PROBLEMS AND SOLUTIONS THAT MAY EMERGE IN THE FOUNDATION AND BODY OF A HOMOGENEOUS FILL DAM ON A WEAK CLAYEY-SILTY-SANDY FORMATION ÇIKRIKÇI DAM Esen Yalım KARADUMAN BAR-SU Eng. & Conc. Inc. Ankara Turkey Mehmet Harun ASKEROGLU BAR-SU Eng. & Conc. Inc. Ankara Turkey Introduction Çıkrıkçı Dam Project, It is located on 2,50 km North of Çıkrıkçı village of Hayrabolu district of Tekirdağ province. The dam is in filled homogeneous type and has been designed in a way that it is at 45m height from the foundation and at 3.65 km crest length. The total dam body volume is 7300000 m³. The reservoir of the dam will filled with the water to be obtained in Hayrabolu and Kumdere stream. Çıkrıkçı Barajı is an off-stream type dam and its active volume is 27.000.000 m³. Upstream slope is selected as 3.5 horizontal / 1 vertical and 5 horizontal / 1 vertical, and downstream slope is selected as 3.0 horizontal / 1 vertical and 5 horizontal / 1 vertical. The typical section of the dam is demonstrated in Image 1 below. The aim of this study is to reveal how to identify the problems that are encountered in the foundation of Çıkrıkçı Dam such as bearing capacity, settlement, consolidation, permeability, internal erosion and liquefaction, and to provide solutions that are suggested within the scope of both the foundation of dam and derivation-outlet conduit. 1 Geology There are pliocene aged clayed-sandy-graveled formations, which are the members of Thracia formation, on the upper parts of foundation at the axis of the dam. There are miocene aged sandstone formations, which are the members of Ergene Formation, on the lower part. Furthermore, gravel and sand lenses that were observed in the upstream-downstream direction and left-right abutment directions within the sandstone formations are available. 2 Geotechnical Works Image 1: Typical Dam Body Section (Maximum Section) 28 boreholes have been drilled in dam site in addition to the ones that were opened in the planning stage. The distribution of such boreholes are as follows: 12 on the axis thru longitudinal section 6 on conduit axis having alternatives 10 in reservoir area
The tests of uniaxial compressive strength, point load strength, elasticity modulus, poisson ratio, triaxial compression strength, unit weight, water content, slave analysis and atterbeg limit have been conducted with the purpose of determining the geotechnical parameters based on the samples of undisturbed and disturbed samples and drilling core samples that were collected from basic borehole by means of on-site tests such as BST, permeability and pressiometer. Drilling places have been demonstrated in the general layout in Image 2. Image 2: Drilling Locations
3 Geotechnical Analysis Results 3.1 Liquefaction Standard penetration tests conducted in boreholes and the laboratory test results performed on undisturbed samples have been used in the calculations of liquefaction. Liquefiable formations are thin and loose materials that are brought by the reservoir area of the tributary of the river bed in right and left abutment regions. As a result of the liquefaction analysis done, the safety factor has been calculated less than 1.0 in thalweg and around borehole locations that were drilled around river bed. There is a risk of liquefaction depending on the sand transitions that take place in silty-sandy formations. The thicknesses of liquefiable formations vary between 4.50m and 9.00m. Image 3: Liquefaction Analysis in the Borehole Opened in Thalweg Image 4: Liquefaction Analysis in the Borehole Opened in the Left Abutment
These formations are demonstrated in Image 2 as scanned with red, and will be removed under the dam body upstream slope. The reason of why the formations are highly heterogeneous the other amelioration and improvement methods have not been taken into consideration. 3.2 Permeability According to the results of hydraulic pressure test and permeability test, it has been identified that the alluvion in the axis of dam and clayey-silty-sandy sedimentary formations demonstrated low-permeable (K=10-5 -10-6 and Lu=1-5) characteristic whereas sandstone formations generally demonstrated impermeable (Lu<1) characteristic in general. However, permeable (Lu=5-25) sandy-gravel levels whose thicknesses reached 2.00 meter have been identified among the clayed formations that were observed high elevations (See Image 6). At the left abutment, the thicknesses of these permeable formations increase at thalweg and at the regions that are close to river bed. Moreover, permeable (Lu=5-25) and high-permeable (Lu>25) sand-gravel lenses are available among the sandstone formations. (See Image 5) Image 5: Left Abutment Borehole Image 6: Right Abutment Borehole
