The Groundwork of Construction: A Technical Guide to Deep vs. Shallow Foundations
The Groundwork of Construction: A Technical Guide to Deep vs. Shallow Foundations
Every impressive structure begins with a robust foundation. It serves as the unsung hero of construction, distributing the substantial weight of a building from its superstructure to the earth beneath. The decision to use either a shallow or deep foundation is a vital engineering choice, influenced by soil conditions, structural loads, and project details. This blog examines the intricacies of both types, providing a straightforward comparison to clarify their respective uses.
Shallow Foundations: Spreading the Load Near the Surface
As the name suggests, a shallow foundation is constructed at the ground's surface. Its primary goal is to distribute the structural load over a large area while maintaining the soil's bearing capacity. When the surface soil is solid and strong enough to support the applied loads, these foundations are typically used.
Key Features:
Depth: Typically reaches a depth that is less than its width, often extending up to 3 meters (10 feet) below the final ground level.
Mechanism: Loads are directly transferred to the soil layers immediately under the foundation.
Cost: Often more affordable due to minimal excavation and simpler building methods.
Suitability: Ideal for low-rise residential buildings on stable soil and other lightweight structures.
A Thorough Examination of Shallow Foundations: Your Guide
In civil engineering, the foundation is the quiet champion. It forms the connection between a man-made structure and the Earth. If the foundation fails, the rest of the building is merely an expensive collection of debris waiting to collapse. Foundations are generally divided into two categories: Deep and Shallow. Today, we will focus on the latter. A foundation is typically regarded as 'shallow' when its depth (Df) is equal to or less than its width (B). Essentially, they convey the building's load to the soil layers that are located just beneath the surface.
Common Types of Shallow Foundations:
1. Isolated (Spread) Footing: This is the most common and uncomplicated type of foundation, utilized to support individual columns. It is a slab of reinforced concrete, which can be square, rectangular, or circular, that distributes the column's load to the soil.
I. Wall Footing
The bottom of this type of shallow foundation is paved with a layer of Brick Bat cement concrete (BBCC) 1: 3: 6. The width of this layer is kept three times the wall thickness. It is also often referred to as a strip footing.
The layer thickness of the BBCC should be twice the projection on both sides.
The width of the lowest layer of masonry is kept twice the plinth level wall thickness.
The height of the bricklayer is 10 cm.
When the wall is under heavy load or the soil bearing capacity is low, then the width of the base of the wall (2 times wall thickness + 2 times projection) is required. So that the width of the masonry increases, and the load can be transferred over a large area of concrete.
ii. Reinforced concrete footing:
The reinforced concrete footing is also one type of shallow foundation. The size of the wall footing is greatly increased when the wall is under heavy load, and the bearing capacity of the soil is low.
In such a case, it is advisable to place a rainforest concrete footing under the wall. Such footing reduces the amount of masonry. Costs also decrease. A layer of concrete 7 to 8 cm thick is laid under the reinforced footing.
iii. Inverted Arch Footing:
An inverted arch is built between two pillars in this type of foundation. The rise of the inverted arch is kept equal to 1/5 to 1/10 of the span.
An inverted arch should be built into the cement mortar.
The lateral pier should be designed to withstand the pressure externally due to arch action. The inverted arch reduces the depth of the base. So this foundation is economical in soft soil.
This method is less commonly used in the foundations of buildings. But more suitable for bridges, tanks, drainage line supports, etc.
An inverted arch footing is more suitable for Soft soil.
iv. Column footing or Isolated footing:
This type of footing is used for separate columns.
It is also called an isolated footing.
Such footing bases are round, rectangular, or square.
Such type footing bases are Simple or Sloped type.
For heavily loaded columns, reinforcement is placed in both directions in the concrete bed. For RCC columns, an offset of 15 cm is maintained on all four sides of the concrete bed.
2. Combined Footing
Sometimes, columns are so close together that their individual footings would overlap. Or, perhaps a column is right on the property line where you can't "spread" the footing in one direction. In these cases, we combine them into one single footing. These are usually rectangular or trapezoidal in shape.
