CENTRIFUGE AND FULL SCALE MODELS OF ......walls. Guler (1990) conducted finite element analysis and showed that lime stabilized cohesive soils can successfully be used as backfill

Emirates Journal for Engineering Research, 8 (1), 15-23 (2003) (Regular Paper)

Emirates Journal for Engineering Research, Vol. 8, No.1,2003 15

CENTRIFUGE AND FULL SCALE MODELS OF GEOTEXTILE REINFORCED WALLS AND SEVERAL CASE STUDIES OF

SEGMENTAL RETAINING WALLS IN TURKEY

E. Guler1 and C. Ocbe2 1 Geotechnical Engineering, Bogazici University, 80815 Bebek, Istanbul, Turkey

2 Geoduvar Ltd., Altinekin Sok No. 7/A 80630 Etiler, Istanbul, Turkey

(Received December 2002 and accepted April 2003)

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2&�� '�<�X��Y��<a��K��+�=+��78��9��b�26�-*.��252 �� =+�>78�2&��IJK� ��+��%9� Theoretical work based on finite element analysis indicated that cohesive soil stabilized with lime can be used successfully as a backfill material in the construction of geosynthetic reinforced retaining walls. In order to understand the behavior better, centrifuge models of geosynthetic reinforced walls were constructed and tested. In order to acquire more experience with this technology, a geotextile reinforced full size wall was constructed, where lime stabilized clay was used as the backfill material. All the experiences gained during these researches were used in the construction of real retaining walls. Several geosynthetic reinforced segmental retaining walls were designed and constructed Turkey. The wall in Istanbul is at its maximum point 10 m high and its reinforcement was instrumented with strain gages. The second wall was constructed in Antalya and is a tiered wall, which has two sections with an overall height of 6 meters. The foundation soil under this wall was very soft clay. The third major wall was constructed in Bursa and has a maximum height of 15 m. In order to measure the deformations four inclinometers were installed at different sections. The extremely short construction time of a wall for a highway project in Izmit is also reported. Most recently walls were constructed for private houses. In all these walls as backfill material marginal soils were used.

1. INTRODUCTION

The geosynthetic reinforced retaining walls with concrete block facings are used in the construction industry frequently. They cost much less than the traditional reinforced concrete retaining walls and are also more easily constructed. The experience with reinforced earth walls, where metal reinforcement is used, showed the fact that the corrosion problem exists and threatens such walls on the long term although measures were taken to prevent corrosion. Therefore geosynthetic reinforced retaining walls are more preferred. For the construction of high and long retaining walls for highway or railway projects, geosynthetic reinforced retaining walls are extensively used in America and Japan with success. Also the appearance of geosynthetic reinforced retaining walls are much more aesthetic than its competitors. Therefore they are preferred not only for road construction projects, but frequently also for residential areas.

Research showed that also cohesive soils can successfully be used as a backfill material in geosynthetic reinforced retaining wall constructions. The use of cohesive soil as a backfill material is very important, because in this way the transportation cost of high quality material is not necessary anymore and the locally available soil can be used. This naturally reduces the construction costs very much. Since there is no corrosion threat for the geosynthetic reinforcement, the use of cohesive backfill does not lead to long term stability problems that would exist with metal reinforcements.

2. PREVIOUS RESEARCH Task Force 27 (1990) foresees that the backfill material for reinforced soil structures must be a granular soil. However in many countries such materials are rare and very expensive. Barrett (1992) showed that the largest economy will be achieved, by using locally available soil as a backfill. In many situations it is possible to find a soil that has a large

Emirates Journal for Engineering Research, Vol. 8, No.1, 2003 16

Figure 1. Failure planes obtained from centrifuge tests.

amount of fine particles but still has an internal friction angle of 30O or more. Such soils are also used in the construction of embankments.

