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The text ONLY of the Paper is given below; but it will probably be more useful to read the .pdf file that includes illustrations and can be found in the Library.  The text is given below simply so that Google and other "crawl engines" can find the text.


Tunnel Collapses  in Tunnel 3A of the T.A.G. Motorway, Southern Turkey

by

Clay, Robin B. Dar Al Handasah, Turkey

Takacs, A. Paul Dar Al Handasah, Turkey

Abstract

The Tarsus-Adana-GaziAntep (T.A.G.) section is the current end of the Trans-European Motorway (T.E.M.), the E5. The T.A.G. Contract extends approximately 258 kilometres Eastwards from Tarsus, past Adana, the provincial capital, and Turkey’s fourth most populous city, across the Cukarova plain to Osmaniye, where the route is obstructed by a range of hills. The motorway crosses the divide into the Urfa plain and thence to GaziAntep. The route through this divide extends some forty kilometres, rising to a height of about 900 metres above the surrounding countryside, about 940 metres above sea-level, and crosses the steeply-undulating terrain by a combination of tunnels, cuttings, embankments and viaducts, see Figure 1. There are nominally four tunnels; they are three-lane, twin-tube, and thirty metres apart between centre-lines : T1 - Tasoluk - 376 & 376 m long T3 - Kizlac - 2717 & 2760 m long T2 - Ayran - 536 & 562 m long T4 - Aslanli - 1225 & 1230 m long Tunnel T3 is divided into two sections, T3A to the West and T3B to the East, by an access adit. Construction of T1 proceeded relatively without incident, and the tunnel has been open to traffic for a couple of years. Construction of T2 and T4 was also relatively incident free, and the tunnels are now complete, awaiting road construction. Construction of T3B has also been incident-free, and the second tube holed-through at the end of March 1997. But construction of T3A has been dogged by misfortune. The South Tube advanced without incident for nearly three hundred metres, before a collapse led to the loss of thirty metres of tunnel. The original face was recovered after six months, during which time the face of the North Tube advanced past the face of the South Tube. Within a few days of this recovery, however, a second collapse of the same size brought work to a halt again, and caused distress in the adjacent North Tube. This paper describes the background to these collapses and their process, the immediate action taken to stabilise the tunnel, and the subsequent investigations.

Figure 1: Location Map

1 Introduction

Tunnelling is inherently a dangerous form of construction, and unfortunately collapses are not a rare occurrence [1]. Tunnelling remains an art rather than a science, and is possibly the most primeval of construction operations - Man against Nature at its most basic. Science is used to reduce the risks [7] - but even the use of science is a risk of a sort in itself, as to whether the money and time invested in research will actually pay a dividend. While "you pay for a Site Investigation, whether you have one or not"[8], there are obviously limits to how much it is worth spending for what return. Taken to the extreme, the only way of finding out exactly what lies ahead is to drive a pilot tunnel - slightly bigger, and on the same alignment. Hence the tunneller has to be pragmatic, and accept occasional collapses as part of the price to be paid for the job [2]. During construction of the T.A.G. Motorway, two significant collapses occurred in one tunnel, the second close to, and related to, the first, an occurrence which is fortunately rare. The situation in each case developed slowly, and did not seem particularly significant at the time, and fortunately there was no loss of life nor injury nor even any damage or loss of plant or equipment. Supervision, like Site Investigation, requires a balance to be struck between quantity and cost. Several other tunnels were being driven contemporaneously; supervision was spread, so constant close watch could not be kept on every face. In what follows, deductions have been made (possibly erroneously) as to what happened. Amongst the parties involved were Austrian, British, Italian, Polish, Romanian, and Turkish engineers, so there was fair scope for misunderstanding. The views expressed herein are purely those of the Authors alone, and represent no official opinion, either of the Engineer or of the Client.

