R. Sathish Kumar, Associate professor, National Institute of Construction Management and Research, Hyderabad
So a research work was carried out to study about the material required for the construction of reinforced earth wall and its specifications and also the construction methodology adopted. A cost and time comparison study was also carried out between reinforced earthen wall and conventional retaining wall, and it was found that the cost for the construction of reinforced earthen walls was approximately 20% less than the cost of conventional retaining wall. From the time comparison study it was observed that the time required for construction of reinforced earthen walls was more when compared to the construction of retaining walls. But if the wall height is more the reinforced earth walls can be preferred in the context of their stability, and also in its capacity to reduce the future settlement of pavement by controlling the erosion of soil fill with the help of geotextile placed between the soil fill and drainage aggregate.
IntroductionA retaining structure is used for maintaining the ground surface at different elevations on either side of it. Geo grid reinforced earth technology has almost completely replaced conventional retaining structures with the help of geosynthetics. Over the years, these products have helped designers and contractors to solve several types of engineering problems where the use of conventional construction materials would be restricted or considerably more expensive. There are a significant number of geosynthetic types and geosynthetic applications in geotechnical and environmental engineering. Common types of geosynthetics used for soil reinforcement include geotextiles (particularly woven geotextiles), geogrids and geocells. The combination of improved materials and design methods has made possible engineers to face challenges and to build structures under conditions that would be unthinkable in the past
Literature reviewR.D. Nalawade and D.R. Nalawade (2008) in their paper "Stability and Cost Aspects of Geogrid Reinforced Earth Wall of Flyover" made an attempt to compute the cost and stability aspects of the reinforced earthen walls In this paper methodological design of retaining wall structure using geogrid for flyover near Agriculture College, Pune is tackled through external, internal, wedge and seismic stability. Finally design by metallic strips and reinforced cement concrete cantilever retaining wall was carried out and the cost comparison was made which shows Geogrid RE wall reduces the cost and time required for construction.
7.1. Concrete segmental block: Figure 1
Seiichi Onodera et al (2001) in their paper "Long-term durability of geogrids laid in Reinforced soil wall" made a study on two types of 5m high geogrid reinforced soil walls (gradient V:H=1:0.1) with two kinds of wall facing (wrapping type and L-shaped concrete block type). Trial soil walls were constructed in 1990 with an 8m high vertical reinforced soil wall with concrete block wall facing and in 1995 a 4.5m high reinforced soil wall (gradient V:H=1:0.5) with a steel mesh frame as its wall facing trial soil walls were constructed. From the beginning of the construction stage, wall displacement or strain of the geogrid, the earth pressure, etc. were measured for a long period of time. In 2002, when the first walls were about 12 years old and the second walls were about 7 years old, parts of the four kinds of geogrids that were used as the reinforcement of the embankment and as the wall facing were sampled and underwent tensile tests to study their long-term durability. They were also immersed in various chemicals for a long period time then underwent tensile test to study their chemical degradation. The results confirmed that the geogrids buried in the soil for 12 years or for 7 years retained their original tensile strength.
Xiao-jing Feng et al (2008) in their paper "The Influence of Facing Stiffness on the Performance of Geogrid Reinforced Retaining Walls" stated that as pointed out by various researchers, consideration of the influence of the facing type on reinforcement loads was lacking in current limit equilibrium-based design methods for the internal stability design of geosynthetic reinforced soil walls. Also the displacement of walls and the strain of reinforcement are also related to the facing type. This paper reported the results of the three instrumented model walls. The walls were nominally identical except one wall was constructed with a rigid concrete block face, the other with a hinge joint wood face, and another with a flexible wrapped face. The displacement of wall face added with the increase of the stiffness of wall face under the same surcharge. The strain of the reinforcement was influenced by the facing stiffness, while the relation between them also effected by the loading type, backfill type etc. Under the strip load , the reinforcement strain in stiff-face wall was higher. The ductile of the wall failure was reduced with the increasing of facing stiffness.
