Matteo Oryem Ciantia

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.

Experimental Methodology for Chemo-Mechanical Weathering of Calcarenites

Calcarenite is a soft rock strongly affected by weathering processes that markedly reduce the mechanical rock properties with time. As a consequence, cliffs and underground cavities formed in calcaernites are frequently affected by intense erosion and instabilities. The field and laboratory experimental results mainly show three peculiarities of calcarenite mechanical behavior: a) a marked and instantaneous reduction in strength, up to 60% of the dry initial value, when water fills the pores of the rock; b) a slow reduction in strength after saturation; c) progressive weakening of the material during wetting and drying cycles. In the present work we concentrate on the long term effect of water on calcarenite. In this context, an experimental procedure necessary for the calibration of a strain hardening-chemical softening elasto-plastic constitutive model is presented. Suitably designed tests under controlled “weathering” conditions were performed in order to define the critical variables that can physically explain the variety of phenomena occurring in the material.

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.

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.

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.

Modeling Physico-Chemical Degradation of Mechanical Properties to Assess Resilience of Geomaterials

It is widely accepted that critical properties of geo-materials that play a key role in failure of earth-structures undergo often a substantial evolution induced by non-mechanical processes and variables. That includes: hydro-thermal fracture, thermal collapse, chemical mass removal or accretion (dissolution or precipitation), chemical shrinkage/swelling, drying shrinkage, capillary force evolution during pore water phase change. The properties affected are: strength in all its manifestation, compressibility, permeability, thermal conductivity, to mention just a few. The physical processes involved are either natural or engineered. Their phenomenology is per se a conundrum, as often they constitute a series of parallel or sequential processes. A review of several phenomena leading to geomaterial degradation, and methodology is presented to deal with multi-physical couplings in constitutive modeling. In plasticity, the central constitutive function is a hardening rule. Also in this case, phenomenological observations indicate a chemo-mechanical, two-way coupling. Other degradation phenomena discussed include drying—cracking, and or the role of suction induced hardening in unsaturated materials.

DEM Investigation of Particle Crushing Effects on Static and Dynamic Penetration Tests

A 3-dimensional discrete element method model has been developed to simulate static and dynamic rod penetration test in a calibration chamber. The chamber has been filled with a scaled analogue of Fontainebleau sand. Crushing effects on penetration results have been examined. It has been found that particle crushing reduces penetration resistance for both static and dynamic tests. Microscale observation of crushed particles has been conducted. It is shown that dynamic impact causes more crushing events. The crushed particles are distributed within 2–3 radius from the rod for both tests.