A combined footing is also one of the common types of shallow foundation, which is mostly used.
A combined footing is used for two columns.
They are rectangular or trapezoidal in shape.
The rectangular footing is used when the load on both columns is the same, and the trapezoidal footing is used when the load on both columns is unequal.
Combined footing is used in the following cases:
When two adjacent footings overlap.
When a column is very close to the property line, and a footing is put on the property line, then the footing is eccentrically loaded. In this case, use this footing.
3. Cantilever Footing (Strap Footing)
A strap footing consists of two isolated footings connected by a structural "strap" or beam. The strap doesn't actually sit on the soil; its job is to transfer the moment from an eccentrically loaded footing (such as one on a property line) to an interior footing, preventing the whole structure from tipping over.
Where cantilever footing used?
When the distance between the two columns is greater, and the allowable soil pressure of the soil is higher, a strap footing is cheaper than a combined footing.
4. Raft or Mat Foundation
Imagine a giant concrete "mat" that supports the entire building. Instead of individual pads for every column, the whole basement floor acts as the foundation.
When to use it: When the soil is weak, or the building loads are so high that individual footings would cover more than 50% of the ground area.
The Benefit: It minimizes differential settlement (when one part of a building sinks faster than another).
Where is the raft foundation used?
The soil bearing capacity is low.
The soil is prepared by filling.
Columns and walls are close, and the footing overlaps.
The soil is non-homogeneous with soft soil pockets.
Differential settlement is likely to occur.
There is a lot of variation in the load on different columns.
5. Grillage Foundation
A grillage foundation is a type of foundation that is used in construction to distribute the load of the structure evenly to the soil. It consists of a framework of steel or concrete beams and columns that are placed on the soil and used to support heavy structures.
The Grillage foundation serves as a sturdy platform for large, heavy constructions like buildings, bridges, and towers. It keeps the soil from settling or shifting, which could result in structural damage, by uniformly spreading the structure's load over a broad area of soil.
Grillage foundations are made of steel or concrete beams and columns that are organised in a grid pattern to form a structure.
Bolts or welds are used to join the beams and columns to form a sturdy, solid structure. Concrete footings or pads that are positioned beneath the beams and columns to offer extra stability and support may also be a part of the foundation.
The design of a Grillage foundation is typically carried out by a structural engineer, who takes into account the weight and size of the structure and the properties of the soil at the construction site to determine the appropriate materials and dimensions required for the foundation.
This foundation helps avoid deep excavations as the necessary base area is provided for the transmission load. The depth of the foundation is limited to 1000 mm to 1500 mm.
Types of Grillage Foundation
A grillage foundation can be made from wood, steel, and reinforced concrete materials. The type of material used depends on the type of structure, how tall the building will be, and how much weight it will bear.
The types of grillage foundations (according to materials) include:
i. Timber
ii. Steel
iii. Reinforced Concrete
i) Timber Grillage Foundation
A timber grillage foundation is a construction foundation designed to support buildings or other structures. It is commonly utilized when the soil lacks sufficient strength to bear the weight of the building, or when the structure is situated in areas with high water tables or challenging soil conditions.
The term "grillage" denotes a framework made of timber beams or planks arranged horizontally on top of each other to form a stable foundation. This foundation type is generally employed for small to medium-sized buildings or structures.
Typically, the grillage is built with a layer of stones or other durable materials placed atop the soil to establish a level base.
Subsequently, the timber beams or planks are positioned on the stone layer and secured using bolts or other fasteners. The grillage is designed to evenly distribute the building's weight across the foundation, preventing settling or shifting of the structure.
In summary, timber grillage foundations offer a cost-effective and relatively straightforward solution for building foundations in areas with challenging soil conditions or limited access to conventional building materials.
However, it is crucial to collaborate with a qualified engineer to ensure that the foundation is designed and installed correctly, adhering to local building codes and safety standards.
