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1 INTRODUCTION

Portal parts of road and motorway tunnels are usually the most demanding parts of the tunnel con-struction, because they are usually realized in the area with less or least quality rocks. In areas affected with landslides, whether with surface or deep sliding planes, or with deformation zones, there can be ap-plied almost all methods of a special foundation, from the nailed and anchored walls, through pile and micropile securing with rehabilitation of the envi-ronment by a jet grouting, to drainage wells. Portals are the longest open construction parts, often with several construction phases. They are exposed to weathering effects and to transport loading of the construction. These circumstances require paying an increased attention to the design of portals´ securing even in a pre-investment phase, in the selection of the site and locating of portals, then already at the alignment of the road or motorway itself.

As an example of a demanding portal securing for mined motorway tunnels we present the realized solution of the Prackovice-Prague portal tunnel on the D8 - 0805 Lovosice - Řehlovice construction, which is a part of the motorway route Praha - Ústí nad Labem – state border CR/FRG.

The corridor, by which the motorway passes the České středohoří protected landscape area, on its be-ginning connects in the Lovosice region to the al-ready operated D8 motorway Praha - Lovosice. At the end, it connects to the also operated section of Řehlovice - Trmice. The total length of the 0805 construction is 16,4 km.

The technical solution includes some unique solu-tions of passages through the landscape – as for ex-ample is the Vchynice motorway bridge, covered by a sound protecting tube, the motorway arched bridge crossing the Opárenské valley, built without inter-vention into the valley, eco-bridges – short back-filled tunnels for animals crossover with guiding ve-

getation, two pairs of mined motorway tunnels with the names Prackovice tunnel and Radejčín tunnel and many others.

2 PRACKOVICE TUNNEL – PRAGUE PORTAL

The Prackovice tunnel is a one-directional, two-lane motorway tunnel of the T 9,5 category. It fea-tures 2 independent tunnel tubes with lengths of 270 m (LTT) and 260 m (RTT). It passes through the massif of the Debus hill crest in the top part of the Prackovický quarry above the Prackovice village.

Tunnel tubes have mined parts and at their ends cut-and-cover parts, backfilled in a definitive state.

Tunnel tubes of the Prackovice tunnel pass, in terms of a geological area structure and a landscape configuration, through a very complicated environ-ment. According to ČSN 73 1001, the construction has been classified in the III. Geotechnical category, i.e. a demanding construction in complicated geo-technical conditions.

3 ENGINEERING – GEOLOGICAL CHARACTERISTICS

Zájmové The area of interest belongs to the České středohoří, which has been formed by the complex of smaller and sometimes completely iso-lated volcanic bodies of a different composition and shape. The complicated relief arised at a Neogene and Quaternary denudation and erosion, when there were significantly applied different between resis-tances of rocks. Valleys and cauldrons were formed in ash rocks (tuffs) or soft underlying cretaceous se-diments and larger overlaying rocks (nappes) caused arising of table hills. Smaller bodies gave a rise to conical shapes of different appearances according to the character of the rock, as for example is the area of Prackovice and Dobkovičky basalt quarries. The eastern rims of a volcanic body affect landslides of a

Problems of tunnel portals in slide areas

K. Novosad PRAGOPROJEKT, a.s., Prague, Czech Republic

SUMMARY: Portal sections of road and motorway tunnels are usually the most demanding parts of tunnel construction because they are usually executed in areas with lower or lowest rock quality. Areas affected by landslides, regardless whether surface or with in-depth shear surfaces or deformation zones, then require ap-plication of various special foundation methods, from the nailed and anchored walls, through pile and micro-pile securing with rehabilitation of the environment with jet grouting, to drainage wells. Portals are the long-est open parts of the construction, often with several construction phases. They are exposed to weather effects and by the burden caused by site transport. These circumstances require increased attention to design and se-curing of the portals already in the preparation phases of the design, in selection of position and situation of portals, i.e., even during the road or motorway route selection.

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block type in the area of interest, and interfere into the area of the D 8 motorway. Labe River cut here under the basis of a volcanic nappe into soft creta-ceous rocks and removed from below of block rims extruded Turonian marlites, so that the gradient of slopes degreased and single blocks of vulcanite rocks slided to the Labe valley. During the valley evolution, the rim of vulcanites moved away from the river, the movement speed degreased up to now, when it almost stopped.

The cut-and-cover part of both tunnel tubes in the area of the Prague portal is in terms of a geological structure and terrain morphology in a very compli-cated environment. From the petrographic point of view, there is represented here a wide scale of rocks.

