It’s hard to imagine civil engineering projects without retaining
walls. What is also true is that these retaining walls consume large
amount of resources and space. Any economy in the retaining walls is
therefore, always welcome. Now, the question arises as to how can one
achieve this objective, without compromising on the quality front?
Thankfully, now a solution is available in the form of a revolutionary
technology “Introjected Backfil Retaining Walls”, developed by
engineering consulting firm StrongGid Technologies.
StrongGid Technologies has been formed by a group of researchers and academicians and have been pioneering the inventions of many technologically sound, yet cost-effective solutions in the field of Geotechnical Engineering and Concrete Technology. ”: The technology of “Introjected Backfil Retaining walls” was invented in the year 2001 by Prof. D.R.Phatak (Retd. Professor College of Engineering, Pune) along with his student Prof. S.S.Sabnis, both part of the consultancy firm. The invention, a design innovation, is under Intellectual property rights (I.P.R’s) and is under successful implementation since from 2001. In the last three years, retaining walls worth ` 100 crore have been constructed using this technology benefiting many government and non-government organizations.
The technology has been extensively used by Govt. of Maharashtra (Water Resources Department and MSRDC) in bridge approaches and abutments, roads, irrigation projects, river training works and underground water tanks. For example, the technology has been utilized for a height of 26m by the Water Resources Department, Government of Maharashtra at Sawant-wadi near the Goa border. Another good example is available in the form of a project of Mahindra Vehicle Manufacturers Ltd, Chakan, in 2011, wherein about 3. 5km length of retaining walls with a height variation of 5m to 15m was constructed using the technology. The estimated cost of the retaining walls using conventional technique was ` 24 crore. However, by using the technique of “Introjected Backfil Retaining Walls,” the entire work was accomplished in
` 14 crore, a saving of ` 10 crore.
The technology is gaining in popularity and is being presently used by several private parties, including MNCs and infra companies to reduce the cost of constructing retaining walls. Using this technology, equity can be saved on various earth retaining structures inclu-ding, (1) Port and sea front walls, berthing walls, (2) Guide walls, divide walls in dam structures, (3) Wing walls and returns of bridges, culverts, viaducts and aqueducts, (4) Property confinement retaining walls, (5) Concrete dams, (6) Bridges spanning upto 40 m, (7) Underground oil and water tanks and (8) Underground sewage treatment plants.
Conventional Retaining Walls and their Drawbacks
The most common types of retaining walls practiced are as follows:
1. Gravity
2. RCC Cantilever
3. Counterfort
Gravity retaining walls
The Gravity retaining walls are easy to construct as these involve huge amount of construction material in the form of concrete or stone masonry. The earth retention is done purely by means of the body weight of the wall and hence these walls are bulky. The base width required is 0.55 to 0.65 (Fig. 1) times the height for the wall to become stable.
RCC Cantilever retaining walls
The RCC Cantilever retaining walls retain the load of the sliding earth mass purely by the reinforcement provided in the slender concrete members (Fig.2). These walls are suitable upto about 6 m height and involve huge amount of closely spaced reinforcing bars. The quantity of reinforcement may vary between 70 to 110 kg /cu.m (1 to 1.2% of concrete quantity) for these walls. The base width required is 0.6 to 0.7 times the height for the wall to become stable.
RCC Counterfort retaining walls
The RCC cantilever retaining walls are provided with counterforts when the height exceeds 6m (Fig.3). The support in the form of counterforts facilitates raising the height of the retaining wall above 6m. But all this comes at the cost of more reinforcement and higher grade of concrete. The quantity of reinforcement may vary between 70 to 110 kg /cu.m (1 to 1.2% of concrete quantity). The base width required is 0.6 to 0.7 times the height, for the wall to become stable.
The major drawback of the commonly used conventional retaining walls is that they consume lot of resources and require great space to accommodate them.
