THE ROLE OF MONITORING IN THE REHABILITATION OF PLAVINAS HPP, LATVIA

R. Peter Brenner1 and Richard Guimond1

Juha Laasonen2

Juerg Speerli3

Dzintars Ostanievics4

ABSTRACT

The Plavinas hydro-electric powerplant in Latvia is located within a complex hydro-geologic environment. The powerhouse/spillway structure is founded on a buried valley, filled with glacial till and having pressurized aquifers on its flanks. Pressure and seepage control is by means of drainage blankets and wells in the foundation and relief wells on the downstream right bank. A large number of piezometers monitors the pressure in the various stratigraphic units. For the design of a reserve spillway required to ensure safety against flood events, the existence of highly pressured zones at the contact of the till with the pressurized aquifers on the slope of the buried valley was an important geological consideration. Hydro-geological investigations revealed that these zones are hydraulically connected. The stilling basin of the reserve spillway will be close to the buried valley but construction activities must not interfere with pressurized strata. Real-time monitoring of piezometric heads by means of an automatic data acquisition system will be used as a tool to ensure safety during construction activities.

INTRODUCTION

The Plavinas powerplant is the uppermost hydro-electric station of the Daugava hydro-cascade in Latvia. It is located near the town of Aizkraukle, some 90 km east of the capital city of Riga and about 107 km upstream from the river’s estuary. With its presently 868.5 MW of installed capacity and a maximum gross head of 40 m, it is also the largest of the three powerplants forming the cascade. It was designed by Institute Hydroproject, Moscow, and commissioned in 1966. The scheme consists of a 1282 m long, right wing embankment dam, a 158 m long concrete section consisting of the powerhouse with the spillway discharging over it, the main hydraulic fill dam of 675 m in length, built across the present river channel, and a left wing embankment dam, 1892 m long, as illustrated in Fig. 1. The main dam and both wing embankment dams are founded on dolomite rock, which is partly karstic, whereas the powerhouse-spillway structure, consisting of two adjacent blocks separated by a vertical joint, is situated on a deep buried pre-glacial valley filled mainly with glacial and fluvio-glacial deposits.

GEOLOGICAL AND HYDRO-GEOLOGICAL SETTING AND CONCERNS

Stratigraphy

The powerhouse-spillway structure is the principal component of the scheme. However, its design and later its performance have been strongly affected by the difficult foundation conditions and the regional hydro-geological setting. The Plavinas site lies in the Baltic artesian basin. The Daugava river in the Plavinas area represents a low regional drainage system. The subsurface conditions consist of a series of sedimentary rock strata of Middle to

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Upper Devonian age, covered by Quaternary deposits (Fig. 2). Some of these strata are confined aquifers and are under artesian pressure. The thickness of the sedimentary complex can reach up to about 300 m. The Devonian series is predominantly sub-horizontal and includes sandstones of the Aurikula-Burtnieku and the Gauja-Amata stratigraphic units and dolomite rocks of the Plavinas-Daugava complex. A simplified listing of the stratigraphy is given in Table 1.

Fig. 1 Layout of Plavinas dam and location of reserve spillway

The sandstone and dolomite layers are relatively pervious and act as aquifers. They are interspersed with silty-clayey deposits acting as aquicludes.

The materials filling the pre-Quaternary buried valley are predominantly tills, with fluvio-glacial deposits on top. The texture of the till is mostly clayey sand, typically having the following fractions: clay 14-17 %, silt 30 %, sand 45-50 % and gravel 7-10 %. The Russian designer termed the more sandy till as sandy loam and the more clayey variety as loam. These materials are fairly impervious, but there are also more pervious zones with lenses of gravel and also pebbles. “Entrained” bedrock material lines the slopes of the buried valley. In the Latvian language this material is termed as “Šleif” which has been translated as “Train”. This material, scoured from the valley flanks by the glacier and/ broken off from higher elevations, was dragged along the valley and consists mainly of gravel and rock fragments. It is highly pervious and may serve as a hydraulic link between aquifers (Dislere, 2006). The Devonian rocks along the buried valley are fissured and cracked, particularly the sandstones of the Amata formation are believed to contain numerous vertical discontinuities. Some of these may originate from stress release when the valley was eroded.