Although the formations in left and right abutments of the dam demonstrates low-permeable and impermeable characteristics, the thickness of permeable levels increase at the points that are close to thalweg and river bed. Sandy-clay, sandy formations and the levels involving sand-gravel lenses that are observed especially in sedimentary formations and sandstone formations demonstrated permeable (Lu=5-25) and high permeable (Lu>25) characteristics. The seepages through these formations towards upstream-downstream direction may result in piping under water load. Therefore, it has been thought that it will be appropriate to constitute the grout curtain throughout all the axis and constructing slurry-trench stabilisation in critical piping regions with the purpose of preventing water seepages and piping towards upstream-downstream direction. Having regard to the location of underground water and permeability values it has been determined that the depth of grout curtain will reduce at least 10 meters below underground water and will fulfill the condition of Lu < 1 or the formula of h'=1/2h+c will be calculated. Below cut-off, single-line curtain will be constituted; at 16.00-20.00 meters of depth in right abutment, at 20.00-28.00 meters of depth in left abutment and 28.00 meters of depth in thalweg. Consolidation groutings at both two sides of the relevant single-line curtain and "slurrytrench+grouting" curtain at a maximum section will be applied. Slurry-trench curtain will be inserted into the bedrock 2.00 meters on the foundation ground that may be exposed to piping towards upstream-downstream direction on the foundation. In order to prevent numbered "1" impermeable mateirla to pipe in the graveled zones in the foundation, filter sand material has been put under clay core in slurry-trench upstream. Gravelled zones might be available in the foundation in the shape of lens whereas they might be observed constantly in the direction of upstream-downstream. Therefore, it has been thought that this kind of a precaution may be necessary. Slurry-trench starting point has been decided in a way that it will extend till the locations in which there is not graveled zones in the direction of upstream and downstream. Furthermore, overlap length as same as blanket length, has also been provided in the intersection region. Impermeable material around slurry-trench curtain, will be constituted on 2% wetter part than optimum moisture.[1] In this way, it will be ensured to make the system to act more elastically in case of a differential settlement. Slurry-trench curtain will be extended in a way that it will enter into the body throughout the impermeable core as minimum H/10. In this way, it has been thought that a sufficient amount of friction might be composed around the curtain. Analyzes have been conducted on different kilometers in order to prevent the erosion that may occur on the foundation [1] in case the grout will not be successful (if drill closes itself) because of the fact that the formations in the foundation become different in each drilling or permeability values are lower than 5x10-4 cm/second. The main purpose of the analyzes is to extend the seepage path in the most economical way and to reduce the hydraulic gradient values on the foundation and the rates of seepages on the foundation. In this way, the objective is to ensure the internal erosion towards the direction of upstream-downstream under all circumstances in the young foundation formations in which all the matrixes such as clay-sand-gravel-clayed sand-clayed gravel etc. Since this requirement cannot be fulfilled in the regions where water load is high (~>18.50m), slurry-trench curtain will be formed in order to prevent the impermeability towards the upstream and downstream direction. 3.3 Bearing Capacity and Settlement 3.3.1 Dam Body In order to determine the settlement potential of clay-sand-gravel formations below the dam body against the load that may come from above, the results of pressiometer test, standard penetration test (SPT) and consolidation test have been used. Safe bearing capacity values that were calculated are given in Table 1 for the corrected SPT values at various depths in thalweg and right abutment wellhole.
