# Types of Raft Foundations – Advantages and features

Types of Raft Foundation are chosen based on a variety of criteria, including bearing capacity, applicable loads, site conditions, cost-effectiveness, etc. A raft foundation is a continuous slab resting on the soil and covering the entire area of the proposed structure. This is one of the most commonly used types of foundation in construction. Raft foundation types are classified according to their application.

## What is a raft foundation?

A raft foundation/mat foundation is a solid slab that is placed at a specific depth and spreads across the entire structure. Raft foundations have shear walls and columns to transfer loads from the structure to the ground. These foundations are typically used when the soil’s bearing capacity is low and it becomes challenging for individual footings to handle the loads. The raft foundation aids in transferring the entire load of the structure to a larger area.

Types of Raft foundations – Youtube video

Types of raft foundations – Related articles from vincivilworld

## Types of raft foundation – Principle

The raft foundation distributes the total loads from the structure over the entire area of the structure. When compared to other types of foundations used in civil construction, they can reduce soil stress. Raft foundations differ from other foundations due to this mechanism of stress distribution.

## Soil Stress Calculation

stress = total load coming on the structure + self-weight of raft/ Area of raft foundation

Consider a total load is 300 T and a foundation size

Size : 20 m x 10 m

Stress on the soil = 300/200 = 1.5 t/sqm

The same structure supported with 8 individual footing

Size : 2m x 2 m

Total area = 8 x 4 = 32 sqm

Stress on soil = 300/32 = 9.375 t/sqm

This shows that same load we are getting stresses of 1.5 T/sqm for raft and 9.375 T/sqm for individual foundations.

As the contact area of the raft is more the load is distributed over a larger area and hence stresses coming on the soil are very less.

## Factors influencing the types of raft foundation

Raft foundations are typically preferred over other foundations when one of the following situations arises.

• Individual footing design and pile foundation construction can be expensive when the soil’s bearing capacity is very low.
• When the soil’s bearing capacity is less and it is essential to minimise stresses that have been induced into the soil.
• The columns, shear walls, and so on are so close to each other that individual footings may overlap.
• Any other type of foundation may cover more than 50% of the total ground area beneath the structure.
• When a possibility of unequal settlement exists.
• Preferred for complex equipment foundations.
• Used when the proposed structure includes basements.

Raft foundations are appropriate for basement buildings where the foundation slabs will be subjected to direct live loads depending on the utility of the building. Raft foundations are a better choice because excavations can be finished with the aid of light excavators in areas with poor soil conditions and limited access to heavy excavation equipment.

## Types of Raft foundations

The types of raft foundations are chosen based on a variety of factors, including bearing capacity, applications, cost-effectiveness, and so forth. Raft foundations are broadly categorized as follows.

• Solid Slab Raft Foundation
• Slab Beam Raft Foundation
• Piled Raft foundation
• Cellular Raft Foundation
• Balancing or Floating raft foundation

## Types of raft foundations: Solid slab raft foundation

In a Solid slab raft foundation, the columns and walls are equally spaced, and the load distribution is also equal. Because they are designed as slabs of uniform thickness, these raft foundations are known as solid slab raft foundations. These foundations are reinforced with a bottom layer and a top layer.

Solid slab raft foundations are classified into four types.

• Flat raft mat foundation
• Wide toe raft
• Blanket raft foundation
• Slip plane rafts

### Flat raft mat foundation

Flat raft mats are used for small buildings with uniform column spacing and a foundation that covers the entire structure. These foundations have bottom and top reinforcements.

### Wide-toe raft

A wide-toe type of raft foundation is used when the structure needs to be economical. A full-size solid slab mat foundation may not be required to support the structure’s loads. In that case, a heavily reinforced toe is provided on both sides, as shown in the figure, to handle the loads.

### Blanket raft foundation

Blanket rafts are used when the surface has unequal settlements or nonuniform strata. In this type of situation, stone blankets will be laid as shown in the figure on a compacted surface. The stone blankets and raft shoes help to distribute the load on the structure.

### Slip plane rafts

The slip plane raft foundation has a fully compacted sand bed beneath the raft. To facilitate the transfer of loads, the sand bed size should be slightly larger than the raft size. The sides of the foundation can be filled with any compressible material.

## Slab beam-type raft foundation

Slab beam-type raft foundations are used when the loads are unequally distributed and the foundation is prone to distortions. Beams included with the slabs serve as stiffeners. The raft is reinforced with two layers of mesh, one at the bottom and one at the top. The beams can offer additional stiffness and guard against distortion.