2 Geology

The Nurdag valley forms the Northern end of the Great Rift Valley which extends South through the Dead Sea, the Red Sea and into Kenya. The area is subject to earthquakes. The strata through which the tunnels are driven are principally sandstone and shale, but with zones of crushed and weathered material, and interspersed with faults. The detailed geology in the vicinity of the collapses is very contorted, with the bedding planes constantly changing in inclination, accompanied by fault zones, some with slicken-sides, and with quartzitic and calcitic inclusions. The strata at and above tunnel level at the collapse is thinlybedded dark grey to black shale, which could be excavated by pick and shovel, and can be crumbled in the hand into pieces the size of a large thick biscuit and smaller. There are also plastic zones of clayey mylonites, and some strata of thinly bedded brown to grey sandstone. All strata are intensely fractured, with some clay and iron oxide present in the joints. The shale which caused the problems may be a lens in the sandstone which generally predominates in the area. Although saturated, the shale appears to have acted as an impermeable barrier to natural flow of water through the ground, resulting in a perched water-table which the tunnel pierced. A railway tunnel driven at the beginning of this century diagonally across the line of the motorway tunnel near the portal effectively drained that zone. There is reason to believe that the collapse occurred at the intersection of two fault zones; however, without proper cores of the area, any hypothesis is almost as valid as any other. One of the drawbacks of the shotcrete support method is that the geology is hidden almost as soon as it is exposed, and it is impossible to get the full picture. An assumed longitudinal section is given in Figure 2: Figure 2: Longitudinal Section showing Geology Some cores were taken during the post-collapse investigation, but the ground, the equipment and the techniques were such that core recovery was very low in the zones where most information was required. Cores longer than 100mm were rare, and much of the material recovered would pass a 10mm sieve (see below). Interpretation of bedding direction and such like was therefore not possible. The tunnels are generally very dry, compared to other tunnels, although there are wet patches; and near the portals the connection to the surface is obvious, with increased flow in the days following rain. The various formations and strata encountered were predicted as likely to occur, but obviously precise details were not ascertainable.

3 Tunnel Driving

3.1 Excavation Method

The tunnels are driven using wheel-mounted equipment for drill-and-blast excavation. Trimming for underbreak is performed by a hydraulic breaker mounted on a traxcavator. Support is provided by a system of shotcrete, re-inforced with wire mesh, and rockbolts, and, where appropriate, roofties or steel ribs, a system similar to the New Austrian Tunnelling Method (NATM). There is a subsequent secondary lining of in-situ concrete, and the two are separated by a polythene membrane, with a drainage layer of artificial felt behind [3, Figure 5.8], [7, Figure 14.9]. Excavation is a multi-stage process, see Figure 3. In the best ground, there is a top heading, followed by bench excavation. In worse ground, an invert may be provided to close the ring, and in the worst ground of all, the top heading may be divided into two side-drifts.

Figure 3: Excavation Cross-section

3.2 Support Core

Following his experience in Tunnel T4 which had been driven by the same Sub-contractor, the Contractor chose, even in Rock Class 4, to have a support core, in order to provide support to the face. The support core occupied roughly the middle third of the tunnel width, and half to two-thirds of the tunnel height, see Figure 4. This reduced the amount of work required to advance the face, as advance of the support core could follow later. However, it did restrict access, and also precluded any rockbolts being placed in the front two or three rounds. The theory propounded was that the bolts were not really needed until that far back, to support the ground arching longitudinally, the effects of which were greatly reduced by the presence of the support core.

Figure 4: Longitudinal Section of Tunnel, showing Support Core

3.3 Method of Advance

The system adopted by the Contractor for driving through the rather poor ground where the collapses occurred was to install about thirty grouted forepoles (32 mm diameter re-bar) four metres long at 400 - 500 mm spacing. The ends were supported on the top of the rib of the previous Round, which were themselves buried in shotcrete to their full depth. The face was drilled, then blasted, and the final half-metre or so around the crown was trimmed by hydraulic breaker. A thin sealing layer of shotcrete was applied from the muck pile / support core to the roof and to the top of the face, after which the muck was excavated. Mesh was tied to the mesh of the previous round which oversailed the rib by about 200 mm, and by tying wire wrapped round the exposed forepoling or embedded in the sealing shotcrete. The rib was erected, in three parts bolted together, and located longitudinally by tie bars clipped to the previous rib. Surveyors checked the rib by theodolite, and it was adjusted until it was located correctly, and was the correct shape. The rib at this stage was very flexible, needing to be fixed into position by diagonal ties welded to it and the previous rib. Once the rib had been fixed into position, the ground profile was measured from it. This clearance between rib and ground was used for calculations of overbreak volume; it was therefore in the Contractor’s interest that the quantity of sealing shotcrete be small, so that the amount of overbreak (for which he might claim payment) was accurately measured. The bottom three metres on each side was shotcreted, followed by shotcreting of both shoulders and then the crown, between the ribs only. The half-metre or so length of excavation in front of the rib was not shotcreted, so as to leave the mesh clear for attachment of the mesh of the next round. At the time - late autumn to spring - the shotcrete was cold and damp. Considerable problems had been and were experienced in trying to produce and place in the tunnels shotcrete which could achieve the specified strengths [4]. The footing of the rib was not considered to be of great significance in “normal” ground, as the theory is that the shell of shotcrete enables the ground itself to support the loads. The purpose of the rib is to strengthen the shell.