There can be cases where a suitable soil that satisfies the above given criteria cannot be found. In such a location where only pure cohesive clay is available, one option could be to improve the characteristics of the available pure cohesive soil. The technology of improving the mechanical properties of soils are numerous. The most common and simple procedure to improve the mechanical properties of cohesive soil is adding admixtures. Lime for example, if added to a cohesive soil, produces substantial improvement in the shear strength and a reduction in the deformation. Mitchell (1981) and Brandl (1981) determined that adding lime to cohesive soil increases the shear strength. Bulut (1986) has made experiments comparing the mechanical characteristics of lime treated and untreated clay soils. He concluded that by adding a certain quantity of lime to the clay, an important increase in the ultimate strength was obtained. Iyidil (1988) investigated the hydraulic characteristics of clay mixed with 2 to 6 % lime, by conducting permeability tests. He concluded that lime stabilization causes an increase in the permeability when compared with the permeability of the untreated clay.

Mitchell and Villet (1987) report that clay can be successfully used as a backfill material for retaining walls. Guler (1990) conducted finite element analysis and showed that lime stabilized cohesive soils can successfully be used as backfill material for geosynthetic reinforced retaining walls. He also showed that treating the backfill clay with lime can substantially reduce horizontal deformation of geosynthetic reinforced walls. Based on centrifuge model tests, Goodings (1989) indicated that geosynthetic reinforced retaining walls with cohesive backfill soil can be successfully constructed. Billiard and Wu (1991) constructed a geosynthetic reinforced wall and loaded it up to failure. They concluded that the US Forest Service Method used in the design of geosynthetic reinforced walls is a conservative method.

Figure 2. Prototype failure heights obtained from

centrifuge tests.

3. CENTRIFUGE TESTS Thirteen centrifuge models of geosynthetic reinforced retaining walls were tested. These models were constructed with lime stabilized clay soil as backfill material. Then the models were tested in the centrifuge by increasing the circular velocity, which as a consequence increases the centrifugal force acting on the model. The circular velocity of the rotating arm of the centrifuge is increased up to the level where the increased self weight of the model causes failure. The height of the models was 19 cm. Ten layers of geosynthetic reinforcement were used. As reinforcement a very thin nonwoven geotextile was used in order to retain the scale properties of the model. The tensile strength of the geotextile used as the reinforcement was 0.7 kN/m. Three different lengths of reinforcement were examined. The ratio of reinforcement length to the wall height was chosen as: 0.5, 0.7 and 1. It was determined that the failure mechanism depends also on the reinforcement length. As seen in Figure 1, the failure plane develops as a vertical line either right behind the reinforced zone or the main reinforcement gets ruptured where the overlap ends. This vertical line joins then an inclined sliding surface and causes failure. The origin of the inclined surface lies slightly above the toe. Also unreinforced models were tested to be able to compare the effect of the geosynthetic reinforcement.

Placing geotextile reinforcement into the clay wall essentially improved stability, even if the length of the geotextile reinforcement was only one half of the wall height. It was confirmed by centrifuge model tests that further improvement in wall stability could be obtained, by stabilizing the cohesive backfill soil with lime (Guler and Goodings, 1992).


Figure 3. Overall view of the full size experimental wall.

The prototype collapse height could be increased nearly up to the doubled value, by treating the cohesive soil with lime as can be seen in Figure 2. As seen in the same Figure one can reach a prototype wall height of approximately 30 m with a lime treated cohesive backfill and one should remember this is achieved with a very weak reinforcement.

4. THE FULL SCALE EXPERIMENTAL WALL

Based on the experience gained from finite element analysis and the centrifuge tests, a 6 m a high, geotextile reinforced experimental retaining wall was designed and constructed at the new development area of the Bogazici University, the Kilyos Campus. A picture of the experimental wall is given in Figure 3.

The front appearance of the wall has a trapezoidal form where the width at the base is 18.2 m and at the top 8.8 m. The width has been chosen long enough to ensure that the central portion will be able to show a two dimensional behavior. As the backfill material locally available clay mixed with 4 % lime was used. After the clay was mixed with the proper amount of lime with the help of a loader, the treated soil was then compacted in layers of approximately 25 centimeters. Layers were compacted in the first two meters with a small cylinder, which had a smooth rolling surface. Despite the fact that the construction took place during the winter season the natural moisture content of the clay allowed a proper compaction. The smooth-cylinder had two disadvantages: (i) the compaction energy is rather low and (ii) since there is no barrier at the edge of the wall, it became more risky as the wall became higher. Therefore for the remaining part of the wall a rammer type compactor was used. The compaction of the rammer was very efficient in operation and also provided high compaction energy. The geosynthetic

reinforced wall was constructed in one month with a 7 man crew and one compaction apparatus. Obviously the construction was delayed at many points due to installation of measuring gauges.

