Design and Analysis of Approach Embankment Bridges over Soft Soils

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1 Design and Analysis of Approach Embankment Bridges over Soft Soils Carmo Cardoso *, Department of Civil Engineering, Instituto Superior Técnico, Universidade Técnica de Lisboa, Portugal Abstract The growth of urban areas near watersides induces the need of construction over compressible soils. This paper presents a study on the behavior of approach embankments bridges over clay soils with special focus on its foundations. In order to achieve results several projects were developed and the results were analyzed and achieved based on the comparison between software Plaxis 2D modeling and the experimental embankments. An equal set of base data was considered for all the projects. The underlying data is equal to the embankment AE1 data, in order to represent the same real problem. The AE1 is one of the three experimental embankment led to breakage, developed within the frame of the Magnani doctoral work in Four models were projected ranging from constructive methods: from the more deformable - conventional embankment with georeinforcement at the base and foundation treated with vertical drains built-in stages - to the more rigid- staked embankment - passing on the intermediate - embankment over stone-columns and embankment over jacketed stone-columns. Keywords: Soft soils, Embankment, Wick drains, Stone Columns. Introduction In large metropolitan areas, the soil occupation took place essentially in coastal areas due to the strategic location a ease of communications and transport together with the economic point of view. However, these areas are surrounded by soils shaped essentially by sedimentation, soils eroded and carried by water and by alluvial deposits of soft clay. These soils were so far avoided due to their diminutive mechanical characteristics, less favorable to groundwork. The construction on soft clay soils remains a real problem today and it is targeted in studies, given the need to build new urban infrastructures and transportation on their surface, as the urban centers continuously grow. Presentation of the embankment AE1 The experimental embankment AE1 was settled along with two others, AE2, AE3 over a soft clay deposit, located in the interior of the south bay of the Santa Catarina island, in the city of Florianópolis, State of Santa Catarina, in southern Brazil, presented in Fig.1. Fig. 1 Location [1] * carmo.cardoso@ist.utl.pt Master Student 1

2 The embankments were constructed on the scope of the doctoral thesis of Henrique Magnani de Oliveira [2]. The main objective was to study the behavior of reinforced embankments executed over a very soft clay deposit in a regime rapid construction and are part of the Via Expressa Sul project, a road that links the downtown area to the southern part of the island of Santa Catarina. The location of the experimental embankments was determined to obtain the lowest possible thickness of hydraulic embankment, to simulate the most critical situation, looking for the area of maximum possible homogeneity both on geotechnical and geometric level, to favor the understanding of the observed data, and finally, as close as possible to the construction site to ensure greater representation and less distance to transport the materials which would be used. The obtained location is presented in Fig.2 Side edges were also planned to reduce the three-dimensional effects of embankments due to its limited length, to minimize potential disruptions and displacements in the longitudinal direction to sections, and also to prevent the disruption of a landfill that could influence the conditions of stability given the vicinity. The pre-assembled vertical drains were carved using the spiking of a keyless chuck with 15 cm diameter, with a horizontal spacing between drains of in a triangular pattern and length in order to cross the full layer of clay. The AE1 embankment featured a length of m, plus the edges, with of cross section and slopes with an 1,5:1,0 (H:V) inclination. The embankment stopped rising in height when it reached the rupture, which occurred at. The performance is presented in Fig. 3. Fig. 2 Location of experimental embankment [2] The embankment AE1 was built with synthetic reinforcement in the base and the foundation treated with vertical drains, to examine the influence of short-term drains on the performance of reinforced embankments. The main function of the reinforcements and drains are to improve stability of the embankment body and reduce the post building constructive settlements, respectively. Under a layer of hydraulic embankment measuring and thick of soft clay, based on a dense sand stratum. Fig. 3 Performance of the embankment AE1 [2] 2

