Riccardo Castellanza

Experimental Study on the Water-Induced Weakening of Calcarenites

Carbonatic rocks, such as calcarenites, are very often subject to damage processes, causing a progressive degradation of their mechanical properties. In nature, in some cases, this phenomenon can cause the collapse of cliffs and underground cavities, with dangerous consequences for the anthropic environment. In this paper, the results of an experimental campaign, intended to both clarify and quantify the mechanical consequences of this process, are illustrated. To achieve such a goal, suitable physical and geotechnical indices are introduced and different time scales to describe the physical/chemical reactions induced by the water saturation of the material are taken into consideration. In particular, the authors have observed: (1) a short-term marked and instantaneous reduction in strength when water fills the pores of the rock; (2) a long-term dissolution; and (3) a progressive chemically induced reduction in the grain size. To describe the degradation processes induced by the material water saturation, owing to the complexity of the hydro-chemo-mechanical phenomena taking place within the material, suitably designed tests under controlled “weathering” conditions were also performed and discussed.

Novel Approach for Health Monitoring of Earthen Embankments

AbstractThis paper introduces a novel modular approach for the monitoring of desiccation-induced deterioration in earthen embankments (levees), which are typically used as flood-defense structures. The approach is based on the use of a combination of geotechnical and noninvasive geophysical probes for the continuous monitoring of the water content in the ground. The level of accuracy of the monitoring is adaptable to the available financial resources. The proposed methodology was used and validated on a recently built, 2-km-long river embankment in Galston (Scotland, United Kingdom). A suite of geotechnical probes was installed to monitor the seasonal variation of water content over a 2-year period. Most devices were calibrated in situ. A novel procedure to extrapolate the value of water content from the geotechnical and geophysical probes at any point of the embankment is shown. Desiccation fissuring degrades the resistance of embankments against several failure mechanisms. An index of susceptibility is ...

Mechanical effects of chemical degradation of bonded geomaterials in boundary value problems

ABSTRACT The paper presents a constitutive approach to describe the effects of rock weathering processes in boundary value problems. The term rock weathering is used to refer to a number of chemical and physical phenomena that continuously transform a rock mass into a granular soil. From an engineering point of view rock weathering can be interpreted as a generalised decay of the mechanical properties of the original material. It acts at a constitutive level essentially by reducing the strength-of the bonds joining the grains together. Such a material degradation can occur in a time scale which is comparable to the average life of engineering structures. Weathering can induce subsidence on shallow foundations resting on soft rocks layers or it can be crucial for what concerns the stability of slopes or abandoned underground mines. In the first part of the paper, it is shown how the progressive destruction of the intergranular bonds due to weathering has been modelled satisfactorily by extending a strain hardening elastoplastic model. Such a model has been corroborated by means of special oedometer tests on soft rock specimens in which a progressive chemical debonding has been induced through the exposition of the rock to a uniform flow of an acid solution. In the second part of the work, three different boundary value problems in which weathering effects cannot be neglected are presented. The numerical analyses performed with the proposed constitutive model refer to: i) the weathering-induced subsidence of a circular foundation; ii) the stability of a slope subject to weathering from the ground surface; and, iii) the effects of the progressive collapse of pillars in an abandoned underground mine.

Evaluation and Remediation of an Abandoned Gypsum Mine

The remediation measures taken for an abandoned gypsum mine in the Italian Alps are presented. The mine consists of four levels, excavated by the room-and-pillar method in an evaporitic formation. The two lower levels were flooded by water coming from adjacent strata after mining ended about 30 years ago. The dissolution process affecting the submerged pillars can jeopardize the long term stability of the entire mine. An overview of the problem is given, and experimental results of dissolution tests are shown. An analytical model developed to simulate the laboratory data is applied to determine the time to failure of the mine. It is demonstrated that the expected collapse time is relatively short. Remedial measures consisting of chamber filling with an appropriate mixture of cement, water, and silty clay are proposed. A theoretical analysis of their effects in delaying time to failure and minimizing its consequences is presented. The analysis allowed the optimal mechanical characteristics of the filling mixture to be determined. The results of a full-scale field test to check the validity of the remedial action are highlighted.

Modelling weathering effects on the mechanical behaviour of rocks

The paper presents an experimental, theoretical and numerical approach in order to describe the effects of rock weathering on bonded geomaterials. The term rock weathering is used to refer to a number of chemical and physical phenomena that continuously transform a rock mass into granular soil. From an engineering point of view, rock weathering can be interpreted as a generalised decay of the mechanical properties of the original material. It acts at a constitutive level essentially by reducing the strength of the bonds joining the grains together. Such a material degradation can occur in a time scale which is comparable to the average life of engineering structures. Weathering can be crucial for what concerns the stability of slopes, cliffs and abandoned underground caves. In the experimental part of the paper micro and macro experimental investigations performed on calcarenite, a natural soft rock, are shown; short- and long-term debonding processes are identified as characterising rock weathering. A large set of hydro-chemo-mechanical data are recovered to properly characterise debonding processes by means of specific weathering tests. In particular, to reproduce long-term rock weathering in laboratory time, the progressive chemical debonding has been induced through the exposition of the rock to a uniform flow of an acid solution. In the theoretical part, it is shown how the progressive deterioration of the intergranular bonds due to weathering has been modelled satisfactorily by extending a strain hardening elastoplastic model by means of multiscale approach coupling hydro-chemo-mechanical processes. Such a model has been corroborated by simulating some tests of the previous part. Finally, in the numerical part of the work, some boundary value problems are presented, in which weathering effects cannot be neglected.

