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Geotechnical features in the construction of "eco-berms" on soft river sediments

Introduction

Many dam walls at river power stations were built several decades ago and now need to be upgraded and raised due to increased flood protection requirements. Dam walls are usually raised by widening them on the air side, which in many cases requires a comparatively large amount of land and encroachment on the air-side floodplain forest (often FFH areas). Overall, this results in complex approval procedures and comparatively high costs.

Against this background, an innovative approach to ecological dam reinforcement on the water side was investigated under the leadership of Lechwerke Wasserkraft GmbH (formerly Bayerische Elektrizitätswerke GmbH, BEW) with the INADAR project, which was funded by the European Commission as part of the LIFE+ programme. The geotechnical support for the project was provided by the Geotechnical Centre of the Technical University of Munich.

The project focuses on the so-called "eco-berm", which contributes to the overall dam safety by incorporating an additional sealing element and provides the basis for widening or raising the dam on the water side. In addition, the introduction of structural elements such as dead wood, groynes or islands significantly improves the ecological situation in the riparian zones.

The construction of eco-berms on impoundment dams is suitable for all river sections where the discharge cross-section of the watercourse does not play a decisive role in flood protection, such as in the area of reservoirs upstream of power plants.

From a geotechnical point of view, such dam widenings on the water side are a special case when they are to be constructed on soft river sediments in silted-up areas of reservoirs. In order to investigate the technical feasibility, stability and ecological effectiveness of the "eco-berm" construction method in this case, following positive experiences with initial small test fields (each approx. 20 m in the reservoir of the Günzburg barrage on the Danube), a 500 m long test section was constructed in the reservoirs of the Oberelchingen (construction Nov. 2016) and Offingen (construction April 2017) (see Figure 1 and Figure 2).

Fig. 1: Location of the Oberelchingen and Offingen Danube barrages with test sections for testing the "eco-berm" construction method

Fig. 2: Location of the test section for testing the "eco-berm" construction method in the reservoir of the Offingen Danube barrage

Structure of the "eco-berm" and geotechnical issues

The existing retaining dams of the barrages on the Danube and Lech rivers (see Figure 3) are mostly of a comparatively moderate height of approx. 2.5 m to approx. 6 m and are sealed on the water side with concrete slabs (1st sealing element).

Fig. 3: Example of an existing dam

Large areas of the above-mentioned reservoirs often show silting up with soft, mushy river sediments that have been deposited over the past decades (see Figure 4). Figure 4 also shows the structure of the "eco-berm" construction method (bentonite-sand mat, geogrid over filter fleece, gravel/armour stone) investigated in the test sections on the soft, muddy river sediments.

Mushy-soft river sediments exhibit high compressibility and low shear strength even under comparatively low loads, which gives rise to the following geotechnical issues/challenges for the construction of the "eco-berm" on soft river sediments, as explained in Figures 5 to 7:

Can the "eco-berm" be constructed on the soft, mushy river sediments, and is the initial stability of the undrained river sediments sufficient to allow the fill to be produced?

Will the large deformations expected in the soft, mushy river sediments settle, or will undesirable settlement creep occur? (see Figure 5)

Is the cross-section of the "eco-berm" on the soft, mushy river sediments permanently stable? (see Figure 6) Is the slip resistance of the second sealing element on the concrete slope slabs sufficient, and is the second sealing element permanently effective even in the event of large deformations? (see Figure 7)

Fig. 4: Initial cross-section and structure of the "eco-berm"

Fig. 5: Geotechnical challenge: large settlements/deformations in the river sediments as a result of the load from the new fill (consolidation settlements and settlement creep)

Fig. 6: Geotechnical challenge: delicate stability (ground failure under the new fill on the water side in the undrained initial and drained final state)

Fig. 7: Geotechnical challenge: positional stability and permanent sealing effect of the second sealing element even with large deformations in the river sediment, slip resistance of the new second sealing element on the embankment concrete slab

Preliminary investigations

The soil mechanical properties of the soft, mushy river sediments, which form an important basis for deformation and stability calculations, were examined in advance at the respective locations for the test sections in situ and on soil samples taken in the soil mechanics laboratory at the Technical University of Munich.

In addition to classification tests (grain size distribution, plasticity limits, water content), determination of organic components and density determinations, compression tests were also carried out to investigate the deformation and creep behaviour, frame shear tests to investigate the shear strength in the final state and tests to investigate the permeability, as well as in situ vane shear tests to investigate the undrained shear strength of the river sediments in their unconsolidated initial state.

As a further aspect of particular importance for the system, the contact friction behaviour between the sealing element used (combined bentonite-sand mat, Bentofix BZ 13-B from NAUE GmbH & Co. KG) and the slope concrete slabs was investigated in advance by means of numerous inclined plane tests in accordance with DIN EN ISO 12957-2. Such tests are particularly important for the system under consideration here, as the contact friction behaviour can vary greatly depending on the sealing element product used and the surface of the concrete slabs. Since the sealing membrane was covered with gravel material in the construction method used here, and therefore special protection of the sealing membrane against the coarse bulk material was required, the aforementioned combined bentonite-sand mat was selected.