Foundation seepage control

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The prevailing hydro-geologic regime in the Plavinas area was a decisive design consideration in the realization of the powerhouse. To ensure the stability of this structure, comprehensive measures in seepage and pressure control had to be provided, namely (see also Wieland et al., 2006):

Connected with the powerhouse structure: an upstream apron with drainage, a multi-layer filter blanket and drainage wells below the foundation slab, and a downstream apron with drainage blanket and relief holes

On the downstream right bank: a large number of relief wells, arranged in several rows to drain the Amata sandstone and a drainage gallery in the Plavinas dolomite.

Fig. 2 Geological section through buried valley along the dam axis

Table 1 Stratigraphic units at Plavinas dam siteAge Stratigraphy

(designation)Type of material Approximate

thickness (m)Quaternary to Recent

al-Q2 & f-Q3 River alluvium with gravel and pebbles more than 100 m in buried valley

gl-Q2 Glacial till of variable composition

Upper Devonian Daugava(D3dg)

Dolomite, “dolostones” 10 to 15

Upper Devonian Salaspils(D3slp)

Dolostones and dolomitic marls 10 to 13

Upper Devonian Plavinas(D3pl)

Dolostones, dolomitic marls, carbonate clays, and silts

21 to 32

Upper Devonian Upper AmataLower Amata(D3amt)

Sandstones, weakly cemented, with intercalations of sand and clay

17-27

Upper Devonian Gauja(D3gj)

Sandstone, weakly cemented 50

Middle Devonian Burtnieku(D2br)

Sandstones and siltstones > 60

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The discharge of the relief wells varies widely. Some of them have to be pumped while others are free flowing. Some wells also eject sand. Different types of well screen designs were employed. Usually, they consisted of a wire-wound steel bar skeleton covered by a slotted steel pipe or a wire mesh, embedded in a gravel pack. Abundant presence of mica particles tends to clog the gravel pack in the vicinity of the screens thus requiring re-development and, after some years, even re-construction. The locations of the relief wells on the right bank and of piezometers on both the right and the left bank are shown in Fig. 3. In addition, a large number of piezometers are installed below the powerhouse.

The control of the uplift pressure through drainage and relieving is critical for the stability of the powerhouse. There is an upward trending flow of the groundwater within the pervious and across the impervious sediments. Within the rather impervious morainic soils filling the buried valley, very high upward hydraulic gradients exist for the continuity of the groundwater flow. The gradients are higher in the clayey silty, less pervious variety of the morainic soil. Various incidents have occurred in the powerhouse foundation. These were briefly described by Wieland et al. (2006).

Hydro-geological singularities

The total discharge of the relief wells on the right bank amounts to about 160 liters/s. Of particular interest is a cluster of three wells which are rather close, i.e. about 50 to 60 m from the downstream edge of the powerhouse (see Fig. 3). These wells (1201, 1221 & 1302) have a very high yield, i.e. about 60% of the total discharge. They have been termed the High Yield Wells (HYW). The reason for the high discharge of these wells is not clear but it is believed that these wells are in contact with a larger fracture in the Amata sandstone providing a hydraulic connection with deeper aquifers. The filter screens of two of these wells are in the Train material, while the screen of the third well is in the Amata sandstone. These three wells can be considered as a singularity of the right bank relieving system. Actually, some years ago an incident similar to a blowout occurred in the vicinity of these wells.

