condition. The measured shear strength obtained from both drained and undrained conditions increased with increasing shear velocity. All tests showed a large fragmentation using rough surfaces compared to the smooth surfaces of the device. The grain crushing was significant during shearing, even when normal stress was not applied. For a given shear velocity, surface roughness is an important feature in determining the shear strength of granular materials.

Experimental study on fibre reinforced sandy soils behaviour under static loadings - drained and undrained conditions

The effect of the addition of 0.5% of randomly distributed polypropylene fibres in a round grained sand, composed mainly of silica, is presented on laboratory scale. The samples were compacted with two void ratios (0.75 and 0.63), corresponding to values of relative density of 50% and 90% respectively. 24 monotonic triaxial tests were performed, with initial effective mean pressures of 20, 100 and 200 kPa. Half of samples were tested in drained conditions, and the other half in undrained conditions. For the samples subjected to drained conditions, the effect of the fibres was identified at the post-failure stage, by showing sustained increase of strength. The positive effect of fibres could also be observed through the increase of material friction angle. In the samples subjected to undrained tests, the addition of fibres reduced the contractive behaviour, mainly for those more compacted.

Stress-Dilatancy for Soils. Part II: Experimental Validation for Triaxial Tests

Different forms of the stress-dilatancy relations obtained based on the frictional theory for the triaxial condition are presented. The analysed test data show that the shear resistance of many soils is purely frictional. The angle Φ0 represents the resistance of the soil as a combined effect of sliding and particle rolling on the macro-scale during shear at the critical frictional state. The stress-plastic dilatancy relations differ not only for triaxial compression and extension but also for drained and undrained conditions. The experiment investigated shows the correctness of the frictional state theory in the triaxial condition.