## Piled raft foundation

Rafts are supported by pile foundations in this type of Mat Foundation, as illustrated in the figure. When the loads on the structure are extremely high, the soil bearing capacity is very low, and the water table is very high, these foundations are used. Piled raft foundations are ideal for high-rise buildings, and heavy industrial structures such as high-rise RCC chimneys, silos, and storage tanks that are typically supported by a single foundation element. Due to their high cost, they are not commonly used in residential applications. Piled raft foundations eliminate the need to design a very heavy raft foundation or a very conservative pile foundation with larger depths.

Instead, they opt for a combination of an optimised raft foundation and a pile foundation capable to share the loads. Over the pile foundation, the raft foundation floats. Typically used in structures such as chimneys, silos, bunkers, and overhead storage tanks where even minor soil settlement may cause the structure to fail.

## Cellular raft foundation

A cellular raft is made up of two-way foundation beams with a solid slab on the ground below and a suspended slab on top. The upper and lower slabs are joined by intermediate beams, transforming the foundation into an I-beam structure.

For covering the top slab, precast soffits can be used. The top slab is cast using precast soffits or other types of permanent formwork or sacrificial formwork, and it is filled with lightweight infill blocks.

Typically used in areas subjected to heavy mining activity and with poor soil-bearing capacities. The foundations must withstand massive bending moments. They are the preferred option in these cases. Cellular rafts are used when removing overburdens resulting in increased bearing capacity. Cellular rafts can be used to control soil uplift pressure.

## Balancing or floating raft foundation

Balancing rafts or floating foundations are used in areas where the soil’s bearing capacity is very low and the soil settlements must be kept within an acceptable range.

The floating foundation operates on the principle that the total weight of the soil and water removed from the excavated area must equal the weight of the proposed structure.

## Advantages of the Raft foundation

• Raft foundations are a safe and cost-effective alternative to other shallow and deep foundation types.
• Raft foundations are preferred in areas with low soil bearing capacity, uneven settlement, and mixed soil types. The load-bearing capacity of these foundations is achieved by distributing stresses over a larger area.
• In densely populated urban areas, access to the sites is frequently restricted, making it impossible to mobilise heavy equipment for foundation construction using other techniques. Raft foundations can be built with very little equipment because of their low heights.
• Raft foundations, when compared to other isolated foundations, provide a much-needed option for designers in terms of limiting settlement limits within the codal provisions.
• When deciding on settlement values, the designers have the option to choose higher values when compared to standard foundations. The raft foundation prevents uneven settlement.
• Raft foundations are a very flexible design option that can be customised to the soil conditions and workability.
• The execution of a raft foundation is simpler than that of individual footings. This, in turn, can speed up the project.

Most of the time, raft design is not considered economically when the soil conditions are extremely poor. Complex raft foundations consume a large amount of concrete and steel and necessitate precise professional/technical supervision and workmanship. As a result, the structure is more expensive than any other alternative foundation. The soil beneath the foundation, especially near the edges, must be preserved.

# Raft foundation types and features

Raft foundation is a continuous slab that completely covers the entire site of the proposed structure and rests on the soil. Depending on the applications and design loads, there are various raft foundation types. The choice of raft foundation type is influenced by a number of variables, including bearing capacity, loads, site circumstances, etc.

## Raft foundation – Definition

A raft foundation or mat foundation is a solid slab that is put at a predetermined depth and covers the entire structure. Raft foundations have shear walls and columns for distributing loads to the ground. These foundations are appropriate for places with low bearing capacity, wherein individual footings struggle to traverse the stresses. Moreover, the raft foundation aids in the transfer of the structure’s total load to a larger area. In comparison to other forms of foundations used in civil construction, raft foundations can minimize soil stress levels. This mechanism of stress distribution distinguishes raft foundations from other types of foundations.

## Determining soil stress

Stress = total load acting on the structure + raft self weight / raft foundation area

Assume a total load of 300 T and a foundation size of 20 m x 10 m

Soil stress = 300/200 = 1.5 t/sqm

The same structure is supported by 8 separate footings. 2 x 2 mtr 8 x 4 = 32 sqm total space

Soil stress = 300/32 = 9.375 t/sqm

This demonstrates that for the same weight, we get stresses of 1.5 T/sqm for the raft and 9.375 T/sqm for the individual foundations.

Because the raft’s contact area is greater, the load is distributed across a larger region, resulting in less stress on the soil.

