Pile slab foundations

June 5, 2024 Roberto Magalhães

Pile slab foundation is a combination of pile foundation and slab foundation. After the geotechnical study and layout of the structure, a section is created for the pile slabs used.

Article on Pile Foundations and Slab Foundations could be studied to know more about the design and construction of these types of foundations .

What is a raft?

A pile foundation is a foundation constructed by combining piles and a slab foundation. First the piles are built and then the foundation is placed combining all the piles.

The interaction between pile, soil and slab is the key factor in the design of pile slab foundations. Efficient use of this interaction leads to cost-effective design.

Otherwise, as explained in the last part of this article, economic construction would not be possible.

Therefore, the following interactions are integrated into the design.

Pile-soil interaction
Pile-pole interaction
Raft-bottom interaction
Stack-plate interaction

Factors affecting the behavior of pile slab foundations

The following factors are important and should be considered when designing pile slab foundations.

Number of batteries to be used
Diameter of piles
Battery length
Stack spacing ratio
Battery location
Stiffness of batteries
Raft stiffness
Raft dimensions
Soil type and soil stiffness
Load distribution along the pile plate

Why raft?

To transfer the loads from the superstructure to the underground, we usually construct a pile foundation or a slab foundation separately.

In beams on piles, the piles and the beam act as a single unit and support the loads.

Let's look at the advantages of a pile in a pile slab foundation.

The addition of the pile reduces settlements and settlement differentials.
Most of the time the pile offers its full load capacity and the balance is provided by the raft.
Improves lateral load resilience.
Typical pile slabs can be observed in tall buildings where very high axial loads act.
If the piles cannot be anchored into the rock due to lack of fresh rock, the pile slabs are considered load-bearing.

Construction Problems with Pile Slab Foundations

According to the technical document “Pile Foundations: Design and Applications” by HG Poulos, the following aspects must be taken into consideration during construction.

Maximum load capacity for vertical, lateral and momentary loading
Maximum revenue
Differential settlement
Raft moment and shear forces for raft static calculation
Pile loads and moments for static pile design

Attention should be focused on the key factors mentioned above and the ferry behavior they represent. Furthermore, necessary changes must be made during the design to control design issues.

Design philosophies for pile slab foundations

Randolph (1994) defined his design philosophies for pile slab foundations based specifically on the load distribution between the piles and the slab.

Normal approach

Piles are designed as a group to support most loads and at the same time carry some of the load through the slab foundation.

Scary Posts

Piles are designed to operate with a working load at which significant creep begins, typically between 70% and 80% of the maximum load capacity. Sufficient piles are included to reduce the net contact pressure between the base slab and the soil to a value below the pre-consolidation pressure of the soil.

Differential settlement control

The piles are strategically positioned to reduce differential settlement rather than significantly reducing the overall average settlement.

The above three approaches can be discussed in more detail based on the load distribution between the piles and the slab. Furthermore, the settlement load curve is used in different cases to select the most suitable type of foundation system.

To create an economical project, it is very important to know the above settlement load curve. Based on the two strategies, the conventional approach and the creeping pile method, the above diagram was developed.

Turn 0

It only represents raft behavior and shows overpopulation .

Turn 1

It is the traditional design approach in which most of the loads are carried by the piles. The behavior of the pile slab is determined by the behavior of the pile group.

Turn 2

Represents the behavior of the creeping pile. The piles are operated with a lower safety factor and the slab supports more load than in curve 1.

Turn 3

It is the strategy of using piles to reduce settlements. In this case, the full bearing capacity of the pile is used when the design load is applied.

Furthermore, the entire foundation system has an adequate safety factor and also meets the settlement criteria.

Therefore, the behavior of the pile row shown by curve 3 is more appropriate and also more economical compared to the other behaviors.

Influence of soil conditions on the behavior of pile slabs

As explained at the beginning, the various interactions between pile, soil and pile are the key factors that determine pile behavior. Therefore, the condition of the soil has a significant impact on the bearing capacity of the pile.

