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Geological Researches

Antecedents, Research, Technical Considerations

Geological survey and investigation of Budapest and, implicitly, the areas impacted by Metro alignment is based on the data continuously collected from the middle of 1800s. Geological investigation of Metro line 4 started in 1967, and most of the borings were carried out in 1977 by applying F-62 double walled core barrel and full geophysical surveying measurements.

Prior to launching the newest research programme, based on existing data and earlier surveys, experts carried out geological reliability tests in order to choose the place of new borings with the following considerations:

  • Soil characteristics on long sections potentially having decisive impact on the technology of construction to be chosen.
  • Characteristics existing in short sections (several hundred meters), requiring special technology in addition to the building technique applied.
  • Local problems, e.g. geological curvatures or faults, which can be overcome by a slight modification of the building technology.

The new borings were carried out with up-to-date "Geobor S" core barrel, combined with Wire Line boring technology and polymer sludge attaining 98% TCR (Total Core Recovery in terms of %) value of 102 mm diameter core samples, as well as preserving the "in situ" state of the core material. Large diameter tri-axial tests were carried out on core samples extended to determine the rock physical characteristics of residual status.

On site surveys of soil mechanics, rock physics (rock pressure metering), hydro-geological and geophysical tests (well logging, cross hole and down hole) supported the success of research works. In the course of study preparation, the results of earlier surveys and tests were evaluated and correlated to the results of new, more accurate and detailed surveys.

A study on earthquake risks and preliminary environment impact for the Metro line has been prepared according to regulations, which used the basic data of geological, hydro-geological, geo-hydrological and engineering geological concise expertise.

Geological structure, engineering geological conditions in the environment of alignment

Geological structure and high scale tectonics were known for geological experts. Recent researches have specified high scale data along the alignment, or even confirmed it around the passage under the Danube.

The area concerned can be divided into three main geological and engineering geological units:

  • Unit 'A', line section in Buda: from the Kelenföld pályaudvar to Szent Gellért tér
  • Unit 'B', passing section under the Danube: Szent Gellért tér to Pest rakpart
  • Unit 'C', line section in Pest: Fővám tér to Keleti pályaudvar.

Engineering Geological Unit 'A', line section in Buda: Kelenföld pályaudvar to Szent Gellért tér

Along the section Kelenföld pályaudvar to Szent Gellért tér, medium and inferior Oligocene clay or clayey marl are found in several hundred meters thickness under loose piedmont deposits of Pleistocene origin.

Fig.1:Ist and IInd engineering geological longitudinal sectionsFig.1:Ist and IInd engineering geological longitudinal sections
Fig.1:Ist and IInd engineering geological longitudinal sections

By a boring technology applied in 1997 and 1999 research borings and determination of RQD, TCR values revealed the following geotechnical subdivision of Tertiary formations:

  • Expanded zone, which can be divided into a seamy, fissured superior and a cracked inferior section,
  • Section beyond expanding effect, a sound rock mass.

In the course of geological processes, when the Oligocene surface has become a continent, a demolishing of several hundred meters occurred with an expansion, breaking down of the rock surface. Strength characteristics of the rock decreased until the depth of such expansion. The rock surface demolished has promoted this weathering process continuing until today under the influence of the ground water flowing through Holocene, Pleistocene layers.

This exposition seeming a bit too detailed is justified by the fact that the tunnel to be constructed will pass and the restructuring stresses will occur in an expanded zone.

In a rock rigidity diagram prepared by Bieniawski (1974) we have presented the specific location of the zone as RSR (31) together with tunnel diameter (6 m) and stability data (Fig. 2). It is obvious that the tunnel passing entirely through this zone cannot have the required stability without any active support.

Fig.2: rock rigidity diagram along the section in Buda, in function of RSR and tunnel diagram (Bienawski, 1974).
Fig.2: rock rigidity diagram along the section in Buda, in function of RSR and tunnel diagram (Bienawski, 1974).