## Raft foundations – Where to use them?

When the following circumstances exist, these foundations are typically favoured over other foundation types.

• When the bearing capacity of the soil is extremely low, building individual footings and executing deep foundations such as pile foundations becomes prohibitively expensive.
• When soil stresses must be reduced since the soil’s bearing capacity is lower.
• The columns, shear walls, and so on are so close to each other that individual footings may overlap.
• Any other type of foundation may cover more than 50% of the total ground area beneath the structure.
• When there is a possibility of unequal settlement.
• Likewise, Raft foundations are Preferred for complex equipment foundations.
• When the proposed structure has basements, raft foundations are preferred

For basement constructions where the foundation slabs would be subjected to direct live loads raft foundations are preferred based on the utility of the structure. Raft foundations are also a preferable option for sites where soil conditions are poor and access to major excavation machinery is limited, wherein raft foundations excavations can be accomplished with the help of light excavators.

## Types of raft foundations

Raft foundations are classified into several types based on soil conditions, structure functionality, and so on. The following are some of the most popular raft foundation types used in civil construction.

• Solid Slab Raft foundation/ Flat plate type foundations
• Slab beam Raft foundation
• Piled Raft Foundation
• Cellular raft foundation
• Balancing or floating raft foundation

## Solid Slab Raft Foundations

Flat plate raft foundations are made up of a reinforced concrete slab of uniform thickness covering the entire bearing area. In this type of raft foundation, the columns and walls are equally spaced, and the load distribution is also equal.

These raft foundations are known as solid slab raft foundations because they are designed as slabs with uniform thickness. This is ideal when the columns are evenly spaced and have equal and minor weights. Steel mesh reinforcement is offered in both directions of the slab. Two meshes are reinforced at the top and bottom of the slab to balance upward and downward bending stresses.

The following are the various types of solid slab raft foundations that are commonly used based on design requirements.

• Thickened flat plate type raft foundation
• Wide toe raft foundations
• Blanket rafts
• Slip plane raft foundations

### Thickened flat plate type raft foundations

When the column loads are extremely heavy, the flat plate type foundation is inadequate. To make it suitable, the slab thickness must be increased. The substantial loads on the column cause negative bending moments and diagonal shear in the slab. A full-size solid slab mat base is not required to negotiate the design loads. To compensate,  a section of the slab beneath the column should be thickened. The placement of a pedestal beneath the column without increasing the slab thickness also assists in receiving heavy loads.

### Wide toe raft foundations

A wide toe raft is used when the ground conditions require an unusually thick concrete slab to provide the necessary load support, which would be quite expensive. For negotiating the design loads,  a full-size solid slab mat base is not necessary. A substantially reinforced toe is provided on both sides of the structure, as illustrated in the image, for economizing the structure.

### Blanket raft foundations

When the construction site comprises small areas of weaker soil or diverse soil types with unequal settlements or nonuniform strata, a blanket raft may be the best option. Before laying the raft foundation, the surface is compacted and stone blankets are spread in layers on the prepared ground, as shown in the figure. However, despite the build’s footprint’s flaws, the raft foundation and stone blanket work together to provide even load support.

### Slip plane raft foundations

A preliminary layer of sand is put across a slightly larger surface than the required raft foundation for slip plane rafts, and the gap around the raft is filled with packed material. A thoroughly compacted sand bed supports this type of foundation beneath the raft.   The foundation’s sides can be filled with any compressible material.

## Slab beam type raft foundations

When the loads are unequally distributed, there seems to be a lot of space between the columns, and the foundation is susceptible to distortions, slab beam raft foundations are preferred. Beams are set in perpendicular directions in this scenario, and they are all connected by a raft slab. The beams incorporated with the slab act as stiffeners and prevent distortions. Columns are precisely located on the intersections of raft foundation beams, as illustrated in the figure. The raft’s reinforcement consists of two mesh layers, one at the bottom and one at the top.

## Piled raft foundation

Pile foundations are used to support the slab in the case of piled raft foundations. When the loads on the structure are exceedingly high, the soil bearing capacity is very low, and the water table is exceptionally high, this method is commonly used. Piled raft foundations are ideal for high-rise buildings and large industrial structures such as high-rise RCC chimneys, silos, and storage tanks that are typically supported by a single foundation element. Because of their exorbitant costs, these foundations are not generally used in residential applications. Designing a very heavy raft foundation or a  very conservative pile foundation with greater depths is avoided by using a piled raft. Instead, they opt for a hybrid of an optimal raft foundation and a pile foundation capable of supporting the structural loads.

