

Decisionmaking analysis for excavation construction methods of building foundations
Department of Civil Engineering, College of Engineering, National Cheng Kung University




 

Excavation constructions of building foundations often cause accidents that bring about substantial fatalities or injuries of workers, competition delay and cost overrun, thus safety is a primary consideration in selecting an appropriate construction alternative among the foundation excavation methods. Although safety is the most important criteria, other important decision criteria such as cost and duration also need to be considered in the process of selecting an optimal excavation method. To avoid or reduce failure risk, the primary task is to accurately plan and determine the most appropriate deep excavation method for the associated site environment. However, the process for determining the most suitable alternative contains a great deal of complexity and uncertainty due to numerous decision factors or criteria, so that it is difficult to correctly make a right decision.
The fuzzy AHP (Analytic Hierarchy Process) model, initially developed by Buckley, has been used in alternative selections. Nevertheless, no fuzzy AHP application was found regarding the selection of foundation construction method. However, Buckley’s model requires tremendous computational time because of complicated fuzzy arithmetic. Moreover, In Buckley’s method, the element of the negative judgment is treated as an inverse and reversed order of the fuzzy number of the corresponding positive judgment. Thus, it requires careful checks to avoid errors arising from such tedious manipulations while constructing a reciprocal matrix. This paper presents a fuzzy AHP approach to deal with the problems. In the proposed method, each negative reciprocal element is characterized by its own representative fuzzy number. The proposed also approach employs triangular and trapezoidal fuzzy numbers and the αcut concept to deal with the imprecision inherent to the process of subjective judgment. The value ofα is between 0 and 1. α= 0 andα=1, signify the degree of uncertainty is greatest and least, respectively. The analysis steps of the approach including (1) construction of hierarchy, (2) evaluation of fuzzy pairwise comparison, (3) calculation of element weight, and (4) aggregation of weights.
To demonstrate its capability in effectively evaluating alternatives, a foundation construction project executed in Kaohsiung area, Taiwan, was applied. At the project planning stage, the project contractor intended to determine the most appropriate excavation construction alternative among four candidates, slurry wall, sheet pile, bored pile, and soldier pile. A decisionmaking group was formed which was made up of eight domain experts who were in charge of the project and had worked on numerous similar foundation projects in Kaohsiung for a minimum of ten years. The hierarchy of the decision problem was constructed based on the experts’ suggestions. As shown in Fig. 1, the top level and the lowest level of the hierarchy represented the overall objective and the candidates, respectively. The three main criteria, namely safety, duration, and cost were included at the second level. The main criteria were further broken down into subcriteria.
Fig 3. The hierarchy for selecting the most appropriate excavation method
Once the hierarchy was established, each expert performed a pairwise comparison to indicate his or her preference for each criterion. Based on the assessment result and by the use of the proposed approach, the weights for safety, duration, and cost yield (0427, 0.241, 0.293), (0.454, 0.274, 0.278), and (0.544, 0.211, 0.267) regarding α = 0, α = 0.5 and α = 1, respectively. The results indicate that safety and cost are the two most important main criteria for selecting a foundation construction method in this study, whereas project duration is the least one. Similarly, the overall weights of subcriteria and alternatives can be derived as shown in Table 1. The results in Table 6 signifying that under α = 0 are constructability (0.178), construction cost (0.136), and undergroundwater condition (0.133) are the three most important subcriteria. The overall alternative weights can be estimated as shown in Table 2. It can be found in the bottom row of Table 2 that the weights for slurry wall, sheet pile, bored pile, and soldier pile methods regarding α = 0, 0.5, and 1 are (0.39, 0.26, 0.19, 0.16), (0.42, 0.27, 0.20, 0.16) and (0.43, 0.28, 0.21, 0.16), respectively. The result suggests that slurry wall method is regarded as the most appropriate method. The result also reflects the fact that this operation is the most commonly used exaction method in Kaohsiung area.
To justify the effectiveness of the approach, Buckley’s model was used to analyze the same case problem. The weights of criteria and alternatives estimated by the two models are pretty similar; however, the proposed model is easier and faster than Buckley’s model. The proposed method provides a structured and systematic approach for identifying the preferred foundation construction techniques. It may be applied for different areas of construction management and solving a largescale decisionmaking problem. Accordingly, the result demonstrates the applicability of the proposed model that can assist project contractors to effectively evaluate foundation construction methods.
Table 1. Weights of the subcriteria under α = 0, 0.5, and 1 Table 2. Overall alternative weights estimated by the proposed model



  