Table 1 The calculated minimum bearing capacities of thalweg and right abutment boreholes have been given in Table 2. According to this table, considering the depths of stripping excavation and net loads based on such depths, it has been identified that the foundation could not carry the given load. Therefore, the total settlements on the foundation have been calculated for the depth of stripping excavation determined, and it was controlled whether the settlements on the foundation remained acceptable or not. Accordingly, the settlements have been calculated as 1% or 2% less than the height of the dam from foundation and they are suitable with the limits given in the literature. [2] Having regard to the fact that the construction of the dam will take at least 4 years and maximum permissible settlement amounts, it has been identified that the sudden and consolidation settlement amounts in question have remained within the limits. Drilling Place Right Abutment Height from Foundation (m) 19.50 %1xH (cm) Depth of Excavation (m) 19.50 3.50 Load (kg/cm²) 4.09 Minimum Bearing Capacity (kg/cm²) Maximum Settlement (Settlement+Consolidation) (cm) 1.70 5.78+0.10=5.79 20.50 20.50 3.50 4.52 1.87 3.86+5.10=8.96 24.50 24.50 5.00 5.16 2.98 8.90+0.20=9.10 Thalweg 30.00 32 3.50 6.43 3.05 8.82 Left Abutment 23.85 24 6.00 5.12 3.20 23.00+2.20=25.20 21.10 21 3.50 4.54 3.16 6.86+9.80=16.66 21.55 22 4.50 4.64 3.95 8.90+0.20=9.10 17.90 18 3.50 3.96 2.92 14.12+8.80=22.92 12.75 13 3.50 2.66 4.12 2.14+8.20=10.34 Table 2 The condition of soil's being a foundation has been determined as SPT-N > 20 [3] and the formation which do not fulfill this condition have been removed below the foundation and as a result of the liquefaction, the determined sandy formations have been removed below the dam body upstream slope. Cut-off trench excavation will be done at 2.00 m. of depth. The width of cut-off trench has been selected as ΔH/2 in a way that the minimum width will not be less than 6.00 meter. [4] ΔH indicates the difference between maximum water level and trench bottom elevation.
Furthermore, in order to prevent any potential differential settlements to cause an crack in the dam body or a concentrated flow in the direction of upstream-downstream, [5] the necessary tests have been conducted and the result is given in Table 3. Kilometer Angular Settlement Maximum Differential Settlement % 0.5H (cm) % S < % 0.2 0+000~0+600 %0.01 9.75 > 5.79 0+650~0+950 %0.01 10.25 > 3.17 1+000~1+200 %0.07 12.25 > 0.14 1+550~1+750 %0.08 15.0 16.4 1+750~1+800 %0.17 11.0 > 8.54 1+800~2+100 %0.03 11.0 > 7.56 Table 3 According to the calculation results given in Table 3, all the differential settlements are within the limits suggested in the literature. Furthermore, throughout the axis of the dam, piezometers will be placed on the foundation and dam body with frequent intervals, and any potential excessive pore water pressures and potential settlements will be controlled by means of such devices. In this way, the speed of the construction will be decided in the construction stage. 3.3.2 Derivation Conduit The maximum load is ~8.00 kg/cm² on the conduit. The bearing capacity values of pliocene aged sandy-clayed formations on the sandstone formations vary between 2.00 kg/cm² and 4.00 kg/cm². Since these formations are poor in terms of bearing capacity, with the purpose of preventing the problems that may arise from this insufficiency, it has been determined to remove them completely below the conduit, and concrete has been filled above the miocene aged claystone-sandstone formations that are able to carry the maximum load. The potential settlements below on the foundation of conduit have been made uniform by means of the concrete fill to be done under the conduit. According to the results of uniaxial compressive strength tests and point load tests that were conducted upon the samples received from the drillings cores on the axis of the conduit, the bearing capacity values at the bottom of the conduit concrete fill has been found as minimum 7.79 kg/cm² and maximum 9.21 kg/cm². It has been understood that the maximum 8.00 kg/cm² of load on the conduit may not lead to a problem concerning bearing capacity or settlement on the foundation of the structure (considering the fact that the load decreases under the upstream and downstream shells) according to the safe bearing values given above. Furthermore, after the stripping excavations, with the purpose of rehabilitating, the weaknesses that may arise in sandstone formations remained under the conduit foundation, consolidation grouting will be conducted on the pattern of 2.00x2.00m and twofold of the foundation width. Conduit trench excavation slopes have been selected as 3 horizontal / 1 vertical under the dam body. Because the steeper excavation slopes may result in settlements based on the difference between the compactibility in itself and the compactibility between the fill and foundation, it may also lead to hydraulic fracture in the fill on the conduit originating from internal erosion.[6] Moreover, in the excavations to be done throughout the conduit, the sharp edges that will remain under fill may cause the fractures within the fill to improve in time and also may lead to stress concentrations. Therefore, the excavations have been conducted without berm.
