Category Archives: geotechnical

sheet pile

Sheet pile -Sheet piling types, sheet piling advantages

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A sheet pile is a type of driven pile that uses sections of sheet materials with interlocking edges. We generally install Sheet piles for lateral earth retention, excavation support, and shoreline protection operations. They are typically made of steel, but can also be made of vinyl, wood, or aluminium. Sheet piles are installed in sequence to the design depth along the excavation perimeter or seawall alignment. The interlocking sheet piles provide a wall for permanent or temporary lateral earth support with little groundwater inflow. We use Anchors strategically to provide lateral support Anchors.

We frequently use Sheet piles for seawalls, retaining walls, land reclamation, and underground constructions. Underground constructions include parking garages, and basements, in marine locations for riverbank protection, seawalls, cofferdams, and so on.

  1. Sheet piling method
  2. Sheet piles – Applications
    1. Retaining walls
    2. Coastal protection:
    3. Cofferdams:
    4. Underground structures
  3. Advantages of sheet pile
  4. Sheet piling types
    1. Steel Sheet piles
    2. Vinyl sheet pile
    3. Wooden sheet pile
    4. Features of wooden piles
    5. Concrete sheet pile
    6. Aluminium sheet piles
    7. Composite sheet piles
    8. Cellular sheet pile
    9. Cold-formed sheet piles
  5. Conclusion

Sheet piling method

Sheet piles can be temporary or permanent. Permanent steel sheet pile design demands a long service life. Often we install Sheet piles using vibratory hammers. If the earth is too hard or dense, we perform the installation with an impact hammer. Hot-rolling and cold-forming are the two major methods for creating sheet piles. Manufacturing of Hot rolled piles takes place at high temperatures, and the interlocks appear to be stronger and more durable.

We install Sheet piles by driving them into the ground with an impact hammer or vibratory driver and connect them to one another by a number of interlocking mechanisms. This includes tongue-and-groove, hook-and-grip, and clutch-bolt connections. Sheet piles, once erected, form a continuous barrier that resists lateral pressure from soil or water, avoiding soil erosion, landslides, and other soil failures.

sheet piles

Sheet piles – Applications

Piles find frequent utilisation in the following construction projects:

Retaining walls

Sheet piles help to construct retaining walls that hold back soil or water while also providing lateral support for excavations.

Coastal protection:

Sheet piling can protect coastal areas from erosion, waves, and storm surges. They can also be used to construct breakwaters and jetties.

Cofferdams:

We use Sheet piles to build cofferdams, which are transient obstructions in water to facilitate the construction of piers, bridges, or other water-based constructions.

Underground structures

We use Sheet piles to construct underground constructions such as basements or underground parking garages. They support the lateral structure and restrict soil or water intrusion.

Sheet piles have various advantages, including their versatility, ease of installation, and durability. Moreover, they offer an affordable option for projects that need lateral earth support. However, adequate design and installation are essential for guaranteeing the sheet pile wall’s stability and safety.

Advantages of sheet pile

Sheet piles provide several advantages in construction projects that require lateral earth support. Following are some of the key benefits of sheet piles:

  1. Versatility: Sheet piles find applications in a variety of construction projects, including retaining walls, shoreline protection, cofferdams, and underground structures.
  2. Speed of installation: We can install Sheet piles quickly and efficiently using impact hammers or vibratory drivers, which can reduce project timelines and construction costs.
  3. Durability: Since the material of construction Sheet piles is steel or other durable materials that can withstand harsh environmental conditions, including exposure to water, corrosion, and extreme temperatures, they are highly durable.
  4. Cost-effectiveness: Sheet piles generally prove to be a more affordable alternative to other types of foundation systems for projects requiring lateral earth support since they need less excavation and backfilling.
  5. Minimal disturbance: Sheet pile installation creates minimal disturbance to the surrounding soil and structures since we drive the piles into the ground without the need for excavation or other site preparation.
  6. Reusability: Sheet piles offer easy removal and reuse in other projects, making them a sustainable and eco-friendly alternative.
  7. We use Sheet piles for temporary and permanent structures and are available in a wide range of lengths, sizes and steel options.
  8. We can install Sheet piles rapidly using silent and vibration-free methods. The installation is easier and faster than secant walls.
  9. We can construct Cofferdams in almost any desired shape. Provide a close-fitting joint to form an effective water seal. Joints are designed to withstand the high pressure necessary for them to be placed in place. A little maintenance is needed above and underwater
sheet piling

Sheet piling types

Sheet piles are broadly classified as follows based on the material used for driving.