3 The Approach Embankment Bridges The approach embankment bridge is the structures that connects the bridge with the ground, with the main objective of reaching the required quota and make the connection in safe and comfort conditions. The occurrence of settlements in these infrastructures in the procedure phase do not represent by itself risks reaching the Ultimate Limit State, though generates high maintenance costs to ensure the Limit of Use State. Selecting the most appropriate constructive method is conditioned by several factors such as the deposit geotechnical features, use of the area and its surrounding area, construction deadlines and costs involved. The issue in question has no restrictions on the available space and difficulties within the neighborhood. Regarding the duration of construction it was agreed on a maximum of 18 months (a year and a half). The purpose of this site is to reach a height of above ground level, a quota and wide, for later construction of a two-way communication via fixed and their respective edges, with both ramps having equal inclination, 2:1 (H:V), as shown in Fig. 4. Fig. 4 - Section type, in meters As in the case of experimental embankment, it is considered that the body of the embankment is executed with the same hydraulic fill sand, with parameters presented in Table 1, and modeled in the software model Plaxis Hardening Soil material. Table 1 Characteristic of Fine Sand - Hardening Soil model [1] Bulk weight γ [kn/m3] 17,5 Initial void ratio e 0 0,6 OCR 1 Vertical permeability 10-4 Horizontal permeability 10-4 Effective Friction angle [ ] 33,8 Dilatancy angle [ ] 3,8 Effective Cohesion c [kpa] 0 (*) Stiffness Modulus [kn/m 2 ] Stiffness Modulus [kn/m 2 ] Power for stress-dependency level, 0,65 (*) In the modeling of unsaturated embankment was adopted c = 1 kpa. Along with the sand of the draining mattress, applied on the basis of all embankments, modeled with the Hardening Soil material model, with the characteristics presented in table 2. Table 2 Characteristic of Sand draining mattress Hardening Soil model [1] Bulk weight, γ [kn/m3] 17,4 Initial void ratio, e 0 0,5 Vertical permeability, 10-4 Horizontal permeability, 10-4 Effective Friction angle, [ ] 33,8 Dilatancy angle, [ ] 3,8 Effective Cohesion, c [kpa] 0 Stiffness Modulus, [kn/m 2 ] Poisson coefficient, ν' 0,30 The clay stratum is modeled with the material model of Soft Soil, which parameters are presented in table 3. The groundwork layer is modeled with Mohr-Coulomb model and material with the characteristics presented in table 4. 3

4 Table 3 Characteristic of Soft Soil - Soft Soil model [1] Bulk weight γ [kn/m3] 13,7 OCR 1 Vertical permeability 10-9 Horizontal permeability 10-9 Initial void ratio e 0 3,5 Effective Friction angle [ ] 30 Dilatancy angle [ ] 0 Effective Cohesion c [kpa] 5 Modified compression index λ* 0,1565 Modified swelling index k* 0,0344 Table 4 Characteristic of Bottom Sand Mohr-Coulomb model [1] Bulk weight γ 17,5 Initial void ratio e 0 0,5 Vertical permeability 10-4 Horizontal permeability 10-4 Effective Friction angle [ ] 33,8 Dilatancy angle [ ] 3,8 Effective Cohesion c 0 Stiffness Modulus Poisson coefficient, ν' 0,30 Projects In all projects developed, such as the AE1 landfill, it was agreed to put a geo-reinforcement on the foundation as a backup step. Conventional Embankment The conventional embankment project is built in stages and foresees the placement of vertical drains and overhead in order to speed up primary repressions and anticipate secondary repressions. In the pre-sizing phase, through analytical calculation, the primary settlements for the properties due to phased construction. To achieve this the equation of repressions for a fixed quota of landfill was applied considering the submersion of the layer, by [3], EQ. [A]. ( ) [A] Having reached a rating of to a embankment of height. The secondary settlements calculation based on the methodology proposed by Martins (2005), exposed in [2], was applied and reached an estimation from Eq.[B] a value of. ( ) ( ) [B] The conclusion was that it is necessary to build a embankment of height to compensate the repressions in the completion,. The overhead is estimated so that the height of the embankment overhead causes a primary discharge equal to or greater than the total discharge previously estimated. It has been estimated an overload of, using the equation of repression by primary densification, EQ. [C], to be removed at the end of the 18 months construction phase. [ ( ) ( )] [C] The assessment was to use the same system of drains that were used in embankment AE1. Mattress sizing draining made according to Cadergren methodology (1967) exposed in [2], follows with this spacing of drains and with thick layer of draining able to set seven horizontal drains spaced by. simplest situation were forecasted, i.e., it was not considered the improvement of the foundation 1 smear is the effect of compaction in the soil surrounding the drain, caused by their spiking. 4 It is estimated the active tractive effort in strengthening through the methodology proposed

