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Applications of Remote Sensing Techniques in the Prediction of water gushing in the Dayaoshan Tunnel

Wang Yuming
Information Centre of Aerial Surveying and
Remote Sensing Techniques of Railway
23 Road Xijiaominxiang, Beijing, China


Abstract
The Dayaoshan Tunnel, 14,295km in length, is the longest double-track railway tunnel in China. Located in northern Guangdong province, the tunnel passes through the Nanling Mountains. Dense jungles, thick overburdens and concealment of most geological features brought great difficulty in the engineering survey of the tunnel, but applications of multiple kinds of space and aerial remote sensing images in comparison, interpretation and analysis provided a scientific basis for the evaluation of the hydro geological conditions of the tunnel. During the survey of the Dayaoshan Tunnel, drilling did not reveal substantial groundwater, but the application of macroscopic features of remote sensing images in analyzing the geological structure in the central part of the tunnel indicated that the geological structure was complex and had adequate water-gushing conditions, thus furnishing information for the construction of the tunnel. There were six localities where there was significant water gushing, of which four were situated within the area extent of remote sensing prediction. Therefore the application of remote sensing techniques in the Dayaoshan Tunnel yielded notable results.

Introduction
The Dayaoshan Tunnel, 14.295 km long, is the longest double-track railway tunnel in China. Lying between pingshi and leeching in northern Guangdong Province, the tunnel cuts through the Nanling Mountains. Dense jungles, thick overburdens and concealment of most geologic features brought great difficulty in the tunnel survey. The construction started in 1981 and the tunnel was open to traffic in 1988 (Fig.1)

In order to check the result of the application of remote sensing techniques in thick forest zones of southern China, the Dayaoshan Tunnel was specifically selected as a test area for the application of remote sensing techniques. Beginning in 1981, large -scale black-and-white aerial, colour infrared, natural colour and black-and-white infrared photography, infrared scanning and multispectral scanning imagery were carried out in the Daysoshan Tunnel area, and Landsat imagery and small-scale aerial photos were collected. In the study of the application of remote sensing techniques in the Dayaoshan Tunnel, eight spaceand serial remote sensing images were used. Through interpretations and analyses, more than 300 lineaments were determined with in the limits of 420 sq km in the tunnel area, and more than 40 lineaments were determined with in the limits of a 5km wide zone along the axis of the tunnel, of which 28 cut through the tunnel. The interpretation of remote sensing images provided a scientific basis and abundant geological information for analyzing the hydro geological condition in the tunnel.

In the course of the survey of the Dayaoshan Tunnel, drilling did not reveal substantial water, but macroscopic features of remote sensing images showed that the Bangu'ao trough in the central part of the tunnel had complex geological structure and adequate water-gushing conditions, thus providing information for the construction of the tunnel.

Topography and Geology
The "bowstring" portion (Fig.1) of the Wushui valley through which the Dayaoshan tunnel passes has a topographic relief of 300-1100 m. The Bangu'ao area in the central part of the tunnel is an elongate trough, 1-2km wide, with elevations of 400-600 m abovesealevel. This trough is flanked by lofty mountains. The Fangjinshi watershed on the east side and the Zhangzixian watershed on the west side both have an elevation of more than 1000m above sea level.

The strata exposed in the region consist mainly of Sinian and Cambrian (Z+e) low-grade metamorphic elastic rocks, such as quartz sandstone, slate and carbonaceous shale, which are distributed in the medium low mountains at the entrance and exit of the tunnel. The Middle and Lower Devonian (D1-2gt) quartz Sandstone and coglomerate is distributed in the watersheds on both sides of the Bangu'ao trough in the central part of the tunnel, and the Middle Devonian (D2d) limestone and dolomitic limestone are distributed in the central part of the Bangu'ao trough.

From an analysis of geological structure, the Bangu'ao trough in the central part of tunnel is synclinal structure. The Core of the sycline is represented by Middle Devonian (D2d) limestone, while its two limbs consist of middle and lower Devonian (D1-2g1)quartz sandstone. The Jiufeng fracture transverses the tunnel along the centre of the Bangu'ao trough and then stretches southwards along the axial zone of the syncline. The fracture, as long as 30 km, is the largest fracture in the region.

Interpretation of Remote Sensing Images
Through an interpretation of multiple space and aerial remote sensing images coupled with an analysis of regional geological data, it was considered that the hydro geological conditions in the Bangu'ao trough in the central part of the tunnel were complex and that the trough was key sector of consubstantial water-gushing would be encountered in the sector. Here the grounds for predicting the water gushing are given as follows.


Fig 1 Map showing the geographical location and geological structrue of the Dayaoshan Tunnel
  1. Interpretation of Landsat Images

    From an analysis of U.S. Landsat images, it was found that the Dayaos has Tunnel is located in the central part of the Dayaoshan block . The Dayaoshan-block is a raised rock body, bounded mostly by fractures with ductile space as the boundaries 9Fig.2 and 3). The Dayaoshan Tunnel penetrates the central part of the block and does not penerate the vunerable sector of the geological structure on the peripheries of the block, which is a complex sector of the geological structure and also a vunerable sector of engineering geology.


