Why Use a Group of Piles Instead of a Single Pile in Foundations: An In-Depth Look

When it comes to constructing robust and reliable foundations for large structures, the decision to use a group of piles instead of a single pile is crucial. Pile foundations are essential in transferring loads from buildings to the ground, especially in areas with weak or unstable soil. This article explores the reasons behind using a group of piles over a single pile, highlighting the advantages and technical considerations that make this approach superior for many construction projects.

Understanding Pile Foundations

Pile foundations consist of long, slender columns made of materials such as concrete, steel, or timber, driven deep into the ground to reach stable soil or rock layers. They are used to support structures with heavy loads or in areas where the surface soil is not strong enough to bear the load on its own.

Advantages of Using a Group of Piles

1. Load Distribution

One of the primary reasons for using a group of piles is the effective distribution of loads. A single pile might not be able to bear the entire weight of a structure, especially if the load is substantial. By using multiple piles, the load is spread across a larger area, reducing the stress on each individual pile and enhancing the overall stability of the foundation.

2. Increased Load Capacity

A group of piles can collectively support much heavier loads than a single pile. This is particularly important for large buildings, bridges, and other structures that exert significant pressure on their foundations. The combined strength of a pile group ensures that the foundation can handle the load without risk of failure.

3. Mitigation of Settlement Issues

Settlement occurs when the ground beneath a foundation compresses under the weight of the structure, potentially leading to uneven or excessive sinking. A group of piles minimizes settlement by distributing the load more evenly and reaching deeper, more stable soil layers. This reduces the likelihood of differential settlement, which can cause structural damage over time.

4. Improved Stability in Lateral Loads

Structures often face lateral loads due to wind, earthquakes, or other forces. A group of piles provides better resistance to these lateral forces compared to a single pile. The collective action of multiple piles enhances the foundation’s ability to withstand horizontal movements, ensuring the structure remains stable and secure.

5. Redundancy and Safety

Using multiple piles introduces redundancy into the foundation design. If one pile fails, the load can be redistributed among the remaining piles, reducing the risk of catastrophic failure. This redundancy is a crucial safety feature, especially for critical infrastructure and high-rise buildings.

Technical Considerations

When designing a group of piles, several technical factors must be considered to ensure optimal performance:

Pile Spacing: Proper spacing between piles is essential to prevent negative interactions such as pile-to-pile load transfer and ensure that each pile can carry its share of the load.
Pile Cap Design: A pile cap is a thick concrete mat that sits on top of the pile group, distributing the load from the structure above to the piles below. The design of the pile cap must accommodate the load distribution and ensure stability.
Soil-Pile Interaction: The interaction between the piles and the surrounding soil plays a critical role in the foundation’s performance. Soil testing and analysis are necessary to determine the appropriate pile length and diameter.
Construction Techniques: The method used to install the piles, such as driven piles or drilled shafts, affects the foundation’s effectiveness. Each technique has its advantages and limitations based on the soil conditions and load requirements.

Conclusion

Using a group of piles instead of a single pile in foundation construction offers numerous benefits, including better load distribution, increased load capacity, reduced settlement, enhanced stability against lateral loads, and added redundancy for safety. These advantages make pile groups an essential component in the design and construction of durable, reliable foundations for various structures. By considering technical factors such as pile spacing, pile cap design, soil-pile interaction, and construction techniques, engineers can optimize the performance of pile group foundations and ensure the longevity and stability of the structures they support.

For more insights on foundation engineering and construction techniques, stay tuned to our blog, where we delve into the latest advancements and best practices in the industry.

If you need a spreadsheet for Pile Group calculation please follow this link

Pile Group Analysis For Rigid Pile Cap Spreadsheet

Backfilling in Foundation: Types and Procedures

Backfilling is a critical process in construction, particularly in laying foundations for buildings. It involves refilling an excavated site with soil or other materials to provide support and stability to a structure. Proper backfilling ensures the longevity and integrity of the foundation, making it a vital aspect of construction projects. This article explores the types of backfilling materials, the procedures involved, and the importance of backfilling in foundation construction.

What is Backfilling?

