Pile foundations are constructed when it is not possible to build the structure on shallow foundations. Depending on the type of structure and other reasons, the selection of pile foundations is carried out as described in the article.
Let's focus on the main topics in this article.
Pile foundations – overview
Pile foundation design
Pie construction
Stack Test
Let’s start by understanding…
What is a pile foundation?
It is a type of foundation that is buried deep in the ground and usually uses circular profiles in its construction.
Shallow foundations rest on the ground and transfer vertical loads directly to the ground. The soil capacity is given as the allowable bearing capacity. If the applied pressure is less than the allowable bearing pressure, the geotechnical design is OK.
However, other methods and other parameters are used for pile foundations.
The design takes into account the surface friction of the soil (positive and negative), the surface friction of the weathered rock, the surface friction of the rock and the ultimate support force of the rock.
Why when piles need to support the structure
- When vertical loads acting on the foundation cannot be absorbed by surface foundations due to their low load capacity.
- If there are weak soil layers, such as peat, in the soil
- To absorb the tensile forces acting on the foundation. To absorb tensile forces, piles can be anchored into the rock.
- To absorb lateral loads (pressure) acting on the foundation. An inclined pile is erected that can absorb compressive and tensile forces.
- When there are very high vertical loads, especially in tall buildings, the bearing capacity of the soil is not sufficient to support such loads. We need bets.
Factors affecting the design and construction of pile foundations
- Superstructure loads
- Soil texture. Skin friction varies depending on the nature of the soil. For soil layers such as peat, negative surface friction must be taken into account in the geotechnical design of the pile.
- Rock State. The RQD and CR values determined by borehole investigations have a great influence on the load capacity of the pile.
- Construction costs are also an important factor in selecting piles as a support system.
- The accessibility of the website must be checked.
- Distances to limits must be checked
- Limitation of vibrations and sound levels must be checked. Excessive vibration can cause damage to adjacent buildings.
Types of pile foundations
This categorization was made based on the type of material used in the construction of the pile and the type of construction.
- Bored piles/in situ concrete piles
- Driven piles/precast piles
- Micropiles
- Sheet pile walls
- wooden posts
- piles of screws
Bored piles or in situ concrete piles
The most common and widely used type of battery. Board piles are found in most structures built on pile foundations.
The post is embedded in the rock. The depth of incorporation into the rock varies depending on the type and amount of filler.
Furthermore, the number of piles required to support a column depends on the bearing capacity of the pile and the applied load.
First, we determine the geotechnical and structural load capacity of the pile. The minimum value of these values is then considered as the load capacity of the pile.
As the applied load is known, the number of piles can be calculated.
Depending on the applied loads, bored piles are constructed as individual piles or group piles. Cluster piles are generally required to support shear cores, shear walls, uplift cores, etc.
Driven piles/prefabricated piles
These are prefabricated piles.
They are erected when the applied load is relatively low compared to bored piles.
Furthermore, prefabricated piles are not driven into the rock, but embedded or inserted into a hard layer of soil. There should be a dense layer of soil to support the pile and serve as a final storage location.
These piles are mainly friction dominant piles, although an end bearing is also present.
Driving can be done manually, throwing dough onto the pile or using a vibrating pile driving machine.
The posts are available in various sizes starting from 400 mm. Furthermore, smaller sizes can also be produced depending on the type of construction.
Furthermore, pile foundations of this type are often used for low-rise buildings when they cannot be constructed with shallow foundations.
Micropiles
Micropiles have become very popular in the construction of low-rise buildings.
If the condition of the soil is poor and the bearing capacity is insufficient to withstand the loads of the superstructure, a deep foundation must be constructed.
With this in mind, when considering the available options, we must choose the type of foundation between bored piles, precast piles and micropiles.
Of these, bored piles are generally more expensive than the other two types.
Depending on the type and nature of superstructure loads, the type of pile is selected.
Furthermore, when constructing this type of foundation, it is advisable to seek the recommendation of a geotechnical engineer.
The design must be based on the parameters specified in the soil test report and these must be verified through the necessary tests after construction.
A micropile is a steel casing filled with concrete. If necessary and as the diameter of the micropile increases, a reinforcing cage can also be inserted into the pile to improve its load capacity.
