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Article Excerpt
Introduction
A model is a representation of a real-world system in order to investigate and analyse that system. Models vary widely in their precision of representing the real world systems according to the level of details considered in these models. Different types of models include but are not limited to: abstract, conceptual, schematic, graphical, mathematical, iconic, pictorial, and real models based upon some sets of modeling and interpretive rules that are particular for each type. Models of various types are essential in solving real world problems. Simulation is a technique that depends mainly upon models. Therefore, if the simulation model is detailed enough to represent the entire aspects of a real world problem, simulation might be a good technique that tackles this problem.
Continuous Flight Auger (CFA) is a piling technique that is also known as Auger Cast-In-Place (ACIP) piles in US and other countries. Zayed, (2005), in developing a deterministic productivity model for CFA piles, stated that subsurface obstacles, lack of contractor experience, and site layout design greatly affect the CFA pile installation process. The factors that affect pile installation productivity can be summarized as follows:
1. Pile equipment characteristics;
2. Operator skills;
3. Soil type;
4. Site layout, pre-investigation, and conditions;
5. Contractor experience;
6. Management conditions;
7. Disposal of excavated spoil system;
8. Concrete pouring system; and
9. Rebar installation method (Zayed & Halpin, 2004a, 2004b; Zayed, 2005).
These factors have to be studied and analyzed in order to accurately predict productivity of CFA process; plan and schedule CFA projects; and study the bid of such projects.
The analysis of productivity and cost of the CFA piles does not receive sufficient attention from researchers (Zayed, 2005). All reviewed literature is only deals with design, such as Mure, Scott, Seward, Quayle, Clayton, and Rust, (2002). Also, Zayed, 2005) investigated CFA piles by developing deterministic models in order to predict CFA productivity. To develop more accurate productivity and cost models for CFA pile installation processes, uncertainty in the various CFA pile activities has to be considered, which are present in most CFA installation activities, such as drilling, pouring and rebar installation.
Research Objectives
The presented research in this paper aims at:
1. Designing a stochastic simulation model for the CFA process;
2. Experimenting with that model to assess the CFA pile's productivity and cost using simulation;
3. Analyzing the results to build productivity and cost assessment tools; and
4. Verifying the designed model.
Background
CFA piles are constructed by drilling a CFA auger into the ground and pumping concrete down through the auger while it is steadily withdrawn (Mure et al., 2002; Zayed, 2005). During withdrawal, the auger passes through an auger-cleaner to remove the spoil soil (Zayed, 2005). The CFA piles, also known as auger cast piles, have become increasingly popular because it is installed considerably quicker and cheaper than other alternative pile types (FHWA, 2006; Zayed, 2005).
Zayed, (2005) reported that the CFA piles are available in various sizes: 20.32, 25.4, 30.48, 35.56, 40.64, and 45.72 cm diameter; however, they might have larger depth (i.e., 30 m). This technique is appropriate for most ground conditions including soft rocks because it has no open hole. It is greatly convenient for non-cohesive sands, gravels or silts because other piles need long casings or slurry to prevent soil falling from sides (Zayed, 2005).
The CFA Construction Method
Das, (2004); DFI, (1994); DFI, (2003); Coduto, (2001); Fleming, Weltman, Randolph and Wilson, (1992); Peurifoy, Ledbetter, and Schexnayder, (2002); Weele, (1988); Zayed, (2005) summarized the installation stages for this kind of piles (Figure 1) as follows:
Stage 1: Auguring the Hole
The auger is used to excavate the hole with the proper size. When the CFA piles are formed, the hole in the ground does not need to be stable or stand open. The auger however, should be kept full of soil, so that the surrounding soil will be supported.
Stage 2: Completion of Auguring and Start Shaft Formation
After the auger has reached full depth, the temporary closure plate below the central stem is pushed away by the concrete or mortar (grout). Usually the auger is for that purpose lifted without rotating and the start of concrete pumping should do the rest.
Stage 3: Shaft Formation
During the withdrawal of the auger, the concrete or mortar (grout)...
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