  • Steel sheet pile
  • Vinyl sheet pile
  • Wooden sheet pile
  • Concrete sheet pile
  • Composite sheet piles
  • Cellular sheet pile
  • Cellular sheet pile
  • Cold-formed sheet pile

Steel Sheet piles

Steel sheet piles are long, thin sections of steel that are driven into the ground to construct a retaining wall or a barrier. The most popular material for sheet piles is steel since we can lengthen it either by welding or bolting and has great water tightness as well as good resistance to severe driving stresses. They find extensive applications in civil engineering and construction projects to provide structural support for excavations, bridges, highways, and other structures.

Steel sheet piles are primarily made of hot-rolled steel and are available in a variety of shapes and sizes. We can link them together to form a continuous wall that acts as a strong barrier against the soil or water pressure. Steel sheet piles should endure heavy loads and give structural stability. Corrosion prevention techniques including coating and cathodic protection help increase the durability of steel sheet pile.

We frequently use Steel sheet piles in foundation work and deep excavations because they offer high resistance to lateral stresses and enable quick and simple installation. They are an eco-friendly option for temporary constructions because we can recycle them.

Overall, steel sheet piles are a versatile and cost-effective solution for a wide range of civil engineering and construction projects.

There are four basic forms of steel sheet piles, Normal sections, Straight web sections, Box sections and Composite sections.

Vinyl sheet pile

A vinyl sheet pile is a form of plastic sheet pile that finds applications in civil engineering and construction projects for a variety of purposes such as seawalls, bulkheads, flood walls, and retaining walls. Vinyl sheet pile primarily comprises polyvinyl chloride (PVC), a lightweight and long-lasting polymer that is resistant to corrosion, chemicals, and weathering. Because of its minimal maintenance requirements, simplicity of installation, and long-term durability, vinyl sheet pile is becoming more and more common in construction projects. Vinyl sheet pile, unlike traditional materials such as wood, steel, or concrete, does not require frequent maintenance or coating, making them a more cost-effective alternative in the long run.

Vinyl sheet pile is also environmentally friendly because it is reusable and does not leak dangerous chemicals into the soil or water. Because of its lightweight qualities, it is simple to transport and install, necessitating minimal use of heavy machinery and labour. Overall, vinyl sheet pile is a versatile and cost-effective solution for a variety of construction and civil engineering projects. Its durability, low maintenance requirements, and environmental benefits make it an appealing choice for contractors and engineers.

An effective alternative to steel sheet piling for bulkheads, seawalls and cutoff walls. They are also superior to alternative materials like concrete and wood. The main advantage of vinyl sheet piles is the superior corrosion resistance when exposed to seawater, where no oxidation occurs.

Vinyl sheet piles are lightweight and resistant to corrosion and chemical damage. They are often used in projects where environmental impact is a concern.

Wooden sheet pile

A wooden sheet pile is a type of retaining system comprising timber planks or boards. We commonly employ them in construction and civil engineering projects with a requirement for a retaining wall, either temporary or permanent. Hardwood sheet piles are a more environmentally friendly and long-lasting alternative to steel or concrete sheet piles. and they are widely utilised in places where environmental impact is a concern. In excavation work, we utilise them for braced sheeting and temporary structures. It must have some sort of preservative treatment for its utilisation in permanent structures above the water table. Even after treatment with a preservative, a timber sheet pile has limited life. Timber sheet piles are bonded using tongue and groove connections.

Features of wooden piles

Timber piles are not suitable in strata that contain gravel and boulders. Hardwood sheet piles are ideal for shallow excavations and we frequently utilise them in building projects where noise and vibration are a concern. They are lightweight and easy to handle, making them a popular choice for jobs requiring speedy installation. In comparison to other retaining wall materials, wooden sheet piles are also more affordable. Yet, there are significant drawbacks to using hardwood sheet piles. They are not as robust as steel or concrete sheet piles and require frequent maintenance to prevent rot and insect infestation. They may also be prone to warping and deformation if exposed to dampness for a lengthy period of time.