Construction of the test sections and installation of the measuring equipment

To create the "eco-berm", the water level at the Oberelchingen and Offingen barrages was lowered by approx. 1 m so that the surface of the river sediments became visible and the construction work could be carried out "in dry conditions". After excavating the approx. 0.5 m deep binding trench for the bentonite-sand mat and roughly cleaning the concrete slabs on the embankment, the bentonite-sand mats were laid on the concrete slabs on the embankment, starting from downstream and working upstream, with an overlap of approx. 0.7 m. The embedding trench was then refilled with sediment, the geotextile fleece and the geogrid required to stabilise the foot of the new fill were laid, and the first layer of gravel was applied (see Figure 8).

In order to investigate the issues described in section 2, three types of measuring equipment were provided at numerous locations on the two test sections at the Oberelchingen and Offingen barrages and installed during the construction of the "eco-berm".

Fig. 8: Construction of the test sections in the sections without measuring equipment

4.1 "SP" measuring device:

Settlement measuring gauge for measuring settlement in the bed surfaces of the fillings

To measure the settlement occurring in the sediments as a result of overfilling, settlement gauges were installed at 10 locations (Offingen) and 6 locations (Oberelchingen) in the test sections. The settlement measuring levels were made of steel profiles that are sufficiently stable to be covered with gravel material. Figure 9 shows the design, location and support of one of these settlement measuring levels. The measurements were taken geodetically by a surveyor from the start of the filling process.

Fig. 9: Installation of settlement gauges to measure the settlement of the new fill

Measuring device "MS": Measuring rods for measuring relative displacements between concrete slabs and bentonite-sand mats

Special measuring devices were installed at four locations in each of the test sections to measure the relative displacements occurring between the concrete slabs and the bentonite-sand mat as a result of the backfill. These consist of a smooth metal rod inserted into a sheath tube (measuring rod = metal rod + sheath tube) and two metal rods drilled into the concrete slope (see example system sketches in Figure 10 and Figure 11 and photo montage in Figure 12).

Fig. 10: Schematic sketch (floor plan): Design and arrangement of the measuring device for measuring relative displacements between concrete slope slabs and bentonite-sand mats (measuring rods)

Fig. 11: Schematic sketch (cross-section in fall line): Design of the measuring device for measuring relative displacements between slope concrete slabs and bentonite sand mat

The lengths of the measuring rods were adjusted in the test section depending on local conditions. The nail plates were anchored to the sand mat lying above the bentonite sand mat. The immediate area around the measuring rods and nail plates was covered with 0/4 mm concrete sand for protection.

The relative displacements between the ends of the metal rods drilled into the front of the concrete slope slab and the ends of the metal rods welded to the nail plates were measured by a surveyor (see Figure 12).

Fig. 12: Installation of measuring rods to measure the displacements between the bentonite sand mat and the concrete slope slab

"SÜ" and "SM" measuring devices for measuring seepage water

To check the system tightness (seepage, back seepage) of the newly installed second sealing element (bentonite sand mat), measuring devices for measuring seepage water were installed at six locations (Offingen) and five locations (Oberelchingen) in the test sections. These measuring devices were arranged both in the overlap area of two "SÜ" sealing membranes and in the middle of an "SM" sealing membrane. The measuring points were positioned so that there was as little hydraulic connection as possible to one of the mostly leaky concrete slab joints in the filter sand area.

The measuring device consists of filter sand/fine gravel with a hydraulic connection to two filter pipes, which were placed on the concrete slabs of the embankment before the sealing membrane was installed (see Figures 13 and 14). To measure the impermeability, a data logger is installed in one of the filter pipes to record the temporal progression of the seepage water rise in the drainage system. The second filter pipe is used to pump out the drainage system as a start for the measurement.

To define the boundaries of the measuring point area, it was sealed on each side with a strip of bentonite paste between the concrete slope slab and the clay sealing membrane. In order to investigate whether additional sealing of the lower end of the sealing membrane with bentonite paste would significantly improve the sealing effect, two variants of the measuring device (with and without bentonite paste at the lower edge) were implemented.

Fig. 13: Schematic diagram (floor plan): Design of the measuring device for seepage water measurement (leak test) in the overlap area of two clay sealing membranes (variant "oB" without bentonite paste and "mB" with bentonite paste at the lower edge of the clay sealing membrane)

Fig. 14: Schematic sketch (cross-section parallel to contour lines): Design of the measuring device for measuring seepage water (leak test) in the overlap area of two clay sealing membranes with lateral sealing using bentonite paste

The photos in Figure 15 and Figure 16 show the installation of the measuring device for measuring seepage water.

Fig. 15: Installation of measuring devices for measuring the through-seepage and back-seepage of the bentonite-sand mat (2nd sealing element) on the concrete slope slabs

Fig. 16: Installation of measuring devices for measuring the seepage through and behind the bentonite-sand mat (2nd sealing element) on the concrete slabs on the embankment

After installing the measuring points, the test section was completed (see Figure 17).

Fig. 17: Completion of the test section by installing the gravel material in layers

Measurement results

Figures 18 to 20 show examples of the results of measurements taken with the measuring equipment described in section 4.