Peter Janopaul et al (1991) in their paper "Retaining Wall Construction And Block Therefor" stated that in general, a block and retaining wall formed by a number of such blocks are interconnected between courses by a plurality of Z shaped anchored elements having an upper and lower body part of substantially rectangular cross-section. The offset of one course of blocks relative to the course beneath will be a predetermined by a fixed amount determined by the offset of the body parts of the interlocking Z-shaped anchor elements. A tie-back arrangement includes means for attaching a sheet of geosynthetic material to the embedded end of a block so as to leave the open cells within and those formed between the blocks unobstructed from the above and available for filling with pea gravel or other drainage fill material.
7.2. Geo grid: Figure 2 7.3. Geo textile: Figure 3
Ennio M. Palmeira et al (2008) in their paper "Advances in Geo synthetics Materials and Applications for Soil Reinforcement and Environmental Protection Works" explained about the usage of geo synthetics materials in construction elements. Geosynthetics have been increasingly used in geotechnical and environmental engineering for the last 4 decades. Over the years, these products have helped designers and contractors to solve several types of engineering problems where the use of conventional construction materials would be restricted or considerably more expensive. There are a significant number of geosynthetic types and geosynthetic applications in geotechnical and environmental engineering. Common types of geosynthetics used for soil reinforcement include geotextiles (particularly woven geotextiles), geogrids and geocells.
Dwight A Beranak, P.E. (2002) in their paper "Use of Geogrids in Pavement Construction" focused how Engineers are continually faced with maintaining and developing pavement infrastructure with limited financial resources. Traditional pavement design and construction practices require high-quality materials for fulfillment of construction standards. In many areas of the world, quality materials are unavailable or in short supply. Due to these constraints, engineers are often forced to seek alternative designs using substandard materials, commercial construction aids, and innovative design practices. One category of commercial construction aids was geosynthetics. Geosynthetics include a large variety of products composed of polymers and are designed to enhance geotechnical and transportation projects. Geosynthetics perform at least one of five functions: separation, reinforcement, filtration, drainage, and containment. One category of geosynthetics in particular, geogrids, has gained increasing acceptance in road construction. Extensive research programs have been conducted by the U.S. Army Engineer Research and Development Center (ERDC) and non-military agencies to develop design and construction guidance for the inclusion of geogrids in pavement systems.
Ragui F. Wilson-Fahmy et al (1994) in their paper "Experimental Behavior of Polymeric Geogrids in Pullout" stated that the increasing use of polymeric geogrids in reinforced soil walls and steep slopes warrants special attention to all details including their anchorage behavior. Because of the open structural nature of geogrids, their performance was different from other sheet-like reinforcing materials such as metallic strips and geotextiles. They derive their anchorage capacity through both friction and bearing resistances. This paper focused on the structural behavior of geogrids under a pullout loading condition. An experimental investigation was conducted using three different geogrids tested at three different lengths. The load-displacement response at different locations along the geogrid was monitored during pullout. The experimental results were compared with predictions using a previously published finite-element model simulating soil-geogrid interaction and taking into account the deformation of the geogrid structure. Tension in the geogrid, as well as friction and bearing components of resistance, were presented in relation to geogrid length, pullout load magnitude, and distance from the clamped end of the geogrid. The results emphasize the fact that the success of a geogrid in fulfilling its anchorage role is directly related to its structural composition and material specific characteristics.
Han Yong Jeon et al (2002) in their paper "Assessment of long-term performances of polyester geogrids by accelerated creep test "viewed that Geogrids are widely used as the reinforcement materials in geotechnical and civil engineering fields. In this study, accelerated-creep tests at elevated temperatures to predict longer-term creep behavior of polyester fabric geogrids were examined using the time–temperature superposition principle. Creep tests were generally performed to calculate the partial factor of safety during the service time of polyester geogrids and two types of geogrids, having different design strengths ranging from 8 to 15 t/m, were used in this study. The creep tests were carried out at various temperatures and loading levels of 40, 50, and 60% of short-term design strengths. Also, the creep tests were made at temperatures between 20 and 50°C to take into consideration the real environmental conditions of geogrids. The results indicated the applicability of the conventional procedures in prediction of longer time creep strain and material dependency of creep strains.