Hardwood sheet piles may not be suited for usage in places with high water tables or salinity in the soil, as these variables might accelerate the decomposition of the timber. Overall, hardwood sheet piles are an efficient and environmentally friendly option for small-scale building projects and temporary retaining walls. Yet, their durability and susceptibility to deterioration and warping make them unsuitable for long-term or large-scale applications.

wooden sheet piling
Wooden sheet pile

Concrete sheet pile

Concrete sheet piles are retaining walls constructed from precast reinforced concrete sections. We frequently employ them in civil engineering and building projects with a requirement for long-term retaining structures.

We must handle and drive the piles carefully, and provide the necessary reinforcement. The most common application of Concrete sheet pile is in deep excavations where soil conditions are unfavourable and we require lateral support. They are impermeable and can withstand hydrostatic pressure, making them excellent for usage in places with high water tables. We provide a capping to the heads of the piles by casting a capping beam, while we cut the toes with an oblique face to make driving and interlocking easier. They are relatively heavy and thick, and while driving, they displace significant amounts of the earth.

The driving resistance rises as a result of the considerable volume displacement. Concrete sheet piles are also resistant to weathering, corrosion, and erosion, making them a durable solution under extreme conditions. Concrete sheet piles are available in a range of dimensions and we can interlock them to create a continuous wall. We can place them in a variety of ways, including driving, vibrating, and pushing. The method of installation depends on the accessibility to the site, the depth of the installation, the state of the soil etc.

concrete sheet piling

Concrete sheet piles are a strong and long-lasting alternative, but their installation may be more costly and time-consuming than that of other retaining wall materials. However, installing them requires large machinery, which can be difficult in places with restricted access or space. Overall, concrete sheet piles are a viable option for permanent retaining walls in deep excavations and severe soil conditions. They are a preferred option for projects involving coastal protection and flood control due to their strength and resistance to water and erosion.

Aluminium sheet piles

Aluminium sheet piles are lightweight, strong, and corrosion-resistant, making them an ideal choice for projects that require a lightweight and durable material.

Composite sheet piles

We manufacture Composite sheet piles from a combination of materials, such as steel and concrete, to provide additional strength and durability. They often find applications in projects that require high load-bearing capacity.

Cellular sheet pile

We usually design Cellular sheet pile with hollow sections that allow for increased strength and load-bearing capacity. They find application in projects that requires a high degree of lateral support.

Cold-formed sheet piles

Cold-formed sheet piles are made by bending steel sheets into a desired shape. They find application in projects requiring lower strength and load-bearing capacity.

Conclusion

Each type of sheet pile has its own advantages and disadvantages, and the choice of material and design will depend on the specific requirements of the project. Proper design and installation are essential to ensure the stability and safety of the sheet pile wall, and consultation with an experienced engineer is recommended before selecting a specific type of sheet pile for a project.

geotechnical, RAFT FOUNDATION

Types of Raft Foundations – Advantages and features

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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.

  1. What is a raft foundation?
  2. Types of raft foundation – Principle
  3. Soil Stress Calculation
  4. Factors influencing the types of raft foundation
  5. Types of Raft foundations
  6. Types of raft foundations: Solid slab raft foundation
    1. Flat raft mat foundation
    2. Wide-toe raft
    3. Blanket raft foundation
    4. Slip plane rafts
  7. Slab beam-type raft foundation
  8. Piled raft foundation
  9. Cellular raft foundation
  10. Balancing or floating raft foundation
  11. Advantages of the Raft foundation
  12. Disadvantages of raft foundations

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 foundation
Types of Raft foundation

Types of Raft foundations – Youtube video

Video of Raft foundation- Types and Advantages

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.

Types of raft foundation - Flat raft mat
Types of raft foundation – Flat raft mat

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.

Types of raft foundation - Wide-toe raft
Wide-toe raft

Blanket raft foundation

blanket raft foundation
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.