Ground water is here and there very aggressive, and the concentration of sulphate ions can be as high as 6000 to 7000 mg/litre.

Engineering Geological Unit 'B', section under the Danube: Gellért tér - pesti rakpart

Geological, hydro-geological and geo-hydrological peculiarity of the new Metro line is the section passing under the Danube where the tunnels are led near Triassic warm karst springs. Longitudinal section of this unit is presented in Figure 3.

Fig.3: Geological longitudinal section III
Fig.3: Geological longitudinal section III

The area is full with tectonic formations. The Triassic dolomite forming the majority of the Gellrt hill is step by step desends to 800 or 900 meters from the surface, and that within a horizontal distance of not more than 400 to 500 mm, thus, a specific asymmetric and tectonic uplifted block structure has been formed. Near the tunnel alignment, passing from the Buda side to the Pest side, the clayey marl of the medium-inferior Oligocene is first substituted with loosen clay-dolomite aggregate of Eocene era along the geological faults. In this section of 40 to 50 meters and on the inferior section of the tunnel, a Triassic surface appears at the bedrock with dolomite dust and cataclastic Triassic dolomite development, the latter being disintegrated under mechanical effects to polyeders. In this section, the tunnel will directly touch the reservoir of warm karst waters existing in Pannon basin or, respectively, its natural free and positive outlet along tectonic lines. The pressure of this karst water system is always higher by 1 to 1.5 m than the prevailing Danube level, i.e. the karst water cannot be mixed or contaminated by Danube water. Temperature of the karst water or medicinal water is around 40C, its quality is determined by the content of calcium, magnesium and hydrocarbonates, as well as a considerable fluoride ion and metasilicic acid content.

Leaving this environment behind, the tunnel will pass through Eocene marl impregnated by silicic acid under the hydrothermal influence of ascendant warm karst waters. Approaching the Pest side, Oligocene clayey marl is substituted for Eocene marl along a definite structural line, which becomes step by step a medium Oligocene clayey marl, then near the Pest river side fine sand, sludge and clay layers of superior Oligocene appear with slight carbonate binding around the tunnel alignment.

A special case of the Metro line envisaged and is worth public opinion in the section under the Danube. One of the reasons is the concern of environment protection revived by approaching the thermal springs. The other reason is the river bed, as a natural formation burdened by artificial elements, since it has an outstanding importance out of hydrological and navigation viewpoint. Due to complex and complicated geological, hydro-geological and geo-hydrological characteristics, special requirements should be satisfied by the technology of construction.

A basic interest is to avoid water ingress with reasonable and safe counter-pressure at the workface and reliable waterproofing at the shell. Amount of this counter-pressure, i.e. the hydrostatic pressure to be considered is the prevailing Danube water level + 2 meters + required technological safety.

A percentage distribution of the rock groups along the vertical axis of the right tunnel is presented in Figure 4.

Fig.4: Percentage distribution of the rock groups along the vertical axis of the right tunnel, from top to bottom, on the geological section 'B'.
Fig.4: Percentage distribution of the rock groups along the vertical axis of the right tunnel, from top to bottom, on the geological section 'B'.

According to the results of laboratory tests, the strength and rigidity characteristics of rocks are defined rather by the tectonic and hydro-thermal effects than by their age or depth. Thus, the strength and rigidity of materials is 2 to 5 times higher than that on the Buda side.

  • Eocene basic rubble: this rubble - and the component basic gravel - comprise a minor quantity of binding material. Due to the friable features of this material it is not suitable for rock mechanic tests.
  • Buda marl: its colour is light brown-grey, no stratification and internal lamination. It is a hard material, being considered as marl due to its lime content. The rock mass exempt of fissure is watertight, however, the fissured rock is able to transport flowing or warm karst water both at the covering and foot side.
  • Tard clay: the alignment envisaged is located north from the earlier surveyed and tested alignment. Data of the former borings reveal that hydro-thermal influece did not affect, or affected just slightly that area, thus, a comparison of rock physical characteristics is not reasonable.
  • Kiscell clay: such a clay can probably appear in a short section, between the left side of the Danube bed and the Pest river embankment.
  • Superior Oligocene: rocks of the superior Oligocene with clayey development and more or less sand content can be expected near the Pest river embankment. In earlier surveys several tests were made to this depth.