Backfilling refers to the process of refilling an excavated area around the foundation of a structure. It is performed after the foundation has been laid and involves placing soil or other materials around the foundation to provide support and stability. The purpose of backfilling is to protect the foundation from various environmental factors and to ensure that it remains stable over time.

Types of Backfilling Materials

The choice of backfill material depends on several factors, including the type of foundation, soil conditions, and the specific requirements of the project. Here are some common types of backfilling materials:

Coarse-Grained Soil: Includes sand and gravel, which are excellent for drainage and compaction. Coarse-grained soil is ideal for foundations that require good drainage.
Fine-Grained Soil: Comprises silt and clay. These materials are less permeable and can hold water, which might not be suitable for all foundation types.
Aggregate Materials: Crushed stone, recycled concrete, and other aggregate materials provide strong support and are often used in combination with other backfill materials.
Controlled Low Strength Material (CLSM): Also known as flowable fill, CLSM is a mixture of cement, water, and fine aggregate that provides excellent support and is easy to work with.
Rock and Boulders: Used in areas where additional structural support is needed. Rocks and boulders are typically used in combination with other materials.

Backfilling Procedures

Proper backfilling requires careful planning and execution to ensure the stability and longevity of the foundation. Here are the key steps involved in the backfilling process:

Site Preparation: Before backfilling, the site must be properly prepared. This involves clearing debris, ensuring proper drainage, and compacting the subgrade.
Foundation Inspection: Inspect the foundation to ensure it is properly constructed and cured. Any issues should be addressed before backfilling begins.
Layer-by-Layer Filling: Backfill material is placed in layers, typically 6 to 12 inches thick. Each layer is compacted before the next one is added to ensure proper density and support.
Compaction: Compaction is crucial for preventing settlement and ensuring stability. Different types of compacting equipment, such as vibratory plates, rollers, and tampers, may be used depending on the material and site conditions.
Moisture Control: Proper moisture content is essential for compaction. The soil should be moistened if it is too dry or allowed to dry out if it is too wet before compaction.
Final Grading: After backfilling and compaction are complete, the site is graded to ensure proper drainage away from the foundation. This helps prevent water accumulation, which can cause foundation issues.

Importance of Backfilling in Foundation Construction

Backfilling plays a vital role in foundation construction for several reasons:

Stability and Support: Proper backfilling provides essential support to the foundation, preventing it from shifting or settling over time. This ensures the structural integrity of the building.
Drainage: Using suitable backfill materials and proper grading helps facilitate drainage, reducing the risk of water accumulation around the foundation, which can lead to water damage and foundation issues.
Protection: Backfilling helps protect the foundation from environmental factors such as erosion, frost heave, and soil movement. It acts as a buffer, absorbing and distributing loads evenly.
Longevity: A well-executed backfilling process contributes to the overall durability and lifespan of the foundation and the structure it supports.

Best Practices for Backfilling

Use Quality Materials: Choose the appropriate backfill materials based on the project requirements and soil conditions. High-quality materials ensure better compaction and stability.
Proper Compaction: Compaction is key to preventing future settlement. Ensure that each layer of backfill is adequately compacted to achieve the desired density.
Monitor Moisture Levels: Maintain optimal moisture levels during compaction to achieve the best results. Too much or too little moisture can affect the compaction process.
Seek Professional Advice: Consulting with geotechnical engineers and construction professionals can help determine the best backfilling practices for your specific project.

Conclusion

Backfilling is a crucial process in foundation construction that ensures the stability, support, and longevity of the structure. By using the right materials and following proper procedures, you can create a strong foundation that withstands environmental challenges and supports the building effectively. Understanding the importance of backfilling and implementing best practices can significantly impact the success and durability of your construction projects.

What Is Anchor Block Slope Stabilisation?

Anchor block slope stabilization is a technique that stabilizes slopes or existing retaining walls using anchored reaction blocks. The block layout pattern is typically in rows across the slope or wall. The finished anchored reaction blocks resist the movement of the retained soil or wall.

Anchors are slope stabilization and support elements that transfer tension loads using high-strength steel bars or steel strand tendons. Micropile Slide Stabilization System (MS³) is a slope stability technique that utilizes an array of micropiles sometimes in combination with anchors. The micropiles act in tension and compression to effectively create an integral, stabilized ground reinforcement system to resist sliding forces in the slope.

