The original design for the primary lining system for the Driskos Twin Tunnel along the Egnatia motorway in Greece, consisting of a PVC plasticized (PVC-P) geomembrane, called for a nonwoven geotextile, 500 g/m², as a puncture protection and drainage layer.
The nature of the rock with high pressures expected on the concrete vault, as well as the huge amount of water that was encountered during excavation, was making it necessary to substitute the geotextile with a high-performance drainage geocomposite. It needed to be capable of guaranteeing an effective drainage under high pressure for the whole design life and to drain the water at 360° within the plane to allow efficient drainage even during concentrated water flows. This article describes the evaluation of the design state of stress on the geocomposite, the required drainage capacity, and the waterproofing performances chosen as the best technical solution. After more than 15 years, the drainage lining system is still working perfectly and no problems have been encountered.
It is well known that excavation of tunnels crossing a phreatic surface without a lining system reduces the water pressure to zero along its edge. This causes a water flow from the aquifer to the tunnel. The effect of this lack of waterproofing can be a subsidence in the area surrounding the tunnel, the drying of wells and springs (Figure 2).
Apart from the damages that can result in the surrounding area, the presence of water in the tunnel can create a risk situation in the tunnel itself. This is mainly due to the possibility of water (or ice in the winter ) on the road, ice stalactites on the roof (obviously only in very cold regions and close to the tunnel entrance), and corrosion of the steel of the concrete vault. These risks can be avoided by using a properly selected geomembrane that guarantees the waterproofing of the vault.
For this application, geomembranes made with flexible PVC (PVC-P) are widely used, thanks to their flexibility, high mechanical resistance and easiness of application. These types of geomembranes must be specially formulated, designed, certified and CE marked according to the main European standards for specification. During the design phase, it is important to consider mechanical, physical and chemical factors that can affect the life of the geomembrane in relation to the life required for the project in question. The European harmonized standard for geomembranes used for applications in tunnels is the EN 13491 “Geosynthetic barriers – Characteristics required for use as fluid barrier in the construction of tunnels and underground structures.”
In general, PVC-P geomembranes used in civil engineering application have a thickness of 2.0 mm or higher, depending on the specific conditions of the project.
Design and selection of geosynthetics
The typical excavation sequence in rock calls for use of a gunite layer over the rock itself to reduce the roughness of the profile and smooth its surface. Considering the very high loads that could be applied to the concrete vault, the geomembrane must be protected from puncture, as the gunite is a rough surface (Figure 1).
The waterproofing layer prevents contact between the water and the concrete vault. The water coming from the gunite should be drained away, not to cause a concentrated excessive water pressure on the structure that could cause an excavation in the gunite layer. For this reason, it is recommended to install a protection-draining layer between the gunite and the geomembrane.
The main requirements for this product can be summarized as follows:
- High compressive strength (> 500 kPa)
- High discharge capacity
- Resistance to puncture
- Flexibility and easiness of installation
The market of geosynthetics can propose a wide range of product; not all of them fully cover all the requirements listed above.
The original proposal consisted of a nonwoven geotextile (500 g/m²), which is very flexible and effective against puncture but inadequate for drainage of high flows and unable to resist against compressive stresses. Furthermore, this kind of product may experience severe clogging, as the solid part is contained in the water.
A good alternative is to use drainage geocomposites with a cuspated geonet (Figure 3) as a core. These products meet all the requirements previously listed, are flexible, and are able to guarantee a very high draining capacity even under very important compressive stresses because of their very high compressive resistance and excellent puncture resistance.
Cupsated geonet has a flat edge and a cuspated edge. The cuspated edge is in contact with the gunite layer. A nonwoven geotextile, 500 g/m², was bonded to the flat edge to be in direct contact with the PVC-P geomembrane to help the geonet in its puncture-protection function. One of the main advantages in using this kind of product is that a cuspated geonet has comparable characteristic flows in both machine and transverse direction.
The data available for design included a detailed structural and hydraulic survey of the twin tunnel containing information about the lithology of the rock, the presence of fractures, diaclases and faults, with an estimation of the rock mass rating (R.M.R.) and consequently a definition of the required support system (bolts with wire mesh, shotcrete and steel sets).
