Performance of Two Low Piled Embankments with Geogrids at Rio de Janeiro

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1 The First Pan American Geosynthetics Conference & Exhibition 2 5 March 2008, Cancun, Mexico Performance of Two Low Piled Embankments with Geogrids at Rio de Janeiro M.S.S. Almeida, COPPE-UFRJ, Federal University of Rio de Janeiro, Brazil M.E.S. Marques, Military Engineering Institute, Rio de Janeiro, Brazil (previously at COPPE-UFRJ) M.C.F. Almeida, Polytechnic School of Engineering, Federal University of Rio de Janeiro, Brazil M.B. Mendonça, Terrae Engenharia Ltda, Rio de Janeiro, Brazil ABSTRACT The piled embankment supported on piles with geogrids has been considered as an advantageous alternative for some soft soil deposits. This paper presents the experience of two cases of constructions where this construction technique was used in very soft soils. Some important features of these two case histories are the low ratio between embankment height to pile cap span compared with most of the literature cases, and the great compressibility of the soft clay which may result in separation between the bottom of the piled embankment and the soil underneath. The performances of the two piled embankments are presented and discussed. Design and construction details are suggested in order to improve the performance of the technique in future applications under similar conditions. 1. INTRODUCTION The conventional solution used to build an embankment on Rio de Janeiro s soft soils has mainly been stage construction with vertical drains, berms and reinforcement. This solution requires long schedules due to the stage scheme and high costs due to the large volumes of fill necessary not only to compensate for primary and secondary settlements, but also for the lateral berms. For the stabilization of a 3m high embankment over a 10m soft soil deposit in Barra da Tijuca soft clays, a fill 6 to 8m high would be necessary. Just secondary settlements in these soft clays are on the order of 5 8% of the soft clay thickness during the lifetime of the projects. In these cases, embankments supported on piles with geogrids are sometimes used as a solution (e.g., Sandroni and Deotti, 2008) with a number of advantages. Also, this technique is more suitable from the environmental point of view, since it demands lower fill volumes than conventional solutions. This paper presents the experience of the authors with two low embankments supported on piles with geogrids, built at Barra da Tijuca, west of Rio de Janeiro, close to the 2007 Pan American Games Village. 2. SITE CHARACTERISTICS The soft soil deposits in the area of interest are very compressible and have low undrained strength, so alternative construction techniques are being developed, including embankments over piles and lightweight fills. The soft soil deposit of the site was described in detail by Almeida (1998) and Almeida et al. (2001). Table 1 presents a compilation of the main characteristics of this deposit. Table 1 Typical parameters of the site s soft soil deposit. Characteristics and parameters Values Water content, w % Plasticity index, I P % Average unit weight, γ 12.5 kn/m 3 Undrained strength, S u 4 18 kpa Compressibility, C c /(1+e o ) 0.50 Average coefficient of horizontal consolidation, c h 6.5 x 10-8 m 2 /s The main design components of a piled embankment with geogrid platform are: compacted fill height (h), pile space (s), pile cap dimensions (b), a = (s - b) is the distance between pile caps, as shown in Figure 1. Table 2 presents the main characteristics of the two embankments, both with pile caps disposed in a square grid. At Barra da Tijuca District, where the two piled embankments are located, the ground level is at elevation +0.5m. The thickness of the working platform, necessary to access the sometimes flooded areas, varies from 0.60m to 1.00m. The final elevation of the embankment is in the range 2.60m 3.00m. As a consequence, the height of the piled embankment is generally lower than 1.50m. This is an important characteristic of the cases described here. As shown in Table 2, the relationships between fill height h and pile caps span a are lower than unity (in the range ) in both cases. 1285

2 Table 2 Main characteristics of the two piled embankments. Characteristics Embankment 1 Embankment 2 Construction year Number of piles 1,900 10,000 Pile space, s (m) Square pile cap dimension, b (m) Distance between pile caps, a = s b (m) Embankment height above pile cap, h (m) Ratio h/a Geogrid characteristics Fortrac R, polyester, biaxial Fortrac, PVA, biaxial Nominal geogrid strength (kn/m) and 240 Geogrid modulus (kn/m) and 4400 Fill height below the pile cap (m) Soft soil deposit thickness (m) h geogrid embankment b preliminary fill pile caps piles soft clay s residual soil Figure 1. General scheme of the piled embankment. 3. CONSTRUCTION DETAILS 3.1 Piled Embankment 1 Experimental Site Embankment 1 was built in 2004 to be the Headquarters of the national commerce institutions SESC and SENAC. At this site, an experimental area was built (Spotti, 2006), where the behavior of the standard 3D layout was compared with a 2D layout (continuous beams instead of the single pile caps). In both cases, the layout was tested for two pile cap distances. Also, two configurations of the support condition below the geogrid was studied (excavated and non-excavated layouts). At the excavated layout a 1 m deep excavation was carried out in between the pile caps, under the geogrid, in order to induce rapid load transference to the geogrid. The 2D layout will not be discussed here and can be seen elsewhere (Almeida et al., 2007). Table 3 presents the main field observations for square pile cap grids and pile centre distance of 2.5m. It can be seen that measured settlements, along a period of six months, vary from 0.1 to 0.35m. For the same embankment height, settlement values of conventional embankments can be 10 times higher. 1286