Moreover, the timber grillage foundation is beneficial in soils with low bearing capacity, typically less than 50 kN/m2.
The wooden planks have a width ranging from 8" to 12" and a thickness of 2" to 3". Each layer of grillage is oriented perpendicular to the adjacent layer.
ii) Steel Grillage Foundation
A steel grillage foundation is a construction foundation type designed to support buildings or other structures. Its design resembles that of a timber grillage foundation, but it utilizes steel beams or sections instead of wooden beams or planks to establish a stable base.
These foundations are frequently employed in locations where the soil lacks the strength to bear the weight of the structure, or where challenging soil conditions, such as a high water table, exist.
They are also prevalent in industrial or commercial construction, where the loads imposed by buildings are generally heavier than those found in residential projects.
The steel sections in a grillage foundation are usually welded together to create a framework that evenly distributes the building's weight across the foundation. This framework is then positioned on a layer of stones or other solid materials to form a level base.
The benefits of steel grillage foundations include their strength, durability, and resistance to moisture and pests. Additionally, they are relatively straightforward to install and can be tailored to suit a variety of building loads and soil conditions.
Nonetheless, steel grillage foundations may incur higher costs compared to other foundation types, and their installation necessitates specialized equipment and expertise.
It is crucial to collaborate with a qualified engineer or contractor to ensure that the foundation is designed and installed correctly, adhering to local building codes and safety regulations.
iii) Reinforced Concrete Grillage Foundation
A reinforced concrete grillage foundation serves as a construction foundation designed to support buildings or other structures. Its design is akin to that of timber or steel grillage foundations, but it employs reinforced concrete beams or slabs to establish a stable base.
These foundations are frequently utilized in construction due to their strength, durability, and resistance to moisture and pests. They are particularly beneficial in locations where the soil lacks the strength to bear the weight of the building or where challenging soil conditions, such as high water tables, exist.
To construct a reinforced concrete grillage foundation, a layer of stones or other solid materials is initially laid on the soil to form a level base. Subsequently, reinforced concrete beams or slabs are positioned atop the stone layer and secured with steel rebar and concrete.
The grillage is generally engineered to evenly distribute the building's weight across the foundation, thereby preventing settling or shifting of the structure.
The benefits of reinforced concrete grillage foundations include their strength, durability, and resistance to moisture and pests. Additionally, they are relatively straightforward to install and can be tailored to support a diverse range of building loads and soil conditions.
Nonetheless, reinforced concrete grillage foundations may incur higher costs compared to other foundation types, and their installation necessitates specialized equipment and expertise.
It is crucial to collaborate with a qualified engineer or contractor to guarantee that the foundation is designed and installed correctly, adhering to local building codes and safety regulations.
Deep Foundation:
A deep foundation is a type of foundation that transfers building loads to the earth farther below
from the surface than a shallow foundation does to a subsurface layer or a range of
depth Generally constructed depth over 3m.
Purpose:
Deep foundation is used for the following purposes:
• When the foundation is supposed to carry loads at depth.
• If the soil beneath the structure to be constructed is weak, compressible soil.
• When the soil is unsuitable, Deep foundations are used to transfer the load to a
deeper strata containing strong soil or rock.
Types of Deep Foundation:
The following are the types generally used in construction:
1. Basement Foundation.
2. Buoyancy Rafts Or Hollow Box Foundations.
3. Well Foundation Or Caissons.
4. Pier Foundation.
5. Drilled Shaft Foundation.
6. Pile Foundations.
1. Basement Foundation:
These are constructed when there is some space below ground level required. These substructures provide workingplace or storage space for various purposes. Designing method depends on the purpose of the structure rather than resistance to external earth (Pressure of earby soil) and hydrostatic pressure. If they’re built in the correct way, they will hold up betterin longevity than the other types of foundation.
Advantages:
• More, Cheap Square Footage.
• Seasonal Living Space, i.e., warm in winter and cool in summer.
• Easy-Access for Repairs.