Shear strength parameters of the soil were measured at three different time periods. Prior to construction the shear strength parameters were determined on samples prepared in the laboratory from the materials to be used in the construction of the wall. The parameters during the construction period and later were determined by taking samples directly of the wall and testing these samples in the laboratory. A summary of all results is represented in Table 1.

Table 1 Change of shear strength properties of full size wall.

During Construction

1 Month Later

8 Months Later

φ (o) 26 19 23 c (kPa) 55 95 43 ω (%) 18 20 24

As reinforcement a very thin nonwoven geotextile was used. The reinforcement had a tensile strength of only 5,9 kN/m. No safety factor was used for the tensile strength of the reinforcement in the design.

The U.S. Federal Highway Administration (F.H.W.A.) design method (Christopher, et al. 1990) was used for the design of the experimental reinforced retaining wall. The design method was selected because of its popularity in practice. All the Safety Factors in the design were taken as unity, so that the wall could be brought to collapse simply. However the lengths of the reinforcements were selected longer than the sufficient anchor lengths because slip of the reinforcement was not desired. A cross section of the wall and the reinforcement is given in Figure 4. To determine vertical and horizontal pressure distribution in the wall Gloetzl type pressure cells were installed. Also the strains of the reinforcement were measured.

To bring the wall to failure or at least to deform the wall to a large extent, a surcharge load was applied to the wall. This surcharge load was applied by the placement of concrete blocks on top of the wall.

In order to enforce a two-dimensional failure, the loaded area was limited to the central portion of the wall. Blocks were placed layer by layer as seen in Figure 3 and 5, so to gradually increase the load on the wall. The goal of the surcharge load was to cause failure in the wall or cause excessive deformations. The surcharge load could not be increased beyond a certain height, since the stability of the blocks themselves became critical after a certain height.


Figure 4. Cross-section of experimental full size wall.

Figure 5. Saturation of an experimental full size wall.

Burwash and Frost (1991) indicated that the saturation of the clay, which is used as backfill material, causes a loss in the strength and also a considerable decrease in the pullout strength. Following this idea, two large holes at the top of the wall has been excavated and filled continuously with water as shown in Figure 5. The goal was to increase the water content of the backfill clay and cause a reduction in shear strength, so that the wall could be brought to collapse. However this did not happen. The water escaped immediately through the first layer of geotextile reinforcement. It was observed that because

of the transmissivity of the geotextile it functioned not only as reinforcement but also as drainage. Thus no pore water pressure was developed and no saturation at the lower layers arose. As a result it was not possible to cause failure in the wall or even to deform it.

This study showed that cohesive soils can be used as backfill material for geotextile reinforced retaining walls efficiently. Then the wall was left under natural atmospheric conditions like snow, heavy rain, strong wind, strong sunshine and high temperature conditions until the next summer. The surcharge was left for 6 months and was removed.

Measurement from the instrumentation took place during the construction of the wall and during a period of two years after the construction of the wall. From the end of the construction to the first stage of the surcharge loading the measured pressure values varied, resulting in the increase of the average value. Although not all measured values increased with same trend they showed generally an increase of the pressure. This was explained by an adjustment of the soil around the pressure gauges. Measured lateral pressure values showed nearly constant values with depth. This type of constant lateral pressure distribution is also in agreement with the lateral pressure distributions obtained from finite element analysis results. The reason of measuring higher lateral pressures than expected at the top levels can be attributed to the lateral pressure that is locked into the soil during the compaction process. On the other hand the relatively lower horizontal pressures measured closer to the base can be explained with the effect of the reinforcement. As a conclusion it can be proposed that in the analysis of geosynthetic reinforced walls this trend in the horizontal pressure should be used instead of the triangular pressure distribution.

The deformations remained below the sensitivity level of the measuring gages. So it was understood that the deformations were minimal and that it could be assumed that practically no deformation occurred. Since the deformation measurements were started after the construction was completed, it was concluded that deformations took already place during the construction phase.