3 Table 3. Monitoring results of the experimental site. Characteristics Value Fill height above pile caps (m) Settlements (m), measured at the centre between pile caps, at pile cap level 0.10 Settlements (m), measured at the centre between pile caps, at pile cap level, excavation under the geogrid Geogrid deformations (%) Figure 2 presents settlement measurements and r/b relationship (settlement/pile cap dimension) of excavated and nonexcavated areas. As expected, the settlements of excavated areas were higher than settlements of the non-excavated areas. Theoretically, settlements of a non-excavated area, when the fill below looses contact with the geogrid, will achieve the same stabilized value of an excavated area, since primary and secondary settlements are expected to occur with time. This construction was finalized by 2004, but later on, maintenance works took place due to further piled embankment settlements. embankment height (m) settlements (m) SP03 (excavated sector) SP04 (non-excavated sector) time (days) End of construction r/b h/b 2h/a Figure 2. Settlement measured at the centre of excavated and non-excavated regions 3D layouts (sector 1) Almeida et al. (2007). Measured strains of the geogrid at the face of the pile cap (DG01 DG03), as well as data in between piles (DG05 and DG09) are plotted against normalized embankment height 2h/a in Figure 3. It is observed that strain values measured in between piles are smaller than strains measured near the face of the pile caps. Rogbeck et al. (1998) obtained the opposite behavior but that difference in performance may be due, in the present study, to the lower h/b value that leads to lower values of soil arching mobilization. Figure 4 compares the evolution of the measured strains versus the normalized embankment height 2h/a at the centre of four pile caps. It is observed that the range of measured values is similar up to 2h/a equal to 1.0, but thereafter the strains measured by DG06 and DG10 are slightly larger than the strains measured by DG07 and DG08. Loads in the reinforcement geogrid were calculated using Kempfert et al. (1997). For maximum measured deformation, of ε a = 2%, and surcharge = 0, the calculated load was T = 42 kn/m. The maximum mobilized load value estimated at field, considering ε a = 2% and modulus J = 1400 kn/m (of geogrid), results in T = 28 kn/m. It seems that the comparison 1287

4 between measured and calculated values was adequate. However the period of monitoring was relatively small and the reinforcement load continued to increase with time, as expected Strain (%) DG01 DG02 DG03 DG05 DG h/a h/b Figure 3. Measured strains in the geogrid in points at: (a) face of the pile cap, (b) half distance between caps. Almeida et al. (2007). (a) (b) Strain (%) DG06 DG07 DG08 DG10 (a) (b) h/b 2h/a Figure 4. Measured strains in between caps: (a) face of the pile cap, (b) half distance between caps. Almeida et al. (2007). 3.2 Piled Embankment Site characteristics Embankment 2 was built in an area at the north border of Embankment 1, in order to provide grounds for the SESC High School. Considering that the soft soil layer thickness is 10 to 12m, and due to schedule restrictions and stability problems resulting from the proximity of a river, the owner decided to build the embankment on piles, over an area of m 2, surrounding the piled buildings. The possibility of constructing the buildings and the piled embankment simultaneously was one of the main reasons to use a piled embankment. Discussions about the interface of piled buildings and piled embankments are presented elsewhere (Almeida et al., Construction supervision Due to its special characteristics, importance and interfaces, this kind of construction demands constant supervision by a specialized consultation team. The present authors supervised the embankment construction during the period Nov/2004-Oct-2005 verifying the construction process and providing quality control of all stages during this period. At the 1288