When analyzing the role of civil engineers who use conventional retaining walls, it is seen that the site engineers have an inherent tendency to make some economy while using conventional retaining walls on the site. The economy is routinely achieved on the site by means of the following methods:
- Use of stone masonry instead of concrete
- Use of plums or greater MAS to make concrete
- Change of the geometry of the walls
These routine methods of achieving economy results in a limited economy of 2 to 3 % and involve greater supervision and maintenance. Stone masonry is suitable only upto a height of 3 to 4 m as the wall demands more space for accommodation. The maintenance of the stone masonry wall may also go up in a span of 5 to 6 years. Additionally, some times the local people are also tempted to remove the stones from the constructed masonry wall for other uses. The use of plums and greater mean aggregate size to make concrete results in limited economy but at the same time the mix design of concrete has to be affirmed for its effectiveness. The method of change of geometry of walls to achieve economy results in cluster of calculations which are to be checked and re-checked for accuracy and often involves skepticism of the higher authorities before giving approvals. Often it is seen that the economy in such cases also does not exceed 3 to 4 %.
When all these factors are taken into consideration, it leads to a million dollar question- “whether any technology or methodology of retaining wall design is available, which will miraculously change the economics of the project?” Fortunately, the answer is available in the form of ‘Introjected Backfil Retaining Walls.’
Aqueduct Project in Sawantwadi
A relatively recent project is a testimonial to the efficacy of the technology, both in terms of obtaining quality construction, and also in terms of cost-effectiveness. The project shown in Fig.4 is an aqueduct which is carrying water from Tilari dam to Sawantwadi town and is situated on the right bank canal. The water from the Tilari dam flows in the right bank canal; passes through a tunnel of length of 1.5 km in the mountain as shown. Immediately after the mountain is a valley (in Fig. 4 the truck is standing in the valley). After the valley, again there is a hillock. Since the canal flowing through the tunnel in the mountain cannot be brought down in the valley, an aqueduct, as shown, was proposed in the year 2007. The aqueduct portion got constructed and even the tunnel face was opened and completed. The problem remained of the connecting canal between the aqueduct and the tunnel mouth. For this were required wing walls of height 26 m which are nothing but retaining walls, which would retain earth and ultimately the earth will support a canal through it.
The wings walls by conventional method (gravity type) required a base width of about 18 m. It was getting difficult to accommodate the 18 m base width of the conventional retaining walls because any excavation near to the mountain was making it unstable. Hence this technology was proposed by the Irrigation Department of Government of Maharashtra. The “Introjected Backfil Retaining Walls” technology required a base width of only 6.5 m. The stability parameters like factors of safety in overturning and factor of safety in sliding were more than 4 both in Static and Seismic conditions. The grade of concrete proposed in the wall body was M15, as the durability considerations demanded the minimum grade as M15. All the designs satisfied the Indian Codal provisions.
Not only did the technology ensure cost savings to the tune of about 27% over conventional retaining walls, it also solved the problem of space restriction. The structure has been operational from the last four years.
Salient features of the technology
Base Width: The base width of the wall is ½ to 1/3 of the conventional re-taining walls. The wall requires simultaneous backfilling during the construction process of the wall.
Concrete Grade: The “Introjected Backfil Retaining Walls” can be constructed in a concrete grade of M15. Only if the durability considerations demand a higher grade of concrete then only the grade of concrete needs to be upgraded for these walls, otherwise a concrete grade of M15 is suitable for the construction of there wall.
Reinforcement: The quantity of reinforcement required in “Introjected Backfil Retaining Walls” is between 8 to 20 kg per cu.m of concrete only.
Stability: The walls are safe both in static and seismic conditions and satisfy the Codal provisions for stability in both static and seismic conditions. The “Introjected Backfil Retaining Walls” are more stable than conventional retaining walls for the same height. The factors of safety in static conditions and seismic conditions are nearly double that of conventional retaining walls.