Based upon the R.M.R. classification indicated in the drawings, the geologists have identified different categories. For every sector, the length, the Bieniawski category proposed, and a short description of the rock type, has been indicated. This information is used to quantify the normal thrust on the tunnel vault. The vaults to be waterproofed are 36 feet (11 m) wide and 26 feet (8 m) high. An extensive monitoring program has been done on both the right and left tunnels. A precise measurement of the water coming into the tunnel has been conducted, indicating for every section whether the water is coming from the top, the left part, the right side or from the whole vault. Different situations have been found from humidity to drops of water. In certain areas the water can be measured as 12 to 15 m³/h. (Figure 4).
It is important to notice how the situation derived from the survey could change in the future, after the construction of the concrete vault, and areas where only a few drops of water were found could be affected by higher flows, due to the change created by the construction of the concrete vault. The values obtained by the study are nevertheless important design input that should be used for the whole tunnels. Three critical sections have been found (top, left/right, all surface).
The measurements obtained are divided by the section length to have a required flow per running meter of tunnel (Table 1).
The draining layer should be able to guarantee for every section the drainage of the water that is crossing the section considered. As shown in Table 1, the critical sections were the LEFT/RIGHT.
The behavior of a draining geonet is strongly influenced by the normal pressure applied over it. The earth pressure over a tunnel and the load conditions depend on several different factors (morphology, lithology, stratigraphy, mechanical and geotechnical properties of the soils or rocks involved in the excavation). Usually, a good estimate of the load can be made by considering the theoretical-empirical formulations based on vast practical experience and on a deep knowledge of the theoretical principles that rule the soil mechanics. The most used (and universally recognized) classification of the soils and of the consequent load conditions in tunnel excavations is Terzaghi’s classification (Terzaghi, 1946).
Based on the geological survey, calculation of the earth pressure acting on the vault was accomplished by considering a stratified and jointed rock. The load, in this case, can be conventionally expressed as the equivalent load of a fraction of the total height of the covering soil over the tunnel. It strictly depends on the dimensions of the cavity (B and Ht) and on the intrinsic characteristics of the material (internal friction angle, cohesion, unit weight, water content).
Calculation of the compressive strength and flow rate
The tunnel has been excavated in flysch. The geotechnical parameters of this material are very low, particularly in terms of internal friction angle. This fact can heavily reduce the capacity of the tunnel vault to generate the arching effect, typical of stratified sedimentary rocks; consequently, the earth pressure over the tunnel can increase in a considerable way, reaching values of the load height comparable to those typical for blocky and seamy rocks. The empirical formulation used to estimate the vertical and horizontal stress level to consider for the dimensioning of the drainage system of the tunnel has the following shape:
σv =γk (B+Ht)
vertical pressure over the vault
σh = γk Ht (B+Ht) tan2(45° -ϕ/2)
horizontal pressure at the base of the tunnel where:
γ = unit weight of the rock mass [kN/m³]
ϕ = friction angle of the rock mass [°]
B = width of the tunnel [m]
Ht = height of the tunnel [m]
k = coefficient for the calculation of the equivalent load height [-]
The values for the coefficient K and for the friction angle of the rock depend on the jointing level of the rock mass. They can be estimated with reference to the Geomechanics Classification or the Rock Mass Rating (RMR) system, published by Bieniawski (1976 and 1989). The rock present in the project was classified into three categories. The values used for the definition of the stresses are shown in Table 2.
The joint friction angle and the K coefficient have been determined after Bieniawski (1989). Under the specified load, each section of the geonet should guarantee the discharge of the water coming from the rock vault and from the upper part of the geonet. The vault has been divided into 12 sectors, six on each side. Five sectors, on the curve part of the tunnel, have length equal to 6.1 feet (1.87 m); the bottom part, vertical, has a length that is higher (8.3 feet [2.53 m]) (Figure 5).
The maximum flow measured, as shown in Table 1, is equal to 11.25 m³/h/m (3.13E-03 m³/sec/m).
