The construction of tunnels in urban areas inevitably results in movement of the ground around them. It may have a significant environmental impact, due to the possible occurrence of accidents or important damage on the infrastructures on the surface or on the subsurface, as well as due to noise and vibration, especially during the construction process. Therefore, it becomes essential, from a point of view of design and planning, to develop rational methods aiming to minimize the associated hazard or damage (Sousa 1998; Burland et al. 2002).
One of the important requirements is related with the assessment of the bearing capacity of buildings in the vicinity of the tunnel, since these structures are sensitive to settlements, to horizontal strains and to differential displacements. The quantification of the damages in the buildings is a highly subjective issue, which can be affected by various factors, such as the field experience, the approaches adopted by the design engineers and by insurance companies. Since there is the possibility of occurrence of damages due to movements induced on the surface, these must be subject to classification. The types of damages in buildings can have the classification as follows: i) visual or aesthetic; ii) functional; and iii) stability-related. In visual terms, 6 damage categories are defined, ranging from 0 (minor) to 5 (highly significant) (Burland et al. 2002).
There are various criteria that relate the values of settlements of a zone with the damages caused in buildings. A group of criteria relates the damages in the works with the maximum deflection of the deformation occurred in the foundation of the structure. However, other criteria relate the damages with the occurrence of movements in the ground, assuming that these result from the maximum strain developed in the structural walls, which depends not only on distortion, but also on horizontal deformations (Boscardin and Cording 1989). Burland established a damage criterion, by a relationship of damage category to deflection ratio and horizontal tensile strain. That approach is illustrated in Figure 24, in which the building is represented by a rectangular beam of length L and height H. Different solutions are presented depending on either the structure is located on a sagging or hogging zone.
Figure 24 shows two extreme modes, bending only about a neutral axis at the lower fibre and shearing only. In the case of bending, the maximum tensile strain occurs in the upper fibre and that is where cracking will initiate. In the case of shear, the maximum tensile strain are inclined at 45- giving rise to diagonal cracking. The movements induced on the surface involve not only sagging and hogging profiles, but also significant horizontal strains. As an alternative to calculating tensile strains, charts are available, like in Figure 25, relating damage categories directly to deflection ratio and to horizontal strain. Figure 25 is related to L/H 1 and for hogging mode. In order to assess the evaluation of the effects induced by the tunnel on the surface buildings, it is not usual to consider their stiffness.
Nevertheless, the approach developed by Potts and Addenbrooke (Cost C7 2003) involves the use of design curves that modify the damage parameters conventionally calculated. Boscardin and Cording (1989) developed the concept of different levels of tensile strain using case records of damage cause by subsidence on a variety of buildings. They showed that categories of damage could be related to ranges of tensile strain as indicated in Table 3. Figure 26 schematically represents three modes of building movements for the transverse settlement. The longitudinal settlements produced close to the excavation front and behind the front may also bring damage to buildings, but is more difficult to quantify. The excavation of tunnels may also affect the subsurface structures, such as existing tunnels and service installations, namely sewers, gas mains, electric cables and pipelines (Figure 27).
In terms of serviceability state, effects of long-term groundwater flow and consolidation should be considered. The new tunnels may act as a drain, causing a permanent lowering of the phreatic level and consequent surface settlement on the ground surface above, as Figure 28 shows. It may affect other buildings outside the initial zone of influence of the tunnel. The cases of instability of tunnels, which have been analyzed in detail, in the first chapters, cannot be accurately predicted, being possible to use for the purpose the analyses of observation results, namely the alarm and warning criteria (Sousa 2001). The instability of the tunnel may be transmitted to the surface. Figure 29 shows two situations, one with insufficient resistance from the abutments to maintain stability of the lining and another, where insufficient face pressure has been maintained.
In many of the illustrated cases, it was possible to identify possible reasons for resulting excessive ground movement and damage. Establishing the causes allows counteract methods to be implemented, which are related to a good construction quality and protective measures. The causes of excessive ground movement are mainly related to inadequate initial investigations, poor design and analysis and poor control of construction works (Cost C7 2003). A number of protective measures are available. The in-tunnel measures include actions taken inside the tunnel during its construction in order to reduce the magnitude of displacements. Improvement in the TBMs had led to greater control of ground movements through controlling pressures at the face and grouting behind the lining.
In the case of open-face tunneling, typical measures can be related to tunneling method, face support measures, excavation by parts, pilot tunnels, mechanical precutting. Ground treatment measures are also adopted in order to reduce and modify the ground movements. Methods of permeation grouting and compensation grouting have become popular in recent tunneling projects. Structural measures are yet adopted by increasing the capacity of the structures, by increasing the ability of the foundations in order to resist the predicted movements, by stiffening the structure in order to modify the predicted movement, by making the structure less sensitive or by controlling the building movements by isolating it from the foundation.
Finally, a very interested case history is presented referring to the Baixa-Chiado station of the Lisbon Metro, where it was necessary to reinforce the rock mass by compensation grouting so as to minimize the damage for the buildings in the neighbourhood (Barreto et al. 1999). The Baixa-Chiado station was developed beneath the historical area of Chiado that was classified by UNESCO as patrimony of mankind. It comprises a considerable group of buildings that presented some anomalies resulting from the alterations occurred during their lifetime. That station started operation recently and serves two metro lines. Due to its strategic location it is presently one of the stations with highest passenger traffic.
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