GEOTECHNICAL ASPECTS OF RAILWAY TUNNEL CONSTRUCTION NEAR RUDOLTICE GEOTECHNICAL ASPECTS OF RAILWAY TUNNEL CONSTRUCTION NEAR RUDOLTICE

The article presents a project of railway tunnel from the point of view of potential short term or long term geotechnical risks; it means risks in the course realisation and operation time. There had been elaborated many variant projects for the new tunnel on main railroad between the cities Krasíkov and Česká Třebová in the Czech Republic in the leg Třebovice v Čechách – Rudoltice v Čechách. We would like to present some positive and negative aspects of one project variant presented by the company Metroprojekt, and, another one is “Study of Project Modification” presented by consortium Krasíkov.


Introduction
The article presents a project of railway tunnel from the point of view of potential short term or long term geotechnical risks; it means risks in the course realisation and operation time. There had been elaborated many variant projects for the new tunnel on main railroad between the cities Krasíkov and Česká Třebová in the Czech Republic in the leg Třebovice v Čechách -Rudoltice v Čechách. We would like to present some positive and negative aspects of one project variant presented by the company Metropro-jekt, and, another one is "Study of Project Modification" presented by consortium Krasíkov.  From the engineering-geological aspect the "Study of Project Modification" is more advantageous in comparison with the "Metroprojekt Tunnel Design", as there is only a little trenching of Miocene's unstable clayey layer.
Both presented variants of railway track leading of shaft sinking tunnel will infringe hydro-geological conditions in close geological environment. Deeper excavation into a rock mass on a considerably longer section should be great disadvantage of the first variant. A substantial part of the first variant new railway tunnel is situated in line of the old tunnel, which is draining surrounding rock environment, but, because of its smaller dimensions, it is less influential. In the presented geological environment, the water flows more through quarternary permeable soil layers than impermeable Miocene's clays. There should be taken into consideration the hydraulics impacts to the surrounding territory after being built-up a large scale barrier element, represented by the excavated tunnel.

Geotechnical Risks of Tunnel Construction
The scope of the second variant, presented by consortium "Krasíkov", is to reduce the tunnel length in order to minimize the impact to unsuitable layer of Miocene's clays. There are real fears from this geological formation, which have originated in construction difficulties of previous "Třebovice Tunnel" as well as from many foreign negative experiences with construction of tunnel in squeezing and swelling rock environment. It has been generally known that the tunnel construction in swell and squeezed rock materials is the most difficult task as an underground construction.
Based upon the existing state of tunnelling know-how the objective geological space might be classified into one of the groups: • swell rock environment with a swelling based upon physical and chemical reaction with water and not upon a mechanical principle. There are very well known problems with osmotic swelling of clayey minerals group of smectites or clayey mixture-layered minerals. Further relevant swelling environments are anhydrite formations. These formations may produce high pressure on bottom tunnel vault; • hardly squeezed rock formation occur mainly in faulty zones in the case of the tunnel excavation, where then acts all-round high rock stress on the tunnel lining; • swell-squeeze rock environment, where swelling is of a mechanical character, i.e., in its deconsolidation, respectively, in plastic stress strain deformation of rock.
As it may be seen, the term "swelling" may be attributed to two various mechanisms of rock volume increase. As there has not been found any information concerning the presence of smectite minerals in the layer of Miocene's clay, it might be presumed that no chemical swelling mechanism in the course of construction "Třebovice tunnel" would occur. The authors Klepsatel, Kusý, (2003) state that possible reason of the swelling process in the old "Třebovice tunnel" had been haeving of organic matters, present in geological formation in substantial amount.
The precise composition of these substances is not known, thus, it is very important to understand the swelling mechanism for being able to forecast potential geotechnical risks, especially in a long-term horizon. If speaking about a short-term one -the construction will be in any case very demanding. From the long-term point of view, we are not able to evaluate exactly all geotechnical risks at the current state of information. When speaking only about mechanical swelling, with an adequate project of the tunnel, the first version would not bring any higher risk than the other one in the long-term horizon. It is presumed that on the basis of present knowledge of potential behaviour of Miocene's clay, it would be better to trace the railway tunnel line in accordance with the second version. Tracing the railway line above the layer of The follow-up calculation of pile retaining wall and its external stability against sliding has been done; the calculation evaluated also the depth of fixation of double-row piled wall. Precise dimensioning and calculation of the other parts of construction should be solved in a final realization project. The basic idea of the projected second version presented by the consortium "Krasíkov" is shown in cross-section in Fig. 2.

Interference of New and Current Structures
The first version of the tunnel project will have to solve the crossing of the railway-line across a recultivated waste dump, analysis of the embankment stability and speeding-up consolidation will be next tasks. At this line section reinforcing and hardening of subsoil must be solved by appropriate geo-technology or the layer of anthropogenous artificial soils will be excavated to an adequate depth. The following technical task of project will be a stability analysis of the high embankment and evaluation of consolidation time of the embankment on soft soils. The project must involve also a calculation of consolidation velocity, according to a realization project, and, a designed distance of sand drains or geodrains (PSK, Alimac, Membradrain …) or, there may be used trenching lime, cement or rock columns that would improve subsoil deformations properties, too. The second version should solve strengthening of rock environment around the tunnel excavation at the crossing of a new railway line over the old tunnel (inclined micropiles or use the appropriate manner) and the next back-filling of the old tunnel and build-up the drainage canal.

Geotechnical and Environmental Aspects
When considering the geotechnical point of view, the locality for construction of the tunnel must be taken as a very problematic, especially because of the existence of very thick Miocene's clays strata. Undoubtedly, a more convenient solution, when taking into consideration technical and economical aspects, would be the version of the consortium "Krasíkov" that avoids trenching of the Miocene's layer, as mentioned above, because of the stability problems of cuts and plastic bulging of embankments after construction of the tunnel.
When comparing the presented versions, the latter one has a substantially less negative impact on the hydro-geological conditions in the locality, as well as on the surrounding fauna and flora.

Comparison of Versions
Positives and negatives of the first version "Metroprojekt Tunnel Design" may be summarized in the following Table 1.

Tab. 1
Positives and negatives of "Study of Project Modification" presented by the consortium Krasíkov are shown in Table 2.

Tab. 2
Having considered all the impacts and factors that could infringe the construction of the tunnel, we would like to recommend the tracing in the sense of the second version, submitted by the consortium "Krasíkov".