5 end of 2005, when the supervision contract finished, there was still a large amount of work to be done, not only at the centre of the embankment, but mostly at the interface areas: i.e., at the interface with the buildings, with the internal lakes and channels and at the neighbor border, although most of the piled embankment area had already been constructed. It was estimated that 10% of the embankment area was yet to be finished, which only occurred at the end of Settlements measured until the end of 2005 are presented in Table 4. Most of these settlements are due to the settlement of the working platform without piled embankment construction, most of which actually occurred in the last three months. Table 4. Results from about 500 days of monitoring end of Characteristics Fill height above pile caps (m) Settlements (m) of the working platform Value Below 1.50 m Below 0.40 m The authors warned the owner that it would be of extreme importance that the remaining work was to be supervised by a geotechnical team, for this pioneering kind of solution. Also, it should be said that at the interface of the piled embankment and buildings, three different contractors were at work, each one with different schedules and execution problems due to this kind of subsoil. The lack of good geotechnical supervision at this later stage had some undesirable consequences. 4. ACQUIRED EXPERIENCE Based on the performance of these two piled embankments, and other piled embankments built nearby, a number of lessons have been learned, which are described next. 4.1 Working platform and piles When the top of the soft soil deposit is at ground level and the water table is high, the first stage of the piled embankment construction is to build a working platform in order to provide access and support for pile driving equipment. The usual thickness of the working platform is of 0.6m; however, when the ground level is low and the area is flooded or there is peat at the surface, the thickness can be of about 1m. Generally, a woven geotextile is used (T = 30kN/m) at the interface of the soft soil embankment, in order to improve bearing capacity and to avoid fill material loss. At both sites, the criteria for the quality control of the embankment piles were the same as used for the building piles. The quality control of the precast piles upon arrival and the control of the piles during driving were carried out by the supervision team. The refusal of the piles was measured in all piles in both projects and dynamic and static tests were also carried out (Avelino et al., 2006). Some geotechnical designers suggest that it is acceptable to use shorter (stiff) piles in a piled embankment, thus having a much lower factor of safety than piles in standard buildings, aiming for a more economical design, as the cost of the piles is an important component in piled embankments. Their justification is that piled embankments with geogrids may be quite complacent in tolerating pile settlements and that these may occur during construction. However, the experience is that with these shorter (stiff) piles the system is more flexible and the piles are much less resistant to lateral loads which quite often occur during construction and are not easily predicted at the design phase. Lateral construction loads could happen due to a number of reasons such as: eccentric loads due to very small pile inclination, fill material placed on the top of the piled embankment, and asymmetric loads due to traffic of construction vehicles. There are reports that the consequences of the use of shorter stiff piles were quite dramatic and then led to the failure of a number of piles. Therefore, it is suggested that the standard design recommended in Foundations Codes of Practice should be used in piled embankments. The above discussion is restricted to stiff piles (e.g., precast concrete piles), as the use of compressible (granular) piles, which may result in smaller differential settlements, is out of the scope of the present paper Pile caps Pile caps can be cast inside the fill as adopted for Embankment 1 or built above the working platform, as adopted for Embankment 2 (Figure 5). Both alternatives have advantages and disadvantages. The improvement in schedule is the main advantage of pile caps constructed above the working platform (Figure 5a), besides a better concrete casting control, since it is executed with moulds. However, its main disadvantage is the decrease of the fill height above pile caps, which is of the order of the pile caps height, about 30cm. This height decrease is a disadvantage for lower fill heights, since it decreases the arching effect and increases the membrane effect. The pile cap cast inside the working platform has the disadvantage of a longer schedule and it is also more difficult to control the geometry and the finishing. 1289

6 A corner bad finishing could result in corners with fingers, which could damage the geogrid, particularly after the settlements of the working platform. An investigation performed at Embankment 1 showed that this is a real concern, as shown in Figure 6. a) Pile caps constructed above the initial fill. Figure 5. Details of pile caps construction. b) Pile caps cast inside the initial fill. 1290

7 Fingers at the corner of the pile cap due to bad finising Figure 6. Pile cap with a finishing problem. It is common to use an elevation of about 5cm at the centre of the pile cap, as shown in Figure 7. This is an attempt to provide a smoother pre-deformation of the geogrid, thus minimizing the high stress levels in the geogrid at the cap orners. A cut at the caps corners was also introduced at the pile caps of Embankment 2, as shown in Figure 7, in order to minimize the corner effect. However, these procedures were not fully efficient, as will be explained hereafter. detail Figure 7. Detail of the pile cap. 4.3 Settlements of working platform The working platform will settle and these settlements are not negligible for the soft clay deposits studied here. The primary settlements are relatively fast, particularly if a superficial peat layer is present. In this case, the smaller the working platform thickness the better, thus allowing the settlements to occur more rapidly since the consolidation coefficient at these low stress levels is higher. Figure 8 shows the settlement of the working platform surrounding the pile cap, for the cases of pile caps cast inside the fill or pile caps above the fill. In any case, and in order to avoid settlements, the geogrid should be installed soon after the pile cap construction. If settlements occur before geogrid installation, it will be necessary to level the fill before the geogrid installation to ensure that it is well stretched. 1291