7.4 Foundation and levelling pad: Figure 4
P. Bataille et al (2004) et al in their paper "Mechanical properties and permeability of polypropylene and poly (ethylene terephthalate) mixtures" studied that the synthetic membranes currently used for soil stabilization and road construction that are mainly made of polypropylene and of polyesters. This paper reported on the mechanical properties and the permeability of mixtures of polypropylene (PP) and polyethylene terephthalate (PET). The elastic modulus of the mixture was at a minimum for a 50/50 mixture. For the other compositions, the moduli gave a positive deviation as compared with the additivity equation results. This was probably due to the fact that pure PET has a fragile behavior at the temperature at which the mechanical tests were run. This 50/50 composition corresponds to the domain where a phase inversion occurs. The yield stress increased, however, indicating that we had a better adhesion and that the copolymer seems to have a certain emulsifier effect, increasing the quality of the dispersion.
|Table -1 Specifications of Geo grid material|
|Polymer||High strength polymers yarns|
|Property||Test methods||Unit||GX40||GX60||GX 80||GX100||GX 130||GX 160|
|Ultimate tensile strength(MD)||ASTM
|Ultimate tensile strength(TD)||KN/m||40||30||30||30||30||30|
|Elongation at break(MD)||%||<11%||<11%||<11%||<11%||<11%||<11%|
|Creep reduced strength 120 years||KN/m||27||41||55||68||89||110|
|Long term design strength(LTSD)||21||34||46||58||75||92|
J. Engrg. Mech. (2004) in his paper "Analyzing Dynamic Behavior of Geosynthetic-Reinforced Soil Retaining Walls" stated that an advanced generalized plasticity soil model and bounding surface geosynthetic model, in conjunction with a dynamic finite element procedure, were used to analyze the behavior of geosynthetic-reinforced soil retaining walls. The construction behavior of a full-scale wall was first analyzed followed by a series of five shaking table tests conducted in a centrifuge. The parameters for the sandy backfill soils were calibrated through the results of monotonic and cyclic triaxial tests. The wall facing deformations, strains in the geogrid reinforcement layers, lateral earth pressures acting at the facing blocks, and vertical stresses at the foundation were presented. In the centrifugal shaking table tests, the response of the walls subject to 20 cycles of sinusoidal wave having a frequency of 2 Hz and of acceleration amplitude of 0.2g were compared with the results of analysis. The acceleration in the backfill, strain in the geogrid layers, and facing deformation were computed and compared to the test results. The results of analysis for both static and dynamic tests compared reasonably well with the experimental results.
7.5. Compaction of soil, Blocks Erection and Drainage material filling: Figure 5
|Table -2 Specifications of Geo textile material|
|Property||Test method||MARV Value|
|Mass per unit area||ASTM D 5261||155 g/ sq.m|
|Thickness 2k Pa||ASTM D 5199||1.5mm|
|Grab tensile strength md||ASTM D 4632||690 N|
|Elongation at break md||75 %|
|Wide width tensile strength. (ave)||ASTM D 4595||11.5 kN/m|
|Elongation at break. md||ASTM D 4595||75%|
|CBR Puncture strength||ISO 12236||1750|
|Apparent opening size (095)||ASTM D 4751||0.25 mm|
|Rod Puncture strength||ASTM D 4833||310 N|
|Permittivity||ASTM D 4491||2.71/s|
Case StudyStudy area
Construction of Eight lane Access Controlled Expressway as Outer Ring Road to Hyderabad City in the State of Andhra Pradesh, India in the stretches from Patancheru – Shamirpet from km.23.700 to km.61.700 (Northern Arc) Package-I from Km 23.700 to Km 35.000 Patancheru-Mallampet.