4 Previous Incidents

The tunnels had been advancing well (see Fig. 5) at one round per day through Rock Class 4 with a round length of two metres, when, on the morning of 30th September at Round 156, an inflow of water of about half a litre per second issued from the crown after excavation. In the evening, there was distortion of the steel rib at the left hand side, even before the first stage of shotcrete had been placed to the bottom three metres on each side. Deformation of the rib was of the order of 250-300 mm, which may have been due simply to the rib having been nudged by the excavation plant, or by movement at the footing. A change to Rock Class 5 was proposed, with the installation of nine metre long rock bolts.

Figure 5: Progress

However, it appeared to be a false alarm; there was no noticeable change to the inflow of water, nor was there any sign of distress indicated by the deformation measurements. Radial drainage holes were instructed, and drilled, after some delay. The Rock Class was not changed, but the round length was reduced to 1.7 metres fro Rounds 158 and 159, and then reduced again to 1.5 metres for Rounds 160 - 163, after which it was reduced still further to 1.2 metres. About a week later, when the face was at Round 163, settlement was observed in Rounds 153 to 159; it was noticed that a rockbolt had broken, and that the washers on several rockbolts in the area had become concave under load. All rockbolts were intended to be fully grouted; yet the bolt had broken at the nut. It was concluded that the deformation was probably due to inadequate footing to the ribs. It was agreed that extra bolts be provided along the right side of the tunnel from Round 152 to 160. Water flow declined, with only drips from the roof from Round 158 onwards, and Round 165 was almost dry. An improved detail was proposed for the foot of the ribs, to give better support. From 11th October, deformation readings were taken daily at the Sections set up at km 41+812.5, at km 41+818.5, and at a new one to be set up at Rounds 165. The Tunnel Designer decided to apply extra support to Rounds 163 to 169.

5 First Collapse

5.1 Sequence of Events

On the evening of 11th October 1995, after sealing shotcrete had been placed on the Left hand side of the face, but before any had been placed on the right or in the crown, the ground fell away to form an overbreak chimney of some ten to twelve cubic metres from the right-hand side of the crown of Round 165, and extending past the rib to the face which had ravelled to be over a metre in advance of the rib, see Figure 6. This stabilised some three metres above the tunnel crown level, and an attempt was made to fill the cavity with sprayed shotcrete. This was not easy to attempt, because there were two layers of mesh at that point, resting on the rib, and the shotcrete nozzle was pushed through the mesh. The shotcrete was thus being sprayed from some distance away, with not much control, nor was it possible for the nozzleman to see clearly what was happening [5]. Due to the awkwardness of the position, much of the shotcrete fell back as rebound; shotcrete could be most easily placed on the face side of the cavity, but little was probably placed on the back face of the cavity, due to problems of access. More than one full day was spent spraying this cavity. Unfortunately, too, there were considerable delays in the shotcrete supply, occasioned by a breakdown of the local shotcrete batching plant, which had necessitated the use of a batching plant some distance away. This situation had been operating for over a week, and had been sufficient for normal requirements.

