ABSTRACT: Grouting is a key element in hydropower construction, as it enhances the long-term stability of a conduit. Grouting for pressure tunnels and shafts can be grouped into rock mass and structural grouting operations. Structural grouting refers to either contact grouting focused on a structural enhancement of the cast concrete lining or to more complex pre-stressing operations, focusing on coupling of the lining and rock mass. An economical structural grouting system relies on a continuous realization, with standardized work sequences and cycles, which depends on rock mass features and the characteristic of structural interface. Typical grouting criteria are not applicable for structural grouting works, which potentially leading to severe technical, economical, operational and contractual implications during execution. The characteristic features of the interface in between the lining and rock mass must be outlined within tender specification for a fair tender process and an economical operation.

1   INTRODUCTION

Grouting is a key element in hydropower construction with the intention to sustainably restore the bedding conditions of the lining and to preserve rock mass conditions in the close vicinity of the conduit over the lifetime of the structure. From a technical point of view, the grouting works can be separated into rock mass and structural grouting operations. Rock mass grouting seals and strengthens fractured rock masses. Structural grouting is intended to assure the bearing capacity of the lining during operation and maintenance. Structural grouting covers inter alia contact grouting and passive pre-stressing of a concrete lining. In addition, skin grouting and filling operations belong to the group of structural grouting operations. Contact grouting refers to any filling at low pressure (< 5 bars) of unintentionally created voids resulting from shaft and tunnel excavation and/or lining methods (AUA 2003). The intention of the contact grouting is to preserve the long- term bedding conditions of the lining. Opposed to the passive pre-stressing operations, which focuses in addition on a long-term coupling of the lining and the substrate. Durable grout is injected into a gap, which has formed as result of the shrinkage processes. In addition, pre-stressing of a concrete lining by high pressure grouting allows for the compensation of thermal losses during the filling operation and creeping processes (Lauffer 1989 and Krenn et. al 2013). The stiffness of the rock mass and concrete,

Operational analyses of pressure tunnel and shaft grouting operations

Helmut Wannenmacher Gaia Ground Engineering, Widnau, Switzerland

Andreas Heizmann Marti Geotechnik GmbH, St. Leon-Rot, Germany

Sewerin Sabew Marti Geotechnik GmbH, St. Leon-Rot, Germany

EUROCK 2015 & 64th Geomechanics Colloquium. Schubert & Kluckner (ed.) © ÖGG  

the characteristics of the dissolved gap developed, as well as the desired grade of filling are the influencing factors governing structural grouting. The overall grouting volume is related to the deformation rate, the characteristics of the gap and the present joint volume in the vicinity of the tunnel. The effective grouting volume will further depend on the roughness of the substrate and the likelihood of imperfections of the lining, superimposed by shrinkage, creep and thermal losses. The operational grouting pressure, which is normally measured shortly prior to the packer system placement, is the sum of the effective grouting pressure and the frictional losses in the remaining hose length and the losses in the course of the grout spreading in the dissolved gap. The effective grouting pressure represents the pressure of the suspension at rest reflecting the already achieved filling grade, the degree of cross-linking of grout lenses and the cohesion of material, which is equivalent to the pressure filtration of the water within the grout. Observations of the effective grouting pressure from the NTFP (Wannenmacher et. al. 2013) show a loss of 40% to 80% based on effective pressure measurements, where Seeber (1999) reported a loss of 20% to 30% for average conditions. The grouting pressure or solely volume criteria (grout take per unit) are often considered as the main indicator for grouting success. These definitions may be misleading in structural grouting for pressure tunnels and shafts regardless of the lining system if not clearly understood and effectively measured. The effective grouting pressure must comply with the minimum required volume to achieve a sustainable grouting result. In the best case, the deformation of the lining is measured during the grouting operation, to verify the operational success in real time (Krenn et. al 2013).