Single lava extrusions of a whinstone basalt (or re-lated rock types), multiply alternate the volcanic ejects – pyroclastic rocks. There is especially a volcanic ash - tuff with an addition of grains and fragments of ejects (lapilli, eventually with isolated lava stones and stone blocks ejected during explosions). Volcanic tuffs have a character of a fragmentally to lumpily parting rocks with a porous or conglomerate structure (locally similar to a cinder with a content of solid basalt fragments and stones). Spread tuffs have, for their high content of a montmorillonitic compound (from 28 % to 75 %), the theoretical ability to blow up.

Both basaltoid rocks and effusive rocks are, in the area of the surveyed locality, differently affected by weathering processes. Above mentioned rocks could be met as compact or very compact rocks – when they are not significantly affected with a post-volcanic transformation (by hydrothermal solutions). In case of an intensive weathering, the originally very compact volcanic rocks have a character of finely fragmentally slacking, weathered to loamy-fragmentally decomposed rocks. The transformation of a compact rock into a loamy-fragmentally de-composed one has been here both slow and very steep.

The Prackovice quarry was worked out more than 15 years ago. For a disruption of a rock massif (for

the mining itself) there were carried out here also chamber blastings, which strongly disturbed the sta-bility of a rock environment.

Further to the described diversity of a rock envi-ronment, construction works were also complicated by a significant landscape ruggedness of the near surroundings and also by an old mine working (mine tunnels of a field survey).

Slopes at tunnel portals are mainly covered with debris. The debris has mainly a stony and locally boulder character, the filling is mainly loamy sand and generally the debris is a loose one. Atmospheric preci-pitations and other climatic effects will result in falling of rock fragments, stones and sliding of debris cones from adjacent slopes above the future motorway.

There was found a relatively high disruption of rock exposures, especially in a slope above a bottom platform in a place of a portal at the Prague part of tunnels. Fissures are open, steeply inclined, and they are mainly oriented across to the motorway axis.

Complicated geological conditions are also in the whole length of a revetment wall, which connects to the Prague portal on the left side to the slope. There is mainly a volcanic ash – tuff with an addition of grains and fragments of volcanic rocks. Volcanic tuffs are sometimes similar to a cinder with a content of solid basalt fragments and stones. So originally very compact volcanic rocks have in this area a cha-racter of a finely fragmentally slacking, weathered to loamy-fragmentally decomposed rock.

Prior the realizing of excavations and securing works, there was in advance realized an exploration gallery in the right side wall of the left tunnel tube, for verifying of the specific geological and hydro-geological conditions, for verifying of suitability and efficiency of structural members used for the secur-ing of mined tunnels and last but not least for secur-ing of an access and for starting of works at the second northern „Ústí portal“.

Figure 1. Situation of the 0805 construction

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The exploration gallery was building in years 2004 to 2005 as a temporary work. It verified in a detail manner complicated geological conditions and determined characteristics of the rock massif, in-cluding the response to excavating works.

Figure 2. Prague portal before starting of the construction

Taking into account that the realization of the in-

put section of the exploration gallery of the Prague portal was omitted, due to problems with the stabili-ty of the ground in the bench face, the contractor of the tunnel work considered as very important to fill the remaining gap in knowledges of the geological environment by a supplementary survey of the portal section. Near the bench edge, there were carried out four exploratory drill holes, with the aim to check the thickness of the made ground – unstable stony and boulder grounds, filled on the bench face.

The drilling of new core holes was connected with problems. Probes in two cases intruded with a very high effort into the depth of 7 m; one hole crashed in the depth of 4 m, where the drilling tool wedged between basaltic stones.

The drill-hole exploration was for this reason completed with a geophysical indirect exploratory method – a shallow refraction seismic method.

The supplementary IG survey of the Prague por-tal section resulted in a new, more specific location of geological structures of the volcanoclastic com-plex.

The structural scheme of the excavated section behind the portal featured alternating of sub-vertically oriented bodies of more compact rocky blocks (coarse-grained agglomerate tuffs) or less weathered (altered) basaltic veins with limited com-pact to loamy-fragmentally distributed tuff locations. At the portal securing, the non-uniform environment was the reason of problems at a drilling and fixation of anchor elements. The extent of the local loosening was also much deeper than was that given in the do-cumentation of the exploratory gallery. Block sepa-rated, more compact bodies complicated the future

excavation „by driving out“from the deck and face of excavation.

With regard to all these facts, there was paid a special attention to securing of portal slopes (defini-tive and temporary ones), that no activation of old or new local landslides could happen during the con-struction and after the construction.

Figure 3. Prague portal with finished mined tunnels

4 TECHNICAL REALIZATION OF SECURING WORKS FOR THE PRAGUE PORTAL

The extent of trenches and securing works for the Prague portal is approximately from the km 58,280 to km 58,370 (chainage of the motorway route). The cut-and-cover right tunnel will be about 90,0 m long here and the left cut-and-cover tunnel about 87,0 m long.