8 region to be filled with soil pile caps Δh (a) pile (c) Δh (b) pile (d) Figure 8. Settlements of working platform. Figure 9 shows settlement and fill elevation with time from 5 settlements plates installed in the working platform. Two months after the fill heightening (settlement plates 32 and 34) the settlements where not quite different from plates where there was only working platform. Therefore it can be concluded that the main settlements were caused by the working platform which were in the range m and not yet stabilized at that time. Fill elevation (m) Settlements (m) PR31 PR32 PR34 PR35 PR Time (days) Figure 9. Settlements of working platform. 1292

9 4.4. Geogrids and compacted fill The next construction stage is the geogrid installation over the pile caps. As the concrete pile cap is rough, generally a non-woven geotextile is used in order to protect the geogrid against abrasion. As for geogrid installation, the more stretched the geogrid (without pre-stress), the earlier the strength will be mobilized upon embankment loading. In the case of pile caps above the working platform, the space between pile caps must be filled, thus the fill and the geogrid level are the same, and the geogrid could be installed without curvature (Figures 8a, c). This space may be filled with loose soil, so the geogrid strength can be mobilized earlier. The solution adopted for Embankment 2 was a loose soil layer with gravel on the top in order to avoid water capillary ascension inside the embankment. However, any soil layer over the working platform will cause additional settlement, which is another disadvantage of pile caps above the working platform. The use of light material such as shredded tires between pile caps may be a good alternative. The final stage is the completion of the compacted fill, with the standard requirements regarding optimum water content and degree of compaction higher than 95% (Proctor Modified). The first layers should be executed with extra care, so the compaction equipments will not damage the geogrid. However, good compaction of these layers is very important, since it will speed up the load mobilization of the geogrid. 4.5 Settlement of the piled embankment The settlements observed at the top of the piled embankment in the short and long term are basically due to the geogrid deformation, as shown in Figure 10. This figure also shows that a void below the geogrid could be expected in the long term, as a consequence of primary and secondary consolidation. Some inspection trenches opened inside the piled embankment (see next item) have shown that this void may actually occur. In this case, the installation of a geotextile over the geogrid will be an additional benefit, minimizing soil loss by erosion. Some design methods (e.g., Kempfert et al., 1997) consider the reaction of the soil below the geogrid. However, as suggested by Figure 9, for very compressible soft soils, this reaction should not be considered, since in very soft soils the geogrid loses contact with the soil underneath it. Δh t void deformed geogrid Δh if Δh t = settlement of the top of the piled embankment Δh if = settlement of the working platform Figure 10. Settlements of the piled embankment. In order to mobilize geogrid support, it is necessary that the geogrid deformation takes place. This deformation will happen due to both embankment loading and settlement of the working platform, The later is particularly important in very soft clays as in the present case. With this construction technique, the pavement construction should be delayed until the end of civil construction, when most of the geogrid deformation may have already developed. 1293

