What is pile foundation? When and Where pile foundation is Used?
Pile foundations are long cylinders that extends deep into the ground to provide stable footing for structures when shallow foundations are inadequate to resist overturning forces, settlement, or uplift. Pile foundations or simply piles are typically made of steel and reinforced concrete nowadays.
The following are some of the most common scenarios requiring the application of pile foundations:
When the underlying soil is weak
Other types of foundation are not feasible due to cost or other factors
When water table is close to the ground surface and deep excavation is not practical
Site has settlement issues due to liquefaction
The presence of nearby deep-water channels that can cause massive seepage
There are possibilities of scouring or erosion
Mass foundations and deep excavation are not possible due to space restrictions such as the presence of nearby structures
When it is necessary to counter uplift forces due to water table rise or other causes
When counter-measures to overturning and lateral forces are necessary such as in areas with high frequency of seismic activities.
In cases of heavy and non-uniform loads which might cause differential settlement
When the structure is subjected to high level of vibrations, hammer, or other impacts
Classifications of Pile Foundation
Pile foundations are classified according to function, material, and installation process.
I. According to function or use:
Wall Piles. In this application, many individual piles are interconnected to form a wall that counters lateral pressure and minimal vertical forces. Wall piles or sheet piles are used as retainers for loose soil and sometimes water in many temporary and permanent applications such as construction of cofferdams, water ways, shore protection, trenches, and more.
Load-bearing piles. These pile types are used to transfer vertical loads from the superstructure through stratas with poor bearing capacity to a strong layer of soil or rock. Load-bearing piles has two types according to the manner they transmit load:
1. End-bearing piles – the bottom end of the pile is rested or anchored at a strong layer of soil or rock. It acts exactly as a column.
2. Friction piles – it develops stability from the friction created between the entire surface of the pile against the surrounding soil. Friction pile is ideal for grounds such as stiff clay.
Ground improvement piles. Piled at designed intervals, they increase the density and bearing capacity of the soil by compressing or compacting while also acting a a group of friction piles.
II. According to material:
Reinforced concrete piles. Also called concrete piles, they are produced either by pre-casting or casting-in-place. Pre-cast piles are installed by driving while cast-in-situ pile is formed by boring a hole followed by placing fresh concrete. Concrete piles are used as foundation elements that supports almost all types of structures especially in areas where corrosion is certain or highly anticipated. Solid concrete piles are commonly circular and rectangular in cross section. Hollow concrete piles or spun piles has shape like a doughnut with thinner walls.
Steel piles. Steel pipes can be I-shaped steel or hollow pipe and are installed by driving using impact or vibration hammers. Steel pipe piles, which are most common, are easy to drive and can be used as permanent piles in smaller diameters and as temporary caisson or casement for large diameters. Steel piles are favored as end-bearing piles because of their ability to be driven in deep strata due to small cross-sectional area, minimizing soil resistance. Another factor is their weldability to provide longer extensions when necessary. Steel pipes can be driven in open or closed-ends.
Timber piles. Though designed to serve minimal loads and shorter lifespan, timber piles are beneficial because of their availability in remote areas and lower cost.
Composite piles. This is an application that combines different materials to come up with the most efficient and economical pile that adapts to existing environmental conditions. In this system, superiorities and inferiorities of different materials are considered to strategically utilize them in maximum advantage. Below is an example:
1. A composite pile that consists H-section steel or steel pipe at the bottom and concrete pile at the top. This happens in situations where the required pile length for proper anchorage is longer than the longest possible cast-in-situ pile. As a solution, a steel pile can be driven first to the desired strata then proceed with a cast-in-situ bored pile.
III. According to Process of Installation
Driven Pile. Driven piles are also called ‘displacement’ piles because the installation process displaces an amount of soil equivalent to the volume of the driven pile. Driven piles gain geotechnical capacity efficiently by displacing the soil around the shaft and compacting the soils at the toe during installation. Pile driving is generally done by impact and vibratory hammers but in difficult cases, it is done with the aid of some techniques as follows:
1. Water jetting – it involves installation of a water jet device at the pile toe that delivers continuous water to reduce friction throughout the pile driving operation
2. Pre-augering – it employs a continuous flight auger that penetrates the ground in advance to facilitate the entry of the driven pile.
3. Pile driving templates – also called mandrels, they are used as holder and guide for piles to maintain vertically and right axial positioning in difficult pile driving situations.
Bored Pile. These pile types are installed by boring a vertical hole in the ground then filling it with fresh concrete. By way of drilling, soil is taken out to provide space for the pile to be installed which is also labeled as the ‘replacement’ pile. Bored piles can be plain or reinforced concrete. Bored pile installation may include driving a temporary steel pipe as casement to avoid erosion and plugging of the drilled hole. This remains in place in the upper portion of the hole until concrete casting is done. Bored piles are commonly used in foundation systems that carries heavy structural loads. They are also favored in unstable and difficult ground conditions like cohesive and heaving soil, and at sites where vertical clearance is limited and excessive vibration is prohibited.
Screw Pile. Screw piles are steel types with a helix near the toe as a self-penetrating mechanism. A screw pile may be equipped with more helices depending on the soil condition. Generally, the more helices provided, the higher pullout resistance is expected. This pile type is used extensively in railways, roadworks, and many other industries where rapid installation is required and anchorage is not necessarily vey deep.
Criteria for selecting the appropriate type of pile foundation
Pile type selection is a complicated procedure that involves formulation, site validation, and optimization of design. It requires meticulous study and close collaboration of geotechnical and structural engineering expertise. There are many influencing factors that must be considered in this process, they are as follows:
2. Available Equipment
3. Surrounding or environment (i.e. non-residential, urban, or congested city)
1. Soil Properties
2. Type and Magnitude of Super-imposed Loads
3. Type of exposure, (i.e. offshore, onshore, hydraulic structure)
4. Depth of Load-bearing Soil
5. Depth of water level and intensity of ground water flow
While the general aspects can be tackled in a brief client-provider discussion, the technical aspects requires time and analytical & technical examination to get it consolidated. This crucial part demands extensive investigation work that involves borehole construction and exploration, sample collection, series of laboratory analyses & testing, mock-up installation, and multiple calculations.
ESC Group: Piling Solutions Supplier
Foundation work is a highly specialized trade. It is a scope of work which has to be done right the first time because human lives and enormous investment is at stake. For this reason, it is highly recommended to seek help from experts in this field.
ESC is a premium provider and installer of imported and domestically manufactured steel pipe piles, sheet piles, combi-walls, structural steel, and other piling solutions. Please don’t hesitate to contact us at www.escsteel.com for your specific enquiries or concern.