Trenches and areas in front of the southern Pra-gue mined tunnel portal had to secure the area for the cut-and-cover tunnels themselves, construction facilities, the transport of excavated material from tunnels and the filling of material to tunnels. Trenches and the securing of side and face (portal) slopes were gradual and parallel to stages of secur-ing works.

At a stepwise material removing, there was used, as a basic technology, the securing by steel bar anc-hors SN – ground nails. These anchors were formed of profiles of reinforcement steel with a diameter of 25 mm, BSt 500 S quality.

The technology contributed to securing of a sta-bility of the ground or rock body by two basic fac-tors. The filling, except of the interconnection of nails with surroundings, at the same time filled also the system of fissures or cavities near the drill hole and reinforced so the drill hole’s surrounding. Drawbars of nails themselves enabled transfer of forces from surface areas to internal areas of the se-cured slope, i.e. to the location with more friendly tightness. There is the reinforcing of the massif

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(semi-rock) by reinforcing bars, which are, by means of a filling, connected to surroundings along all its length. The force loading of nails results from changes of tightness due to removing of the rock in the area around the inner surface. Nails in this part are tensioned, reducing so deformations and a drop of tightness. Tensions are transmitted into deeper parts of the massif, where the influence of the re-moved material is only small.

This interaction of tension elements - nails with the reinforced environment of the rock massif was not possible to ensure reliably without verifying of the character of distributed layers or without their rehabilitation.

Figure 4. Ústí portal with finished mined tunnels

Rehabilitations were carried out in form of solidi-

fication of the environment by jet grouting along the whole active length of single nails. Only into such prepared environment, there were made drill-holes, installed nails and activated by a grout filling.

With regard to the complicated and non-regular massif structure, there was first possible only esti-mate the extent of drill holes rehabilited by the jet grouting. The more exact extent was determined on-ly after finding of results and after the interpretation of the nails application from the testing field. Ac-cording to the evaluated tests of drilling, grouting, nails activation and results of tension tests, carried out by an independent accredited testing department, there was further determined the extent in other par-tial stages and it was stepwise specified.

The securing of the reinforced portal slope was further complemented with strand anchors of pre-tensioned steel on three levels through steel walers. Anchors were in the length of 26, 24 and 18 m, three-strand anchors, pre-tensioned to 300 kN with the root part of the length 9 m. Walers were of the Larssen III-n sheet piles, with the length of 7 or 3,5 m for three or two anchors, respectively. The contact of walers with the secured portal wall of SC was se-cured after the installation of walers by a concrete

encasement, also of SC. Anchors situated above the contour of future tunnels were shifted in direction so as not to intrude into the tunnel profile.

The rock reinforcing method and the application of the shotcrete on the surface run gradually from the top in coordination with excavating works.

For the design of securing elements gradient and dimensions, there was chosen the portal in the km 58,367 (chainage of motorway axes), where the total height difference was about 25 m.

Figure 5. Prague portal with the stabilizing block and the sup-porting rocky wedge.

The inclination of the portal wall perpendicular to

the axis of tunnels was divided into 3 types: In the bottom part, there is a gradient of 2,5:1 (i.e. 68 de-grees). The steeper gradient is more suitable here for applying of excavation. Above the tunnel level, there is chosen an easier gradient, and 1,75:1 (i.e. about 60 degrees) and on the highest level there is a gradient of 1,25:1, chosen with regard to the totally loosen rock character (i.e. about 50o). The portal wall was released by two benches with the width of 1,5 m.

The density and the length of the reinforcing were calculated. The surface layout was designed in a net from 1,8x1,8 to 2,0x2,0 m. The length of nails was stepped from 8 m at the slope foot, to 12 m at the slope crest. Lengths were determined by means of a fictive slide plane (fissure), which was giving the greatest active forces. Beyond this plane, nails were pulled min. 5 m, considering the usage of nails´ lengths.

The surface of the reinforced slope was secured by a shotcrete C16/20-X0 of the th.20 cm, which was reinforced with a double "KARI" mesh of prof. 6 mm, with loops of 10x10 cm.

Within works stabilizing the removed slope, there was carried out a reinforcing structure for facilitat-ing of the tunnel excavation – protecting umbrella of 114/12,5 mm micro pilots with the length of 20 m above the contour of future tunnels.

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Figure 6. Prague portal with the stabilizing block and the supporting rocky wedge-longitudinal section.