## Cellular raft foundation

A cellular raft is made up of two-way foundation beams with a solid slab lying on the ground below and a suspended slab on top. Between the upper and lower slabs, intermediate beams are provided. The intermediate beam is responsible for transforming the entire structure into an I Beam. The top slab is cast using precast soffits or various types of permanent formwork, sacrificial formwork, or lightweight infill blocks. Cellular raft foundations are often used in areas with significant mining activity and poor soil carrying capacity, where the foundations must withstand huge bending moments. These types of raft foundations are used when eliminating overburdens resulting in greater bearing capacity. Moreover, cellular rafts can be used to control soil uplift pressure.

## Balancing raft or Floating Raft Foundation

Balancing rafts or floating foundations are deployed in situations where the soil bearing capacity is very low and the soil settlements must be kept within an acceptable range.

The floating foundation operates on the idea that the total weight of the earth and water removed from the excavated area must equal the weight of the planned structure.

This process involves huge earthwork excavation. Dewatering systems like well point systems have to be provided when the water table is very high. Likewise, Sheet piles are to be installed for weak soils or soils that may collapse. To protect the nearby structures from any defects due to scoring of soil, temporary retaining walls etc. have to be organized before starting excavation and some of these activities have to go in tandem with excavation. This type of structure is not economical and requires very minute technical supervision.

These foundation types are used for building structures in highly dense areas following all safety precautions to avoid any damage to the nearby structures.

Floating rafts are preferred for building with multiple levels of underground car parking facilities. However, for more details about floating rafts, you can go through our detailed article Balancing rafts or floating raft foundations

## Components of dam – 12 dam components explained

• Raft foundations offer a safe and cost-effective alternative to conventional shallow and deep foundation types.
• Preferable in areas with low soil bearing capacity, uneven settlement, and the presence of mixed soil types. By distributing stresses across a larger area, these foundations are able to achieve load-bearing capacity.
• Used in congested metropolitan locations where access to sites is limited and heavy machinery mobilisation for foundation construction using conventional deep foundations, such as pile foundations, is not feasible.
• Raft foundations can be built using very light machinery due to their low heights.
• Raft foundations offer designers a much-needed alternative to conventional isolated foundations for limiting the settlement restrictions within the codal provisions.
• When compared to regular foundations, the designers have the option of choosing larger settlement values during the design process in the case of raft foundations. Moreover this in turn can avoid an unequal settlement.
• Got flexible design alternatives that can be customized as per soil conditions and workability.
• Comparatively speaking, the execution is simpler than individual footings. This, in turn, can accelerate the project schedule.

• In extremely poor soil conditions, raft foundations are not cost-effective.
• Complex raft foundations demand careful professional/technical supervision and workmanship and consume a  substantial amount of concrete, and steel. As a result, the structure is more expensive than any other alternative foundation.
• The soil beneath the foundation, particularly around the margins, must be protected. Edge erosion is common in raft foundations.
• When the soil conditions are extremely poor, pile foundations are more cost-effective than raft foundations.
• Raft foundations occasionally need further strengthening, which raises the overall cost.
• Compared to other foundations, skilled labour is needed for raft foundations.
• When the mat or raft foundation is under the concentrated(point) load, further care should be taken.

# Balancing Raft or Floating Raft Foundation

<p value="Balancing rafts or floating rafts are used when the bearing capacity of the soil is very low. This <a rel="noreferrer noopener" href="https://vincivilworld.com/2020/05/25/foundation-types-shallow-and-deep-foundations/&quot; target="_blank">foundationBalancing rafts or floating rafts are used when the bearing capacity of the soil is very low. This foundation protects against soil settlements and works on the principle that the total weight of the soil and water excavated for the foundation equals to the total load of the proposed structure. They are called zero settlement structures and are used for high rise towers with multiple basements. This type of foundation is preferred only when the soil is very compressive and soft and even friction piles are not able to support the loads.

## How Floating Raft Works?

The basic concept of a floating raft is countering the weight of excavated soil and water with the total load of the structure resting over it. ie. The same weight of excavated materials is replaced by a structure with the same weight. Because of these balancing floating rafts are also known as balancing raft foundations or compensated foundation. The vertical pressure at the foundation depth remains same before and after excavation.