• Storm Protection.
Disadvantages:
• Increased Cost.
• Potential Flooding.
Note: The best scenario to prevent potential flooding is ensuring a natural path
for drainage.
• Lack of Natural Light.
Use:
Used in Basements.
i)Basement
The basement foundation is an additional floor partially or completely below ground and built with poured concrete walls. It’s the deepest of the common foundation types and matches most or all of the floor space of the level above.
Basements have a lot of benefits, including adding more square feet at a low cost and serving as a shelter from severe weather. However, given that basements are below ground, they’re more prone to flooding. Therefore, basements might not be ideal if your area experiences frequent floods.
ii)Crawl space
This is an enclosed area between the ground and the home’s first floor, which consists of short foundation walls that stand on footings. It provides limited access to things like plumbing, wiring, storage, and other equipment.
This type of foundation is often used in colder climates. It’s less expensive than a full basement because it requires less material and labor. However, it doesn’t provide much protection from inclement weather.
iii)Pier and beam piling
This foundation uses piers that are set into the ground, with beams extending from pier to pier. Pier and beam, or pilings, are often seen in flood-prone areas and coastal towns, or on permafrost. This foundation, however, is not the right fit for areas prone to earthquakes or hurricane-strength winds.[3]
iV)Slab
The slab is a raised perimeter foundation that supports floors and load-bearing walls. A concrete slab is often used, which serves as the bottom floor of the home. These types of foundations work best in climates that don’t experience ground freezing and thawing, because this can lead to cracks in the slab and shifting of the foundation.
Maintenance matters
Even if you don’t know much about foundations, you likely realize that a cracked foundation is not a good thing. Cracks happen when moisture leaves the soil and returns unevenly, which can shift the foundation, causing it to crack.[4] Cracks can lead to interior damage, plumbing issues, and sticky doors and windows.
2. Buoyancy Rafts Foundation:
This type of foundation is constructed over soft and weak soils. When loaded, it acts as a floating (buoyant) structure and is designed in a way that it reduces the intensity of the load on the soil. It is a type of floating foundation. Also known as Hollow Box Foundations,Compensated Foundations, or Deep Cellular Rafts.
These are called Compensated foundationsbecause the soil below the foundation isremoved during the construction process. The weight of the superstructure is equal to the weight of soil and water removed. Like this, the removed weight is compensated for by the weight of the superstructure.
Design Considerations for Buoyancy Raft:
The general design involves following step-by-step procedures:
1. The depth of the excavation is determined initially from the plan.
2. Center of gravity to remove overburden that should be equal to sustain the structural
buoyancy.
3. The basement design decided should be compared with the opinions and needs of the client.
4. Water pressure calculation for checking the flotation.
5. Finally, design details for external walls, floors, and separating walls are prepared. The main design concern is earth pressure, moments, and shear forces. Consideration special flotation design is a must.
Advantages:
• Elevation of a structure protects it from high water levels.
• Soil settlement chances are reduced due to the fact that the total weight of the structure
is equal to the removed soil and water.
Note that a slight increase in loading will not bring any extreme change in the settlement limit.
Disadvantages:
• Due to larger depths of weak soils, excavation also increases, hence
increasing the project cost.
• Catastrophic Movement can occur if excavation beyond the critical depth of
Stability is done.
• Earth pressure from surrounding soft soils may cause settlement of the
support of foundations.
• Space is wasted.
• There are possibilities of leakage of gases into these hollow cells. As these Situations are unaware, they remain unventilated. This would cause unexpected explosions, even if there are small chances of ignition anywhere around.
Use:
1. Areas of low altitude often face high floods, which result in the collapse of houses. Increasing the house elevation is useful with a buoyant foundation.
3. Well Foundation OR Caissons:
Well Caissons or Caissons are box-like or circular or rectangular structures which are sunk from the surface of either land or water to desired/required depth. From inside, they are hollow and can be filled with sand
and plugged at the bottom. They have larger diameter than Pile foundation.