10 4.6 Inspection trenches About 18 months following the completion of Piled Embankment 2, it was decided to open inspection trenches inside the embankment with the purpose of inspecting the level of the geogrid and top caps. Thus, 30 trenches were opened in regions where settlements were greater than expected. It was observed that 3 trenches presented geogrids in an initial stage of damage and 8 trenches presented geogrids with quite a bit of damage. Geogrids in 19 trenches were in good condition. It was noticed that most of the damaged geogrids were located in areas of heavy truck traffic and that the rupture or the tearing had begun at the corner of the cap, apparently due to abrasion caused by cyclic loads. Even though there was a cut at the cap corner (Figure 7), this procedure was apparently not efficient enough to minimize the stress concentration, causing the tear of the geogrid, which was then propagated to the sides of the cap. Based on this, a circular cap could be more effective than the square caps in those cases. Tensile tests carried out on the geogrid collected from the trenches are presented in Table 5 (Metropolitana, 2007). The tests showed that the geogrid strength was adequate, and the lower values could be justified by installation damages, which are always considered in design as reduction factors for installation damage. Table 5. Results from tensile tests carried out on exhumed geogrids taken from trenches. Geogrid Tensile strength (kn/m) Deformation at rupture (%) Laboratory J A (Brazil) J B (Germany) J A (Brazil) J B (Germany) Installing the geogrid not just above the pile cap, but instead with a sand layer in between, may minimize the high stress gradients. However, this would diminish the distance between the geogrid and the embankment top, which would decrease even more the arching effect for low embankments. There are case histories where more than one layer of geogrid was used; however, in the present case, this design would further increase the membrane effect due to the decrease of the distance between the geogrid and the top of the embankment. 5. CONCLUSIONS The piled embankment with geogrid platform can be an alternative with technical, economical and schedule advantages, when compared to the solution of a reinforced embankment with berms on vertical drains used for very soft clays in Rio de Janeiro. The two piled embankments described in this paper were built on these soft clays in an area of about m 2 with low ratio of embankment height to pile caps span. The measured settlements varied from 10 to 40 cm, thus on average one order of magnitude lower than the settlements of an equivalent conventional embankment. The deformations measured in the geogrid were in the range of 0.5 2% and the measured loads in the geogrid were lower than the loads calculated using the methods of Kempfert et al. (1997). Based on the performance of the two piled embankments, it was observed in very soft clays that the working platform causes important settlements, the magnitude of which is not negligible and takes long time to stabilize, also because secondary settlements are quite high. The results presented in the paper suggest that subgrade reaction should not be considered for the design of piled embankments with geogrids.considering the continuous geogrid deformation and the possibility of post construction settlements it is suggested the pavement should be light and flexible, allowing future maintenance works. The pile caps may be buried inside the working platform or placed on its top. A comparison of the two alternatives for the existing conditions of the two sites suggest that burial is preferable, provided careful finishing of the pile cap is ensured to avoid damaging the geogrid. Pile caps adopted in Pile Embankment 2 had a curvature on top and corners cut to decrease stress gradients on the geogrids. Despite these, trenches opened inside the embankment showed damaged geogrids in areas with heavy traffic, apparently due to cyclic loading leading to abrasion of the geogrid against the pile cap, which suggests that, in these circumstances, a soil layer should be placed in between the geogrid and the pile cap. Also, circular pile caps may be a better alternative to square pile caps, as the tearing of the geogrids started at the corners. The use of compressible granular coluns without pile caps as opposed to stiff concrete piles with pile caps may be a better technical alternative in very soft. However the cost of granular columns solution is usually greater than the cost of the stiff pile solution particularly in very soft clays, as granular columns may have to be encased with high modulus geotextile. 1294

11 ACKNOWLEDGEMENTS The authors are grateful to the SESC and SENAC and Construtora Metropolitana for their support of the development of the studies described here. REFERENCES Almeida, M. S. S., Almeida, M. C. & Marques, M. E. S. (2008). Numerical Analysis of a Geogrid Reinforced Wall on Piled Embankment, Paper submitted to the 1 st Pan American Geosynthetics Conference & Exhibition, 2 5 March 2008, Cancun, Mexico. Almeida, M. S. S. (1998). Site characterization of a lacustrine very soft Rio de Janeiro organic clay, Proceedings of First International Conference on Site Characterization, ISC 98, Atlanta, Georgia, Almeida, M. S. S., Ehrlich, M., Spotti, A. P. & Marques, M. E. S. (2007). Embankment supported on piles with biaxial geogrids, Journal of Geotechnical Engineering, Institution of Civil Engineers, ICE, UK, volume 160, issue 4: Almeida, M. S. S., Santa Maria, P. E. L., Martins, I. S. M., Spotti, A. P. & Coelho, L. B. M. (2001). Consolidation of a very soft clay with vertical drains, Géotechnique, 50, n. 6, Avelino, J. D., Almeida, M. S. S., Santa Maria, P. E. L., Almeida, M. C. F. (2006). Provas de carga em estacas em terreno com presença de solos moles, Anais do XIII Congresso Brasileiro de Mecânica dos Solos e Engenharia Geotécnica, Vol. 2, p , Curitiba. Kempfert, H-G., Stadel, M. & Zaeske, D. (1997). Design of Geosynthetic-Reinforced Bearing Layers Over Piles, Bautechnik, 74, Vol.12: Metropolitana (2007). Personal communication from the contractor of Piled Embankment 2. Rogbeck, Y., Gustavsson, S., Södergren, I. & Lindquist, D. (1998). Reinforced piled embankment in Sweden design aspects, 6th International Conference on Geosynthetics, Atlanta, Georgia, USA, Vol. 2: Sandroni, S. S. and Deotti, L.O.G.(2008). Instrumented test embankments on piles and geogrid platforms at the Panamerican Village, Rio de Janeiro. Paper submitted to the 1 st Pan American Geosynthetics Conference & Exhibition, 2 5 March 2008, Cancun, Mexico. Spotti, A. (2006). Monitoring results of a piled embankment with geogrids (in Portuguese). DSc. Thesis, COPPE/UFRJ, Rio de Janeiro, Brazil. 1295