Micropiles are used in the construction of piers and in the construction of bridge piers. The lateral loads acting on the pillar can be transferred to the ground through inclined micropiles.
In the construction of piers, three or six hexagonal-shaped piles are used to support vertical loads.
The main risk of this type of construction is steel corrosion. If the battery is exposed or does not meet corrosion requirements, it may fail.
On the other hand, there is also less risk because the pile is underground and there is less chance of all the components responsible for corrosion getting into it.
If the structure is built in a coastal area, special attention must be paid to protecting the steel cladding.
Micropiles are made from 150, 200, 300 mm steel tubes, etc.
Sheet pile walls
Sheet piling can also be considered a type of pile foundation, although unlike other types of piles, it is generally not used to support structures directly.
For example, sheet piles are used to support the soil around the structure while also acting as a permanent structure. Whether to remove or consider permanent work depends on the type of construction and the condition of the soil.
Additionally, sheet piles are often used in construction to protect the earth during excavations. For deep basement structures, properly anchored sheet piling can be used, as noted above.
Furthermore, it is also useful in the construction of cofferdams.
There are different types of sheet piles depending on the profile and arrangement of the connection. Furthermore, we can select the appropriate sheet piling wall based on the required section modulus according to the project requirements.
The article, Sheet Pile Retaining Wall Discuss the stability design of sheet pile retaining walls.
wooden posts
Better technology was used not only in current buildings but also in old ones.
They knew that if the ground was weak, stakes would have to be erected. Therefore, they used sustainable material for this.
Even today it is possible to see wood being stacked during construction or expansion work.
The buildings and bridges in particular were constructed on wooden piles.
Wooden stakes are durable, economical and sustainable.
Special wood with good durability properties is used.
Please bear the loads of surface friction and end bearings.
Wooden piles are used in construction in very weak areas, where heavy machinery cannot reach.
piles of screws
The post resembles a screw, as shown in the following image.
The type of screw depends on the design.
There are also different types of screw piles.
Screw piles can be used to build connections or other structures, such as in bridge construction.
Pile foundation design
Since piles as foundation type According to the recommendation of the geotechnical investigation report, the number of piles is determined.
So we need the stacking capacity.
To evaluate the bearing capacity of the pile foundation, there are two components in the pile foundation.
We take the smallest one from the bottom.
- Geotechnical project
- Structured project
Geotechnical project of piles
Assessment of the geotechnical performance of the pile is based on the soil and rock conditions when it is anchored into the rock.
The geotechnical capacity of the pile can be represented by the following equation
Qu = Qp + Qs
Where
Qu – Maximum geotechnical capacity of the pile
Qp – Maximum ultimate strength of the pile
Qs – Maximum friction of the pile skin
The allowable capacity (Qall) can be calculated as follows:
Qall = Qu / FoS
FoS – safety factor; varies between 2.5 and 4
In addition, there are several methods for calculating the allowable load capacity of the pile. The method of applying the safety factor may vary from country to country depending on local standards.
Sometimes a separate safety factor is applied for both final storage and skin friction, and the single safety factor is also used.
It is observed that a safety factor as low as 2.0 is also used for skin friction. It is strongly recommended that local standards be taken into consideration when designing.
There are essentially five components associated with the geotechnical capacity of a pile.
- Skin friction from the ground (positive skin friction and negative skin friction)
- Weathered rock surface friction
- Rock surface friction
- Final disposal of the rock
- End ground support
For prefabricated piles, if the pile ends in the ground (hard layer), final in-ground storage is used. If the pile is anchored in rock (bored piles cast in place), the final support in the rock is used to calculate the load capacity of the pile.
The above five parameters are provided by the geotechnical recommendation based on the borehole survey data.
If we know the soil parameters, we can calculate the surface friction values according to the equations.
The following methods are available to calculate the surface friction of soils.
Skin friction in the sand
- Based on overlap and angle of friction between soil and pile
- Correlation with standard penetration testing (SPT)
- Correlation with Cone Penetration Test (CPT)
Rubbing the skin in clay
- λ method
- α method
- β method
- Correlation with CPT
The ultimate bearing capacity of soil can also be calculated using several proposed methods. The following methods are commonly used by planners.