Slip plane raft foundation
Slip plane raft foundation

Slab beam-type raft foundation

slip plane raft foundation
slip plane 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

piled raft foundation
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

cellular raft foundation
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

completed raft foundation.
completed 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.

Disadvantages of raft foundations

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.

geotechnical, RAFT FOUNDATION

Raft foundation types and features

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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.

Raft foundation
Raft foundation

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. 

Solid slab raft foundation
Solid slab raft foundation

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.

Wide toe raft foundation
Wide toe raft foundation

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.

Blanket rafts
Blanket rafts

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.

Slip plane raft foundation
Slip plane

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.

slab beam raft foundation
slab beam raft

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.

piled raft foundation
Piled raft foundation

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.

Cellular raft foundation
Cellular raft foundation

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

Advantages of Raft Foundations

  • 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.

Disadvantages of raft foundations

  • 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.

Geosynthetics, geotechnical

Geosynthetics- Types and applications.

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Geosynthetics are a much-talked-of topic as they are finding a wider range of applications nowadays. According to Fortune business Insights, the global geosynthetics market was valued at USD 27.16 billion in 2018. It is expected to grow at a CAGR of 6.6 percent to USD 45.25 billion by 2026.

The global geosynthetics market is being driven by growing construction activities. The rising applications in erosion management, landfill, and filtration also contributes for that.

All those who are curious about this multi-faceted material have landed in the right spot. In this blog, I will show you full details about geosynthetics. Are you ready to dig deeper about it?

 Geosynthetics Types:

Before we learn about geosynthetics types, let’s have a basic understanding about Geosynthetics material.

  • Geosynthetics are planar products made of polymeric materials. They are used in conjunction with soil, rock, earth, or other geotechnical engineering-related materials.
  • The products’ polymer nature makes them ideal for use in the ground where long-term durability is needed.
  • Geosynthetics are manufactured goods with at least one part made of a synthetic or natural polymer in the form of a sheet, strip, or three-dimensional structure.

The types of geosynthetics are further described in brief as follows:

  • Geotextiles
  • Geogrids
  • Geonets
  • Geomembranes
  • Geosynthetic claylines
  • Geoform
  • Geocells

Now we are good to go to know more about each of these.

Geo textiles

  • A geotextile is a continuous sheet made up of woven, non-woven, knitted, or stitch-bonded synthetic fibres or yarns.
  • These sheets are flexible and permeable. 
  • Separation, filtration, irrigation, stabilisation, and erosion control are all applications of geo textiles.
  • They are used to reinforce earth structures by using fill materials.
Geotextile
geotextile

Geogrids

  • Polymers with wide apertures between individual ribs in both the transverse and longitudinal directions, forming a very open, grid-like configuration. 
  • Produced by stretching rods in one, two, or three directions using traditional textile manufacturing methods such as weaving or knitting machines, or by laser or ultrasonically bonding rods.
  • Used to reinforce soil in retaining walls, and sub bases below roads or structures.
Geogrid
Geogrid

Geonets

  • Made by extruding parallel sets of polymeric ribs at acute angles to each other in a series of steps.
  •  Made up of three-dimensional networks of a rigid polymer or rigid polymer fibres.
  • The design purpose is the containment of a fluid. Hence they are used to transport a variety of fluids.

Geomembranes

  • Geomembranes are impervious polymeric sheets. They are used mainly for the linings and covers of liquid and solid storage facilities. 
  • The second-largest category of geosynthetics.
  • All types of landfills, surface impoundments, canals, and other containment facilities fall under this category. 
Geomembrane
geomembrane

Geosynthetic clay liners

  • GCLs, or geosynthetic clay liners, are an intriguing mix of polymeric materials and natural soils. 
  • They’re thin layers of bentonite clay fabricated in the factory. It is sandwiched between two geotextiles or bonded to a geomembrane. 
  • Needle-punching, stitching, or adhesive bonding ensures the structural integrity of the resulting composite. 
  • Applications

– Geoenvironmental and containment applications

– Transportation

– Geotechnical

– Hydraulic

– As a composite portion underneath a geomembrane. 