The geotextiles, which are embedded in cohesive soil as reinforcement, does not only function as reinforcement but also as lateral drainage. So the stability of the wall increases. Therefore there is a strong proof that geotextile reinforcement can strengthen the clayey soil backfill effectively and increase its stability. The current design methods seem to be conservative. Therefore it was concluded that considering the factors specified above can lead to more economic designs of such walls.


5. CASE STUDIES These experiences were used in the construction of geosynthetic reinforced real retaining walls in Turkey. In the following sections five major geosynthetic reinforced segmental retaining walls constructed in Turkey are reported.

5.1 THE GEOSYNTHETIC REINFORCED WALL IN ISTANBUL

The first Segmental Retaining Wall in Turkey where concrete blocks are used as the facing and the reinforcement is a geotextile has been constructed during the summer of 1997. The project was constructed as part of the Altunizade-Umraniye Highway construction. The highway had interrupted the Nurbaba Street and it had to be elevated to pass over the tunnel portal (Figure 6).

The facing elements were simple concrete blocks and as the backfill a greywacke has been used. The reinforcement was a woven geotextile with an ultimate tensile strength of 40 kN/m. Though it was the first wall of its kind in Turkey, it included tremendous amounts of complexities. These can be summarized as follows:

• The existing road had a mixed cross-section and the retaining structures supporting the fill had deteriorated severely. So they had to be removed from the site. Due to this fact and that the original ground is sloped, the two sides of the road had to be formed on two different elevations as can be seen in the cross section of the structure (Figure 7).

• The foundation of the wall was a heavily weathered rock. Its consistency was similar to that of overconsolidated clay. The foundations of the two walls, each on one side of the road, had to be constructed on different elevations. This fact brought up the concern, that there can be a stability problem on the slope that is created between the two foundation levels. Special concern and analysis was devoted to the foundation of the wall constructed on the crest of the slope created by excavation.

• Provisions were needed for the utility lines. There were four utilities that had to pass from underneath the road, namely water, telephone, natural gas and high voltage electrical power. Since these lines could not be placed side by side, it was not possible to locate the utilities at the center of the road. When the utilities were distributed over the road surface, the reinforcement at the top layers had to be kept short. This problem was solved by considering the top portion as a separate short wall itself and its effect on the lower layers was considered as a surcharge load.

Figure 6. The wall in Istanbul and the Tunnel Portal.

Figure 7. A cross section of the wall in Istanbul.

Figure 8. The staircase that was constructed as part of the geotextile reinforced wall in Istanbul.

• Ladders were needed to provide access to the houses and they were constructed as part of the reinforced soil wall as illustrated in Figure 8.

• At one point the road jumps onto the tunnel portal and at this point the height of the wall suddenly reduces from 10 m to 1 m as shown in Figure 6 and the foundation becomes a rigid structure. To prevent future problems a joint was provided at this point.


Figure 10. The cross section of the wall in Antalya.

The cost of the whole wall was $172,000 where the reinforced concrete alternative would have cost $263,000. So a saving of 35% was achieved.

5.2 THE GEOSYNTHETIC REINFORCED WALL IN ANTALYA

Another wall using the same technique was constructed during the summer of 1999 in Antalya (Figure 9). This wall supports the approach embankment of the new Manavgat Bridge. The bridge is on a motorway and has a midspan of 80 m with a clearance from the river of 8 m. A cross section of the wall is given in Figure 10. As can be seen from the cross section, the total height supported reaches 6m. The wall is a tiered wall and consists of two sections with a one m wide berm in between. Since the area has dense vegetation and is a tourism center, this

berm has been specifically designed to allow some vegetation to grow at mid height of the wall (Figure 9). The design was conducted taking into consideration of the two stage wall. The total wall facing constructed at this project was approximately 5500 m2. On one side of the bridge the motorway approaches the bridge with a curb. The use of small blocks as facing element and geotextile reinforcement allowed the wall to adopt itself easily to the curb.

On both sides of the river, steps were required to allow pedestrians to reach to the bridge. The steps are constructed using the same technology on all four corners of the bridge as can be seen in Figure 11. The use of different colored blocks allowed for various designs as well as writing some monograms like TCK, which are the initials for Turkish Highway Authority.