Engineering Geological Unit 'C', Pest section: Fővám tért to Keleti pályaudvar

Under a few meters of embankment granular sediments of the formerly meandering antecedent Danube are found, grain size increasing downwards. Thickness of this Pleistocene sediment is in average 10 to 12 m, in extreme cases up to 15 m. The ground water flows in Pleistocene layers where it is stored as well.

Under Pleistocene layers Oligocene and Miocene layers are located. Engineering geological longitudinal section of this unit is presented in Figure 5.

Fig.5: Engineering geological longitudinal sections IV and VFig.5: Engineering geological longitudinal sections IV and V
Fig.5: Engineering geological longitudinal sections IV and V

Near the tunnel, 100 to 150 m from Danube embankment, inferior Miocene layers cross-stratified with erosion and angular discordancy are superposed on superior Oligocene rocks. Inferior Miocene layers comprise low content carbonate bound sand, sandy gravel, as well as hard, fine sand and marl clay layers with fine granule size containing 8 to 14% carbonate.

From such an appearance of Miocene layers until the end of the section concerned (Keleti plyaudvar) there are more and more younger, medium and superior Miocene, cross-stratified layers. The inferior Miocene layers are formations of the Miocene beach barrier, and the younger Miocene layers are alternatively transgressive and regressive, however, in respect of the process there are sediments of the see deepening. From an engineering geological viewpoint this means that the sandy layers are continuously decreasing by approaching Keleti plyaudvar, more and more non-consolidated clay, bentonite clay appear, from the size of several meters up to 40 m near tectonic faults.

The confined water stored in Miocene layers is here and there in direct connection with Pleistocene ground water, but in other locations it becomes under pressure. The pressure of the confined water is a few decimeter higher than that of the ground water.

Due to the origin of Miocene layers of the Tertiary era, the rocks range from loosen layers without any cohesion through soft clays to compact clays and lime marl. Figure 6 is an illustration of the percentage distribution of rocks, from top to bottom along the vertical axis of the right side tunnel.

Fig. 6: Percentage distribution of rock groups from top to bottom, along the vertical axis of the right side tunnel.
Fig. 6: Percentage distribution of rock groups from top to bottom, along the vertical axis of the right side tunnel.

Based on classification tests, fine granular or low strength sandstone layers - since the original structure had to be destroyed because of laboratory technique reasons - were considered as sludge or sludgy sandy flour. This was completed by a relatively high carbonate content (16 to 36%), which also could influence tests. Due to the carbonate binding, cohesion could vary between quite wide limits. Maximum cohesion is originated from fine grain sand rock. However, the thickness of such layers is not more than 0.5 m.

Hydrological Environment

In a strictly speaking sense, ground water on the Buda side appears just within the Holocene - early Pleistocene flood plain in a high extension and quantity. Ground water is supplied by precipitation, however, from the line of the 2nd Danube terrace (the limit of terrace 2 and 1 is about in the line of Fehrvri t), on the areas towards the river, the prevailing water head supplies or reduces ground water level or influences flow direction.

One of the main flow directions is the Danube, as a leakage towards erosion basis (the Danube is collecting ground water in a very similar manner to an advance heading). Another main flow direction of the ground water is north-south, matching the Danube valley.

The flow image described above is somehow modified by minor flows following the brooks coming from the hills. Such motions of the terrace water of brooks, towards valleys, will end in the Danube, as an erosion basis, however, in other locations, mainly forming a north-south leakage field.

