Piling pipe is a commonly used foundation engineering material used in the foundation construction of large structures such as buildings, bridges, and docks. Its main function is to transfer the load of the superstructure to the deep hard soil or rock layer through the pile foundation to ensure the stability and safety of the structure. This article will explore in depth how to improve the cost-effectiveness of foundation engineering through reasonable design and selection of piling pipes.
In traditional pile foundation design, a "one-size-fits-all" approach is often adopted, that is, the same type of pile pipe and allowable pile load are used in the same project. Although this approach is simple, it often leads to inefficient design. For example, for projects with multiple structural loads, using uniform pile segments and allowable pile loads may lead to the following problems:
Excessive use of piles with low allowable pile loads: In order to support large structural loads, a large number of piles with low allowable pile loads may need to be installed, which will significantly increase the foundation cost.
Waste of piles with high allowable pile loads: For small structural loads, using piles with high allowable pile loads will result in a waste of resources.
To solve these problems, appropriate pile pipes and allowable pile loads should be selected according to the specific support requirements of the structure, thereby improving the cost-effectiveness of the design.
Using multiple allowable pile loads, types, and segments on a single project is an important means of improving cost effectiveness, depending on the structural support requirements. Specifically, the following points should be considered:
Higher allowable pile loads: Generally speaking, higher allowable loads can result in more cost-effective foundations. This is because longer pile lengths are required when penetrating weaker soils to reach stronger soil layers. The return on "investment" per pile is greater if the allowable load is higher. While the cost of installing piles increases linearly with depth, soil strength/pile resistance generally increases at a higher rate, so pile support costs decrease with increasing depth and corresponding higher allowable loads.
Smaller pile cap costs: Higher allowable loads result in smaller pile caps, which reduces pile cap costs. The increased pile cap thickness that may be required is offset by the reduced pile cap footprint. Additional savings can be achieved if pile cap excavation costs are impacted by support or drainage requirements, disposal of contaminants, etc.
Reduced construction control costs: Fewer, high allowable load piles reduce construction control costs, such as inspection time, dynamic monitoring, etc.
Choosing the right type of piling pipe is the key to cost-effectiveness. Common types of piling pipes include spiral welded steel pipes (SSAW), straight seam submerged arc welded steel pipes (LSAW), and seamless steel pipes (SMLS).
Spiral welded steel pipes (SSAW): SSAW steel pipes are manufactured through a spiral welding process and have good corrosion resistance and pressure resistance, making them suitable for piling projects in deepwater and marine environments. Due to its simple manufacturing process, the cost is relatively low, but its strength and bearing capacity may not be as good as LSAW and SMLS steel pipes.
Straight seam submerged arc welded steel pipes (LSAW): LSAW steel pipes are manufactured through a straight seam submerged arc welding process and have high strength, high bearing capacity, and good dimensional accuracy. It is suitable for high-strength and high-load piling projects, especially in foundation projects that require high bearing capacity and stability. Due to its complex manufacturing process, the cost is relatively high.
Seamless steel pipes (SMLS): Seamless steel pipes are manufactured through hot rolling or cold drawing processes and have excellent strength, toughness, and pressure resistance. It is suitable for extreme environments and high-demand piling projects, such as deep sea, earthquake zones, etc. Due to its complex manufacturing process and high cost, but its superior performance makes it the first choice for high-end projects.
Soil/Pile Setting refers to the phenomenon of driving pile capacity to increase over time. Setting is primarily associated with an increase in vertical bearing capacity and contributes significantly to long-term capacity. Cost-effectiveness and construction schedules can be improved by incorporating setting into the design and installation. Achieving higher pile loads can be achieved by:
Using smaller pile segments
Building the pile foundation shallower
Using smaller installation equipment
Pile design must consider both allowable geotechnical and structural loads. One or the other will dominate the design. If the pile is installed with an allowable geotechnical load that is significantly higher than the allowable structural load, available soil resistance is wasted, which may manifest as unnecessary pile length. If the allowable structural load installed is greater than the allowable geotechnical load, available additional structural support is wasted, which may manifest as unnecessary pile size or material strength. Relatively balanced allowable geotechnical and structural loads can lead to a more economical design.
Choosing the appropriate pile type based on the required allowable load and subsurface conditions can help improve cost-effectiveness. For example:
Tapered pile: suitable for granular soils.
Timber piles, steel single or multiple tube piles: suitable for relatively low allowable loads.
H-type piles: very suitable for conditions where end loads are dominant.
Displacement piles: very suitable for conditions where axial forces are dominant and/or where important settings are presented.
The material used to construct the pile may affect cost-effectiveness. Steel is more expensive than concrete, which is more expensive than wood. From a material cost perspective, all other things being equal, a timber pile is less expensive than a prestressed concrete pile, which is less expensive than a concrete-filled pipe pile, which is less expensive than an all-steel pile.
The material stresses created by the design loads of the pile may affect cost-effectiveness. For example, higher material stresses may be permitted with increased field testing, thereby reducing the required cross-sectional area for a given allowable load.
The drive standard used to install the pile may affect cost. For example, a code, agency, or designer may require different dynamic formulas or wave equation analyses with different safety factors. In addition, the various standards may have different inherent biases, accuracy, conservatism, etc., which may result in different installation lengths for a given required capacity.
Field testing related to pile design includes underground exploration, driven probes/indicator piles, dynamic load testing, rapid load testing, and static load testing. All of these testing options contribute to cost-effectiveness because they allow for lower safety factors to be used with respect to geotechnical and structural allowable loads.
To assess the cost-effectiveness of various foundation options, the concept of support costs can be used. Support costs are defined as the cost of the foundation element divided by its allowable load (expressed in dollars per allowable ton). For example, pile support costs are the cost of installing the pile divided by its allowable load. Other examples include pile cap support costs (the cost of the pile cap divided by the allowable load of the pile cap), and construction control support costs (the cost of the construction control method divided by the total allowable support tons to which it applies).
For pile-supported structural slab or mat foundations, higher pile load allowances can increase the pile spacing, resulting in increased thickness of the structural slab or mat, more reinforcement, and higher cost. In these cases, using support costs, it is possible to work closely with the structural slab or mat designer to determine the combination of allowable pile loads and the relevant structural slab or mat thickness to achieve the lowest overall cost.
The cost-effectiveness of foundation projects can be significantly improved through the proper design and selection of pile pipes. This includes selecting the appropriate pile type and material, properly designing the pile loads and stresses, and optimizing the drive criteria through field testing. In conclusion, a well-designed and implemented pile foundation solution can not only ensure the safety and stability of the structure, but also save costs to a great extent and improve the overall economic benefits of the project.