Interestingly, another singular point was found unexpectedly some 1.7 km downstream in a small tributary valley drained by the Lauce river when on January 6, 1999, a borehole was advanced in combination with a cone penetration test (CPT-6) in the undisturbed morainic soil (see Fig. 4). This test was intended to serve as a reference to the soundings that were performed below the powerhouse. A second borehole, equipped with Ø 146 mm casing, was drilled right next to it for the purpose of soil sampling. When the drilling of this borehole, CPT-6B, had reached Elev. -21.8 m, sampling was no longer possible because the soil was loose with sand and dolomite fragments. A blowout of CPT-6 occurred on January 19, 1999, accompanied by large quantities of coarse sand and gravel. The discharge was estimated to be about 45 liters/s. This discharge had an effect on the High Yield Wells. Their cumulative discharge decreased from 92 liters/s to 88 liters/s in the period January 19 to 20, 1999. Borehole CPT-6 was then plugged between January 27 and February 11, 1999. Starting from February 3, the discharge of the HYW increased again.

Similarly, the blowout also affected the left bank and decreased the piezometric levels. The greatest drop was noticed for the two piezometers with their tip in the Gauja formation. It reached 1.9 m in the vicinity of the powerhouse (piezometer Pz 3101) and 0.5 m some 70 m away from the powerhouse (Pz 3102). The piezometric levels in the Amata formation also decreased by 2 m in Pz 3301 and by 3.7 m in Pz 3302. Drops of around one meter could even be observed in some of the piezometers in the moraine. Hence, the conclusion from this

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observation was that there must be a connection between all these wells and piezometers and that there must be some short-circuit through some very pervious channels.

Exploration hole CPT-6B was plugged later and two new boring were made and transformed into relief wells, namely 5602 and 5603.

Fig. 3 Locations of piezometers and relief wells in the downstream area

Monitoring as a tool to understand the subsurface flow regime

In order to obtain a better insight into the hydraulic connections between the wells and piezometers, a series of relief tests was carried out with the Lauce wells. Meanwhile, the response of the wells and piezometers could be monitored in real time by means of an automatic data acquisition system (ADAS) whose installation was completed in 2001. Such a system is invaluable in the management of a large number of sensors. An ADAS is able to read, quantify, and convert physical information continuously, periodically or on request and transmit it to a remote data processing system thus enabling on-line monitoring of relatively rapidly changing sensor values. The system of data transfer at the Plavinas powerplant is illustrated in Fig. 5 and comprises 15 data loggers with 32 remote multiplexers and a system of nine computers which includes four acquisition computers, four workstations and one server for real time data processing (Popielski et al, 2001). Instrumentation at the Plavinas powerplant now contains more than one thousand measuring points. Out of these, initially 430 were selected for replacement by automatic devices. Particularly, sensors for pressure and flow monitoring were given first priority in the incorporation into the ADAS.

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The ground elevation at the two Lauce wells (5602 & 5603) is around 38 m asl. The wells pass through 7 to 8 m of gravel with rock fragments and sand, followed by some 50 m of morainic material, i.e. sandy loam and loam with inclusions of gravel and rock fragments. At a depth of about 59 m (approx. elev. -20 m asl), the holes encountered highly pervious entrained material (gravel and sandstone fragments) identified as entrained material. Well 5602 penetrates this Train layer by about 13 m (i.e. to Elev. -33.5 m asl) while well 5603 ends at Elev. -23.1 m asl with about 2 m in the Train material. The bedrock was not reached in any of these holes.

Fig. 4 Map showing the locations of the Lauce wells, the High Yield Wells, the boundaries of the buried valley and the axis of the proposed reserve spillway with the stilling basin.

Relieving tests carried out during September 9 to 26, 2001, discharged a total of 120 liters/s (70 liters/s from well 5603 and 50 liters/s from well 5602). The outflow remained closed during the night and weekends. Selected piezometers representing different subsurface strata close to the powerhouse on both the right and left bank, and as well the three High Yield Wells, were monitored by the ADAS. Figure 6 shows the response of the three High Yield Wells on the right bank to the relieving of the two Lauce wells and Fig. 7 presents the response of piezometers in the Upper Amata on the left bank. The graphs clearly demonstrate the effects of the discharges at the Lauce wells at a distance of 1.5 to 1.7 km from the monitored sensors. Also shown in the graphs is the variation of the reservoir water level which varies weekly between about 70.5 and 72 m asl. These reservoir level fluctuations are imprinted on the variation of the well discharges and the piezometric heads. During the period from September 17 to 21, the total discharge from the High Yield Wells decreased by 4.8 liters/s, which includes the effect of the lower reservoir level. From September 17 to 25, the piezometric level dropped by about 0.6 to 0.9 m.