Advantages

Cost-saving solution for landslide repair and slope stability control
Can be designed for permanent or temporary support
Crane-mounted equipment can reach even the most difficult access slopes

https://www.theengineeringcommunity.org/wp-content/uploads/2022/01/Slope-stabilization-Anchor-block-slope-stabilization-.mp4?_=1

What are Deep Foundation? The Common Types of Deep Foundation

1. What are Deep Foundations?

A deep foundation is needed to carry loads at depth or for functional reasons from a structure through weak compressible soils or fills on to stronger and less compressible soils or rocks.

Deep foundations under the finished ground surface are founded too deeply for their base bearing ability to be affected by surface conditions, generally at depths > 3 m below the finished ground level.

When unsuitable soils are present near the surface, the deep foundation may be used to transfer the load to a deeper, more capable strata at depth.

2. Types of Deep Foundation

The types of deep foundations in general use are as follows:

Basements
Buoyancy rafts (hollow box foundations)
Caissons
Cylinders
Shaft foundations
Pile foundations

a. Basement foundation

They are hollow substructures built to provide space below ground level for the work or storage. The structural design is driven by its practical needs rather than by considerations of the most effective method of resisting external earth and hydrostatic pressures. In open excavations, they are set up in place.

b.Buoyancy Rafts (Hollow Box Foundations)

Buoyancy rafts or hollow box foundations also known as the floating foundations is a type of deep foundation is used in building construction on soft and weak soils.

They are designed to provide a buoyant or semi-buoyant substructure underneath which reduces net loading to the desired low intensity on the soil. Buoyancy rafts can be constructed to be sunk as caissons, and can also be installed in open excavations.

Buoyancy rafts are more expensive than traditional forms of foundations. For that reason, their use is usually restricted to sites that are on silts, soft sands and other alluvial deposits that are very deep, or where loads can be kept concentric. Schemes requiring underground tanks or where it’s economical to incorporate deep basements into the design are common.

c. Caissons Foundations

A caisson is a sort of foundation of the state of the hollow prismatic box, which is worked over the ground level and afterward sunk to the necessary depth as a solitary unit. It is a watertight chamber utilized for establishing foundations submerged as in rivers, lakes, harbors, etc. The caissons are of three types:

Open Caissons: Open caissons are of hollow chambers, open both at the top and the bottom. The lower part of the caisson has a bleeding edge. The caisson is sunk into place by eliminating the soil from within the shaft until the bearing layer is reached. Well foundations are special type of open caissons used in India.
Pneumatic Caissons: Pneumatic caissons are closed at the top but open at the bottom. A pneumatic caisson has a working camber at its bottom in which compressed air is maintained at the required pressure to prevent entry of water into the chamber. So, these type of excavations are done in dry.
Floating Caissons: Floating caissons are open at the top but closed at the bottom. These caissons are developed ashore and afterward shipped to the site and floated to where these are to be finally installed. These are sunk at that spot by filling them with sand, ballast, water or concrete to an evened out bearing surface.

d. Cylinders

These foundations are placed when there is required to place only a single cylindrical unit.

e. Drilled Shaft foundations

These foundations are constructed by drilling a cylindrical hole within a deep excavation and subsequently placing concrete or another prefabricated load-bearing unit in it.

Their length and size can be easily tailored. Drilled shafts can be constructed near existing structures and under low overhead conditions, making them suitable for use in numerous seismic retrofit projects.

It may, however, be difficult to install them under certain conditions such as soils with boulders, soft soil, loose sand, and sand under water.

e. Pile foundations

Pile foundations are relatively long and slender members designed by driving preformed units to the desired foundation level, or by driving or drilling in tubes to the appropriate depth – tubes filled with concrete before or during withdrawal or by drilling unlined or wholly or partially lined boreholes filled with concrete after that.

What is Vibroflotation Ground Improvement Method?

Vibroflotation is a technique developed in Germany in the 1930s for in situ densification of thick layers of loose granular soil deposits. Vibroflotation was first used in the United States about 10 years later. The process involves the use of a vibroflot (called the vibrating unit).

