Stability Evaluation of Gravity Concrete Structures
Stability evaluation of concrete gravity structures such as dams and retaining walls may be required because of project modifications, changes in site conditions, enhanced knowledge of site data, or changes in the stability criteria.
Stability refers to resistance against sliding, floatation, limit on the eccentricity of the resultant applied loads, and limit on the bearing capacity of the foundation materials.
Since, changes to enhance the stability of an existing gravity concrete structure for example dams, retaining walls, floodwalls, and pumping stations is costly and expensive. Therefore, a systematic and phased assessment process must be employed to specify whether the structure needs stability improvement or not.
To prevent modifications, which may not be required, all sorts of resisting actions such as vertical friction, side friction, or three dimensional effects should be considered.
If the stability of an existing gravity concrete structure does not meet the specifications of a new construction, it might not be required to improve its stability specifically if the structure does not show any stability issues or its life span is not long.
Procedures for stability estimation and improvement of existing gravity concrete structures is discussed in the following sections.
Fig.1: Gravity Concrete Structure – Hoover Dam
Methods and Procedures for Stability Evaluation of Gravity Concrete Structures
Following are the procedures for stability evaluation of existing gravity concrete structures:
Phase I: Preliminary analysis and evaluation
Phase II: Study, investigation, and comprehensive analysis
Phase III: Advanced analytical studies and reliability analysis
The provided procedures are applied to evaluate the stability of the gravity concrete structure and specify whether it needs to take required measure to tackle stability issues.
It is recommended that, stability assessment producer shall be employed as a guidance and not replace engineering judgment.
The revision of stability evaluation is required when there are considerable modifications in loading conditions, serious damage as a result of accidents, deterioration, aging, and found structural deficiencies, stability criteria revision to make it safer.
Phase I: Preliminary Analysis and Evaluation of Gravity Concrete Structures
Preliminary analysis must be conducted depending on not only available data but also actual conditions of the structure under consideration.
Foundation data, geological data, and entire available information for the building, design plans, as built plans, damage reports, periodic inspection reports, previous modification plans of the building, instrumentation data, movement records, and all other related information should be collected and reviewed properly prior to performing the analysis.
It might be required to inspect and examine the structure to estimate its current conditions. The conduction of two dimensional preliminary analyses is the first step in the stability evaluation of an existing structure.
The influence of uplift pressures, internal friction angle, and cohesion on the safety factor needs to be estimated through parametric study.
Unless the first phase results is acceptable and the structure meet stability and performance requirements, different choices must be studied to reach needed safety and performance requirements.
Phase II: Study, Investigation and Comprehensive Analysis of Gravity Concrete Structures
Plan of works for a detailed stability assessment of gravity structures need to be established after the conclusion of preliminary analysis shows that the structure does not meet safety and performance objectives.
The size of investigation and testing program needed to specify foundation parameters, the analytical program required to determine loading conditions, the improvement technique to be explored, and the size of any extra parametric study have to be outlined thoroughly in the plan.
When required, a program for exploring, sampling, testing, and instrumentation should be set to specify the magnitude and acceptable variation limits for parameters that substantially affect the safety and performance of the structure as determined by parametric study.
The largest practical amount of shear strength data for the foundation and backfill material should be obtained through tests in the laboratory. Both material strength and strength data achieved from sampling and testing program are employed to carry out a complete stability analysis.
If parametric study performed in phase II demonstrated that the structure meets safety and performance conditions, then remedial measure is not to be considered.
Phase III: Advanced Analytical Studies and Reliability Analysis of Gravity Concrete Structures
This phase includes the following:
A) Advanced Analytical Study
If the safety of the structure is in doubt after performing preliminary and complete or though analysis, then it is necessary to undertake advanced analytical study.
In this investigation, two and three-dimension finite element method is employed to understand and clarify the interaction between foundation, backfill, and the structure. It also used to determine the capacity of the structural system to distribute loads to neighboring abutments and monoliths.
Merely in special circumstances, fracture mechanic analysis can be employed to assess cracking and the progressive development of uplift pressure along potential failure planes. This is because neither nonlinear analysis is simple to carry out nor fracture mechanic program are readily available to use.
B) Reliability Analysis
Reliability analysis is utilized as alternate toolsto estimate the stability of existing gravity concrete structures. In this analysis, engineers evaluate the potential for stability failure with the help of both known information about the structure and acceptable ranges of unknowns.
Even though, it is generally used for creating investment decisions about projects competing for major rehabilitation funds, the process may be employed to assess dangers related with structures that, when analyzed by ordinary deterministic procedures, show a stability-failure potential.
C) Vertical Shear
There is numerous gravity earth retaining concrete structures that do not reach the stability conditions but performed satisfactory for many years. It is reported that, vertical shear force is generated in rigid gravity walls constructed on rock due to the settlement of backfill material.
This vertical shear force contributes the stabilization of the gravity retaining wall built on the rock and need to be considered while the stability of existing structure is assessed. The vertical shear force can be computed by utilizing nonlinear finite programs for example SoilStruct as part of the phase III analysis.
Read More: Gravity Dams: Causes of Failure of a Gravity Dam