Sector 1 of the vault (6.1 feet [1.87 m] long) is interested by a flow that can be calculated by means of the equation.
qsector 1 = Q · Lsector 1 / Ltotal = 11.25 m³/h/m · 1.87 m / 11.88 m = 1.77 m³/h/m
Sector 2 (immediately below the first) should guarantee a draining capacity equal to the one required by sector 1 (as its length is the same), plus the water coming from sector 1.
qsector 2 = qsector 1 + Q · Lsector 2 / Ltotal = 1.77 m³/h/m + 11.25 m³/h/m · 1.87 m / 11.88 m = 3.54 m³/h/m
The normal pressure acting on each sector of the tunnel has been calculated projecting the vertical and horizontal pressure (assumed constant for the entire height of the tunnel) along the direction orthogonal to the sector surface.
With θ as the angle between the radius of the considered segment and the horizontal direction, the normal stress can be calculated as:
σn = σv sen θ + σh cos θ
The results obtained for the critical part (LEFT/RIGHT) are summarized in Table 3.
Figure 6 shows flow-rate diagrams for geosynthetics products used, depending on the rock condition. In the flow diagrams, the critical conditions are identified through colored stars: red star = sector 1; blue star = sector 2; green star = sector 4; violet star = sector 6.
The red star should be compared with the curve obtained at gradient 0.10, the blue star with the curve at gradient 0.50, and the green and violet stars with the curve at gradient 1.00. The same flow rate applies to different values of the thrust, depending not only on the sector, but also on the rock class. The flow-rate measurements are performed according to EN ISO 12958 method (Cazzuffi and Recalcati, 2018).
The drainage geocomposite was fixed to the vault through nails and connected at the bottom with a drainage pipe (Figure 7).
The geomembrane is the main functional layer of the waterproofing system. The system adopted in the Egnatia Odos tunnel is composed of a single layer of PVC-P geomembrane FLAGON BSL 2.0 mm applied on the regularization layer and on the geocomposite, fixed by PVC-P disks to the support upgraded by the introduction of compartments and waterstop joints. (Figure 8).
Proper testing on the geomembrane integrity and on the weldings have been performed according to the practices described by Cunegatti (2019).
Conclusions
It is important to properly design the waterproofing and drainage system of a tunnel to preserve its durability but also the safety and the durability of its internal spaces and technological systems. Yet this objective cannot be reached with the use of a single geomembrane alone. Rather, meeting this objective requires a waterproofing system, where the geomembrane is one important component. The choice and the correct design of an appropriate waterproofing system is the basis of a good and safe project.
Another important component in combination with the PVC-P geomembrane is the protection/drainage layer made by a geocomposite that must be compatible and properly work in combination with the PVC-P geomembrane. To ensure a proper draining-protection effect to a tunnel waterproofed with a PVC geomembrane, it is recommended to install a drainage geocomposite between the geomembrane and the gunite layer.
References
Bieniawski, Z. T. (1976). Rock mass classification in rock engineering. In Exploration for rock engineering, proc. of the symp. (ed. Z. T. Bieniawski). Balkema, Cape Town.
Bieniawski, Z. T. (1989). Engineering rock mass classifications. Wiley, New York.
Cazzuffi, D., and Recalcati, P. (2018). “Recent developments on the use of drainage geocomposites in capping systems.” Detritus—Multidisciplinary Journal for Waste Resources & Residues, 3 (2018), 93–99.
Cunegatti, M. (2019). “European practice on the use of PVC-P geomembranes in tunnels and underground structures – special testable and reparable waterproofing systems.” Proc., Pan American Conference on Soil Mechanics and Geotechnical Engineering, Cancun, Mexico, 1279–1285.
Terzaghi, K. (1946). “Rock defects and loads on tunnel supports.” In Rock tunnelling with steel supports, (eds. R. V. Proctor and T. L. White). Commercial Shearing and Stamping Co., Youngstown, OH.
Project Highlights
Egnatia Motorway—Risko Tunnel Double Tunnel 6KM
CONTRACTOR: Hellenic Ministry—EU (Egnatia ODOS S.A)
CONSTRUCTION COMPANY: Aktor S.A
WATERPROOFING AND DRAINAGE SUBCONTRACTOR: Domissima S.A
GEOMEMBRANE: Flagon BSL 2,0 MM (Flag SPA—nowadays Soprema SRL)
GEOCOMPOSITE FOR DRAINAGE: Tenax TDP 700 and 1000—Domodrain 1200 and 1500 (Tenax SPA)