This article will give some background and brief insight on the history of piles, and go over the advantages and disadvantages of Concrete Piles, which can be designed using SkyCiv’s Foundation Design module.
Pile foundations, also known as just piles, are a type of foundation needed to support a structure. Unlike shallow or wide foundations such as Isolated Footings or Combined Footings, piles are driven deep into the ground and are usually much thinner in diameter or width than they are in length. Due to their sometimes incredible length/depth, piles usually carry a higher load capacity than the aforementioned shallow footings. One of the most common situations in which piles are used are when the top layers of soil of the site are very weak and cannot support the structure, usually characterized by wet, clay-like soil. In this case, piles bypass these weak conditions and attempt to transfer the building loads to a more stable layer of soil beneath, like bedrock. The other common use case would be when the resulting building loads are very large and/or cover a small footprint. Examples of this include high-rise structures or skyscrapers, large bridges, and water tanks.
Traditionally, timber or wood can be used to manufacture piles that are commonly used to support small structures or bridges in weak soils. Some larger structures still use wood to support structures, such as jetties, but timber piles bear some limitations. First, timber is prone to deterioration or decay over time, increasing the uncertainty in its lifespan. Another drawback is the limited available length that timber piles can be due to the physically length of the tree that is being cut down. Subsequently, it is not advisable to join two timber trunks together to try to combat this length issue.
With the majority of structures, concrete piles are used. The earliest form of the concrete pile is a cast-in-place pile. A further advancement in construction technology lead to the creation and adoption of precast piles and eventually, prestressed piles. All three concrete piles are viable options in today’s construction.
Steel H-piles became a common substitute for concrete piles. They can withstand heavier loads and may eliminate some installation issues as compared to concrete piles. The process of importing and pouring concrete may not be doable in some environments, so that is when the steel pile is used. However, concrete piles are more preferred to use on locations with corrosive soils than steel piles.
A composite pile may be a combination of two different materials, most commonly the combination of steel and concrete. This type of pile is usually used when the required length for bearing capacity exceeds the capacity of a simple concrete pile. Composite piles are rarely used as the installation is difficult and it is challenging to provide a proper joint between the materials.
Figure 1: Rendering of various pile types
Concrete piles are categorized into two types: cast-in-place piles and precast piles. Cast-in-place piles can be further identified as cased or uncased piles. Conversely, precast piles can either be normal reinforced concrete pile or a prestressed pile.
Generally, cast-in-place piles are more commonly used than precast piles. The use of this type of pile is more advantageous than precast piles due to ease of handling and the elimination of any storage requirements. When moving precast piles, there is a chance for damage upon handling, and they need to be stored on site. Uncased piles offer a more economical type of cast-in-place pile, while cased piles offer a more secure and accurate placement of concrete.
Cased cast-in-place piles use a cylindrical or tapered thin-walled steel tube that serves as the form or mold for the concrete, which lines the bored hole where the pile is housed. This type of pile is more desirable as it allows inspection of the pile before the pouring of concrete. The casing leads to a cleaner and more dependable pour, without irregularities. Cased cast-in-place piles are better in nearly every soil condition. Some common examples of cased cast-in-place piles include:
- Raymond piles
- Mac-Arthur piles
- Union metal monotube pile
- Swage pile
- Western button bottom pile
Figure 2: Breakdown of a Mac-Arthur pile
Uncased cast-in-place piles are more economical and practical than cased piles. However, caution and inspection is needed during installation, because of the direct contact with soil. It is recommended to use only on highly cohesive soils, since with un-cohesive soils, there is risk of the integrity of the bore hole, which could lead to soil or water breaking into it. After pouring, water could seep into the cracks of the concrete and degrade the reinforcement. The following are the different types of uncased cast-in-place piles:
- Simplex pile
- Frankie pile
- Vibro pile
- Pedestal pile
Figure 3: Stages in the formation of a Franki Pile
Precast piles are commonly used where the foundations need to extend above water or ground level. Precast piles are manufactured in a controlled environment with uniform or tapered cross-sections, and may be cast in circular, square, or octagonal shapes. The deep end of uniform cross-section piles are sharply tapered and secured with a cast steel shoe to protect the pile and help penetrate hard strata during driving. Precast piles with larger widths are usually manufactured with hollow cross-section to reduce its weight and increase the effectiveness of the driving of the pile.
Advantages of Precast piles:
- Can carry relatively higher working loads
- Well suitable for marine installations
- Can extend above ground or water
- Manufactured in controlled environment — more certainty on overall quality.
- Highly resistant to biological and chemical actions of subsoil
Disadvantages of Precast piles:
- Requires special equipment for handling and transporting
- Precast piles are heavy and require machinery to transport and move around the site
- Challenging to increase or cut-off excessive length
- Pile length may be limited due to storage or transportation
- High initial cost
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