In the Offingen test section (see Figure 18), with sediment thicknesses of approx. 1.0 m to approx. 1.3 m beneath the new fill, immediate settlements of approx. 2 cm to 20 cm were measured at the settlement measuring levels "SP" during the filling work in the river sediments.

In the course of consolidation settlement (primary settlement), the settlement increased further to values of approx. 3 cm to approx. 23 cm within approx. 100 days. The ecological bank design, approx. 100 days after the construction of the new fillings, triggered further consolidation settlements in the range of approx. 0.5 cm to 3.0 cm due to additional loads (see Figure 18). Taking creep deformation into account, the settlement has increased over time (last measurement to date after approximately 510 days) to between approximately 5 cm and approximately 29 cm. Further measurements in spring 2019 will be taken to determine any creep settlement more accurately.

Fig. 18: Offingen test section – settlement of the new fill measured using 10 settlement gauges "SP"

Figure 19 shows the relative displacements measured in the Oberelchingen test section with 4 measuring rods "MS" in the slope fall line between the bentonite-sand mat and the slope concrete. Overall, differential displacements of approx. 3.5 mm to approx. 11 mm were measured, which is considered harmless for the planned functionality of the sealing mat.

Fig. 19: Oberelchingen test section – relative displacements between the embankment concrete slabs and the bentonite-sand mat measured using four measuring rods (MS)

Figure 20 shows the temporal rise in seepage water measured after emptying the measuring device for leak testing ("SÜ" or "SM") together with the water level in the storage area (Oberelchingen test section).

It can be seen that the measuring points without lower edge sealing (oB), regardless of whether they are located in the overlap area of two sealing membranes or in the middle of the sealing membrane, show a significantly faster rise in seepage water in the measuring device. With lower edge sealing with bentonite paste (mB), the slow rise in seepage water of approx. 0.50 m or approx. 0.70 m in approx. 2 days indicates a good sealing effect of the second sealing element lying on the concrete slabs of the embankment.

Fig. 20: Oberelchingen test section – temporal rise in seepage water in the measuring devices for testing the tightness of the second sealing element (bentonite-sand mat) after emptying (SÜ = measuring device located in the overlap area of two sealing membranes, SM = measuring device located in the middle of a sealing membrane, no hydraulic contact with the overlap area)

Results from finite element calculations

In order to assess the feasibility of the "eco-berm" on soft, mushy river sediments, even for embankments that are higher and wider than those in the test sections, finite element calculations were performed using the Plaxis calculation programme and the "soft soil" material model. The calculation model is shown in Figure 21.

Fig. 21: Cross-section of the Offingen test section – calculation model for the FEM calculation with the Plaxis programme, "Soft Soil" material model

The FEM calculations with variation in the width of the new embankments show that, for comparatively narrow embankments (e.g. < 2.0 m), plastified soil zones only occur in the river sediments and deformations are limited to the foot of the new embankment (see Figure 22 above).

For wide embankments (e.g. 3.5 m), the FEM calculations show disproportionately larger deformations, and the plasticised soil zones already cover large parts of the new embankment at comparatively small embankment heights. A slope failure must be expected here (see Figure 22 below).

Fig. 22: Cross-section of the Offingen test section – results from FEM calculations with variation in the width and height of the new fill

Conclusions

Based on the experience and measurement results from the test sections in the reservoirs of the Oberelchingen and Offingen barrages on the Danube, as well as the results from FEM calculations, the following conclusions can currently be drawn for the construction of "eco-berms" on soft, muddy river sediments:

• The most important prerequisite for the construction of "eco-berms" on soft, muddy river sediments is the long-term stability of the river sediments. Erosion directly jeopardises the stability of the location.
• "Eco-berms" can be constructed on soft/muddy river sediments.
• So far, positive experiences have been gained with sediment thicknesses under the new embankments of up to approx. 1 m, embankment widths of approx. 2.0 m and embankment heights of up to approx. 2.5 m.
• Despite significant deformation, no failure of the new fillings was observed for the aforementioned geometries.
• In the case of significantly greater sediment thicknesses, greater fill widths or greater fill heights, calculations carried out to date indicate that a slope failure is to be expected as a result of large deformations in the soft, mushy river sediments beneath the new fills.
• The additional second sealing element (bentonite-sand mat) installed on the concrete slope slabs (first sealing element) is effective, so that in the case examined here, the load case LF3 "sealing" specified in DIN 19700 Part 13 is effective. sealing element (bentonite-sand mat) installed on the slope concrete slabs (1st sealing element) is effective, so that in the case examined here, the load case LF3 "seal defective" specified in DIN 19700 Part 13 can be classified as extremely unlikely and it is therefore possible to dispense with this mathematical proof.
• With sufficient contact friction between the concrete slabs and the sealing membranes used (must be investigated depending on the product), the tightness of the second sealing element is maintained even with large deformations of the new fillings. Sealing with bentonite paste at the lower edge of the sealing membrane effectively reduces the risk of water running behind the second sealing element.
• The deformation measurements planned for 2019 will provide additional insights into the long-term deformation of this construction method due to creep in the river sediments. Further research is still needed in this area.