The foundation soil of this wall was very soft clay. The bridge piers were constructed on piled foundations. To prevent differential settlement between the bridge deck and the approach embankment, geosynthetic vertical drains have been installed and a surcharge load had been applied to complete consolidation. However these precautions were only taken in the area close to the bridge and a large portion of the wall had to be constructed on untreated soil. The foundation soil was so soft that even small pneumatic loaders had difficulty in operating. However on this soft clay the 6 m high wall was constructed successfully and no problem was encountered. A reinforced concrete retaining wall on such subsoil conditions would require piled foundations. This means that the construction of the geotextile wall was able to provide an incredible saving. It also allowed the wall to have a much better aesthetic appearance.

Figure 9. The tiered wall in Antalya.


Figure 11. The staircase that was constructed as a geotextile reinforced wall in Antalya.

Figure 12. A view of the wall in Bursa.

5.3 THE GEOSYNTHETIC REINFORCED WALL IN BURSA

The largest geosynthetic reinforced retaining wall was constructed in Gemlik, Bursa for the Bursa Free Trade Zone. The land on which the Free Trade Zone had to be constructed was mainly a hill. In order to create a level ground the tip of the hill had to be excavated and the periphery to be filled. It was here at the periphery where the wall was necessary. The length of the wall was 1900 m (Figure 12) and a major portion of it has a height of 15 m. To make the wall more cost competitive it was designed as a tiered wall. The height of the first section was chosen as 8 m and the height of the second level was chosen as 7 m (Figure 13). The width of the berm in between these two sections was chosen as 3 m. At some locations where the geometry allowed only the first portion of the wall was constructed and the rest of the elevation was achieved with a slope. The total surface area of the wall was 22.000 m2. By using the technique of geosynthetic reinforcement, it was possible to provide an economical solution. Since the construction of a 15 m high reinforced concrete retaining wall is practically impossible, the only alternative was the use of metallic reinforcement.

Figure 13. A view of the wall in Bursa.

Figure 14. A view of the wall in Izmit Looking West.

This however would necessitate the import of granular backfill material. Because geotextile reinforcement was used it was possible to use the material excavated from the tip of the hill as the backfill material. This has considerably reduced the cost of the wall. The construction of the wall was successfully completed in October of 2001. Considering the height of the wall inclinometers have been installed behind the wall at four different locations. The inclinometers measurements indicated that after the construction a maximum horizontal displacement of 0.5% of the wall height took place. In the last three months no further deformation was measured.

5.4 THE GEOSYNTHETIC REINFORCED WALL IN IZMIT

Recently a geosynthetic reinforced wall was constructed in Izmit for the Highway administration of Turkey (Figures 14 and 15). The project is part of an overpass construction and at one location the road had to be risen to above the overpass. The maximum elevation to be reached is 10 m and the length along which the elevation is increased from zero to 10 m is


Figure 15. A view of the wall in Izmit Looking East.

Figure 16. A view of the limited space available at the base of the wall.

the approximately 199 m. This means that the total surface area of the wall is 500 m2. On one side of the elevation a reinforced concrete wall was constructed all ready at the time we got involved into the project. The road had to be opened in a short time, so the contractor was in a hurry. We stated that the construction time for the geosynthetic reinforced wall was very short, so that it will be of great benefit for them in terms of schedule. A preliminary cost analysis indicated that the geosynthetic wall was at the same

Figure 17. Top view showing the small distance between the trees and the finished wall.

time 20% less expensive than the reinforced concrete alternative. As a result it was concluded that the geosynthetic reinforced wall should be used instead of the reinforced concrete retaining wall. The construction of the wall was conducted parallel to the road fill and the total construction time was 7 days. This short construction time impressed the engineers very much and they admitted that in the 7 days where the retaining structure and road fill was completed jointly they would only be able to erect the formwork of the reinforced concrete retaining structure.