The device is about 2 m in length. This vibrating unit has an eccentric weight inside it and can develop a centrifugal force.

The weight enables the unit to vibrate horizontally. Openings at the bottom and top of the unit are for water jets. The vibrating unit is attached to a follow-up pipe. The figure below shows the vibroflotation equipment necessary for compaction in the field.

The entire compaction process can be divided into four steps:

Step 1. The jet at the bottom of the vibroflot is turned on, and the vibroflot is lowered into the ground.
Step 2. The water jet creates a quick condition in the soil, which allows the vibrating unit to sink.
Step 3. Granular material is poured into the top of the hole. The water from the lower jet is transferred to the jet at the top of the vibrating unit. This water carries the granular material down the hole.
Step 4. The vibrating unit is gradually raised in about 0.3 m lifts and is held vibrating for about 30 seconds at a time. This process compacts the soil to the desired unit weight.

Stone Column Method For Ground Improvement

Stone column method for ground improvement

Stone column method for ground improvement is a vibro-replacement technique, where the weak soil is displaced using a cylindrical vibrating probe (i.e. vibroflot), thus creating a column that is then filled and compacted with good-quality stone aggregates.

With the inclusion of stone aggregates to the in situ soil, its stiffness and load-carrying capacity increases. It also helps to reduce the static as well as differential settlement of the soils.

Bulging action of the stone columns imparts lateral confinement to the surrounding soils and it also acts as a drainage path accelerating the consolidation of cohesive soils.

These stone columns are generally used for soils that are much more compressible but not weak enough to necessitate a pile foundation. Moreover, for the construction of low-to-medium rise buildings on soft soils, pile foundation sometimes becomes expensive. In such cases, stone columns are preferred.

Stone columns are very useful for the improvement of cohesive soils, marine/alluvialclays, and liquefiable soils. Stone columns have been used successfully for a widerange of applications from the construction of high-rise buildings to oil tank foundation, and for embankment and slope stabilization.

Stone column installation methods

For the installation of stone columns a vibrating poker device is used that can penetrate to the required treatment depth under the action of its own weight, vibrations, and actuated air, assisted by the pull-down winch facility of the rig.

This process displaces the soil particles and the voids created are compensated with backfilling of stone aggregates. The vibroflot penetrates the filled stone aggregates to compact it and thus forces it radially into the surrounding soils.

This process is repeated till the full depth of the stone column is completed. The lift height is generally taken as 0.6–1.2 m for the filling and compaction of the stone aggregates.

Depending upon the feeding of stone aggregates into the columns there are basically two methods for the installation of stone columns:

1- Top-feed method

In the top-feed method, the stone aggregates are fed into the top of the hole. The probe is inserted into the ground and is penetrated to the target depth under its own weight and compressed air jetting. However, jetting of water is also done especially when the soil is unstable. This also helps to increase the diameter of the stone columns and to washout the fine materials fromthe holes.

The top-feed method is suitable when water is readily available and there is enough working space to allow for water drainage. Moreover, the soil types should be such that it would not create messy surface conditions due to mud in water.

The top-feed method is preferable when a deeper groundwater level is encountered.

Stone Columns installation Top-feed Method

2- Bottom-feed method

The bottom-feed method involves the feeding of stone aggregates via a tremie pipe along the vibroflot and with the aid of pressurized air. The bottom-feed method is preferable when the soil is highly collapsible and unstable. However, the stability of holes will also depend upon the depth, boundary conditions, and the groundwater conditions. In areas, where the availability of water and space and the handling of mudin process water are limiting factors, the bottom-feed method can be implemented.

Due to limited space in the feeding system, a smaller size of aggregates is used inthe bottom-feed method compared with that used in the top-feed method. On the otherhand, the flow of stones to the column is mechanically controlled and automatically recorded in the bottom-feed method.

Stone Columns installation Bottom Feed Method

Read More about Stone Columns: What are Stone Columns?

Tunnel Boring Machine Types

There are several types of TBMs. The best TBM for a project is based on the geological conditions of the site and the project’s features.The general classification of the different types of TBMs for both hardrock and soft ground are presented here.