Figure 6: Overbreak in Round 165

At about 22:00 the following evening, 12th October, 1995, work was continuing on filling the cavity, which had been slow, partly because the access was difficult on account of all the rebound material. Bundles of mesh had been pushed up into the cavity, to re-inforce the shotcrete, and to try to help stabilise the free surface of the ground, but this obviously prevented the shotcrete from reaching the face. It was felt that a shotcrete shell could be formed which would suffice to stop the flow of ground into the tunnel. The men were removing the rebound material to improve access, when the small but steady trickle of loose material from the right shoulder of the tunnel, from the back face of the cavity, started to increase rapidly. This flow continued to increase, until it became a flood, and eventually the ribs and shotcrete slowly collapsed over a length of some six rounds, allowing about 650 cubic metres of this loose material to flow into the tunnel, accompanied by considerable quantities of water, estimated at about twenty-five litres per second. When water-logged, this material had the consistency of wet concrete.

5.2 The damage

The extent of the damage included :

• Ribs at the front of Rounds 163, 164 and 165 had totally collapsed, and were buried beneath the rubble.

• Ribs to Rounds 161 and 162 were visible, but had also collapsed and were twisted and lying across the middle of the tunnel, half buried.

• Ribs to Rounds 159 and 160 were still standing on each side, but the crown parts had been torn away. The centre part of the shotcrete had been punched through by a large boulder which was still retained by the remnant of the support.

• The middle five metres of the crown of the shotcrete shell of Rounds 156, 157 and 158 were cracked and spalled, and had come down by about 200 to 300 mm.

5.3 Immediate Measures

As is usually the case, the collapse occurred on the night shift. Fortunately, the inflowing ground and water soon eased off, as the pressure of both was reduced, and the passage was blocked by the material which had flowed in reaching its natural angle of repose. There was, of course, no immediate action which could have been taken to prevent or reduce the inflow. The first requirement was to re-inforce the support which was still standing, by the application of additional mesh and shotcrete, which was applied from before the first signs of distress and cracking, i.e. from Round 152 as far forward as possible. Extra six metre long rockbolts were installed on the left side and in the crown in between the existing ones from Round 150 forwards, to match the extra bolts already installed on the right side, as described above. The next stage was to support the face of the loose material, with mesh, shotcrete and face bolts, and a shotcrete arch rib was formed in the crown of Round 158, extending down to the shoulders, where it intersected the face of the inflow material. From this rib, a steel beam was installed on the right hand shoulder of the tunnel, extending forward to a shotcrete “abutment” formed in the collapsed material where its face slope reached the roof of the tunnel in the middle of Round 162. The inflow of ground had been made worse by a substantial inflow of water, although this appeared to have come from a water pocket, as the flow rate eased off quite rapidly. This flow was measured, and a graph of flow with time appears in Figure 6 below. Additional precise monitoring of deformations was also instituted, on a pattern augmenting the routine measurements, as will be discussed later.

6. Investigations

6.1 Probe drilling

At the same time as the above measures were being put in hand, a programme of probe drilling was started, to try to establish the extent of the area affected by the collapse. Initially, two fifteen metre long holes were drilled vertically in Rounds 154 and 155, and also in Round 155, at inclinations of 30, 45 and 60 degrees from the horizontal longitudinally, and in the crown and both shoulders, all holes extending to a height of eight metres above the tunnel crown. Each hole was grouted after drilling to a pressure sufficient to ensure the hole was filled with grout, but no more, and each hole was subsequently re-drilled to act as a drain. 6.2 Water It was mentioned above that the flow when the collapse took place was estimated at about twentyfive litres per second. All inflow was channelled away to a single drain, and the flow was measured twice a day, as shown in Figure 7.

Figure 7: First collapse - a: water inflow rate; b: total water inflow

The flow tailed off over about a month back to the steady flow of about two litres per second which had existed before the collapse occurred. This indicated that the tunnel had driven into a waterbearing strata below the water table, and that the water table had been drawn down. The total volume of water abstracted was estimated at some twenty to twenty-five thousand cubic metres, suggesting that voids to this extent now existed above the tunnel as pores in the ground, in addition to the one large 650 cubic metre void created by the inflow of ground. An analysis was done of a sample of the water, with the following results :

pH 8.0

Na 10.0 mg/lt

SC3 18.0 mg/lt

Cl 10.6 mg/lt

K 0.5 mg/lt

CO3 3.1 mg/lt

Ca 10.2 mg/lt

Mg 9.8 mg/lt

HCO3 90.0 mg/lt

A Tritium and Oxygen-18 analysis indicated that the water at the collapse was less than 50 years old, according to the Isotope values obtaining in Central Europe - there is no data available for Turkey.