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RESERVE SPILLWAY

Foundations conditions

The reserve spillway is a major component in the overall rehabilitation of the powerplant. Its main purpose is to correct the inadequate flood discharge capacity of the existing spillway to satisfy modern PMF criteria. It is a gated structure with four bays and has a design discharge capacity of about 4000 m3/s. The ogee crest is at elevation 58.10 m asl. The location on the right hand embankment dam was selected among five other locations because it has the least interference with existing structures, utilities in the ground or drainage facilities in the vicinity of the power plant and as well with settlements in the downstream area or private land plots. The position of the axis of the reserve spillway is indicated in Figs. 1 and 5. Most of the structure is founded on the dense and impervious glacial till (moraine). Only the stilling basin at river level is founded on Plavinas dolomite rock. The ground elevation at the river shore line is about 33 m asl while the top of the bedrock is at 27 m asl.

Fig. 5 Components of the automatic data acquisition system (ADAS)

Hydro-geologic concerns and monitoring

A major geologic concern is, that the extension of the spillway axis intersects the buried valley not far from the end of the stilling basin (see Fig. 4). The exact position of the rim of the buried valley is not known and will have to be explored by additional borings. However, at this location the shoreline of the present river bed and the rim of the buried valley almost coincide. The response of a possible interference with the buried valley during construction or during operation of the spillway must be predictable from the present knowledge supplemented by additional investigations.

There are no deep drillholes in the immediate vicinity of the planned reserve spillway. The closest investigations in the buried valley were carried out along a section which lies on the left bank of the present river channel. The section contains five boreholes (BH-1 to BH-5). A sixth borehole, BH-6, was drilled next to the shoreline and is about 500 m from the stilling basin and positioned almost exactly on the extension of the spillway axis. All six holes were drilled to reach bedrock. The purpose for drilling these holes was to find locations with entrained materials below the moraine which would be under high pressure similar to the wells found in the Lauce valley. Except for borehole no. 3, which exhibited material similar to

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entrained material within the depth range 130.0 to 137.6 (Elev. -93.5 to -101.1), there were no traces of entrained material in the other holes (i.e. BH-1, BH-2, BH-4 and BH-5).

Fig. 6 Response of the High Yield Wells to relieving the Lauce wells

Fig. 7 Response of piezometers in the Upper Amata on the left bank (in the vicinity of the powerhouse) to relieving the Lauce wells

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Borehole BH-6 was drilled on the basis of geophysical investigations (vertical and symmetric electric profiling and as well gravimetric surveying). Electric profiling was carried out to a depth of about 65 m which is believed to be the elevation of entrained materials. The area containing the wells in the Lauce valley exhibited a low resistivity and the water from these wells had a conductivity which was two to three times higher than the background values. Hence, prospecting for entrained material zones had to focus on the identification of zones of low resistance. Borehole BH-6 is on the right hand slope of the buried valley. The hole was drilled to a depth of 131 m (Elev. -96.1 m asl), i.e. until bedrock was encountered. Entrained material which was under pressure was found between the glacial till and rocks belonging to the Middle Devonian Burtnieku formation, which is below the Gauja formation at a depth reaching from 97.6 to 116 m below ground level. Subsequently, borehole BH-6 was equipped for conducting relieving tests. The filter was installed in the depth interval of 111 to 120 m, i.e. in the lower part of the entrained material and partly in the upper range of the Burtnieku rock. The pressure was, however, not as high as expected; the piezometric head was only about 22 m. The discharge from this borehole amounted to 19.6 liters/s.

The investigations in the buried valley have demonstrated that the occurrence of entrained material is not persistent, also its thickness may vary. High pressures are expected to exist where the material overlies the Gauja formation or Amata connecting to Gauja through discontinuities. Most of the entrained material may be present on the slopes of the buried valley, but it is conceivable that there are also accumulations at the valley bottom. These may be hard to detect by geophysical means because of the influence of the slopes on the measurements.

