Earth Structures Stability Under Rapid Drawdown Conditions

S t r e s z c z e n i e Artykuł przedstawia analizę stateczności skarp niezmodernizowanego odcinka języka rozdzielczego Groszowice w warunkach zmian poziomu wód, szczególnie po gwałtownym ich obniżeniu. W oparciu o polowe i laboratoryjne badania gruntów przeprowadzono analizę stateczności poszczególnych elementów konstrukcji. Dla górnego języka rozdzielczego wyznaczono stopień wykorzystania wytrzymałości skarpy w oparciu o normę Eurocod 7. Obliczenia wykonano dla normalnego i wysokiego poziomu wód oraz w warunkach gwałtownego obniżenia. Analizę przeprowadzono pięcioma powszechnie stosowanymi metodami przy użyciu programu komputerowego GEO5. Uzyskane wyniki wykazały, że przy niskim i wysokim stanie wód budowla pozostała stateczna, co jednak nie odpowiada rzeczywistym jej zniszczeniom. Utratę stateczności wykazała dopiero analiza w warunkach gwałtownego obniżenia poziomu wody. Dodatkowo dla środkowej części języka rozdzielczego określono położenie krzywej filtracji metodą elementów skończonych MES. Wykazany obliczeniami wysięk wody na skarpie powyżej normalnego poziomu piętrzenia wyjaśnia wymywanie i sufozyjne wynoszenie cząstek gruntowych, jakie ma miejsce przez uszkodzone uszczelnienie skarp oraz drenaż. K e y w o r d s : GEO5 software; Internal erosion; Levee; Pore pressure dissipation; Rapid drawdown; Slope stability. 1/2017 A R C H I T E C T U R E C I V I L E N G I N E E R I N G E N V I R O N M E N T 87 A R C H I T E C T U R E C I V I L E N G I N E E R I N G E N V I R O N M E N T The Si les ian Univers i ty of Technology No. 1/2017 Z . Z i ę b a , M . M o l e n d a , K . W i t e k Unfortunately, the most common reasons for the threats to the stability of hydrotechnical earth structures are design and construction errors. These result from economic factors and limited geotechnical knowledge. It was noted that over 70% of damages and failures of hydrotechnical structures in Poland were caused by the lack of proper analysis of soil properties and groundwater conditions [2]. The statistics quoted hereinabove prove that water movement in the soil medium is an essential issue, especially in the aspect of the negative consequences that it may cause. Seepage forces that occur as a result of water flow have a significant influence on the stability of the solid particles. The main parameter that describes their influence on the behaviour of soil is critical hydraulic gradient. Exceeding this parameter leads to changes in the mechanical and physical properties of the soil medium. Depending on the groundwater conditions, the quicksand, hydraulic uplift, piping, suffosion or clogging may occur [3, 4]. A specific instance of filtration threat for hydrotechnical earth structures is a rapid drawdown. This phenomenon is particularly dangerous for upstream slope stability. The related problems pose a threat to earth dams during fast emptying of the reservoir as well as flood embankments during a rapid drawdown of flood waters. The discussed problem concerns mainly cohesive soils, with low permeability, where pore pressure does not disperse at the same rate with the external water level changes. As a result of the absence of surface water pressure and pore pressure dissipation in the structure body, the shear stress increase and the safety factor of the upstream slope decreases, which may lead to slope failures [5, 6, 7, 8]. This demonstrates that the slopes stability of hydrotechnical earth structures is closely linked to changes in external water level, especially in case of rapid drawdown. A special type of hydrotechnical earth structures that are vulnerable to this phenomenon are separation levees. In this case, water is present on both sides so both slopes are prone to loss of stability. The aim of this paper is to analyse the phenomena that affect the stability of the unmodernised part of the Groszowice separation levee under conditions of rapid changes of external water level. Site inspection revealed the significant damages of the structure. The study is an attempt to explain the reasons of such damage. 2. CHARACTERISTICS OF THE OBJECT The Groszowice barrage is located on the Odra River (km 144 + 600), in the Opolskie Voivodeship. It is the sixth object in the cascade of the canalised section of the Odra River. Its main functions include the production of renewable electric energy and impoundment of water to obtain a transit depth on an long section below the Kąty barrage [9]. The analysed separation levee, as an element of the barrage, protects the lower roadstead against silting and enables safe access to the locks of trailing sys88 A R C H I T E C T U R E C I V I L E N G I N E E R I N G E N V I R O N M E N T 1/2017 Figure 1. Localisation of the Groszowice barrage E A R T H S T R U C T U R E S S TA B I L I T Y U N D E R R A P I D D R AW D O W N C O N D I T I O N S tems. Both on the upstream and downstream side, it is located between the weir and the top and lower roadstead of the canal lock (Fig. 1). Originally, the separation levee was constructed from soil that contains a large addition of lime stone, with slopes secured by stone riprap. However, it was destroyed during the flood of 1997. In spite of subsequent repairs and modernisation, it was still exposed to damages. They were caused mainly by the negative consequences of infiltration in the body of the levee, as well as mechanical damage that occurred as a result of manoeuvring floating objects. Currently, only the lower levee has been modernised (Fig. 2) by means of anti-suffosion protection with nonwoven geotextile and a stone riprap reinforcement. Due to financial reasons, the upper separation levee, as well as central part of the structure was not modernised. The central part had been originally secured with 0.15 m thick concrete slabs laid directly on the ground (Fig. 3a) [9]. However, local inspection showed large expansion joints between the slabs, from which soil particles are scoured, creating cavities (Fig. 3b). Moreover, suffosion displacement of particles through drainage pipeline was observed (Fig. 3c). C I V I L E N G I N E E R I N G 1 /2017 A R C H I T E C T U R E C I V I L E N G I N E E R I N G E N V I R O N M E N T 89 Figure 4. Upper separation levee; Photo Michał Molenda c Figure 2. Modernised lower separation levee; Photo Michał Molenda Figure 3. Central part of the separation levee: a) concrete slab reinforcement, b) expansion joints between the slabs, c) suffosion displacement of soil particles through drainage; Photo Michał Molenda e