Therefore, soil profiles containing hard clay or dense sand are favorable conditions for pile foundations.

However, if the soil profile is subject to the following deformations, the load-bearing capacity may be reduced while the settlement increases at the same time.

Soft tone close to the surface
With loose sand near the surface
With soft, compressible layers at shallow depths
With a soil profile, settlement is expected due to consolidation
Soil profiles are subject to expansion movements due to external influences

Raft project

Designing a raft on piles is a very complex and time-consuming process. Due to the complexity of the analysis and other procedures, the design process developed by Poulos is as follows.

Preliminary design phase

The possibility of using a stake plate and the number of stakes, etc., are being examined.

Second floor

Determine pile location based on general pile requirements and characteristics.

Final phase of the detail project

The ideal number of stakes, location and configuration are determined. In this phase, variations in settlement, bending moments and shear forces of the slab, loads and moments of the piles, etc. are calculated.

In this article we will focus more on the preliminary design phase.

Furthermore, structural design methods can be categorized based on the analysis method used.

Simplified calculation methods

A simplified method is used to evaluate the raft. This is a type of preliminary design, similar to the preliminary design phase described above.

Approximate computer-based methods

The strip-on-feather approach, in which the slab is represented by a series of footings and the soil is modeled with chord elements. Furthermore, the piles are also modeled as spring elements, taking stiffness into account.

Additionally, the plate-on-spring method can also be used in this method. In this method, the slab is supported on plate elements while the piles are modeled with spring elements.

Both methods are approximate and a more complete analysis is needed to determine actual behavior.

More rigorous computer-aided methods

A simplified finite element model can be used to analyze the beam based on the final arrangement of the pile beam. Depending on the type of structure, the complexity of the pile may vary.

Simplified finite element method, three-dimensional finite element analysis, finite difference analysis, boundary element method, etc. These are the best known methods for analyzing pile slabs.

Let's discuss the preliminary design phase, which determines the basic layout and pile beam requirements.

Pre-project phase

The most important phase of pile slab design is the preliminary design phase, although it is also called the preliminary phase.

At this stage most things are completed and we move on to the second design phase and the detailed design phase, where we now have more information about the behavior of the pile driver.

Let's see how this is done.

The following method of utilizing the charges in an in-line pile may be generally observed.

Stack Row – Elastic
Make the most of stackability and elastically raft
Pile and Raft reach maximum capacity

In other words, it can be discussed as follows.

First, the batteries begin to bear loads. As the pile is set, earth pressure under the pile plate gradually develops. At this stage the pile plate is elastic.
Other settlements/loads cause the piles to reach their full bearing capacity. However, the raft has not reached its full carrying capacity, which is why we say that the raft is elastic.
A further increase in settlement causes both the raft and the pile to reach their maximum load capacity.

The following figure shows the behavior of the pile slab foundation described above.

The following steps are followed in the preliminary design phase of pile drivers

Estimate the Stiffness of a Pile Slab Foundation
Calculate the proportion of loads supported by the slab and piles
Develop the trilinear load settlement curve as shown in the figure above.

For further explanation of the load distribution calculation method, see the publication “Piled Raft Foundations: Design and Applications” by Harry G. Poulos.

Preliminary Design Procedure for Rafts

This article explains the method proposed by Randolph and the design principles proposed by Poulos.

There is slight variation in the equations used by Randolph and Poulos, as noted in the design procedure below. The in Red The color corresponds to the Randolph method.

The Kp value can be taken from the method described by Fleming et al. (1992) the proposed formula can be obtained as shown below.

Using the above equations we can calculate the settlement of the pile slab under load. Furthermore, a trilinear pressure-load curve can be developed.

As explained in the technical document “Simplified analysis of pile beams with irregular geometry” by Vrettoc C., there are limitations in using the above method for foundations of tall buildings. This method should only be adopted after careful examination of the background and limitations.

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