Study area detailsThe area in which the works are located is mostly plain to rolling terrain. The Project area is located between 170 11/ 39// - 170 36/ 27.13// N latitude and 780 14/ 15// - 780 41/ 21// E longitude.
General Climatic Conditions
- The variation in temperature in this region is between 100 C and 460 C.
- The annual rainfall in the area is in the range of 790 mm to 1000 mm.
- The works are located in seismic zone II as defined in IRC-6-2000
Materials and their specifications Geo grids (Ref fig-2)Geogrids used as soil reinforcement were GX polyester geogrids of style GX 40/40, GX 60/30, GX 80/30, GX 100/30, GX130/30, and GX 160/50 with the following specifications.
Precast segmental blocks (Ref fig-1&fig 7)
- The facing elements used at the site were segmental blocks having a mass of approximately 35 kg as shown in fig 1.
- These blocks are of length 450mm on the front side and 280mm on the back side.
- The height of each block was 300mm.
- These blocks were manufactured by cold pressing process in automatic block –making machine ensuring consistent quality, accuracy of dimensions and good finish with cement concrete of 35MPa after 28 days curing.
- The blocks were cured for a sufficient length of time as approved by the engineer using potable water. Sufficient care was taken to ensure that blocks are not damaged during handling, storage and transportation.
- Units were acceptable for placement in the structure if the strength at 7 days or before exceeds 75% of the 28 days requirement.
3.3 Drainage aggregate (Ref fig -5)
- The drainage materials were cleaned crushed stone or gravel with particle size in the range of 9.5-19.1mm and % fines <5% or a suitable material.
- The drainage collection pipes were perforated or slotted PVC or HDPE of 150mm diameter.
Geotextile (Ref fig-3)The geotextiles used as filter for the granular drainage bay were meeting the requirements of MORTH specifications for Road and Bridge works, Clause 702.2.3.The geotextile used were meeting the following minimum requirements in terms of minimum average roll values.
Excavation and Foundation preparation
- The site was excavated to the lines, width and grades as shown in the approved construction drawings
- The trench for the leveling pad was excavated to the correct depth and width.
- In the reinforced soil zone the ground was excavated to a depth of 200mm (minimum) below the first layer of geogrid reinforcement.
- Any unsuitable soils if present were removed and replaced by compacted fill, Similarly pits, depressions etc. were filled by compacted fill of approved quality.
Foundation leveling pad (Ref fig -4 &fig-6)
- The centerline for the leveling pad was marked on the bottom of the trench ensuring required setback to accommodate the facing batter as shown on the construction drawings and the side forms are fixed for the leveling pad.
- The leveling pad consists of a plain cement concrete strip footing of 600 mm width and 200 mm thickness.
- Concrete used for leveling pad was with a minimum grade of M 15 and the maximum size of aggregates was limited to 20 mm.
- Concrete was poured and compacted using needle vibrators, and screed to the correct level and finished using wooden floats to flat and smooth finish.
- The leveling pad was casted with a level tolerance of a 5mm and the surface finished using a smooth wood float.
- The leveling pad was cured for a minimum period of 48 hours before erection of segmental units is commenced.
Placement of first course of segmental blocks (Ref fig-4)
- The first course of segmental block was placed to the correct line as marked on the leveling pad.
- A thin layer of stiff cement mortar was provided on top of leveling pad, to ensure accurate placing of leveling blocks.
- The next extremely important step was to place the first course of blocks to the correct line and level.
- Drainage aggregates were then placed and lightly compacted to fill openings between segmental units.
- Soil fill was placed and compacted behind the segmental units and drainage material is in filled up to the height of the block.
Placement of first layer of geo grid reinforcement (Ref fig-4)
- After ensuring the drainage infill between the blocks or slightly above the top of the segmental unit, the debris was cleaned off from the top of the segmental units.
- Position geogrid of the required type and length as shown on drawings (fig) with the longitudinal direction perpendicular to wall face.