A further borehole, BH-1327, was drilled in 2005 about 200 m from the stilling basin of the planned reserve spillway. This is the nearest hole in the stilling basin area drilled to Amata level and it is close to the rim of the buried valley. This drill location was, however, not based on geophysical investigations and the purpose of this hole is not clear. The top of the Amata formation in this borehole is at El. 17.65 m asl, i.e. about 9 m below the base of the stilling basin. There is a clayey stratum of about 3 m thickness separating the Plavinas dolomite from the Amata sandstone. The pressure encountered in the Amata rock of borehole BH-1327 was about 0.2 MPa, i.e. the piezometric level rises to about El. +40 m asl, which is higher than the tailwater level. Further drillholes are, however, needed to establish a reliable pressure profile in the area of the stilling basin. Some of the piezometers could then be connected to the ADAS, at least temporarily during the period of construction.

Not all boreholes drilled into the Amata will encounter pressurized water. Only when drilling into highly pervious material between the base of the impervious moraine and the rock with artesian water will this occur. Such locations do not seem to be frequent because inspite of the large number of boreholes drilled in the area, only very few have encountered such conditions. Therefore, exploration drilling to better define the rim of the buried valley along the extension of the spillway axis may not find such conditions.

From the relieving tests carried out with the Lauce wells, it is expected that even if a high pressure zone were encountered during further exploration or during construction work, its impact on the pressures in the foundation of the powerplant would not exceed a decrease in piezometric head of one to two meters. Still, precautionary measures are required when constructing the stilling basin of the reserve spillway. The contractor who installed the new instruments below the powerhouse, which required drilling through the drainage blanket used

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Dosapro (or pinch) valves, which are able to control pressures up to four bars. (Popielski et al., 2001).

CONCLUSIONS

The Plavinas powerplant is located in a complex and unique hydro-geological environment, involving pressurized subsurface strata in its foundation. To ensure the stability of the powerhouse, changing of the prevailing regime of pore water pressures and uplift pressures must be avoided when rehabilitating the facility or when adding new components to the system.

Comprehensive site investigation, installation of piezometers and possibly also relief wells with real-time monitoring by means of an ADAS can reduce uncertainties and minimize risks related to the hydrologic response to construction activities and to the impact induced by the operation of a new facility.

The paper illustrates how field investigations in combination with monitoring can improve the understanding of the subsurface conditions and the risk involved when a new facility (in this case a reserve spillway) is added to the powerplant system. It facilitates the planning of additional investigations and predicting the response of the hydro-geologic regime to construction activities and operational impact. If significant changes in the pressure regime below the powerhouse are predicted, countermeasures can be invoked during the design phase to minimize or preferably avoid adverse situations.

REFERENCES

Dislere, S. Monitoring as a tool of dam safety improvement at Plavinas HPP. In: Dams and Reservoirs, Societies and Environment in the 21st Century, L.Berga et al., eds., Vol.1, 611-617, Taylor & Francis (2006).

Popielski, A., Rollet, M., Bolmanis, A. and Sulcs, U. Upgrading the instrumentation system at Plavinas, Latvia. Hydropower & Dams, Vol. 8, no. 3, 85-86 (2001).

Wieland, M., Brenner, R.P., Speerli, J., Ostanievics, D. and Bolmanis, A. Safety evaluation of the Plavinas run of river scheme in Latvia. Transactions 22nd Int. Congress on Large Dams, Barcelona, Q.86, R.72, Vol. 3, 1189-1209 (2003)..

_________________________1 Pöyry Energy Ltd., Hardturmstrasse 161, CH-8037 Zurich, Switzerland2 University of Applied Sciences, Oberseestrasse 10, CH-8640 Rapperswil, Switzerland3 Fortum Power & Heat Oy, P.O. Box 1, FI-00048 Fortum, Finland4 Latvenergo Joint Stock Company, Pulkveza Brieza Street 12, LV-1230 Riga, Latvia

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