2   ON THE DEFINITION OF GROUTING CRITERIA AND OBJECTIVES

The purpose of structural grouting is a defined filling of a dissolved gap, leading to a corresponding deformation of the lining and the rock mass (see Fig. 1a). The volume entry hereby directly corresponds with the effective pressure. The culmination of a predefined maximum pressure is therefore not sufficient to guarantee the long-term circumferential deformation of the lining. Especially during low-pressure regimes (< 5 bar), the effective pressure is generally inadequate to activate the circumferential bedding conditions of the lining (Wannenmacher et. al. 2013). Successful structural grouting criteria are deemed as a congruence of a minimum required volume per section and the effective grouting pressure. The examination of the induced deformation will preferably verify the congruence of volume and pressure. The exceedance of the minimum required volume per section indicates other operational influencing factors, which demand further consideration. The intersection of the encasing pressure - volume curve (see Fig. 1b) of the Grout Intensity Number (GIN) criteria (Lombardi 1996) for the grout curve 1, although reaching the effective pressure, does not satisfy the premises of the required volume per section. Vice versa, the grout curve 2 although fulfilling the volume criteria lacks the corresponding pressure. The supplemental adoption of a GIN criteria is therefore also of misleading characteristic for structural grouting. The grouting curve 3 shows the characteristic elements of a structural grouting curve. At low pressure a pronounced filling phase is established, which is followed by a certain pressure rise to level off the effective grouting pressure superimposed by the frictional losses. Effective grouting pressure tests indicate the interaction of the volume with corresponding effective pressure (see Fig. 1c).

Figure 1. (a;b;c) Comparison of grouting success criteria for structural grouting.

Deviation of effective grouting pressure within a pre-defined range of volume indicates undesired grout spreading ahead of the actual grouting section, where the effective grouting pressure will cease in case of an improper filling grade.

3   OPERATIONAL ASPECTS

Structural grouting opposed to rock mass grouting is generally required throughout a pressure tunnel or shaft. The pressure and the volume of structural grouting required to achieve long-term stability of the lining depends on the magnitude of the internal water pressure. The grouting volume is subsequently one of the main factors influencing the operation. The grouting operation consists of a filling with a much shorter pressurized phase. The performance of a structural grouting operation can be determined via cycle times, which consist of the following processes; handling packers, intrinsic grouting and enclosing packer systems. Parallel to the grouting operation, preparation for continual and future installation may occur.

Figure 2. Schematic illustration of cycle times for structural grouting per time increment.

The available grouting time will therefore be effectively used to perform all necessary parallel works ahead “n+1” of the current grouting section “n”, to avoid downtimes during the grouting process. Parallel works are therefore considered as a part of billable pump hours, which are often the basis of a settlement.

4   INSIGHTS IN GROUTING OPERATION

Two different methods for structural grouting of cast in place concrete linings have evolved over time. Borehole grouting is the most common method and preferentially accomplished by relatively short boreholes, protruding no more than 30 cm into the rock, with a diameter typically ranging between 38 mm to 70 mm, depending on the site-specific drilling method. The number of boreholes must be aligned to guarantee a complete penetration of grout within the dissoluted gap. The boreholes are mainly drilled by percussive drilling and sealed with mortar upon hardening of the grout. The second method, which has evolved, is the sleeve hose grouting technique, which is quite often referred to as the TIWAG gap grouting method (Lauffer et. al. 1968). Prior to casting, grouting sleeve hoses are installed on the substrate of the lining. The pipes are laid out along the concrete lining and can be connected to the grouting system.

Figure 3. Schematic sketch of borehole (A) and sleeve hose grouting (B).

The sleeve hose method (SHM) does not require any further perturbations of the lining and can be closed off after grouting by simple inserts. Although both methods do not differ in their intention from a geotechnical point of view, some differences towards crack (dissolved gap) penetration arise. The main impact of the penetration process is related to the substrate preparation. The utilization of debonding agents (lime milk or membranes) avoids adhesion in between the substrate and concrete. Any development of adhesive strength depends on the reactivity of the substrate with the free alkaline water of the concrete and increases over time with an upper limit of the tensile strength of the concrete itself. In the case of borehole grouting, the grouting pressure induces a hoop stress at the circumference of the borehole. The dissolved crack for penetration is nevertheless situated normal to an initiated hydraulic induced crack orientation. The crack initiation and further penetration of an unfavorable oriented joint requires more energy, mainly to overcome the bonding strength.