Investigation, monitoring and modelling of a rapidly evolving rockslide: The Mt. de la Saxe case study

During the last decade, a large rockslide along the NW slope of the Mont de la Saxe (Courmayeur, AO—Italy) showed a significant increase in the rate of activity in conjunction with snow melting periods and more recently with important amount of rainfall. The ca 8 mm3 rockslide affects a heavily tectonised and intensely fractured mass of black schists, for a surface area of about 150,000 m2. The rockslide failure surface is typically located at depths of 60–80 m b.g.l. Because of the highly valuable exposed elements regional authorities commissioned a comprehensive ground investigation and monitoring campaign, as well as theoretical and numerical analyses, in order to assess slope stability conditions, the possible evolution and the suitable stabilization and mitigation countermeasures. Here we present the recent rockslide evolution till spring 2014 when a major seasonal reactivation of a 400,000 m3 mass occurred. Finally, with the aim to develop a possible tool for displacement prediction, the results of a series of 1D visco-plastic simulations are presented, discussing the possible role of each of the main controlling factors (e.g. groundwater recharge and piezometric level oscillation, rock degradation).

Evaluation of the stability of underground cavities in calcarenite interacting with buildings using numerical analysis

Soft and highly porous rocks such as calcarenites are very common in all Mediterranean region. Due to their porous calcareous structure these rocks are prone to water induced weathering mechanisms. Natural onshore and inland underground cavities are evidence of such phenomena. The collapse of cliffs and underground cavities is usually the long-term result of a complex hydro-chemo mechanical process taking place at the micro-scale. Experimental results mainly give evidence of: (a) a marked and instantaneous reduction in strength and stiffness for these porous rocks when macro-pores are filled with water, (b) a slow successive reduction in strength and stiffness occurring in the long-term due to dissolution processes; (c) a more pronounced weakening of the rock material as a consequence of wetting and drying cycles. In the present work a methodological path to cope with deterministic assessment of the stability of natural and anthropic caves will be presented. The following steps will be adopted: (i) experimental study: execution of an experimental campaign to identify the physics of the processes taking place at both the micro-scale and the macro-scale; (ii) theoretical study: extend the concept of strain hardening-non mechanical softening to the time evolution of c-fi reduction; (iii) numerical study: present the 3D numerical results of a real case-study showing the capability of the proposed methodology to cope with risk assessment in complex geomechanical situations concerning weathering, as for underground cavities.

Implicit integration of constitutive equations in computational plasticity

ABSTRACT The paper discusses an extension of the now standard Generalized Backward Euler (GBE) algorithm to a general class of elastoplastic constitutive equations for geomaterials, characterized by mechanical and non-mechanical hardening mechanisms. The resulting integration scheme is well suited for the application to relatively complex, three-invariant yield surface and plastic potential functions. A closed form expression for the consistent tangent stiffness matrix is derived for the general case, extending the work of [TAM 02a] for isotropic- hardening models. The application of the numerical procedure is discussed with reference to a constitutive model for chemical weathering of bonded geomaterials recently proposed by [TAM 02b]. Results from a series of numerical experiments are given to illustrate the accuracy and convergence properties of the algorithm.

A 3D Numerical Approach to Assess the Temporal Evolution of Settlement Damage to Buildings on Cavities Subject to Weathering

The goal of this paper is to show how a recently developed advanced chemo-hydro-mechanical (CHM)-coupled constitutive and numerical model for soft rocks can be applied to predict the temporal evolution of settlement damage to buildings on cavities subject to weathering. In particular, a Building Damage Index (BDI) and its evolution with time is proposed. The definition of the BDI is inspired by the work of Boscardin and Cording (J Geotech Eng 115:1–21, 1989) and uses the surface differential settlements obtained by finite element (FE) analyses to assess how far a building is from a non-acceptable service condition. By modelling the reactive transport of chemical species in 3D and using a coupled CHM constitutive and numerical model, it is possible to simulate weathering scenarios and monitor the temporal evolution of surface settlements making the BDI time dependent. This approach is applied to evaluate the damage evolution of two buildings lying on two anthropic caves in a calcarenite deposit belonging to the Calcarenite di Gravina Formation. Standard and advanced experimental tests are performed on the in situ material, and the results are used to calibrate the constitutive model. The soundness of both constitutive relationship and reactive transport solver is subsequently tested by simulating two laboratory-scale boundary value experiments. The first is a model footing test on dry and wet calcarenite, while the second is a small-scale pillar that, after the saturation-induced short-term water weakening, fails due to a long-term dissolution weathering process. Finally, both 2D and 3D coupled FE analyses simulating different weathering scenarios and corresponding settlements affecting the buildings above the considered cavities are presented. Particular attention is placed on assessing the BDI and its temporal evolution.

Earth pyramids: Precarious structures surviving recurrent perturbations

Earth pyramids are tall tapered spires, slender or stocky, made of rock or soil material. This type of features have been marginally studied. Even if they are often included in parks and geosites. We start from this lack of studies to analyse the geometrical and physical-mechanical characteristics considering some case studies in northern Italy. Because of their geometrical characteristics these elements are intrinsically weak and can be used as indicators of past external perturbations. We performed laboratory characterization and numerical modelling to analyse the involved actions, the stability and the reaction to dynamic perturbations.