- Adjacent roll of geogrid was placed such that they are butting each other.
- Next course of segmental unit was placed in a running bond configuration.
- Segmental unit was moved forward to engage shear key and ensuring proper alignment and set back of the segmental units.
- Geogrid was pulled tight using uniform tension, hold or stake to maintain tension throughout the fill placement process.
- Drainage infill and soil fill was placed on the openings between segmental units and then the fill and drainage in-fill is compacted.
|Height||Width||Length||Thickness||Quantity||Cost (Rs)||Total cost(Rs)|
|Height of wall||Width(m)||Length(m)||Thickness(m)||Quantity(cum)||Cost(Rs)||Total cost(Rs)|
|Height of wall(m)||Length(m)||Average thickness(m)||Quantity(Rs)||Cost(Rs)||Total cost(Rs)|
|Height of wall(m)||Steel required(ton)||Cost/ton(Rs)||TOTAL COST(Rs)|
|Total cost of retaining wall|
|Height of wall(m)||Length of wall(m)||Cost of concrete (Rs)||Steel cost (Rs)||Total cost (Rs)|
|Reinforced earth wall|
|Height of wall(m)||Length(m)||Area in Sqm||Blocks/Sqm||Total blocks|
|Cost of segmental block cost is Rs150. including drainage material. Erection of RE wall includes cost of geogrid, geotextile, aggregate labour and equipment cost|
|Height||Cost of block(Rs)||Total block cost(Rs)||Area in Sqm||Erection cost/Sqm||Total erection cost||Cost of RE wall|
Placement of subsequent courses of segmental blocks and geogrid (Ref fig-5, fig 8 & fig 9)
- Segmental blocks were placed in a running bond configuration. (fig 4.2)
- Proper care must be taken to clean the top surface of the blocks with a stiff brush or broom to remove any fill, drainage aggregate etc., before placing the subsequent course of blocks.
- At each level, blocks were properly aligned and pushed forward to engage the shear key to ensure proper set back.
- Drainage aggregate was placed and lightly compacted to fill openings between segmental units.
- After 3 layers of segmental blocks the second layer of geo grid was laid and this continuous for the entire wall section
Compaction of fill (Ref fig -5)Reinforced fill
The reinforced fill selected is a granular fill with the following properties.
- Peak effective angle of shearing resistance was 320
- % fines (passing 75 micron sieve) was 15%
- The dry density of the compacted fill was meeting the minimum requirements as per IS 2720(part 8)
- The deposition, spreading, leveling and compaction of the fill were carried out in a direction parallel to the facing.
- No plant or equipment with a weight exceeding 1500 kg was allowed to operate within 1.5m from the facing.
- Construction equipment was allowed to move directly over the geogrid, ensuring that there is a minimum soil cover of 100 mm over the strips.
- Abrupt stopping, turning etc. of the equipment was avoided to minimize misalignment of geogrids.
- Care was taken during the deposition, spreading, leveling and compaction of the fill to avoid damage, disturbance or misalignment of segmental blocks, geotextile filter and geogrid reinforcement.
- Fill placed near the facing was ensured, that no voids exist directly below the geogrid reinforcement.
- Fill was placed and compacted in lifts. Thickness of lift was consistent.
Equipment used for compactionCompaction of the fill was carried out using appropriate equipment, which will not induce excessive loads on the facing and at the same time achieves the required compaction. Towards this the following equipments were recommended for different zones:
- Within 300 mm of the facing, the fill/drainage material was compacted by a light-weight plate compactor or by hand tamping.
- Beyond 300 mm and within 1.5 m from the facing the fill was compacted using a walk behind vibratory roller or plate compactor with a total weight less than 1500 kg.
- Beyond a distance of 1.5 m from the facing, the fill was compacted using appropriate rollers of 8-10 MT weight.
- Movement of compaction equipment was in a direction parallel to the wall face, starting near the face and gradually moving away from the wall face.