4.1   High pressure borehole grouting for passive pre-stressed pressure tunnel

Recent observations from a pressure tunnel, with a cast in situ lining highlight some of the problems, reassuring the negative influence of bonding strength on the structural grouting process. A passive pre-stressed lining with short boreholes was planned to restore the long-term stability of the lining. The pressure tunnel, with a length of ~3,300 m was excavated with an open tunnel boring machine (TBM-O) with an excavation diameter of 3.8 m. The lining thickness of the inner lining is 25 cm. Ground support was only used locally in higher fractured zones within the massive schistose phyllites and quartzite’s. In situ testing and numerical back calculation of the adhesion strength show for schistose phyllites as the substrate revealed a shear bond strength of ~ 0.5 MPa for a relative smooth tunnel wall (equivalent joint roughness coefficient (JRC) = 2 to 4) to the inner lining. The relatively low bond shear strength was sufficient to withstand hydrostatic pressure of up to 60 bars in more than 2/3 of all the boreholes during hydro fracturing tests in the borehole. Conversely, hydro fracturing of the bonded gap does not necessarily mean grout take can be expected. An appreciable grout take of more than 20 l were found in only half of the hydraulically active boreholes. A third of all boreholes showed a grout take in the range of the borehole volume, which confirms an overall bonding of the lining; however, it does indicate the ability of the lining to withstand the internal water pressure without cracking. Although the specified GIN was reached in most of the boreholes, insufficient grout take and local deformation measurements, with radial deformation rates of 0.1 mm to 0.3 mm, indicate an inadequate result of borehole grouting and do not meet the structural requirements for a passive pre-stressed concrete lining.

4.2   Low pressure borehole grouting for contact grouting of a shaft

Similar observations and conclusions can be drawn from the grouting works of two recently completed vertical shafts. The excavation diameter of each shaft is ~ 8.0 m with an inner diameter of 7.0 m. The rock types within the project area were mainly comprised of meta- greywacke and ortho- gneisses of good to very good quality. Low pressure contact grouting was foreseen over the entire shaft. The complete shaft was supported with shotcrete. The application of debonding agents was not foreseen. The first shaft showed a grout take for 61% of 1072 boreholes equal to or less than the exact borehole volume. A total of 92% of all boreholes showed a grout take of less than 3 l for contact grouting. The minimum grout take gave reason to investigate the cause in more detail. Back-calculation of a likely gap due to shrinkage of the concrete lining revealed delamination of less than 0.05 mm indicating non-penetrable conditions for the cementous grout (d95 < 40 µm). The massive rock mass conditions contributed the situation.

Grouting tests were undertaken with acrylic glass pipes to investigate the filling process of a tight borehole. Grout inflow in the borehole can be characterized as a continuous sedimentation process. The air within the borehole is pushed by the grout to the end of the borehole, effectively compressing and entrapping the air. An expulsion of the entrapped air via the dissoluted gap is therefore not possible. Gravitational settlement and horizontal separation begins upon drawdown of the pump pressure, which vanishes shortly after closing the valve at the packer and disconnecting the grout line

(effective pressure). The sole design criteria of a limiting grouting pressure and a defined holding phase for the encountered conditions was seen as inadequate.

Figure 4. Longitudinal and cross section of grouting test with acrylic pipes.

4.3   High pressure sleeve hole grouting for passive pre-stressed pressure tunnels

Two double strand pressure tunnels for a hydropower project were recently passively pre-stressed. The tailrace tunnels and the head race tunnels were both constructed with an unreinforced passive pre-stressed concrete lining. The surge pressure tunnel (pi = ~10.5 bar) was characterized by an internal diameter of 8,0 m and thickness of the lining of 40 cm. The two tailrace tunnels (pi = ~17 bar) are characterized by an internal diameter of 5,500 mm and the thickness of the lining was 500 mm. To facilitate grout flow in between the layers a de-bonding agent consisting of lime and water with a solid ratio 8:10 was used. The passive pre stressing was facilitated by sleeve hole grouting, with primary and secondary grout lines evenly spaced at 3,0 m intervals. The secondary grout lines showed a minor grout take of 15%, indicating a continuous filling of the dissolved gap already by the primary lines. The average grout take for the tail race tunnel per bay (length = 6 m) was found to be in the range of 330 l/bay. An average grout take of ~4.3 l/m2 was derived by a moving average analyses of the neighboring bays. The grouting pressure, the volume and the deformation were monitored throughout.