To avoid the falling of stones into trenches from the

ravine with a non-stabile part of a debris cone at and above the right side of trenches, there was carried out, in this part of a ravine, where a platform appears above the portal at a level of about 340 m a.s.l., increasing of the securing by the reinforced-concrete anchored ring beam with the width of 60 cm, the height of 90 cm in the length of about 26 m with a wall of micro-pilots.

The shape of the ring with an irregular arc follows the contour approximately at a level of about 340 m a.s.l. In advance, prior the realization of the ring, there was made from the top a partial back filling of the ra-vine so, that there was made a sufficiently wide plat-form (about 6 m) for making of a micro-pilot wall of MP 114/12,5 mm, with the length of 10 m, per 90 cm in the axis of the future reinforced-concrete ring beam. MP were always alternately vertical and inclined towards the massif under 30°.

The upper part of MP protrudes about 90 cm over the top of a ring and serves as a barrier against sliding of stones. On protruding MP there are welded bars of rein-forcement with a catching fence of Kari meshes upto the height of about 1,5 m. Number of MP is 28 pieces. Af-ter concrete casing of MP into reinforced-concrete ring beam, the ring was behind the back side carefully con-creted around for securing of the contact with a slope and there was behind it realized a drainage trough of a shotcrete. Through the in advance installed bushings, the reinforced-concrete ring was finally anchored by means of eight three-strand anchors with a pretension of

300 kN, with the length of 28 m, with a root of the length 9 m. The gradient from the horizontal was 20°. In the layout plan, anchors form a fan with a pitch on the ring of 2,6 m. After the realization of this upper ring and its activating, there proceeded a securing of a portal wall into lower stages with a stepwise removing of the sup-plementary backfilling.

For side securing of both mined tunnels of the Pra-gue portal, there was in the entry part of the portal wall made rehabilitation (side reinforcing) by micro-pilot fans on tunnel sides of two triples of MP 114/12,5, rea-lized during securing works of the portal from the second working level on the altitude of 326,30 m a. s. l.

During the realization of all these securing works, there run at the same time a geotechnical monitoring, carried out by the „AZ Consult“ company, and which monitored the size and development of portal wall de-formations by a 3D geodetic measuring of selected points. At the same time, there was monitored also the response of deeper parts of the massif by an inclinome-tric and extensometric monitoring. Chosen heads of pre-tensioned anchors were fitted with dynamometers for monitoring of the development and size of pretension changes.

Results of the measurement were regularly submitted and evaluated on meetings of the Council for geotech-nical monitoring and in case of a necessity, there were made some decisions about required measures or ap-pointed a plan of consequent works.

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Figure 7. View of securing works on the Prague portal

Measurements showed a relatively significant re-

sponse for advancing works, especially by a size and development of deformations of the portal wall it-self. The greatest movements were on the edge of a portal top, on the upper plane behind this edge and especially on the plane of a portal slope in the mid-dle between tunnels, and with the size upto 80 mm. Deformations were confirmed also by inclinometric and extensometric measurements and by the increas-ing of a pretension of strand anchors. Drops were most significant above the portal and horizontal de-formations - extrusion of the portal wall to the trench in the centre above tunnels, at which the ex-cavation started.

Deformations were a response to the stepwise removing of the material and securing of a portal, but also relatively significantly to the excavation of tunnels themselves in their entry portal part and to partial stages of an excavation sequence.

In the end of 2008, the development of deforma-tions was gradually degreasing but constant. To se-cure a stabilization of deformations, there was car-ried out an additional reinforcing of the portal slope foot stability between tunnels by means of a fixing of the stabilizing block with concrete between tun-nels on the supporting rocky wedge. In the bottom of the stabilizing block, there were in advance realized inclined micro-pilot trestles.

This measure, after the evaluation of the mea-surement, contributed significantly to the stabiliza-tion of deformations, however these deformations appeared, to a significantly lesser extent, as a re-sponse to other excavation sequences, and to changes of climatic conditions.

Today, there have been finished excavations of both tunnel tubes; there proceeds a concreting of cut-and-cover tunnels and consequent backfilling.

Securing of the Ústí portal at the opposite side of tunnels will be made using the equal technology as on the Prague portal. Though, there were very con-fined space conditions and the access to securing works was for a certain time period possible only through a small profile of an exploration gallery, the portal securing was carried out without problems. The Ústí portal was significantly smaller and practi-cally the same rocks were not so strongly affected with alteration and open mining activities in the past. The minimum area and the portal securing were also enabled due to the ability of a contractor to carry out a breakthrough of the right tunnel prac-tically with a zero overburden.

REFERENCES:

PRAGOPROJEKT, a.s., TUBES spol. s.r.o., 2008-2009, Realizační dokumentace stavby