Improvement of Seismic Safety of Chardara Dam – Kazakhistan

Project Name: Improvement of Seismic Safety of Chardara Dam

Country: KAZAKHSTAN

Finance Source: Government

Tags: Kazakhstan, Local, HEPP, Assesment

Narrative Description of Project:

Seismic Hazard Analyses of Chardara Dam and Evaluation of Seismic Safety Improvement Measures:

The potential for liquefaction to occur at Chardara was recognised in the Feasibility stage of the project and extensive investigations and analysis was carried out. It was indicated that if the potential existed there would be a need to carry out extensive works, with significant cost implications to reduce the risk of failure.

However, it was concluded in the Feasibility Study that further testing and assessment of the seismic conditions would be required to assess the potential.

Within the scope of this project, the necessary geotechnical investigations have been performed and a scrutinized seismic safety evaluation based on these updated in-situ and laboratory data have been performed.

Description of Actual Services provided by TEMELSU:

The Seismic Safety of the Chardara Dam has been performed by using two different methods of analyses:

1. Dynamic Response Analyses for estimating the liquefaction potential of the dam embankment during strong seismic excitations by using three actual time history records modified for the site and seismicity conditions, and,
2. Pseudo-static Slope Stability Analyses by utilizing “Limit Equilibrium Analysis” for estimating the safety factors of the embankment for different load cases that the dam may experience during the service life.

The methodology followed and the results obtained in that course to estimate the vulnerability of the dam for the extreme loading of seismic events at different hazard levels presented below.

Methodology of the Seismic Risk Analysis

Peak ground accelerations corresponding to Operating Base Earthquake (OBE) and Maximum Design Earthquake (MDE) levels were defined in ICOLD Bulletin No: 72 as:

• OBE has a significant probability of occurrence during the service life of the dam facilities. The probability of not being exceedence within the service life of the dam (usually taken as 100 years) is greater than 50 %. Dam safety at this hazard level requires that the dam remains operational after the earthquake.

• MDE is the maximum design earthquake hazard level that probability of not being exceedence is greater than 50 % for a very long return period. The return period of MDE level is usually taken in between 5000to 10000 years depending on the seismicity of the region.
Extensive and possibly irreparable damage to the dam and its outlet works is permissible during the MDE, provided that this does not lead to hazard to life or an uncontrolled release of water from the reservoir.

The basic steps followed for the evaluation of the seismic safety of the dam are:

• Determination of the site specific seismic design parameters (i.e. Peak Ground Acceleration (PGA) for OBE and MDE earthquake hazard levels.
• Adopting design values of material properties based on in-situ and laboratory tests. When not available, information is extracted from the literature given for similar fill dams or formations,
• Slope stability analyses for the load cases summarized below by using “Pseudo-static Limit Equilibrium” formulation,
• Assessment of excess pore pressure build-up (for OBE and MDE earthquake hazard levels),
• Dynamic Liquefaction analyses based on pore pressure – stress coupled Non-linear Time History analyses for OBE and MDE earthquake hazard levels.
• Calculation of permanent displacements of potential sliding masses along the dam
• slopes for OBE and MDE hazard levels,
• Estimation of freeboard reduction during the safety evaluations for earthquake.
• Evaluation of post earthquake safety factors.
• Developing Altermative retrofitting and strenghtening measures to improve the seismic safety of the dam anf for smooth operation:

Alternative – 1
A diaphragm wall extending to the bedrock at the upstream berm, and stone columns of the toe area of downstream shoulder are suggested. Analyses with the critical results have been repeated with the suggested improvements, and the results have indicated settlements of 0.25 m and 0.75m, smaller than half of the freeboard, which are considered as acceptable to prevent overtopping.

Alternative – 2
As a second alternative, buttresses composed of compacted sand and gravel are suggested at the upstream and downstream sides, with relief wells at the downstream toe and a parapet wall on the crest. The settlement of the crest has been reduced to 1.0 and 2.05 m for OBE and MDE load cases, with the use of buttresses and relief wells. These results show that the safety of the dam can be satisfied with the respective improvement system, including a parapet wall with h=2 m height on the crest, by preventing overtopping for the most severe earthquake loadings.