Purpose/Suitability:
• Deep sandy or soft soils.
• Heavy structures on low-bearing-capacity soils.
Advantages:
• Economical.
• Environmentally friendly.
• Can be reached to great depths.
• Minimum handling equipment is required.
Disadvantages:
• Expert engineers are required to design the construction.
• Workers may experience giddiness, breaking of eardrums, bursting of blood vessels
in the nose or ears, and end up in paralytic death, which is known as caisson disease.
• The help of divers can be required.
• Not good for polluted sites.
• The construction process is difficult.
Use:
• Pump house.
• Bridge piers.
• Abutment in rivers and lakes.
4. Pier Foundation:
A pier foundation is a collection of large-diameter cylindrical columns to support the superstructure and transfer large superimposed loads to the firm strata below. It stood several feet above the ground. Also known as “Post foundation”.
Advantages:
• Less labor and materials required.
• Minimal disruption to the soil environment.
• A lot of variety in designs.
• Saves money and time.
• As the diameter of the shafts is greater,
inspection is possible.
• Design can be changed by Engineers is desired.
Use:
Bridges.
5. Drilled Shaft foundations:
Drilled shafts are also called drilled piers, caissons, or bored piles.
These are used to support structures having large axial and lateral loads.
These foundations are constructed by drilling a borehole within a deep excavation, then concrete or some
other pre-fabricated load-bearing units are placed in it.
Length and size can be changed easily. Shaft foundations can be constructed near existing structures
and under low overhead conditions, which allows them to which they can be used in numerous seismic
retrofit projects. It can be difficult to install them in soils with boulders, soft soil, loose sand, and sand under
water.
Construction Steps:
• Excavation of piers.
• Providing support.
• Concreting.
Working:
Drilled shafts are typically designed and constructed to support axial forces through a
combination of side friction and end bearing resistance. They range in diameter from 24” to
120” and can be installed to depths in excess of 300 feet.
Types of Drilled Shafts:
Use:
Drilled shafts are an economic solution that can be installed in a variety of different ground conditions
conditions to accommodate large axial, lateral, and overturning forces.
6. Pile Foundation:
Piles are relatively long, slender members that transmit foundation loads through soil strata of low bearing capacity to deeper soil or rock strata having a high bearing capacity. It is a type of foundation that is supported on piles. It consists of a base called a pile cap (RCC slab) supported at bottoms by piles.
The load to the soil in contact is transferred by piles either by friction only or by friction in combination with bearing at their ends.
Purpose/Suitability:
• When soil is very soft and a solid base is not available at a reasonable depth.
• When grillage and raft foundations prove to be very expensive.
• High building with heavy concentrated loads.
• Construction of a building along the riverbed or the seashore.
• Can also be used to help resist uplift, overturning, and lateral forces.
Type of Pile Foundation:
Some of the main types are mentioned below;
I. End bearing Piles.
II. Friction Piles.
III. Settlement Reducing Pile.
I. End bearing pile:
Piles that terminate in hard, relatively impenetrable material such as rock or very dense sand and gravel.
Most of the carrying capacity is acquired from the resistance of the stratum below the pile at its toe
II. Friction Pile:
Most of the carrying capacity is acquired from
the by skin friction or adhesion.When piles do not reach an impenetrable
stratum at a reasonable depth, but are driven for some distance into a penetrable soil. Their carrying capacity is derived partly from the end bearing and partly from the skin friction between the embedded surface of the soil and the surrounding soil.
III. Settlement Reducing Piles:
Beneath the central part of a raft foundation, these piles
are incorporated in order to reduce differential
settlement to an acceptable level.
Such piles act as a reinforcement beneath the soil below
The raft prevents dishing of the raft in the center.
Uses Of Pile Foundation:
• High groundwater beneath the structure.
• High and non-uniform load of superstructure.
• Presence of highly compressible soil at shallow depth.
• Location of the structure near the river bed or the sea shore.
• Passage of canal or deep drainage systems near the structure.
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