Fig 1. TBM’s Classification

1. Gripper Tunnel Boring Machine

A gripper TBM is a rock tunnel boring machine which generally utilizes roller disc cutters as excavation tools and which moves forward by reacting (i.e., exerting shove forces) against the tunnel walls through a hydraulic gripper reaction system.

It is is suitable for driving in hard rock conditionswhen there is no need for a final lining. The rock supports (rock anchors, wire-mesh, shotcrete, and/or steel arches) can be installed directly behindthe cutter head shield and enable controlled relief of stress and deformations.

The existence of mobile partial shields enables gripper TBMs to be flexible even in high-pressure rock. This is useful when excavating in expanding rock to prevent the machine from jamming.

Fig 2. Typical diagram of an open gripper main beam TBM.Courtesy of TheRobbins Company

2. Double-Shield Tunnel Boring Machine

A double-shield TBM is generally considered to be the fastest machine for hard rock tunnels under favorable geological conditions with installation of the segment lining. It is possible to drive 100 m in 1 day. This type of TBM consists of a rotating cutter head and double shields (Fig. 3), a telescopic shield (an inner shield that slides within the large router shield), and a gripper shield together with a shield tail.

While boring, gripper shoes radially press against the surrounding rock to hold the machine in place and take some of the load from the thrust cylinders. For the motion of the front shield the gripper shoes are loosened, before the front shield is pushed forward by thrust cylinders protected by the extension of the telescopic shield.

Because regripping is a fast process, double-shield TBMs can almost continuously drill. As for the shield tail, it is used to provide protection for workers while erecting,installing the segment lining, and pea grouting.

Fig 3. Typical diagram of a double-shield TBM.Courtesy of The Robbins Company

3. Single-Shield Tunnel Boring Machine

Single-shield TBMs (Fig. 4) are used in soils that do not bear ground-water and where rock conditions are less favorable than for double shields, such as in weak fault zones. The shield is usually short so that a small radius of curvature can be achieved.

Fig 4. Typical diagram of single-shield TBM.Courtesy of The Robbins Company

4. Earth Pressure Balance Machines

Earth pressure balance (EPB) technology (Fig. 5) is suitable for digging tunnels in unstable ground such as clay, silt, sand, or gravel. An earth paste face formed by the excavated soil and other additives supports the tunnel face. Injections containing additives improve the soil consistency, reduce soil stick, and thus its workability.To ensure support pressure transmission to the soil, the earth paste is pressurized through the thrust force transfer into the bulkhead.

The TBM advance rate (in flow of excavated soil) and the soil outflow from the screw conveyor regulate the support pressure at the tunnel face. This is monitored at the bulkhead by the readings of pressure sensors.

Fig 5. Scheme of an EPB machine

5. Slurry Tunnel Boring Machine

Slurry TBMs are used for highly unstable and sandy soil and when the tunnel passes beneath structures that are sensitive to ground disturbances. Pressurized slurry (mostly bentonite) supports the tunnel face. The support pressure is regulated by the suspension inflow and outflow. The slurry’s rheology must be chosen in accordance with the soil parameters and should be carefully and regu-larly monitored.

6. Mixshield Technology

Mixshield technology (Fig. 6) is a variant of conventional slurry technology for heterogeneous geologies and high water pressure. In mixshield technology an automatically controlled air cushion controls the support pressure, with a submerged wall that divides the excavation chamber.This wall seals off the machine against the excess pressure from the tunnel’s face. As air is compressible in nature, the mixshield is more sensitive in pressure control and thus will provide more accurate control of ground settlement.

Fig 6. Overview of a mixshield TBM.Courtesy of Herrenknecht AG

7. Pipe Jacking

Pipe jacking (also called microtunneling) is a micro- to small-scale tunneling method for installing underground pipelines with minimum surface disruption. It is used for sewage and drainage construction, sewer replace-ment and lining, gas and water mains, oil pipelines, electricity and telecommunications cables, and culverts .

A fully automated mechanized tunneling shield is usually jacked forward from a launch shaft toward a reception shaft. Jacking pipes are then progressively inserted into the working shaft. Another significant difference between the pipe jacking method and shield method is that the lining of the pipe jacking is made of tubes and the lining of the shield method is made up of segments. In order to significantly reduce the resistance of the pipes, a thixotro-pic slurry is injected into the outside perimeter of the pipes. The thixo-tropic slurries can also reduce disturbance to the ground while pipe jacking slurry thickness. The thickness should be six to seven times thevoid between the machine and pipes.