Bottom repository
- Meyerhof process (sand/clay)
- Vasic method (sand/clay)
- Coyle and Castello method (sand)
- Correlation with SPT and CPT
Skin friction caused by rock
Rock skin is determined based on the nature and type of rock.
Generally, the maximum surface friction of fresh rock and weathered rock is specified in the geotechnical investigation report.
We need to apply the safety factor to calculate the allowable capacity. If the allowable capacity is specified, we can use it directly.
Point storage of rock (final storage)
Assessment is based on test results. To determine the strength of the rock, the uniaxial compressive strength (UCS) test is generally performed.
The relationship between UCS and final storage is used to determine the final value.
RQD and CR values should also be checked when determining pile capacities and embedment lengths as they reflect the condition of the rock.
In summary, we obtain the required geotechnical parameters such as skin friction and ultimate bearing capacity from the geotechnical investigation report. What we need to do is apply the required safety factor and calculate the geotechnical capacity.
Pile structural design
The allowable stress of concrete in bored piles is considered as follows: 0.25 FCU in most standards. There are only small deviations.
- ACI 318: 0.25 fcu
- EC2: 0.26 fcu
- CP4: 0.25fcu
However, piles should be checked for buckling, especially if they are erected on weak ground. Therefore, a buckling analysis for pile foundations is performed.
Taking this fact into account, structural design or reinforcement design can be carried out.
There are two methods/steps to design a stack.
- Calculate the critical buckling load and check whether it is greater than the applied load.
- Perform more rigorous buckling analysis and design.
Below you will find a summary of the calculation steps. Before planning, you should do more research.
Step 01
Calculate the critical buckling load (Pcr).
Step 02
Determine the effective length (Lcr) based on Pcr, ground springs, rotation at the top of the pile (possibly with some rotational fixation), etc.
Step 03
As we know the applied loads, the effective length and the diameter of the pile, we can size the pile using a traditional method or using software.
The most important factors that need to be considered when designing pile foundations are summarized below.
- Evaluate the geotechnical and structural capacity of the pile and consider the lowest value as the pile capacity.
- Divide the pile capacity by the applied load (column load or applied load; serviceability limit state) to determine the number of piles.
- When you want to size a group of piles, the individual load must be calculated based on the load center and the geometric center of each pile. Loads must be distributed according to the position of the pile.
- If there is more than one pile, the minimum distance between the piles must be 2.5 times the diameter of the pile.
- Increasing the distance between piles does not allow using the truss analogy for Pile Head Construction Therefore, a distance of 2.5 to 3 times the pile diameter is maintained between piles.
- On organic soils, attention should be paid to negative skin friction. Otherwise, the bearing capacity of the pile will be incorrectly estimated.
- In very soft soils, such as B. peat, post buckling should be checked at greater depth.
- Pay attention to RQD and CR The specified values must be indicated when deciding bolting lengths.
- In general, the allowable tolerance for design deviations according to most standards is 75 mm. This must be taken into account when designing the block. Special attention is required for individual cuttings. The moment caused by centrality must be supported by the floor beams. Therefore, this must be taken into account when designing floor beams.
Construction of pile foundations
Let's discuss the basic steps to follow when building piles. The following procedure is explained in relation to cast-in-place concrete piles.
The following tolerances are allowable deviations in construction according to different standards.
| code | Allowed tolerance |
| ACI-336 | 4% of the diameter or 75 mm, whichever is smaller |
| EN 1536 standard | 100mm; for pile diameter (D) ≤ 1000 mm
0.1D to 1000 150mm D>1500 Design for a tilt angle of less than 1 in 15 limited to 20 mm/m Design with a tilt angle between 1 in 4 and 1 in 15 limited to 40 mm/m |
| KP4 – The big goal | 75mm |
| BS 8004 | No more than 1 in 75 vertically or 75 mm
For bored piles with an inclination angle of up to 1 in 4, a deviation of up to 1 in 25 is allowed. |
Steps in pile construction and important aspects to consider
- Implementation of staking
- Start by removing the topsoil down to the rock level. Always try to maintain the position of the post as indicated in the drawings, although there is generally an acceptable tolerance of 75mm.