7 Subsequent Stabilising Work

7.1 Theory

The Tunnel Designer decided against trying to grout the surrounding ground; in his experience in alpine tunnels much time and money had been spent without achieving the desired objective, particularly in cases of high water inflow, and particularly with standard grout, although he believed that polyurethane grout might prove effective. No tests were done on the “groutability” of the ground, although this was suggested. The Tunnel Designer proposed to improve the properties of the collapsed material inside the tunnel by grouting, by installing drainage pipes, and by bolts, wire mesh and shotcrete. His proposal was then to drive a pilot heading through to the previous face; to enlarge to full size; and subsequently to investigate the extent of the loosened zone around the tunnel, and treat it as necessary. The collapse of ground, and the flow of material into the tunnel, had left a void outside the tunnel, the roof of which was unsupported, and could, with time, collapse still further. The Engineer was of the opinion that this void should be re-filled, to provide support to its roof, and prevent further collapses from spreading ever upwards. The Tunnel Designer calculated that such work was unnecessary, and that the tunnel could retain the full over-burden height with eight metres of “stabilised” ground around the tunnel. However, it was agreed that some grouting should be done first, as well as installation of extra rock bolts, to provide such a tunnel structure, although there was some doubt as to the efficacy of bolts anchored in loosened ground [4]. The effectiveness of the grouting was to be checked by core drilling, but the Contractor was unable to obtain suitable equipment in Turkey. The Tunnel Designer held that the effectiveness of the grouting could be checked simply by how efficiently the seepage of water was reduced. Ultimately, all holes drilled following the first collapse were percussion-drilled, and subsequently many of them were inspected by petroscope.

7.2 First Stage

In early December, 1995, two months after the collapse, a programme for advancing the tunnel was agreed. This programme started with the drilling of holes to form an umbrella of perforated pipes (six metres long and 300mm apart) installed at a flat angle over the collapsed part of the tunnel, and grouted with a neat water/cement grout of about 0.3 to 0.4 W/C ratio, with the take and pressure both limited, in order to form a curtain so as to prevent flow into the tunnel of more ground, or of grout to be injected in subsequent stages. This was followed by a second umbrella, similar but at 25° to the horizontal, and a third at 50°. Subsequently, more holes were drilled in the crown of the tunnel. In Rounds 156, 157 and 158, radial holes (vertical longitudinally) six metres long and two metres apart around the arch, were percussion-drilled, and grouted with cement grout (W/C 1:1) to a pressure equivalent to six metres of grout at the mouth of the hole. Extra rockbolts six metres long were installed to the same pattern, in between the grout holes. These drillings confirmed the presence of large open cavities above the tunnel, and another set of inclined holes was drilled in Round 158 as had been drilled in Round 155 as described in 6.1 above, but to a depth of twelve metres, or, in cases where a void was intersected, through to rock the other side of the void. Subsequently, tunnel driving re-commenced, and the tunnel was advanced using methods to be described elsewhere; the position of the old face was recovered on 5 March 1996, and excavation through new ground started, five months after the collapse.

7.3 Second Stage

In accordance with the Tunnel Designer’s proposals, a new campaign was instituted, to seek out and deal with any void still remaining above the tunnel. A pattern of holes twelve to fifteen metres long was drilled in each round, from Round 158 to Round 165. There were three - one central, and one three metres away on either side. On 21st March, the Contractor proposed to drill two twelvemetre long horizontal holes ahead of the face, but this was not done, because by this time the North Tube, thirty metres away, had advanced past the South Tube, and it was felt that it provided sufficient information. Tunnel driving continued slowly through new ground, and the round length was increased from 0.8 metres through the collapsed material to one metre, and then to 1.2 metres for Round 173 and onwards. On 26th March, the Contractor again proposed to drill two twelve-metre long holes ahead of the face, inclined by ten to twenty degrees upwards, but these were also not drilled, because by this time the North Tube had advanced fifty metres past the South Tube, so it was felt that these holes would provide little further information. 7.4 Incident between collapses At 08:00 on 16th March 1995, it was noticed that the shotcrete was cracked in Rounds 138 to 146, and an extra layer of mesh and shotcrete was authorised. By 19th this work had still not been done, and the distressed area had spread to include Rounds 136 and 137, with concave bolt washers and distress to the ribs. The situation in that area did not deteriorate greatly, despite the second collapse occurring at the face at Round 179 at 09:30 on the third of April as described below; this stability was fortunate, as the agreed extra support was not completed until 5th April, by which time the shotcrete in Round 135 had also become cracked. It was agreed that the extra support be extended to Round 134. At the same time, it was noticed that the cracking of shotcrete had extended at the other end to Rounds 147 to 151, and the extra support was authorised for those rounds as well. In addition, the extra bolts which previously had been considered not to be necessary were now authorised. The final situation in this area was cracking of the shotcrete in top heading and bench from Round 134 to Round 143; in the top heading in Rounds 147 to Round 151. Extra support was authorised, extra bolts, mesh and shotcrete, from Round 136 to Round 146 in the bench, on both sides, and from Round 137 to Round 151 in the heading and bench. During this period, no supporting data from the Measurement Sections was offered as evidence of something amiss. The information was available for viewing on the Contractor’s computer screen, but the extensive time taken to plot out the results meant that plotting was not done very often.