Placement of drainage system (Ref fig-5)
- Drainage material was placed to the minimum finished thickness
- Vertical layers of drainage layer material were brought up at the same rate as the adjoining fill material.
- Geotextile filter was provided behind the drainage bay and is wrapped back into the fill with a minimum wrap length of 200 mm.
- Perforated collection pipe wrapped with geotextile filter is installed at the location as shown on the drawings. Discharge exits are provided at required interval
|Height (m)||Length (m)||Retaining wall cost(Rs)||RE wall cost(Rs)|
Coping beamOn the topmost segmental unit, erect form work and cast coping beam to get the required longitudinal profile of the wall.
Cost analysis of conventional retaining wall and reinforced earth wallsThe cost analysis was done between a conventional retaining wall and reinforced earth wall for a wall of height different heights from 4m to 8m for a length of 20m.
Conventional retaining wall
PCC cost per cum – Rs.2600/- (Cost includes cost of M15concrete. labour and shuttering)
Raft cost per cum – Rs.2150/- Wall cost per cum – Rs.3900/-
Steel binding cost per metric ton – Rs.3500/- (Cost of labour, cutting and binding)
Steel cost per metric ton – Rs.41500/-
- Materials used for construction of reinforced earthen walls were geogrids, geotextile, drainage aggregate, drainage pipe, and segmental blocks. For the construction of reinforced earthen walls there is no need of deep foundations (pcc, reinforcement, raft,).It involves the process like casting of foundation leveling pad, erection of facing units, placement and compaction of soil fill to the first layer of reinforcement, placement of the first layer of geogrid reinforcements, placement of next and subsequent lifts of soil fill, erection of subsequent rows of facing units and reinforcements, coping.
- The cost for the construction of reinforced earthen walls is nearly 20%of the cost of retaining wall. The time required for construction of reinforced earthen walls was found more when compared to the construction of retaining walls. We can construct a retaining wall of height 1.2m in a day, and the entire process like (deshutteing, reinforcement binding, shuttering for additional height) takes 3-4 days, where as for the reinforced earthen wall, the construction is done in layers, and each layer of 200mm was filled and compacted up to 98% MDD, so on an average it takes a week days for 0.6 m height. So we can conclude that, the retaining wall can be preferred if the height of the wall is less than 4m, and if the height is more than 4m reinforced earthen walls can be preferred due to their stability, and also due to its capacity to reduce the future settlement of pavement by controlling the erosion of soil fill with the help of geotextile placed between the soil fill and drainage aggregate.
- The workers/ labor required for the construction of retaining wall was found more when compared to reinforced earthern wall, 7-9 workers are required for the construction of retaining wall, where as for the construction of reinforced earthen wall, the blocks are casted by the machine and only 2 or 3 workers are required for the placing and finishing of blocks
- R.D. Nalawade and D.R. Nalawade (2008): "Stability and Cost Aspects of Geogrid Reinforced Earth Wall of Flyover"
- Seiichi Onodera et al (2001): "Long-term durability of geogrids laid in Reinforced soil wall"
- Xiao-jing Feng et al (2008): "The Influence of Facing Stiffness on the Performance of Geogrid Reinforced Retaining Walls"
- Peter Janopaul et al (1991): "Retaining Wall Construction And Block There for"
- Ennio M. Palmeira et al (2008): "Advances in Geo synthetics Materials and Applications for Soil Reinforcement and Environmental Protection Works"
- Dwight A Beranak P.E. (2002): "Use of Geogrids In Pavement Construction"
- Ragui F. Wilson-Fahmy et al (1994): "Experimental Behavior of Polymeric Geogrids in Pullout"
- Han Yong Jeon et al (2002): "Assessment of long-term performances of polyester geogrids by accelerated creep test"
- J. Engrg. Mech. (2004): "Analyzing Dynamic Behavior of Geosynthetic-Reinforced Soil Retaining Walls"
- "Design and Construction of reinforced earthen wall"- GEOSOL ASSOCIATES.