Geofoam 

  • A polymeric product made from polystyrene that has been processed into a foam with several closed cells filled with air and/or gases. 
  • The skeletal design of the cell walls is reminiscent of bone structures made of non-expanded polymeric material.
  •  They are huge, but extremely light, blocks. It can be stacked side by side and in layers to provide the lightweight fill.

Geocells

  • Also called Cellular Confinement Systems.
  • 3-D honeycombed cellular structures. When it is filled with compacted soil, forms a confinement system. 
  • The rigid (and normally textured and perforated) walls of a flexible 3D cellular mattress are extruded from polymeric materials into strips that are welded together ultrasonically in sequence. 
  • The cell-soil interactions produce a new composite object when it is filled with soil.
Geogrid
geogrid

Eager to know about their applications which made them immensely popular? Read on to know more about their uses.

Geosynthetics Applications

Separation

  • Separation is the use of a flexible geosynthetic material. It consists of a porous geotextile, between two dissimilar materials in order to maintain or strengthen the consistency and functionality of each.
  • Applications: Paved roads, unpaved roads, and railroad bases

Geosynthetics for Reinforcement:

  • The introduction of a geogrid, or geocell into the soil or other separated material results in a synergistic improvement of a total system’s strength. 
  • Application: Mechanically stabilised and retained earth walls, steep soil slopes.

Barrier:

  • Some geosynthetics can act as a relatively impermeable barrier. It prevents liquids and gases from mixing between the two soil layers.
  • Because of their low permeability, geomembranes are effective inhibitors.
  • Application: Pavement overlays

Drainage: 

  • Certain geosynthetics may also be used as a drain. They allow any fluid to flow through a low-permeable soil.
  • Non-woven geotextiles can help with drainage, and geocomposite can be used for high flows.
  • To avoid track faults, proper drainage is critical.

Geosynthetics for Protection: 

  • Geosynthetics can provide friction and perforation protection.
  • The geotextile will serve as a cushion over the geomembrane during construction, preventing or minimising damage to the ground floor layer.

Next, let’s study its advantages and disadvantages.

Advantages of Geosynthetics

  • Geosynthetic sheets take up less room in landfills and can fit almost anywhere.
  • Since geosynthetic materials are processed in a quality-controlled workshop, they are homogeneous.
  • In comparison to traditional materials, it increases soil properties.
  • When compared to aggregates and soils, geosynthetic materials are less costly to buy, ship, and install.

Disadvantages of Geosynthetics

  • To ensure the long-term efficiency of the formulated resin in geosynthetics, additives like antioxidants, ultraviolet screeners, and fillers must be used.
  • Since geosynthetics are polymeric, their exposed lifespan is shorter than when they are unexposed.
  • For certain soil types (Loess soils, fine cohesionless silts, extremely turbid liquids) clogging of geotextiles, geonets, and/or geocomposites is a difficult design challenge.

What do you think of these synthetic materials which has a range of applications? Let us know in the comments.

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geotechnical, Retaining wall

MSE Retaining Walls – Components & advantages.

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MSE retaining walls means mechanically stabilised earth or reinforced earth. This is an earth retaining system where compacted granular soil is reinforced with horizontal layers of steel strips or geo-synthetic materials. This compacted earth is held together with thin facing elements made of Precast concrete, shotcrete or weld mesh reinforced panels. They are used extensively for constructing retaining walls, bridge abutments, highway wall systems, dykes, etc. MSE retaining walls cost almost half what a concrete structure would have cost for similar uses.

Components of a MSE retaining wall

a) Reinforcing element

b) Back fill materials

c) Fascia element

Typical section of MSE Retaining wall
TYPICAL SECTION OF MSE RETAINING WALLS

Reinforcement element

The reinforcements are used to reinforce and provides the requisite tensile strength to hold the soil together. Two types of reinforcing elements are used in MSE walls. They are metallic and polymeric reinforcements. Metallic reinforcements (In-extensible) include Galvanised iron ribbed strips (50mm-100mm) or ladder strip arrangements.

GALVANISED IRON RIBBED STRIPS
GALVANISED IRON RIBBED STRIPS AND LADDER STRIPS

Polymeric reinforcement (Extensible reinforcement) could be geo-grids or geo-textiles, which are preferred in corrosive environments. For any vertical and horizontal obstructions, reinforcements are bend at an angle, not more than 15 degrees.