The construction of “Introjected Backfil Retaining Walls” is straight and does not require any batter to any face of the wall. It is the experience of the Inventors that the time required to construct “Introjected Backfil Retaining Walls” is less than conventional retaining walls of the same height. The technology can be implemented to unlimited heights.
No concerns in implementation of the technique: The material of construction remains the same as the conventional walls with no special demand for formwork or backfill material. The wall design fits as per the tender requirements with no special demand for any new item of construction. The wall design is flexible enough to adapt to the items of the existing tender and hence successfully implemented in many projects which were incomplete either due to technical reasons or due to inadequate funds.
Base width remains same even after change of height: It is seen in conventional walls that the base width changes whenever there is a change in the height of the retaining wall. The construction procedure for conventional retaining walls becomes clumsy when there is abrupt change in the height of the walls in shorter distances e.g. Wings walls/return walls of bridges. In such cases, the “Introjected Backfill Retaining Walls” maintains same base width which helps in ease and speedy construction.
Uniform Pressure Distribution below Walls: The wall maintains uniform pressure distribution. This is explained in Fig. 5.
Hence these walls are extremely stable in areas of low bearing capacity also.
Economy: The “Introjected Backfil Retaining Walls” give an economy between 25 to 35% over the conventional retaining walls without compromising any strength or stability parameters still satisfying the codal provisions. Interestingly, as the height of retaining walls increases the economy due to “Introjected backfil retaining walls” also increases. This is for the reason that the increase in the wall section and reinforcement is nearly proportional to the height of retention in case of conventional retaining walls. This proportionality is not followed by the “Introjected backfil retaining walls”.
On an average, the conventional retaining walls require a B/H ratio (Base width to height ratio) of about 0.65 and reinforcement in the range of 70 to 110 kg/cu.m of concrete, whereas for “Introjected backfill retaining walls” the ratio of B/H is between 0.25 to 0.35 and the requirement of reinforcement is between 10 to 25 kg/cu.m of concrete (i.e. 0.2 % of concrete quantity).
Cost Economics of “Introjected Backfil Retaining Walls”: The graphical representation in Fig.6 below shows the variation of cost between “Introjected Backfill Retaining Walls” and conventional retaining walls.
Another major achievement of the technology has been the fact that till date it has been found to be ideally suited to nearly all types of strata.
StrongGid Technologies has been formed by a group of researchers and academicians and have been pioneering the inventions of many technologically sound, yet cost-effective solutions in the field of Geotechnical Engineering and Concrete Technology. ”: The technology of “Introjected Backfil Retaining walls” was invented in the year 2001 by Prof. D.R.Phatak (Retd. Professor College of Engineering, Pune) along with his student Prof. S.S.Sabnis, both part of the consultancy firm. The invention, a design innovation, is under Intellectual property rights (I.P.R’s) and is under successful implementation since from 2001. In the last three years, retaining walls worth ` 100 crore have been constructed using this technology benefiting many government and non-government organizations.
The technology has been extensively used by Govt. of Maharashtra (Water Resources Department and MSRDC) in bridge approaches and abutments, roads, irrigation projects, river training works and underground water tanks. For example, the technology has been utilized for a height of 26m by the Water Resources Department, Government of Maharashtra at Sawant-wadi near the Goa border. Another good example is available in the form of a project of Mahindra Vehicle Manufacturers Ltd, Chakan, in 2011, wherein about 3. 5km length of retaining walls with a height variation of 5m to 15m was constructed using the technology. The estimated cost of the retaining walls using conventional technique was ` 24 crore. However, by using the technique of “Introjected Backfil Retaining Walls,” the entire work was accomplished in
` 14 crore, a saving of ` 10 crore.
The technology is gaining in popularity and is being presently used by several private parties, including MNCs and infra companies to reduce the cost of constructing retaining walls. Using this technology, equity can be saved on various earth retaining structures inclu-ding, (1) Port and sea front walls, berthing walls, (2) Guide walls, divide walls in dam structures, (3) Wing walls and returns of bridges, culverts, viaducts and aqueducts, (4) Property confinement retaining walls, (5) Concrete dams, (6) Bridges spanning upto 40 m, (7) Underground oil and water tanks and (8) Underground sewage treatment plants.