    Fig. 2 Landsat images of the Dayaoshan Tunnel


    Fig. 3 Interpretation of the Landsat photo of the Dayaoshan Tunnel

  2. Analysis of Regional Geological Structure

    On the 1:200000-scale geological map (Fig.1) it is clear that the jiufeng fracture cuts through the centre of the Hydong sycline at a place 3.5 km from the axial line of the tunnel on the right after the fracture transverses the tunnel in the Bangu'ao trough. This place is an intermontane basin where surface water and subsurface water in its surroundings gather. As shown clearly by remote sensing images, in limestone strata in the centre of the syncline karts is well developed and there is a river on the surface, which fully demonstrates that this place is a syclinal structure with very plentiful groundwater. On the other hand, the ground elevation of this place is 190 m higher than the designed elevation of the tunnel, with a hydraulic gradient 0f 6%, and in addition the place is connected with the tunnel by the jiufeng fracture, thus creating good conditions for the Water-gushing in the tunnel. Moreover, the Luopqunzhai sycline south of the hydong sycline is also cut by the jiufeng fracture; therefore groundwater in the Luoqunzhai sycline is likely to be connected with that in the Hudong sycline, thus bringing substantial water-gushing in the tunnel

  3. Interpreation of Aerial remote Sensing Images

    From an analysis of aerial remote sensing images, there occur a series of lineamentzones parallel to the Bangu'ao trough in the central part of the tunnel, for example RF14, RF15,RF17,REF18 and RF19, but at a distance of 3 km on the left side of the axial line, all the above-mentioned lineament zones are cut off by an oblique structure (RF42) , except RF15 (Jijfeng fracture ) which passes the structure.


    Fig. 4 Interpretation of serial remote sensing images of the Bangu'ao


    Fig. 5 Diagram showing the inflow in a parallel pilot tunnel at DK1994+213

    Therefore the Jiufeng fracture becomes a bundle of fractures in the tunnel area, of which RF15 is the major fracture of the Jiufeng fracture bundle (Fig.4) Apart from the above-mentioned fracture structures parallel to the Bangu'ao trough, there are also tranverse fratures, forming a criss-cross pattern of lineaments, thus providing favourable conditions for interconnection of groundwater between various structural zones. Therefore, it was predicted that water-gushing would occur at several localities in the construction of the tunnel
Verification of the prediction of water gushing in the construction of the tunnel
  1. Water-gushing in the Shaft

    On April II, 1985, when the tunnel was driven from the shaft in the Bangu'ao trough towards Guangzhou to the mile post DK 1994 +213 , a water flow of 4175 t/d happened suddenly six drill holes in the face of a parallel pilot tunnel.

    As the water contained 20% silts, pumps failed to drain the water containing voluminous silts; thus groundwater rose from the bottom of the shaft to the elevation of

    405 m and inundated the shaft. This water-gushing spot was only 57m from the RF16 structural zone predicted by remote sensing and was also the first water-gushing spot predicted by remote sensing (Fig. 5)

  2. Ground Regimes

    When the shaft was inundated, a series of ground collapses, drying up of springs, ground cracking and subsidence of building foundations happened with pumping of the water gushing. What are interesting are that these ground changes were mostly regularly distributed in RF15, RF16, RF17 and RF31 that were recognized by remote sensing and that ground collapses all appeared on the right side of the axial line of the tunnel (Fig. 6) . The above fully shows that the water -gushing of the tunnel was derived from the centre of the hudong syncline on the right side of the axial line of the tunnel and that groundwater's in these structural zones were connected. This fully verified what was predicted before.



    Fig. 6 Distribution of ground collapses in the Bangu'ao through

  3. Verification of the Prediction of the water gushing in the construction of the tunnel

    After water flowed into the shaft of the tunnel, a parallel pilot tunnel was driven from the entrance of the tunnel. With the progress of the construction of the tunnel, water gushing were also encountered in RF17 , RF18 and RF19. During the construction of the Dayaoshan Tunnel, there were six localities where substantial water gushing happened, of which four were situated within the limits of remote sensing prediction. This fully demonstrated that the interpretations of remote sensing images could furnish precise information of prediction (Table1)

    Table 1 Verification of the Water-gushing Predicted by Remote Sensing in the Dayaoshan Tunnel.
    Serial
    No.    		 Remote sensing prediction Verification by construction
    Mileage Case Mileage case
    1 DK 1994+156 RF 16 structural zone DK 1994+213 Inflow 4175 t/d
    2 DK 1994+604 RF17structural zone DK 1994+591-638 Inflow 8200 t/d
    3 DK 1994+947 RF18 stuctural zones DK 1994+840 Inflow 17000 t/d
    4. DK 1995+056 RF19structural zone DK 1995+065 Inflow 10000 t/d
    5     DK 1995+271 Inflow 3000 t/d
    6     DK 1995+713 Inflow 5000 t/d
Features of water gushing in the dayaoshan tunnel
The Dayaoshan Tunnel is 14.295 km long, but the localities with substantial water gushing were mostly distributed within about 1 km of the hanging wall of the Jiufeng fracture. This is because the Jiufeng fracture (RF15) is a compressive fracture, which is well cemented and impermeable. All the ground springs that are located in the hanging wall of the fracture have dried up because of the water-gushing of the tunnel, while all the ground springs that are located in the footwall of the fracture have not (Fig.7); therefore the Jiufeng fracture (RF15) is conspicuous cut-off interface. The water-gushing of the tunnel concentrate in the hanging wall of the fracture, which is a typical water gushing feature encountered in the construction of long tunnels in China(Fig.8) as well as case to which special attention should be paid when a hanging wall water rich zone of the fracture is encountered in the future construction of long tunnels.

From above, it is concluded that a abundant information can be obtained by utilizing macroscopic features of remote sensing images. In particular, the full application of remote sensing techniques in areas with dense vegetations, thick over burdens and difficult conditions of engineering surveys is of major significance for the evaluation of the stability of engineering works.


Fig. 7 Distribution od dried-up spring in the Bangu'ao though


Fig. 8 Section showing the geological structure of the Bangu'ao though in the Dayaoshan Turnel area