GEOGRIDS
GEOGRIDS

Appropriate connections hooks are embedded behind the fascia walls for the anchorage of geo-grids and metal reinforcements.

Back fill materials

The select back fill materials have to be cohesion-less and have to meet the criteria of gradation, plasticity, organic content, and electrochemical properties, The materials should be free drainage and with minimum fine content. The soil friction angle has to be checked by shear tests. The angle of interface friction between compacted fill and a reinforcing element shall not be less than 30 degrees when measured as per IS 13326 part 1.

FLYASH BACK FILL
FLY ASH BACK FILL

Fly ash can also be used as back fill and shall be as per the standards. The select back fill lift should be placed parallel to the wall and starting approximately three (3) feet from the back of the wall panels. The back fill should be placed in 6″ compacted lifts. Soil materials can also be placed without reinforcement between the stabilised zone and natural surface of the ground called retained back fill.

Facing elements

Facing elements are used to retain the filled materials, prevent local slumping of steeply sloping faces, and to be in line with the structural and design, and aesthetic requirements.
The facings are made of precast reinforced cement concrete, plain concrete hollow blocks forum filled, etc.
The embankment area has to be provided with a suitable drainage system to avoid water logging. (Ref fig) Drainage layers have to be provided on the backside of reinforcement zones for enabling proper drainage of water.
A Drainage layer of around 2-3ft width is provided on the backside of the facing wall using free-draining material to enable proper drainage of water.

WALL DRAINAGE
WALL DRAINAGE

Jointing and filling materials

Rubber or wooden bearing pads are used between horizontal joints of facing elements so that there shall not be any concrete to concrete joints. The interior panel joints are sealed with geotextile filler cloth in the horizontal and vertical directions as shown in fig. This is done to ensure that no interior back fill materials sweep through the joints.

RUBBER PADS AND GEO TEXTILE LAYER
RUBBER PADS AND GEO TEXTILE LAYER

Advantages of MSE walls

Advantages in terms of economy, ease of construction and rapid and speedy construction with minimum disturbances to traffic and other services makes MSE walls one of the most favourites and preferred retaining wall system. A variety of materials and customisation options in terms of design and construction made it one of the most popular earth retaining system. The fascia elements, the back-fill, and the reinforcing system combine to form a gravity retaining structure that relies on the self-weight of the reinforced soil mass. This self-weight resists the lateral pressure from the earth and the service loads, seismic loads, and hydro static pressure.

  • They can be designed to take extremely heavy loads like bridge abutment footings, crane loads, service loads, etc
  • MSE walls can resist seismic and dynamic forces and transfers the bearing pressure to a wide area.
  • Faster construction than conventional retaining walls.
  • Less site preparation is required and can be constructed in confined areas where other retaining walls are impossible to construct.
  • There is no supports, finishes and curing time.
  • The granular back filling enables free drainage of water through the exposed panel joints and reduces hydro static pressure.
  • The fascia walls are lightweight and are precast and conveyed to the site and lifted using simple lifting equipment. These walls can be made to any height and can resist unequal settlements
  • They can be customised to any geometry and the construction process is very simple. They do not need any heavy types of machinery and specialised workers.
  • The fascia can be customised for designs and logos and gives superior and elegant finished and aligned walls.
  • Any obstructions inside the back filled areas can be managed by adjusting the angle of the reinforcing elements.
  • They possess a very good service life in extreme loading and complex applications.

Disadvantages of MSE retaining walls.

  • MSE retaining walls require granular material in huge quantities. Areas where there is a scarcity of granular material the construction cost increase and make the structure uneconomical.
  • The corrosion or reinforcement and deterioration of geo-grids on exposed to sunlight has to be addressed. The reinforced component must be designed to withstand erosion and corrosion processes which can highly deteriorate the mechanical behaviour of the composite structure.
  • Proper drainage system should be provided.
  • The wall must obtain a minimum width in order to acquire adequate stability

Also read

INSTALLATION METHOD OF MSE RETAINING WALLS

geotechnical, Retaining wall

Soil Nailing – Installation,advantages and applications

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Soil nailing is a slope protection technique for supporting unstable natural slopes and over steeping of existing slopes. Soil nails are reinforcing passive elements drilled and grouted sub-horizontally in the ground. They are used to support excavations in soil, or soft and weathered rock and slope protection works.