The most common types of retaining walls practiced are as follows:
1. Gravity
2. RCC Cantilever
3. Counterfort
Gravity retaining walls
The Gravity retaining walls are easy to construct as these involve huge amount of construction material in the form of concrete or stone masonry. The earth retention is done purely by means of the body weight of the wall and hence these walls are bulky. The base width required is 0.55 to 0.65 (Fig. 1) times the height for the wall to become stable.
RCC Cantilever retaining walls
The RCC Cantilever retaining walls retain the load of the sliding earth mass purely by the reinforcement provided in the slender concrete members (Fig.2). These walls are suitable upto about 6 m height and involve huge amount of closely spaced reinforcing bars. The quantity of reinforcement may vary between 70 to 110 kg /cu.m (1 to 1.2% of concrete quantity) for these walls. The base width required is 0.6 to 0.7 times the height for the wall to become stable.
RCC Counterfort retaining walls
The RCC cantilever retaining walls are provided with counterforts when the height exceeds 6m (Fig.3). The support in the form of counterforts facilitates raising the height of the retaining wall above 6m. But all this comes at the cost of more reinforcement and higher grade of concrete. The quantity of reinforcement may vary between 70 to 110 kg /cu.m (1 to 1.2% of concrete quantity). The base width required is 0.6 to 0.7 times the height, for the wall to become stable.
The major drawback of the commonly used conventional retaining walls is that they consume lot of resources and require great space to accommodate them.
When analyzing the role of civil engineers who use conventional retaining walls, it is seen that the site engineers have an inherent tendency to make some economy while using conventional retaining walls on the site. The economy is routinely achieved on the site by means of the following methods:
- Use of stone masonry instead of concrete
- Use of plums or greater MAS to make concrete
- Change of the geometry of the walls
These routine methods of achieving economy results in a limited economy of 2 to 3 % and involve greater supervision and maintenance. Stone masonry is suitable only upto a height of 3 to 4 m as the wall demands more space for accommodation. The maintenance of the stone masonry wall may also go up in a span of 5 to 6 years. Additionally, some times the local people are also tempted to remove the stones from the constructed masonry wall for other uses. The use of plums and greater mean aggregate size to make concrete results in limited economy but at the same time the mix design of concrete has to be affirmed for its effectiveness. The method of change of geometry of walls to achieve economy results in cluster of calculations which are to be checked and re-checked for accuracy and often involves skepticism of the higher authorities before giving approvals. Often it is seen that the economy in such cases also does not exceed 3 to 4 %.
When all these factors are taken into consideration, it leads to a million dollar question- “whether any technology or methodology of retaining wall design is available, which will miraculously change the economics of the project?” Fortunately, the answer is available in the form of ‘Introjected Backfil Retaining Walls.’
Aqueduct Project in Sawantwadi
A relatively recent project is a testimonial to the efficacy of the technology, both in terms of obtaining quality construction, and also in terms of cost-effectiveness. The project shown in Fig.4 is an aqueduct which is carrying water from Tilari dam to Sawantwadi town and is situated on the right bank canal. The water from the Tilari dam flows in the right bank canal; passes through a tunnel of length of 1.5 km in the mountain as shown. Immediately after the mountain is a valley (in Fig. 4 the truck is standing in the valley). After the valley, again there is a hillock. Since the canal flowing through the tunnel in the mountain cannot be brought down in the valley, an aqueduct, as shown, was proposed in the year 2007. The aqueduct portion got constructed and even the tunnel face was opened and completed. The problem remained of the connecting canal between the aqueduct and the tunnel mouth. For this were required wing walls of height 26 m which are nothing but retaining walls, which would retain earth and ultimately the earth will support a canal through it.