5 in 1985, by Rowe and Sodermenn exposed in [2], developed to evaluate the tensile forces mobilized in strengthening from the value of predictable deformation,, in function of a dimensionless parameter Ω, which relates from a abacus. Through the analysis we projected a strain for a geo reinforcement presenting a tensile stiffness modulus in direction, to 5% deformation (J 5% ), de approximately [4], we may concluded that the maximum traction which can be mobilized in the reinforcement is in an order of magnitude of, much lower than the. A predication was made on how long it takes for the primary densify to occur due to construction of embankment considering densification with purely radial drainage,, due to the effect of vertical drains. It wasn t considered the effect of smear 1 and changes of the values of the coefficients of permeability due to phased construction in order to simplify and to compensate the non-consideration of vertical drainage component. We decided to make a Plane strain modeling. Obtaining the model's figure. [5],Fig.6. The obtained permeabilities are in the following table. Table 5 Permeabilities of Soft Clay Vertical permeability Horizontal permeability Horizontal permeability in the smear zone Plane strain horizontal permeability Plain strain horizontal permeability in the smear zone Fig. 6 Transformation of permeability [5] By this means and after running the model in Plaxis 2D software, it was possible to get the offsets at the top of the embankment which is presented in graphic form in Figure 6, where it is visible a near blistering when the overload is removed. It was found that the georeinforcement has a maximum axial effort. Fig. 5 Transverse profile of the Conventional Embankment: 1 - Embankment; 2 - Draining Mattress; 3 - Hydraulic Embankment; 4 - Clayey Stratum; 5 - Stratus Bottom Sand To build the model in Plane strain and to represent the smear effect, the change of 2D and 3D problem altered the permeability along drains, by applying the methodology recommended by Fig. 7 Progress of the settlement on the top of the embankment and a height of embankment constructed time-related 5

6 Fig. 7 shows that the construction in stages produced settlements smaller than estimated analytically and that more time is needed to stabilize due to the densification of clayey stratum. Jacketed Stone-Columns Embankment The Jacketed Stone-Columns Embankment has as an advantage regarding conventional embankments: shorter construction time and added control over the repressions, causing an increase in resistance of the clay due to the drainage reached and the increased strength of the foundation. It was agreed that the columns would measure length, to be built-in the layer, diameter, forming a square weave of side, in order to ensure the absence of differential settlements at the top of the embankment. The criteria applied was the one proposed by [6] which presents the EQ. [D]. Table 6 Characteristic parameters of the materials of the Stone-Column - Mohr-Coulomb model Soil unit weight above phreatic level γ h [kn/m3] 18 Soil unit weight below phreatic level γ sat [kn/m3] 20 Vertical permeability 1,16x10-4 Horizontal permeability 1,16x10-4 Initial void ratio, e 0 0,5 Effective Friction angle, [ ] 38 Dilatancy angle, [ ] 12 Effective Cohesion, c [kpa] 5 Stiffness Modulus E [kn/m2] 25x10 3 index n 0,30 In the modeling process the unit cell concept was used with equivalent diameter, using the option of symmetry, Axisymmetry model. [D] Where and soil. is the diameter on the top of the column is half the highest horizontal distance of Imposing height,, with diameter getting chose to stipulate, common value in this type of works, for drainage issues. The granular material used has the same properties of common sand, modeled following the model of material Morh-Coulomb and with the characteristic parameters shown in table 6. All other materials remained identical.fig.8. Fig. 8 Unit cell and finite element mesh of the Jacketed Stone-Columns Embankment Finding the curve of the settlements at the top of the embankment on the non-jacketed and jacketed cases, making it clear the contribution of the coating, Fig.9. 6

7 Table 7 Characteristic parameters of Concrete Bulk weight,, γ [kn/m3] 25,0 Stiffness Modulus, [kn/m 2 ] 30x10 6 Poisson coefficient, ν' 0,00 Vertical permeability, 10-4 Horizontal permeability, 10-4 Fig. 9 Progress of the settlement on the top of the embankment and a height of embankment constructed time-related for 1 Stone-Column and 2 - Jacketed Stone-Column It was observed that the stake had a vertical displacement of and there are no differential settlements at the top. Embankment over piles In this solution it was considered the embankment on stands, spiked with, and with circular capitel with diameter, forming with a quadrangular mesh aside, this dimension guarantees the non-existence of differential settlements at the top of the embankment, as in the project of the embankment on granular columns, respects the criteria proposed by Kempfert in 2004 present at [2] and ensures that the stresses in the reinforcement are admissible. As in the case of embankment on granular columns, the modeling of the embankment on stilts resorts to a unit cell, Fig.10. However, the axisymmetric modeling does not consider the anisotropy of the geo-reinforcement and even allows checking the stress concentration in the corners of the capitals. To bypass this limitation, in the sizing phase it was foreseen the construction of circular capitals without angular areas. The concrete stake was designed with a linear elastic behavior, with the properties shown in the following table. All other materials had their properties and models unchanged. Fig. 10 Unit cell and finite element mesh of the staked embankment Economic Analysis In table 8 the activities and quantities for each constructive option method is described and at Fig.11 is present the. The quantities shown are accounted for by embankment and longitudinal metro as a result, the final prices also refer to /(underground longitudinal embankment ). The unit price values applied to each task are representative of the average market values of the Portuguese market. In the proposal for the conventional embankment the construction price for the transition paving 7