Fig 7. Typical Components of a Pipe Jacking Operation

8. Partial-Face Excavation Machine

Partial-face excavation machines (Fig. 8) have an open-face shield andcan sometimes be more economical in homogeneous and semistable ground with little or no groundwater. In boulders layer, the open-face can deal with boulders much easier than closed shield machines. In cavity ground, the open-face can avoid the risk of falling down into the bottom of the cavity. Thanks to their simple design and that the operator workplace is close to the open tunnel face, these machines can easily be adapted to changing geological conditions. Good excavation monitoring can also be carried out.

Fig 8. Two kinds of partial-face excavation machines.Courtesy ofHerrenknecht AG

Soil Nail – Helping Combat Climate Change with Extraordinary Geotechnical Techniques

Pollution caused global warming which threatens our climate, our Earth. There have been significant changes in the behavior of Earth’s top layer and climate.

The risks of climate change require swift and deep reductions in emissions of heat-trapping gases and investments to prepare for now unavoidable impacts. Geo engineering is just one measure to combat the daunting challenge of keeping the rise in global temperatures in check.

Geo engineering or Climate engineering is the intentional large-scale intervention in the Earth’s climate system to counter climate change. It includes techniques like removing CO2 from the atmosphere, and steps to rapidly cool the Earth by reflecting solar energy back to space.

Geotechnical Engineering is a part of geoengineering that involves the application of soil and rock mechanics as well as engineering geology to solve engineering problems. These are design of foundations, slopes, excavations, dams, tunnels and other Civil, Mining and Environmental engineering projects relating to the mechanical response of the ground, and the water within it. It deals with many types of infrastructure – tunnels, bridges, dams, buildings, roads, railways, ports and landfills – that are built on the ground.

Soil nailing and its advantages

Soil nailing implies using grouted, tension-resisting steel elements (nails) to reinforce in situ soils and creating a gravity retaining wall for permanent or temporary excavation support.

Common uses

Stabilize slopes and landslides
Support excavations
Repair existing retaining walls

Advantages

Equipment is small enough to use in areas with restricted access
Often a more cost effective and faster solution for excavation support
Can be installed from crane-suspended working platforms for existing steep slopes, such as bluffs or existing retaining walls
Allows excavation to start at the same time as the shoring system is being installed

Precast Prestressed Spun Concrete Piles

Precast prestressed spun concrete piles are closed-ended tubular sections of 400 mm to 600 mm diameter with maximum allowable axial loads up to about 3 000 kN.

Pile sections are normally 12 m long and are usually welded together using steel end plates. Pile sections up to 20 m can also be specially made.

Precast prestressed spun concrete piles require high-strength concrete and careful control during manufacture.

Casting is usually carried out in a factory where the curing conditions can be strictly regulated.

Special manufacturing processes such as compaction by spinning or autoclave curing can be adopted to produce high strength concrete up to about 75 MPa. Such piles may be handled more easily than precast reinforced concrete piles without damage.

This type of piles is generally less permeable than reinforced concrete piles and may be expected to exhibit superior performance in a marine environment. However, they may not be suitable for ground with significant boulder contents. In such cases, preboring may be required to penetrate the underground obstructions.

Spalling, cracking and breaking can occur if careful control is not undertaken and good
driving practice is not followed

Precast Reinforced Concrete Piles

Precast reinforced concrete piles are not common nowadays.

These piles are commonly in square sections ranging from about 250 mm to about 450 mm with a maximum section length of up to about 20 m. Other pile sections may include hexagonal, circular, triangular and H shapes. Maximum allowable axial loads can be up to about 1 000 57kN.

The lengths of pile sections are often dictated by the practical considerations including
transportability, handling problems in sites of restricted area and facilities of the casting yard.
These piles can be lengthened by coupling together on site.

Splicing methods include welding of steel end plates or the use of epoxy mortar with dowels.

This type of pile is not suitable for driving into ground that contains a significant amount of boulders or corestones.