## Concerns of Floating Raft/Balancing Raft

Execution of a floating foundation demands massive earthwork excavation in high water table areas. Open dewatering may not be sufficient to control a large amount of water flowing in the excavated areas. Wellpoint dewatering systems have to be run simultaneously with the excavation to lower the water table.

Excavation of floating foundations can trigger soil collapse and soil scoring from the bottom of existing foundations. Sheet piles have to be erected to protect the building against any possible damage during excavation.

Materials required for temporary retaining walls like sandbags, PCC blocks, etc has to be organised and mobilised before starting excavation. Temporary retaining walls have to be constructed in tandem with excavation works.

Heave formation can always happen while doing deep excavations because of the weight of soil on both sides of the excavation pit. Heaves formations cannot be avoided in any case but have to be controlled or else it may initiate foundation settlements. Heave formation can be minimised by further lowering the water table to a depth below the founding level of the structure. Installation of friction piles or micro piles on partially completed excavation surfaces can also control heave formation to an extent. Concrete bands are provided at the bottom level to control heave formation.

Balancing raft foundations are adopted for high rise buildings with multiple basements. Execution of these foundations requires highly skilled professionals and micro-level supervision supported by an experienced and skilled workforce. All safety precautions have to be done to protect the nearby structures from any damage.

# Foundation types- shallow and deep foundation

Foundation is the most significant part of any structure/building which transfers the total loads of the structure and its components to a competent surface on the ground. Foundations are broadly classified into two types. ie. Shallow and Deep Foundations.

Foundation is the last part of the structure which touches the ground. The area of contact with the ground is called the foundation bed.

Every structure is divided into:

## b) Super structure

Components of a structure that are coming below the ground level are called substructure, and above ground level is called superstructure. Foundations are coming in the substructure category. Foundations are responsible for transferring loads of superstructure components to the ground.

## HOW TO FIX TYPE AND SIZE OF FOUNDATIONS?

The selection of foundations depends on the bearing capacity of the soil and the purpose of the structure. Geotechnical engineering is a field of Civil Engineering, which analyses the physical and chemical properties of soil and furnish designers with the inputs on the soil properties and proposed types of foundations. The Safe bearing capacity of the soil determines the foundation type and dimensions.

## SAFE BEARING CAPACITY OF SOIL

Bearing capacity is the capacity of soil to support a structure without settlement or failure. To keep the structure safe, the bearing capacity has to be calculated at different locations. The ultimate bearing capacity has to be divided by a factor to derive the safe bearing capacity of the soil. Safe bearing capacity is defined as the maximum load per unit area soil can withstand without settlement and failure. The safe bearing capacity is determined by conducting field tests or soil investigations.

## QUALITIES OF A WELL DESIGNED FOUNDATION

A well-designed foundation is supposed to possess the following qualities.

a) Have to distribute the total load on the structure to a larger area.

b)Have to counter unequal settlement in case of any displacement.

c) Has to prevent the structure from lateral moments.

d) Foundations are responsible for the total stability of structures.

## SHALLOW FOUNDATION

Shallow foundations transfer the load laterally to the soil. It is also called stripped foundations. The depth of a shallow foundation is less than its width.

## Characteristics of shallow foundations

Shallow foundations are adopted when the load acting on a structure is reasonable and has a competent soil layer capable of negotiating the loads available at a shallow depth or shorter depth.

Shallow foundations are placed on the surface of the ground. The depth of a shallow foundation can range from 1 meter to 3.5 meters and sometimes more.

The width of the shallow foundation is greater than the depth. Shallow foundations are very easy to construct and do not require highly skilled manpower and professional supervision. These foundations can even be done with the help of medium-skilled workers. A shallow foundation is very economical when compared with a deep foundation. Shallow foundations are end bearing type foundations that transfer loads to the end of the foundation.

Shallow foundations are considered as the most preferred option when the safe bearing capacity of the soil is reasonable and the structural loads are within the permissible limits.

## DEEP FOUNDATION

Characteristics of deep foundation

The width of the deep foundation is less than the depth. The depth can even go up to 60 meters or more depending on the design, loads, and availability of capable strata.

Deep foundations require technical expertise, sophisticated equipment, and highly skilled manpower for interpreting and executing works.

The deep foundations are costly due to their way of execution involving the infusion of quality materials, skilled labor, professional engineering support, and equipment

Deep foundations do not rely only on end bearing for transferring the loads. The skin friction developed between the foundation surface and the soil surrounding it may also be considered in the design stage.

The deep foundations can resist uplift pressure much more than shallow foundations and hence the chances of failure are less compared to shallow foundations.