8 Second Collapse

8.1 Situation of tunnel faces

At the time of the second collapse, the faces were situated thus :

 

8.2 Sequence of events

One month after the old face had been recovered, after driving a distance of seven metres through collapsed material and seven metres of virgin ground, the face was at Round 179 when a second, virtually identical, collapse occurred at 09:30 on 3rd April 1995, when about 600 cubic metres of ground flowed into the tunnel. In this instance, the mesh had been fixed and rib 179 had been erected, but not fixed. Sealing shotcrete had been applied to the left-hand side, but not to the right because of a delay in supply. At about four o’clock in the morning, while the surveyors were checking the rib, they became aware of noise and movement, and exercised their discretion. Radial movement of Rib 174 occurred, last measured at about 300 mm. Shotcrete in Rounds 173, 174 and 175 started to crack and spall away. Some pressure relief holes were drilled, but is soon became apparent that another collapse was occurring, and two loads of spoil were dumped near the face to act as a dam to limit the inflow. The collapse thus started on the right hand side, back from the face, but extended progressively forward to the face, backwards to Round 172 and across the full width of the tunnel. Water followed the ground, starting on the left hand side, and reached an estimated flow of about 25 litres per second, reducing to 3 litres per second within four days, and to less than half a litre per second after ten days. A sample taken from a drainage pipe installed in the south side of the tunnel on 25th April gave the following : Half a litre per second, at 16.5°C, conductivity 0.2 mS/cm, dissolved solids 134 mg/litre Figure 8: Second collapse - a: water inflow rate; b: total water inflow 9 Subsequent events “New” cracks in the shotcrete were observed nearer the Portal, extending from Round 68 to 77. How long these cracks had been there is obviously unknown. An extra layer of mesh and an extra 50 mm of shotcrete was applied. On 10th April, core drill-holes were requested, and five exploratory holes, fifteen metres long, were to be drilled from the crown of Rounds 137 to Round 147. As mentioned above, the results from the core drilling were disappointing and inconclusive, and so core-drilling was discontinued, and the remaining drill holes were all drilled with percussion equipment, which was much quicker and gave no less information. One disappointment was that no cores were obtained of grouted material. Three samples of ground were taken from the inflow material in mid-July, and sieved : Sieve Analysis of material at Tunnel 3A second collapse: Average of three samples totalling 11 kg

The particle size distribution of the material (D15 approx. 0.21 mm) is such that this material could not be grouted with Ordinary Portland Cement (D95 approx. 0.07mm) as the ratio is about 3, whereas for groutability this ratio should be about 11 [6,7]. Many holes were drilled and grouted to refusal at the specified pressure, which was kept low to avoid extra loading on the tunnel. It was assumed that grout was stabilising the material, when in fact probably little penetration of the ground occurred at all, which is why the quantities injected were small. All that was happening was that the sides of the holes were being lined with grout. The proposed water-pressure tests were not carried out, as there was no desire to increase the loading over the tunnel; the only tests carried out were thus the rather inconclusive petroscope examinations. Probe drilling continued, and eventually a large void was identified some 30 metres above the tunnel, and 500 cubic metres of concrete were pumped into this void. Subsequent drilling indicated further voids, though some of the drilling results were somewhat suspect. 10 Could the collapses have been foreseen? 10.1 Deformation Measurements As routine, Deformation Measurement Sections are established at intervals along the tunnel, and a pattern of readings is taken at defined intervals. This pattern involved the installation of five defined points (pins) in the crown, at the foot of the top heading, and at the foot of the bench. The measurements taken were the level of each point, to give settlement, the distances from the crown point to the other four, and the horizontal distance between pairs of points at the same nominal level, to give convergence. This method was used with useful results during construction of Tunnels T1, T2 and T4, and was also in use in Tunnel T3 at the time of the collapse. The following Measurement Sections were installed:

As may be seen, only the Section installed in Round 154 was available for monitoring before the first collapse. The settlements (not deformations) recorded at this section are plotted in Fig. 9: Fig. 9: Settlements at Round 154 Information given by this Section was used as a guide for the application of the additional support in Rounds 153-160, but (in hindsight) there were significant indications that something unusual was happening. Learning from the experience of the first event, additional measurement Sections were installed in Rounds 142, 147 and 150 during the advance through the collapsed area. 10.2 Comments on results from the measurement Sections Round 142: No measurements were made from the end of January until the collapse in March. In this interval deformations increased from 7mm to 19mm. Within a short time after the collapse an additional 5mm of deformation were recorded. Round 147: As with the previous section only the horizontal convergence H1 was measured until the collapse. In the fourteen weeks from installation until the beginning of March, 9mm of convergence were measured. In the following month this increased by an additional 8mm. This increased speed of deformation was not considered critical because bench excavation was being carried out in the North Tube, and some also in the South Tube, as well as excavation of the top heading of the South Tube, and so this information was discounted as being “accounted for”. Round 150. This section is installed in a part of the tunnel where no bench excavation has yet been done. The shape of the deformation/convergence curves has almost the same shape as the measurement Section in round 147. The significance of the increased speed of deformation recorded after the beginning of March had not been appreciated. From the middle of November to the beginning of March, 8mm deformation were recorded, with another 4mm taking place in March, 2mm of that immediately after the second collapse. Round 154. This was the only Section in place before the first collapse. As may be seen from Fig. 8. no measurements were taken except the settlement of the right side of the top heading. Based on this, additional rock bolts were installed which resulted in a reduction of the speed of deformation, and was considered as a successful solution. Round 157. Only a settlement in range of 11-19mm was recorded during the period of overcoming the first collapse until the second occurred. After that no significant changes were recorded. Round 160. At this Section at the end of March a sudden settlement of 13mm was recorded of the left side of the tunnel in just few days, but this was considered to be due to measurement errors, because in the same period no change in horizontal convergence was noticed. Round 162. This Section was installed after the second collapse. Until middle of June this Section did not reveal any significant movements. However, there is a strange fluctuation in all measurements in the period from the middle of August to the middle of September. Round 165. This Section was also installed after the second collapse, and here, too, unusual deformations were recorded in the one month period between the middle of August and the middle of September. The same strange behaviour but of less amplitude is noticed in Sections at Rounds 157 and 150. No explanation has yet been established for these movements. In that period the only activities under way were the filling of the void in the South Tube in the middle of June 1996, and advance of the side drift in the North Tube, more than 30m away from the collapsed tunnel face. The shape of the movement (increase and decrease) appears to indicate a ground movement which may have been caused by a collapse above an undiscovered void above the tunnel.

10.3 Conclusions to be drawn from the results of the Measurement Sections.

Amplitude of deformations is not only an effect of the geology and support. Progress rate has a much more important influence than is commonly thought. This could be due of two things: much deformation occurs before measurements can be initiated; and the support is more rigid and less deformable. See Figure 10. 2. It would seem that Sections installed well back from the face give more complete information regarding the behaviour of the rock mass. The measurements from Sections close to the face are useful for the transversal arching effect, but the longitudinal arching effect which may lead to collapse may be shown only by the Sections which are more than about one-and-a-half tunnel-diameters back from the tunnel face. But since at that distance there is almost always some bench excavation in progress, this may still not give the complete picture.