5   TECHNICAL AND OPERATIONAL INFLUENCES

An inadequate consideration of all aspects of structural grouting may lead to severe complications and downtimes during structural grouting operations. Especially the abdication of debonding agents may lead to adhesive bonding of a chemically reactive substrate. The strain induced by shrinkage is not able to delaminate the layers and if so introducing a minor tensile strength within. The inability for structural grouting prevents for the compensation of thermal losses, which do occur during the first operational filling. The thermal contraction finally leads to a delamination of the layers. In general the allowable tensile strength of the concrete lining is exceeded by far, introducing cracking in the lining, effectively leading to a change in the hydro mechanical regime (Schleiss 1985). Beside the structural impacts, the non-ability of gap penetration holds some major impacts for the execution of works. Finding the borehole volume as the grouting volume, the average grouting time will be in the range of one minute.

The necessary time for changing packer’s equals in such a case the time for grouting, impeding the execution of parallel works. This leads to an interruption of the complete process and either prolongs the actual construction period or demands for additional workmanship for compensation.

The common contractual procedure considers the actual time for grouting as the payment unit (ÖNORM B4454, DIN 18309). The unit rate for the operating time per borehole, considers hereby if not otherwise stated in the contract, all additional works such as packer disconnection, grout preparation, etc. Any unsteady grouting process accompanied by a shortfall of the actual grouting time, negatively affects the calculation basis. If the actual grouting time is far below the anticipated time for grouting, the assumed parallel works are not compensated and further hamper the grouting process. Figure 5 shows the cycle time with constant time for enclosing packers. All additional works are in this case compensated by additional workmanship. Otherwise, the time for enclosing packers will rise, leading to longer interruptions and subsequently to a longer construction time. The decrease of billable grouting time leads indirectly to higher costs for the compensation of the additional works.

Figure 5. Schematic illustration of cycle times of a disturbed construction sequence for structural grouting.

6   SUMMARY

Structural grouting for cast in situ lined shafts and tunnels is a challenging operation, depending on adequate design and implementation of works. Especially the lack of detailed understanding of the grouting process within a defined crack, leads inevitably to misconception with potential tremendous impacts on technical, economical, operational and contractual aspects of the construction project. Sleeve hole grouting systems with debonding agents (regardless if lime milk or membrane) are favored over borehole grouting, since crack widening and crack coalescence requires only a portion of the energy of crack opening. Not only from the perspective of grouting criteria, the beforehand specification of the required grouting volume per section and definition of the effective pressure is an essential requirement for fair tender specifications and allows for detailed work preparation and optimization. Deviation of such processes need to be understood and should be associated with the employer’s risks, or be compensated within the contract. The beforehand definition of the grouting parameters further allows computer aided steering of the grouting process, eliminating human related failures in the intervention of the grouting process.

REFERENCES

AUA. 2003.Guidelines for backfilling and contact grouting of tunnels and shafts, American Underground Construction Association, Thomas Telford Publications, London.

DIN 18309. 2012-09. Vertragsbedingungen für Einpressarbeiten VOB Vergabe- und Vertragsordnung für Bauleistungen - Teil C: Allg. Techn. Vertragsbedingungen für Bauleistungen (ATV) – Verbauarbeiten.

Krenn, H., Roner, M., Bauert, M. & Wannenmacher, H. 2013. Deformation measurement and long-term behaviour of passively prestressed pressure tunnels through the example of the Niagara Tunnel Facility Project. Geomechanik Tunnelbau, 6: 398–406. doi: 10.1002/geot.201300042.

Lauffer, H., Seeber, G. & Kaindl, F. 1968. Verfahren und Einrichtung zum Auskleiden od. dgl. Tiroler Wasserkraftwerke AG in Innsbruck, Österr. Patentamt, Patentschrift Nr. 284014.

Lombardi, G. 1996. Selecting the grout intensity. The International Journal on Hydropower and Dams, Issue 4, pp 62 -66.

ÖNORM B4454. 2001. Geotechnical engineering (foundation engineering) - Soil- and rock-grouting - Testing, Österreichisches Normungsinstitut, Austrian Standards plus GmbH, Wien II. Fachnormenausschuss A 023 Geotechnik. 2001.

Schleiss, A. 1985. Bemessung von Druckstollen (Dissertation I+II), Mitteilungen der VAW, Nr. 78 + 86. Seeber, G. 1999. Druckstollen und Druckschächte – Bemessung– Konstruktion – Ausführung. Stuttgart, Enke

im Thieme Verlag. Wannenmacher, H., Krenn, H., Bauert, M., Komma, N. & Engel, F. 2013. Improved Pressure Tunnel Lining

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