- Start rock drilling and monitor rock drilling depth. It is important to ensure that drilling is carried out in fresh rock and not weathered rock.
- It is measured on site using samples, penetration rate, well log data and other pile depths if applicable.
- Since it is difficult to locate fresh rock, the first layer is dropped closer to a well. Then the other parameters can be evaluated. Based on this, we can proceed to the pile foundation.
- The quality of the rock is checked through visual observation.
- Additionally, testing methods such as point load testing can be used to check rock strength. Point load test results can be correlated to determine the ultimate load capacity of the pile. If the results are not satisfactory, rock core drilling should be carried out until intact rock is found. The article Testing techniques for construction materials For more information about testing, you can contact us.
- Once drilling into the rock is completed according to the embedment lengths, cleaning is carried out.
- The main purpose of cleaning is to remove mud, sand, etc. of bentonite. This is also known as rinsing.
- To ensure that the stack is clean enough, some parameters must be checked. The following figure shows the limits. These values change from specification to specification.
- As soon as the bentonite content in the excavation pit reaches the specified limit values, washing is stopped.
- The contractor's pipe is then inserted into the excavation pit.
- Then the concrete is slowly poured into the contractor. Once filled, the contractor is lifted slightly to allow the concrete to drain.
- This concrete will gradually rise along with all the mud and contaminants at the bottom of the pile. Then the hopper is filled with concrete again and the concrete is allowed to flow.
- This ensures that the contractor's end of the pipe is always in the fresh concrete. This means that fresh concrete can always mix with fresh concrete and the top layer of concrete can gradually move upwards.
- Furthermore, it is very important to control the concreting speed to avoid lifting the reinforcement cage. At a higher speed, the basket rises.
- Repeat this process until the concreting is completed.
Testing pile foundations
Unlike other foundations, we can't see what's going on underground.
Nothing visible…
This will help you determine if we built the stack correctly.
- Adequate reinforcement coverage
- No constriction
- No bulge
- No concrete mixes with bentonite
- No cavities (like honeycombs) in the concrete
- No mud at the bottom of the pile
- Etc…
Therefore, we need to test the stack to ensure it is built correctly.
It is the contractor's responsibility to carry out pile testing in consultation with the project consultant and an independent testing agency.
Stack Test Method
There are essentially four types of pile testing methods.
- Pile Integrity Test (Low Strain Integrity Test)
- Dynamic load test (high load dynamic test)
- Static Load Test
- Cross bore sound test
Stack Integrity Test
The easiest way to predict stack health.
This test can be used to predict bulges, constrictions, indentations, etc.
This is the best method to identify the broken file, but the stack capacity cannot be evaluated.
It provides an initial warning about whether the post is defective.
Pile integrity testing is used to identify piles to be tested using other methods such as dynamic pile testing and static pile load testing.
Furthermore, this testing method is cost-effective compared to other tests. Additionally, all piles are tested using this method.
Dynamic Load Test
The most commonly used method for determining pile bearing capacity in current construction.
Unlike a static load test, results are provided immediately. The load capacity can be determined directly after on-site testing. However, to obtain accurate answers after analysis with software such as CAPWAP, additional analyzes are performed.
We can determine the friction of the pile skin and the ultimate support developed for the test load.
First, the pile test is simulated using software and the hammer drop height is set so that no tensile stresses arise in excess of the allowable forces or those supported by the pile reinforcement.
This is called wave equation analysis (WEAP). With this method there is no need to apply the hammer load multiple times until we find the test load.
WEAP establishes the relationship between test load, compressive stress, and tensile stress development.
Therefore, testing can be done very easily.
Static Load Test
This is the most reliable and conventional method of pile testing. Since all measurements are done manually, we have an idea of what happens as the load increases.
We increase the load on the pile to the test load specified in the pile design and then gradually reduce it.
The deformation of the pile is monitored and verified that it is within limits.
Cross bore sound test
This test checks the battery status. This allows you to check the condition of the work in question within the holes drilled in the stake.
The cables are placed in the stack. The test device is then placed on the stack and tested. Transmitter and receiver are used to check the battery status.
Based on wave speeds, the condition of the pile is predicted. For more information about the test method, see the Wikipedia article Cross-Bore Sound Measurement .