It makes no sense speaking of ground water in the section below the Danube, but the effect of the prevailing Danube water level may not be ignored. Average ground water level is in the Danube's direction, as long as in case of a high water level the Danube water refills or impounds ground water. This range, where refilling height is more than a normal average water level oscillation, amounts to 400 m. This can be confirmed by the 1965 flood flow data, as well as a study of the Miskolc University, which has investigated the impact of the diaphragm wall section of Metro line north-south on ground water.

On the Pest side, a specific border line separates two areas with ground water of special character, origin and motion from each other. This is the supposed line of the eastern Danube embankment in the Early Holocene era, as cut in into the Pleistocene Danube terrace. This digging of the Danube transformed the western border of the terrace into a quasi vertical river embankment. The area filled up with Holocene flood plain sediments of the antecedent river extends towards the Danube from this antecedent river bank (lower terrace levels), and eastwards from this line one can find higher terrace levels.

The ground water is mainly consisted of infiltrated precipitation waters, and just a small part is supplied by side flows of the same direction as the consequent slope direction of the Tertiary underlaying rock, as well as some live water flows (Szilas brook, Rkos brook), respectively. Erosion basis of the area is the Danube, a hydrological element of generally good recipient capacity. At the same time, ground water is supplied by high water levels and, on the contrary, it is lowered by low water levels in the river.

Out of water quality viewpoint, the Metro line can be divided into two parts:

  • The first part (Budars station to Móricz Zsigmond tér) is the area of expansion of slope ground waters and valley ground waters,
  • The second part (Móricz Zsigmond tér to Újpest station on the Pest side), where the ground water of Danube flood plain appears.

Monitoring system during and after construction

  • Monitoring of the effects of intervention in natural environment before, during and after construction is necessary in order to maintain this environment in its original status. That is why such systems should be developed and operated already before commencement of construction (measuring '0' status), which enable the monitoring of ground, confined and karst water.
  • Ground water oscillation should be established by systematic surveys, which could also become a basic document of possible changes.
  • In the monitoring pits renewed, instead of earlier non-systematic and seldom surveys, water level measurements should be carried out every three days.
  • In order to establish ground water quality, water samples should be taken and analysed.
  • The investor should construct one monitoring pit in each section between 29 and 31, respectively 39 and 45 km for the survey of water quality, at the location where the highest SO4 content can be expected.
  • Two monitoring pits are envisaged within the investment, at the environment of each station to be constructed, having the function to record ground water changes. Surface connections of the stations and any station facilities may also locally traverse the ground water storing layer. The local expansion of these may affect ground water flow. In restricted areas this may involve an increase of the ground water level, which should be at once compensated by technical solutions.
  • Near Szent Gellért tér station a new monitoring pit should be installed for the systematic monitoring of karst waters. This pit has the function:
    • to reach and monitor the main karst water storing rock,
    • to test the pressure level, water quality and temperature of the so called covered karst water.

Ground water monitoring should be continued during construction as well, within a system as described above.

Monitoring of confined water during construction should mainly refer to water quality. For this purpose water sampling and analysis should be carried out every 50 meters in case of open front construction in respect of the component specified for ground water.

The full monitoring process should be continued one year long following completion in compliance with the rules set out for construction period.

With respect to the monitoring of under-surface waters, (standard) monitoring pits should be implemented that are also suitable for water sampling (by pumping). The design of such monitoring pits also includes a plan for instrumentation, as well as monitoring instructions for the full pit network.

Engineering geological longitudinal sections published in the Concise Geotechnical, Engineering Geological And Hydro-Geological Report for Section I of Metro line 4 in Budapest (data for soil mechanics not included). (c) Copyright: DBR Metr-Geovil Kft. Preliminary Environment Impact Study for Section I of Dl-Buda - Rkospalota Metro line (4) in Budapest (complemented) (Volume III, Summary, June 2002)