3.4 Piping and Erosion Image 7: Abutment Typical Section The precautions will be taken against three different kinds of erosion having regard to the fact that the material on foundation at the size of gravel may be in the shape of sand lenses and that the material at the size of sand may be in the direction of upstream-downstream constantly in several sections. [5] 1. Backward erosion 2. Contact erosion 3. Internal Stability (Geometrical and Hydraulic Conditions) Seepage models have been composed for different impermeable upstream blanket lengths and formations underneath the dam body (clayed-sand and sandstone) and seepage velocities and hydraulic gradient values have been calculated. In order to achieve this objective, analyzes were conducted on 20 different model. A typical model that was used is given in Image 8. Image 8: Typical Seepage Model At the Abutments The selected and calculated critical hydraulic gradient values have been compared and the upstream blanket lengths have been determined in right and left abutment. The calculated blanket heights are demonstrated in Table 4 below. The matters have been tested in the calculations such as whether the thicknesses at different sections belong to formations (silt, sand) in foundation that are suitable for piping, granulometry and frequency of the material in foundation, and whether the fine material inside granulometry of the material in foundation will be able to erode or not. Kilometer H (m) L (m) 0+100 3.80 40 0+200 6.55 60 0+500 16.50 120 0+700 18.00 160 3+100 7.95 80 3+200 6.60 65 3+500 5.00 55 Table 4
Sand filters have been constituted at 2.00 m of width at 2 vertical / 1 horizontal slope in both upstream and downstream inside the fill with the purpose of creating a dryer zone in downstream and keeping the phreatic line under control by reducing it. Sand filter has been thought to be a significant protection measure against the risks of; piping that may appear throughout the conduit, internal erosion in transverse cracks that may arise throughout the dam axis or potential backward erosion. The continuity of this filter has been ensured throughout the thalweg and the leekage water will be collected in the drainage wells that were designed at 150 meters of intervals at the downstream toe. In this way, drainage will be ensured. Filter sand material has not been designed on the sections above the minimum water level in upstream shells since the height of the dam is not so high for the rapid drawdown condition. 4 Conclusion and Recommendations Geotechnical works have been conducted in the project to design the Çıkrıkçı Dam in order to reveal whether the problems such as bearing capacity in weak formations, settlement, consolidation, liquefaction and piping are existing or not. The following matters have been determined as a result of the calculations and analyzes that were conducted based on these works: a)thicknesses of liquefiable formations, b)depth of the foundation at which the dam will be designed (bearing capacity, settlements and consolidation will be evaluated together) c)precautions that will be taken in the foundation and body of the dam against permeability and internal erosion. River bed material having the risk of liquefaction will be completely removed under upstream slope and underneath the maximum section, they have been placed on clayey-silty-sandy formations until the depth (SPT- N > 20) that is the condition of soil to be the foundation of the dam. Since the coarse gravel-sand formations that were cut in drillings are permeable to high-permeable, grout curtain has been designed throughout the entire axis in order to prevent the potential water seepages and piping in the direction of upstream and downstream. Slurry-trench stabilization has been implemented on the top in critical piping regions. Furthermore, it has been thought that the clay blanket that will be implemented in the upstream of dam body will be an effective precaution against backward erosion and contact erosion at the higher elevations of the abutment. References 1. Engineering For Embankment Dams, p.189, Balkema Publishers 2. Training Aids for Dam Safety, Evaluation of Embankment Dam Stability and Deformation, p. III-9, Asok K. Chug,USBR. 3. Water Structures, volume-1, Dams And Small Dams, 3 rd Edition, p. 93, Turgut Sungur, DSI Publication. 4. GeotechnicalEngineering of Dams, 2nd Edition, RobinFell, 2014, CRC Press, p. 999 5. Bulletin 164, Volume-1: Internal Erosion Processes and Engineering Assessment ;ICOLD, b.3.2.3, p.26 6. Tecnical Manual: Conduits Through EmbankmentsDams, FEMA, 2005, p. 118. 7. Tekirdağ-Çıkrıkçı Dam Final Project Geotechnical Report, Jemas, 2015. Writers Esen Yalım KARADUMAN graduated from İstanbul Technical University and has been awarded with the title of Civil Engineer. He took part in the stages of final project and implementation project of several projects in Turkey including small-scale and large-scale dam, regulator and irrigation projects. Currently, he works at Bar-Su company as the project manager. Mehmet Harun ASKEROĞLU has been awarded with the title of bachelor's degree and master's degree from Gazi University. He took part in the stages of final project and implementation project of several projects in Turkey including small-scale and large-scale dam, pond and regulator projects for 15 years at Bar-Su company and he works at the same company as the project manager at the moment.