Independant study on Reinforced soil retaining wall

DEPARTMENT OF CIVIL ENGINEERING

Dr B R AMBEDKAR NATIONAL INSTITUTE OF TECHNOLOGY

JALANDHAR – 144011

Reinforced Soil Retaining Walls

Independent Study Work

Supervisor-Dr A K AgnihotriDr Akash Priyadarshee

Submitted by-Gurjeet Kumar 14217009

Retaining wall Retaining walls are used to retain earth or

other materials which have the tendency to slide and repose at a particular inclination.

They provide lateral support to the earthfill, embankments or other materials in order to hold them in a vertical position.

Retaining walls also have application in buildings and bridges such as basement, foundation wall, bridge abutment etc.

Reinforced soilReinforced earth is a combination of earth and linear reinforcing strips

that are capable of bearing large tensile stresses.

The reinforcement provided by these strips enable the mass to resist the tension in a way which the earth alone could not. The source of this resistance to tension is the internal friction of soil, because the stresses that are created within the mass are transferred from soil to the reinforcement strips by friction.

Idea of Reinforced SoilIT ALL BEGAN LIKE A GAME, when Henri Vidal, a highway engineer and architect, was trying to built a sandcastle on the beach. But the sand kept on falling off and this led to the idea of reinforcing the construction with pine needles. That is how the general principle of Reinforced Earth came about.

Reinforced Soil Retaining Wall (RS-RW)

Evolution of RS-RW

Load Transfer MechanismThe flexible reinforcement interacts frictionally with the soil resisting

the shear stresses in the soil massThe shear stress at the interface of the soil and the reinforcement

generates strains in the reinforcement and a tensile force is mobilised in the reinforcement

If this tensile force exceeds the tensile capacity of the reinforcement, rupture failure occurs – Tensile failure

If deformations are high or if the interface is smooth, it is likely that a slip occurs between the soil and reinforcement – Pullout Failure

For stability, Tensile failure and Pullout failure to be examined

PrinciplesIf a vertical stress (v) is applied on a soil element, it undergoes a

vertical compression ( v) associated with a lateral deformation (h).If a reinforcement is added to the soil in the form of horizontal layers,

the soil element will be restrained against lateral deformation as it acted by a lateral force.

Contd.

Modes of Failure

Components of RS-RW

SoilSkinReinforcement

SoilIt should be granular, cohesion less material, not too much silt or clay

having particle size not more than 125 mm.

Not more than 10 percent of the particles shall pass 75 micron sieve & the earth reinforcement coefficient of friction to be either higher than or equal to 0.4 & Plasticity Index < 6.

The soil must have a moisture content suitable for compaction.

SkinSkin is the facing element of the reinforced soil wall.

These elements support the backfill and keeps the reinforcement at a desired elevation in the reinforced soil wall and also protect the granular at the edge falling off.

Made of either metal units or precast concrete panels.

ReinforcementA variety of materials can be used as reinforcing materials such as-SteelConcreteGlass fibreWoodRubberAluminium Geosynthetics

Construction Procedure1. Place first row of panel/block on levelling course.

Contd.2. Insert next row of panel/block, align and clamp them.

Contd.3. Spread and compact backfill upto point of reinforcement.

Contd. 4. Lay and fasten the reinforcement.

Contd.5. Spread and compact backfill upto next point of reinforcement.

Contd. 6. Repeat steps 2-5 until structure is completed.

Cost Comparison

Seismic Stability

High performance during the 1995 Kobe Earthquake of a GRS RW of this type that had been constructed at Tanata validated its high seismic stability

Contd.

Some Finished Structures

Contd.

Contd.

Contd.

Applications1. Roads2. Airfields 3. Railroads 4. Embankments 5. Retaining structures

Benefits of using RS-RWLateral thrust on the wall is almost eliminated due to the development

of soil-reinforcement interface friction.Thin wall element known as skin is found adequate to retain the

backfill resulting in considerable economic savings.Simple construction. Faster construction than traditional concrete walls. Can be built in confined areas or areas where a concrete wall is almost

impossible to be constructed. High seismic load resistance. Various shapes and forms can be made.

Literature ReviewYear Author Summary of work1992 Swami Saran et al. Unattached reinforcing strips, embedded in the

cohesionless backfill behind a rigid retaining wall, are effective in reducing the lateral earth pressure(upto 50%) on the wall.

1998 K.G. Garg Retaining wall with geogrid reinforced earthfill was constructed only at 79 per cent of the cost of the retaining wall with conventional earthfill.

2000 Robert M. Koerner 35,000 RS-RW exists and they cover the entire range of practical wall heights and it is seen that geosynthetic reinforced walls are the least expensive of all wall categories and at all wall heights.