11. Could either collapse have been prevented?

Obviously, the answer to that question is “Almost certainly”, in the same way that road accidents could be almost totally eliminated if no-one drove faster than five k.p.h. In both cases, the extreme is unacceptably “expensive” in terms of money or time, and we take some risk for some extra benefit, knowing that we may lose the gamble.

11.1 If so, how?

The most dangerous risks in tunnelling are generally those associated with inflows of ground, of water or of gas, from the zone around the tunnel or ahead of the face. The more knowledge that there is available regarding this zone, the lower the risk, and geological prediction is therefore the most powerful - possibly the only - weapon in the tunneller’s armoury that can enable him to avoid or to prepare for problems which he may encounter during the tunnel drive. Well before construction starts, a geological prediction has to be made, in order to establish the viability of any tunnelling project. In the case of the earlier TAG Motorway tunnels, the advanced prediction made by the Tunnel Designer was found, during four-and-a-half years’ of tunnelling, to give a reasonably accurate assessment of the ground. However, during tunnelling, a more conservative Rock Mass Quality Classification has been applied, in order to reduce the perceived risk. In the case of Tunnel T3A, however, this prediction started off accurate, but the ground deteriorated unexpectedly after some two hundred metres. The only prior incident was an overbreak of some twenty-two cubic metres which occurred at a fault which had been predicted. Drilling of probe holes ahead of the face may frequently be justified, in reducing the risk; but these have a cost in both money and time. Percussion drilling is cheaper than core drilling, but gives only very limited information, except where water is encountered, in which case the information can be very useful.

12 Summary of specific possible aggravating factors

In an attempt to reduce the risks of a collapse, we present here a list of factors which may have aggravated the situation, and which may profitably be addressed for future projects :

1. Insufficient immediate supervision

2. Delay in applying full thickness of shotcrete

3. Delay in delivery of shotcrete

4. Failure to apply shotcrete ahead of the last rib

5. Use of forepoling

6. Weakness of shotcrete

7. Failure to grout bolts fully

8. Failure of the grout to penetrate the ground

 

13 Conclusion

There can be no argument that tunnelling is still very much an art rather than a science. Each interpreter’s perception of the ground will differ from every other’s, to a far greater extent than is generally the case in other forms of construction. At the same time, tunnelling is very much a team endeavour; but a “team” concept does not lend itself to “Specification”, and is very greatly affected by the varying personalities of the individuals involved, each of whose own previous experience, knowledge and understanding will be quite different; and then the ground actually encountered differs again. Every tunnel project will be, to a greater or lesser extent, subject to a “learning curve”, as the Engineers gain in experience of that particular series of ground formations. It is inevitable, then, that in every tunnel that is driven, there are likely to be instances where the conditions lie outside the collective understanding of those involved, and a collapse becomes more likely, and this is one of the risks that must be taken and accepted, although every endeavour is made to reduce it.

 

Acknowledgements

The Authors acknowledge with thanks the co-operation and permission of Dar Al Handasah and of the Turkish Department of Highways. The entire responsibility for all errors, and for all opinions expressed herein, remain those of the Authors alone.

 

References

[1]Safety of New Austrian Tunnelling Method (NATM) Tunnels The Health and Safety Executive, London

[2]Takacs, A.P. (1996) May we prevent a collapse? - searching for answers. Proceedings of the Second National Conference for Underground Structures, Romanian Tunnelling Association, Bucharest.

[3]The Institution of Civil Engineers, London - Design & Practice Guide Sprayed Concrete linings (NATM) for tunnels in soft ground (1996) Thomas Telford Publishing, London

[4]Clay, R.B. and Takacs, A.P. (1996) Contractual aspects of testing shotcrete and rockbolts. International SCA/ACI Conference Proceedings, E&FN Spon, London.

[5]Code of Good Practice Sprayed Concrete Association, Aldershot

[6]Henn, Raymond W. (1996) Practical Guide to Grouting of Underground Structures ASCE Press, New York, and Thomas Telford Publications, London ISBN 0 7844 0140 3

[7]Whittaker, B.N. and Frith, R.C. Tunnelling - Design, Stability and Construction The Institution of Mining and Metallurgy, London

[8]Professor Stuart Littlejohn, (Feb 1991) Introduction to Inadequate Site Investigation Institution of Civil Engineers, London




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