Soil nail walls are used as permanent earth-retaining structures in highway projects. They can also be constructed as temporary structures in roadway work when used as shoring of deep excavations.

Components of soil nail and its function

Components mainly constitutes installing passive reinforcement without any post tensioning in existing ground know as nails. Soil nails are later grouted if they are installed in drilled holes. Soil nails using solid bar drilling system or hollow bars (sacrificial hollow bar system which drills and grouts simultaneously ) need not need any grouting. Let us go in detail each and every component of soil nailing system.

Typical cross section of soil nail

a) Tendons

They are the ground reinforcing elements and are equivalent to steel bars. Tendons can take care of tensile stress developed during the lateral movement and deformation of retained soil and also the external loads in the service stage such as surcharge loads and traffic loads.

There are two methods of fixing soil nails or reinforcement bars.
a) Holes are drilled and pressure grouted with fully threaded bars embedded inside.
b) Using sacrificial drill bits where drilling and grouting will be done simultaneously and the sacrificial drill bit is converted to rebar.

b) Grout

Normal OPC cement mixed with water is used for grouting. The function of grout are

a) Transfers shear stress between the ground and tendons and

b) Corrosion protection for rebars.

c) Installation of facing 

Soil nail construction is done from top to bottom. Every nail is installed with anchor plates or bearing plates.
First, a single row of soil nails is installed after excavating the surface. Excavate further and install the second layer of soil nails as per design. After completing a reasonable height at which soil can free stand ( 1-2mtr) for at least 2 -3 days the next phase of the shotcreting process will start.

soil nailing

d) First face shotcrete on soil surface

A geotextile drain matting is placed over the soil and is followed by welded wire mesh as shown in the figure. Rebar stiffeners are sometimes provided to strengthen the shotcrete against punch shear forces. On completion of the first phase of shotcreting bearing plates with bevelled washers are installed over the shotcrete surface.

e) Second phase of reinforced concrete

If required as per design the first layer is covered with a second phase of reinforced concrete layer as shown in the figure. This concrete covers the nail head.

Applications of soil nails

Soil nails are one of the most economical and feasible tops to bottom constructed retaining walls system. They are technically feasible, fast, and reliable slope protection and earth retaining system. Soil nails offer a perfect cost-effective system for temporary retaining walls for deep excavations..

  • High way cut excavation of hilly areas
  • Road widening under an existing bridge end.
  • Repair and reconstruction of existing retaining structures.
  • Temporary or permanent deep excavations in urban areas.

Feasibility of soil nail

Before confirming the soil nail system please ensure the following parameters at the site.

  • Soil should be able to free stand at a height of around 1-2 mtr unsupported for a minimum of two days.
  • All soil nails within a cross-section will be above the water table.
  • If the soil nails are not located above the groundwater table, the groundwater should not negatively affect the face of the excavation, the bond between the ground and the soil nail itself.
  • They can be used for almost all types of soils including stiff/sandy/hard clay, dense sand and gravel areas, evenly weathered rocks.
  • Avoid using soil nails in dry, poorly graded cohesion-less soils, soils with a high groundwater table, soils with cobbles and boulders, soft to very soft fine-grained soils, highly corrosive soils, weathered rock with unfavourable weakness planes, etc.

Advantages of soil nails over other retaining systems

  • Soils nails require very little workspace comparing with any other retaining systems.
  • The operations are mostly light and silent and there is no disturbance to the traffic and people residing nearby.
  • Soil nails do not need any foundation or any structural whaler beams at the bottom like cantilever and anchored retaining walls.
  • With the soil nailing method, we can reduce the duration of work, and fewer materials are consumed in this process.
  • They are so flexible and easily customizable and nail location can be easily adjusted on encountering any obstructions.
  • Small equipment is used for soil nailing works.
  • They can accommodate differential settlements and deflection of soil nails are usually within tolerable limits.
  • They are cost-effective than any other retaining wall system because shotcrete of minimal thickness is used than heavy structural walls in the case of other retaining wall systems.