The wings walls by conventional method (gravity type) required a base width of about 18 m. It was getting difficult to accommodate the 18 m base width of the conventional retaining walls because any excavation near to the mountain was making it unstable. Hence this technology was proposed by the Irrigation Department of Government of Maharashtra. The “Introjected Backfil Retaining Walls” technology required a base width of only 6.5 m. The stability parameters like factors of safety in overturning and factor of safety in sliding were more than 4 both in Static and Seismic conditions. The grade of concrete proposed in the wall body was M15, as the durability considerations demanded the minimum grade as M15. All the designs satisfied the Indian Codal provisions.
Not only did the technology ensure cost savings to the tune of about 27% over conventional retaining walls, it also solved the problem of space restriction. The structure has been operational from the last four years.
Salient features of the technology
Base Width: The base width of the wall is ½ to 1/3 of the conventional re-taining walls. The wall requires simultaneous backfilling during the construction process of the wall.
Concrete Grade: The “Introjected Backfil Retaining Walls” can be constructed in a concrete grade of M15. Only if the durability considerations demand a higher grade of concrete then only the grade of concrete needs to be upgraded for these walls, otherwise a concrete grade of M15 is suitable for the construction of there wall.
Reinforcement: The quantity of reinforcement required in “Introjected Backfil Retaining Walls” is between 8 to 20 kg per cu.m of concrete only.
Stability: The walls are safe both in static and seismic conditions and satisfy the Codal provisions for stability in both static and seismic conditions. The “Introjected Backfil Retaining Walls” are more stable than conventional retaining walls for the same height. The factors of safety in static conditions and seismic conditions are nearly double that of conventional retaining walls.
The construction of “Introjected Backfil Retaining Walls” is straight and does not require any batter to any face of the wall. It is the experience of the Inventors that the time required to construct “Introjected Backfil Retaining Walls” is less than conventional retaining walls of the same height. The technology can be implemented to unlimited heights.
No concerns in implementation of the technique: The material of construction remains the same as the conventional walls with no special demand for formwork or backfill material. The wall design fits as per the tender requirements with no special demand for any new item of construction. The wall design is flexible enough to adapt to the items of the existing tender and hence successfully implemented in many projects which were incomplete either due to technical reasons or due to inadequate funds.
Base width remains same even after change of height: It is seen in conventional walls that the base width changes whenever there is a change in the height of the retaining wall. The construction procedure for conventional retaining walls becomes clumsy when there is abrupt change in the height of the walls in shorter distances e.g. Wings walls/return walls of bridges. In such cases, the “Introjected Backfill Retaining Walls” maintains same base width which helps in ease and speedy construction.
Uniform Pressure Distribution below Walls: The wall maintains uniform pressure distribution. This is explained in Fig. 5.
Hence these walls are extremely stable in areas of low bearing capacity also.
Economy: The “Introjected Backfil Retaining Walls” give an economy between 25 to 35% over the conventional retaining walls without compromising any strength or stability parameters still satisfying the codal provisions. Interestingly, as the height of retaining walls increases the economy due to “Introjected backfil retaining walls” also increases. This is for the reason that the increase in the wall section and reinforcement is nearly proportional to the height of retention in case of conventional retaining walls. This proportionality is not followed by the “Introjected backfil retaining walls”.
On an average, the conventional retaining walls require a B/H ratio (Base width to height ratio) of about 0.65 and reinforcement in the range of 70 to 110 kg/cu.m of concrete, whereas for “Introjected backfill retaining walls” the ratio of B/H is between 0.25 to 0.35 and the requirement of reinforcement is between 10 to 25 kg/cu.m of concrete (i.e. 0.2 % of concrete quantity).
Cost Economics of “Introjected Backfil Retaining Walls”: The graphical representation in Fig.6 below shows the variation of cost between “Introjected Backfill Retaining Walls” and conventional retaining walls.
Another major achievement of the technology has been the fact that till date it has been found to be ideally suited to nearly all types of strata.
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