8 stone was not introduced since this appraisal was not viable per meter of longitudinal embankment. After obtaining the costs for each submitted proposal it is reasonable to conclude that the more flexible solution - Conventional Embankment - is in fact the more cost efficient, with a cost of 3.061,00 per meter of longitudinal embankment. However it is also the solution that requires more time - 18 months - for the stabilization of compressible stratum. The time factor should be analyzed in sets with other factors such as the time of construction of other infrastructure or other interventions that the area will suffer. The waiting time between stages of construction could be used, in its entirety, for the construction of other infrastructure. The most expensive solution is the proposed Embankment on Jacketed Stone-Columns, with a cost of 5.690,00 per meter longitudinal embankment, showing no advantage for the solution of the Landfill staked. The proposed Embankment staked is the most rigid and more reliably. In several projects the incline of slopes did not change, 1:2 (vertical: horizontal) for being one initial condition. However, it is more likely that more rigid solutions may be executed with steeper slopes, which would translate into less earth volume that in the end would reflect significantly on the final cost. Table 8 Resume of the Economic Analysis Description Conventional Embankment Jacketed Stone-Columns Embankment Embankment on piles Quantity 1. Foundation treatment including raw material price and all work necessary for its proper application. 392,0 m 120,0 m 52,0 m 2. Installation geo-renforcemt including raw material price and all work necessary for its proper application. 48,0 m 2 42,0 m 2 41,0 m 2 3. Construction of the embankment, including on the price raw material, quantity of water and its transport, all the necessary work to compaction and testing. 270,0 m 3-33,8 m 3 170,5 m 3 154,0 m 3 Total 3.061, , ,00 Fig. 11 Section type of each solution: A - Conventional Embankment; B - Embankment over Jacketed Stone-Columns; C - Embankment over piles. 8

9 Conclusion Embankments on soft soils can be constructed successfully if the solution was taken in account the complex behavior of clayey soils. The solution of the Conventional Embankment, as the experimental embankment AE1, explores at most benefit of consolidation accelerated by combining the drains and overhead. It is noted that the first settlements occur identically, i.e., the curve of the settlements at the beginning of the loading exhibit similar trends and values. However, it was observed that the progress of experimental embankment AE1 compared to conventional embankment modeled showed lower levels of pore pressure due to the temporal spacing, allowing for their dissipation and consequently to maintain the stability of the foundation. The traction strain measured at the geo-reforcemt before break was T = 32kN, while in the model the maximum axial strain measured was T = 12.2 kn. With the modeling it was verified that the clay stratum obtained significant improvements, because of phased construction, resulting in smaller settlements and consequently reducing the permeability coefficient, which causes the need for more time for the dissipation of pore pressures, i.e., more time to stabilize. The economic analysis was performed with the intention of compare the cost of each solution and was used the mean value of the Portuguese market. However, the values contains prices of skilled labor higher than those applied in South America, which means that in the accounting of the time factor requires special attention. of the peculiarities overcome in each solution is a critical factor, in other words if that project can be extremely careful but the prospect does not allow the correct geotechnical characterization, the final result will not be satisfactory. References [1] Sonney, R. (2013). Numerical analysis of 3 test embankments on soft ground: effect of basal reinforcement and prefabricated vertical drains. Master thesis COPPE - UFRJ. Rio de Janeiro, Brasil. [2] Magnani, H. O. (2006). Comportamento de aterros reforçados sobre solos moles levados à rutura. Doctoral dissertation COPPE - UFRJ. Rio de Janeiro, Brasil. [3] Almeida, M. S., & Marques, M. E. (2010). Aterros sobre solos moles: Projeto e desempenho. Sao Paulo: Oficina de texto [4] HUESKER. (s.d.). Stabilenka - Woven Fabrics for Reinforcement and Separation [5] Indraratna, B., Rujikiatkamjorn, C., Sathananthan, I., Shahin, M. A., & Khabbaz, H. (2005). Analytical and numerical solutions for soft clay consolidation using geosynthetic vertical drains with special reference to embankments. [6] Filz, G. (2012). Column-Supported Embankments: Settlement And Load Transfer. Geotechinical Engineering State Of The Art And Practice - Keynote Lectures From Geocongrees, (p. 54). The importance of a good geological and geotechnical characterization is the key factor for the good project performance. The interpretation 9