5.5 THE GEOSYNTHETIC REINFORCED WALL IN ISTANBUL

The wall was constructed to build a support to the garden of a house. There was an existing masonry wall with inadequate cross-section and as a consequence the wall had collapsed. The base of the wall is a rock so the reason of the failure of the wall was definitely not a foundation failure. However due to the presence of the masonry wall with inadequate cross section the space on which the wall had to rest was considerably small (Figure 16). So as a result to construct a proper reinforced concrete wall was impossible without further excavation, which would at the same time necessitate the cutting of old tress that were in a way the jewels of the garden (Figure 17). Considering the strength of the base rock, a design was prepared to allow for a reduced reinforcement length at the lower layers. So the wall was constructed without harming the trees in the garden (Figure 18).

6. CONCLUSION The previous studies and the research summarized in this paper leads to the conclusion that geotextile reinforced retaining walls can be constructed with marginal backfill soils. Also the case studies of geotextile reinforced walls where the facings are concrete blocks as reported above have shown that such walls can be constructed successfully.


Figure 18. A view of the finished wall

In Turkey, enough confidence was gained with the concept of reinforced soil technology. Many walls have been successfully constructed without any problems. The first geosynthetic reinforced wall constructed in Turkey is already six years old. Savings in construction time and cost has been demonstrated when compared to reinforced concrete retaining walls. The recently constructed geosynthetic reinforced structure with modular block facing has gone even one step forward and became very popular. The engineering community of Turkey has admired the easy construction technique, the tremendous cost saving and the aesthetic advantage of the geosynthetic reinforced modular block faced wall. It is anticipated that similar projects with wider scale will be constructed in the near future for commercial and governmental projects. As a summary it can be stated that the future of the geosynthetic reinforced soil retaining structures in Turkey seems to be promising great success.

7. REFERENCES 1. Christopher, B.R. et al. Reinforced Soil

Structures, Vol.1, Design and Construction Guidelines, FHWA-RD-89-043, 1990.

2. Barrett Retaining Walls. A design Primer: Geotextiles and Related Materials, Industrial Fabric Association International, 1992.

3. Billiard und Wu Load Tests of a Large Scale Geotextile Reinforced Retaining Wall. Proc. Of the Geosynthetics ’91 Conference, Atlanta, Vol. 2 pp.537-548, 1991.

4. Brandl Alteration of Soil Parameters by Stabilisation with Lime. Proc. Of 10th ICSMFE, Stockholm, Vol. 3 pp.587-594, 1981.

5. Bulut Determining the Mechanical Properties of Lime Stabilised Clay. M.Sc. Thesis, Bogazici University, 1986.

6. Burwash und Frost Case History of a 9 m High GeogridReinforced Retaining Wall Backfilled with Cohesive Soil. Proc. of Geosynthetics ’91 Conference, Atlanta, Vol.2 pp.485-493, 1991.

7. Goodings Effects of Poorly Draining Backfill on Geotextiles for Earth Reinforcement of Vertical Soil slopes. Final report to the Maryland Department of Transportation, Bureau of Research, 1989.

8. Guler Lime Stabilised Cohesive Soil as a Fill for Geotextile Reinforced Structures. Proc. of the 4th Int. Conf. On Geotextiles, Geomembranes and Related Products. The Hague, Vol. 1 pp. 39-44, 1990.

9. Guler und Goodings. Centrifuge Models of Clay Lime Reinforced Walls. A.S.C.E. Geotechnical Special Publication No: 30, Vol.2 pp.1249-1260, 1992.

10. Iyidil. The effect of Lime on Geotextiles. M.Sc. Thesis, Bogazici University, 1988.

11. Mitchell Soil Improvement, State of the Art Report. 10th ICSMFE Conf., Stockholm, Vol.4 pp.509-565, 1981.

12. Mitchell und Villet Reinforcement of Earth Slopes and Embankments. National Cooperative Highway Research Program, Report No. 290, pp.323-330, TRB, 1987.

13. Task Force 27. In Situ Soil Improvement Techniques: Design Guidelines for use of Extensible Reinforcements (Geosynthetics) for Mechanically Stabilized Earth Walls in Permanent Applications. AASHTO / AGC / ARTBA Joint Committee Report, 1990.

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CENTRIFUGE AND FULL SCALE MODELS OF ......walls. Guler (1990) conducted finite element analysis and showed that lime stabilized cohesive soils can successfully be used as backfill