G. Scarpelli

Excavation works in stiff jointed clay material: examples from the Trubi formation, southern Italy

The construction work of the DG22 mega lot, an 11-km segment of the new Jonica national road (southern Italy), mainly involves the Pliocene deposits belonging to the Trubi stiff jointed formation. This paper focuses on the geotechnical studies carried out to explain the kinematics of the instability processes which were observed during excavation work. Two different cases are presented. The first one refers to the movements caused by a 20-m-high slope excavated for a cut and cover tunnel. The second one describes the kinematics of the failure of a shallow tunnel which caused the subsidence of a widely extended area. In both cases, the response of the soil mass to the excavation was governed by the system of discontinuities that characterizes the Trubi clay formation. The influence of the structural features, beyond the knowledge of the mechanical behavior of the soil at the scale of the laboratory sample, was an essential ingredient to find a convincing explanation for both phenomena observed.

A knowledge-based approach for the structural assessment of cultural heritage, a case study: La Sapienza Palace in Pisa

The full knowledge of the morphological evolution of an historical masonry building, defined more as ‘structural aggregate’ than as ‘single construction’, together with the analysis of the architectural, structural, geological and geotechnical aspects, allow the assessment of the static safety and seismic vulnerability of the complex and the design of retrofit interventions. In the present paper, a Knowledge-Based-Approach is applied to the historical building ‘Palazzo La Sapienza’ in Pisa, allowing to provide reliable results concerning the actual structural condition of the building avoiding the strong computational effort usually associated to the execution of refined numerical analyses. In case of complex buildings, characterized by a high heterogeneity of materials, structural typologies, geometries and so on, the adoption of a global model is not always useful to represent the effective structural behaviour. The proposed approach shows how a deep multidisciplinary knowledge of the construction can limit the use of cumbersome numerical modelling and analysis, however reaching reliable and accurate results usable also in the current practice.

Slope Instability Problems in the Jonica Highway Construction

During the construction of the south portal of the tunnel Baldaia I along the SS106 motorway in Calabria, unexpectedly large and continuous displacements of the earth retaining structures occurred leading to a precautionary suspension of the excavation works. To study the kinematics of the observed instability phenomenon, inclinometer probes were used together with the topographic monitoring of the portal structures; it was found that the unstable mechanism was deep-seated, block-type and extending over the entire slope. In this paper the site investigation activities carried out to understand the origin of the instability process are presented. The interpretation of the observed behaviour is then given together with the description of the remedial measures suggested to safely complete the construction of the tunnel. The link between the time evolution of the instability and the sequence of the excavation works, which turned out to be an important issue in the process, is also discussed in some detail.

Numerical modelling for clays with internal discontinuity surfaces

A numerical model capable of performing deformation analysis of a medium containing discontinuity surfaces is presented. The discontinuity can be either a crack, which can be open or closed, or a shear band. The model consists of two separate numerical algorithms, which are coupled together by means of the principle of superposition. In particular, an integral equation scheme based on the theory of dislocations is adopted for modelling the discontinuity, while a finite element discretization is used for the continuous medium. In this paper the discontinuity modelling is illustrated in detail, together with the specific formulation of the principle of superposition adopted, and some simple examples of application are presented. The well-known modelling approach based on Fracture Mechanics theory is also briefly discussed. The two models are compared and some advantages and drawbacks of each are pointed out, comments are made regarding their applicability in the specific case of soil mechanics, and conclusions are drawn as regards the conditions under which one or the other is appropriate. Finally, a full-scale example of deformation analysis using the proposed model is presented.

Small-Strain Stiffness Values for a Reconstituted Soil from Southern Italy

In numerical modelling of soil response it is often required to define an accurate value for the stiffness at small strains (G0). Such values are usually obtained from tables or charts available in the literature, rather than being directly measured in tests. These will however be sufficiently accurate only if a similar soil has already been studied and described. In the present paper, the results of a series of laboratory tests conducted on a soil from southern Italy are reported. In these tests, a well-established empirical relationship linking small-strain stiffness values to state parameters was calibrated, allowing the determination of accurate G0 values for the specific soil, and contributing to the collection of data in the literature. The bender element technique, in conjunction with standard triaxial testing on reconstituted soil samples, was employed. A series of isotropic compression tests was used for the calibration, and the resulting power law was then checked through shear testing. The proposed equation compares well with results reported in the literature for different types of soils. On approaching failure a change in behaviour is evident; this is in accordance with observations already reported in the literature for a variety of soils and is subject to several interpretations, but does not substantially interfere with the general validity of the proposed correlation, which can therefore be adopted as a useful empirical equation for determining G0 values for this soil.

Parameter Calibration and Numerical Analysis of Twin Shallow Tunnels

Prediction of displacements and lining stresses in underground openings represents a challenging task. The main reason is primarily related to the complexity of this ground–structure interaction problem and secondly to the difficulties in obtaining a reliable geotechnical characterisation of the soil or the rock. In any case, especially when class A predictions fail in forecasting the system behaviour, performing class B or C predictions, which rely on a higher level of knowledge of the surrounding ground, can represent a useful resource for identifying and reducing model deficiencies. The case study presented in this paper deals with the construction works of twin-tube shallow tunnels excavated in a stiff and fine-grained deposit. The work initially focuses on the ground parameter calibration against experimental data, which together with the choice of an appropriate constitutive model plays a major role in the assessment of tunnelling-induced deformations. Since two-dimensional analyses imply initial assumptions to take into account the effect of the 3D excavation, three-dimensional finite element analyses were preferred. Comparisons between monitoring data and results of numerical simulations are provided. The available field data include displacements and deformation measurements regarding both the ground and tunnel lining.

Sheet Pile Quay Wall Safety: Investigation of Posttensioned Anchor Failures

This paper examines the collapse of an important port structure and analyzes the causes of failure. In particular, a length of newly constructed anchored sheet pile wall exhibited very large displacements at the crest, which clearly indicated that a failure condition had been reached within the retained ground. On removal of the failing soil and inspection of the structure, all the barrel/wedge anchors were found to be devoid of the strands they should have been holding. Observations and testing on such components led to the conclusion that sliding of the wedge system within the barrel had actually been inhibited by two main causes: on one hand, corrosion of the metal surfaces, which is in part predictable in a marine environment and, on the other, the presence of cement mortar which blocked the wedge components in place.

A Closed-Loop Controlled Equipment for Soil Consolidation Tests

A novel equipment for consolidation tests of soil samples is described. The equipment consists of a press and an oedometric cell equipped with a closed-loop control architecture. Closed-loop control is used for regulating the compression process of a soil sample according to the dynamical behaviour of the interstitial pressure. in such a way to emulate natural conditions. The equipment allows to perform consolidation tests of a new kind and to collect data for deducing Consolidation Curves with accuracy and, in comparison with classic methodologies, in a very short time.