Year Author Summary of work2004 Satyendra Mittal et

al.Unattached reinforcing strips, embedded in the cohesionless backfill behind a rigid retaining wall are effective in reducing the lateral earth pressure on wall. The extent of reduction in the resultant pressure will depend on the amount of reinforcement present in the backfill. The optimum length of reinforcing strips is found to be around 0.6–0.8 times the height of wall for most practical cases.

2005 Hoe I. Ling et al. Earthquake performance of RS-RW improved by increasing the length of top reinforcement layer, reducing vertical reinforcement spacing and grouting the top block to ensure firm connection to the reinforcement.

2006 Iqraz Nabi Khan and Swami Saran.

Presented an experimental study on a model reinforced earth wall with sand backfill with aluminium reinforcing stirps and found that Rankine’s theory of earth pressure was close to the observed values.

Year Author Summary of work

2007 G. Madhavi Latha and A. Murali

Krishna

Damage to RS-RW will be more in case of stronger seismic events if the backfill is not properly compacted.

2007 Iqraz Nabi Khan and Swami Saran.

Presented a model study, different lengths of bamboo strips and geogrids were used for strengthening the backfill. Results indicated decrease in moments up to about 65 percent due to reinforcing the backfill.

2012 H. Ahmadi and M. Hajialilue-Bonab.

Presented a study focuses on the experimental and analytical investigations of small-scale physical model tests. Experimental results shows that the bearing capacity of footings located on the backfill can be increased significantly by including geotextile layers on the top of the backfill.

2013 Christopher Y. Tuan.

Presented Full-scale explosive test data from 4.6-m-high and 24-m-wide reinforced soil walls and found that the use of RS-RW wall systems for protective structures has received attention for their energy absorbing capability and blast resistance

Gaps in StudySeveral additional studies should be carried out to further the knowledge and use on reinforced soil retaining walls and these are mentioned below:

1. Settlement of the backfill while performing different analysis or behaviour of surface of backfill under different conditions.

2. Replacing backfill with some waste material like pond ash and fly ash and introduce reinforcements in the same backfill.

3. Wrap face backfill with geotextile behind a rigid wall need to be studied.

4. Inclusion of geocell and tire chips in the backfill.5. Reinforcing the backfill with two or more reinforcements.

ConclusionReinforced soil retaining walls have evolved as viable technique and

contributed to infrastructure in terms of speed, ease of construction, economy, aesthetics etc.

It is a technology that needs to be understood well in terms of its response, construction features etc. Failures of RE walls have also been noted in a few places due to lack of understanding of behaviour of reinforced soil walls.

FHWA, NCMA guidelines need to be studied in detail for seismic stability and deformation issues.

References1. Swami Saran, K. G. Garg and R. K. Bhandari. “Retaining Wall with Reinforced

Cohesionless Backfill”. Journal of Geotechnical Engineering, Vol. 118, No. 12, December, 1992.

2. Hoe I. Ling, Yoshiyuki Mohri, Dov Leshchinsky, Christopher Burke, Kenichi Matsushima and Huabei Liu. “Large-Scale Shaking Table Tests on Modular-Block Reinforced Soil Retaining Walls”. Journal of Geotechnical and Geoenvironmental Engineering, Vol. 131, No. 4, April 1, 2005.

3. Christopher Y. Tuan. “Ground Shock Resistance of Mechanically Stabilized Earth Walls”. International Journal of Geomechanics, Vol.14, 2014.

4. Robert M. Koernera and Te-Yang Soong. “Geosynthetic reinforced segmental retaining walls”. Geotextiles and Geomembranes 19 (2001) 359–386.

5. G. Madhavi Latha and A. Murali Krishna. “Seismic response of reinforced soil retaining wall models:Influence of backfill relative density”. Geotextiles and Geomembranes 26 (2008) 335–349.

Contd.6. Iqraz Nabi Khan and Swami Saran. “A Model Study on Retaining Wall with Reinforced

Backfill”. Journal - The Institution of Engineers, Malaysia (Vol. 68, No. 2, June 2007).7. Iqraz Nabi Khan and Swami Saran. “A Model Study on Metallic Strip-Reinforced Earth

Wall”. Malaysian Journal of Civil Engineering 18(1) : 38-45 (2006).8. H. Ahmadi and M. Hajialilue-Bonab. “Experimental and analytical investigations on

bearing capacity of strip footing in reinforced sand backfills and flexible retaining wall”. Acta Geotechnica 7 (2012) 357–373.

9. K.G. Garg. “Retaining wall with reinforced backfill-a case study”. Geotextiles and Geomembranes 16 (1998) 135-149.

10. Satyendra Mittal, K. G. Garg and Swami Saran. “Analysis and design of retaining wall having reinforced cohesive frictional backfill”. Geotechnical and Geological Engineering 24 (2006) 499–522.

Thank You