Setúbal's Alegro Shopping Center Geotechnical Solutions for Peripheral Walls and Foundations

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1 Setúbal's Alegro Shopping Center Geotechnical Solutions for Peripheral Ana Teresa Martins Rodrigues Instituto Superior Técnico, Universidade de Lisboa October 2014 Keywords: Shallow Foundations; Deep Foundations; Peripheral walls; Modelling; Back Analysis; Optimization Abstract : The continuous expansion of urban areas has implications in terms of town planning and management. Thus, there is a need to use underground spaces, and to put existing buildings to new uses. Civil Engineering, in particular, Geotechnics, has played a key role in the development of technologies that meet structural, geological and geotechnical challenges, as well as the constraints of excavations in urban areas. In this paper it was intend to conduct some analyses that address these issues, as a contribution to the study of flexible structures behaviour, whose structural performance is conditioned by the geological and geotechnical context of each construction site. The aim of this thesis is to study the behaviour of peripheral retaining structures in urban areas, and shallow foundations, semi-deep foundations and deep foundations, as applied to a case study, which consists of the rehabilitation and expansion of Alegro Shopping Center in Setúbal. Although several types of retained structures have been executed in the said construction work, given its large scale, the solution of bored piles is the most representative and the one which presents more structural interest, inasmuch as it serves as a support for the excavation, near the existing building. The executed foundations solutions consist of footings, footings over piers and piles, using continuous flight auger. For the numerical study of the behaviour of the peripheral retaining structures and foundations, a modelling was performed with a 2D analysis of finite elements software. This modelling was based on a back analysis, in order to assess the geotechnical parameters that best approximate the model to the real situation. For this purpose, instrumentation was used to approximate, as much as possible, the real displacements to the values estimated through the model to the real displacements. Once the model was calibrated, the behaviour of both solutions was studied and a direct analysis of the structures displacements and stresses was made. Considering the geotechnical solutions suitability, an alternative optimized solution for foundations and earth retaining structures was designed. The modelling was developed using the aforementioned software, and the structural, functional and economic viability of the solution was analysed. This analysis was followed by a damage risk assessment of the excavation in the adjoining building and the structure itself. Introduction Nowadays, the European construction industry is experiencing a severe crisis. With new projects running very scarce, the remodelling and recovery of existing buildings becomes ever more important. Such interventions often involve the use of underground spaces, in order to maximize the buildings capacity. In projects of this kind there are space limitations for the construction of underground structures, leading to solutions of vertical excavation in order to occupy the smallest possible area. Due to the proximity of the excavation to existing buildings, those works require an extra care to prevent any damages. Another limitations inherent to these projects are the local geological and geotechnical constraints. The latter constitute challenges that Civil Engineering, and mainly Geotechnics have been able to surmount by developing new soils and structures stabilization techniques and technologies. The aim of this thesis is to study the behaviour of flexible earth retaining structures and shallow and deep foundations, using as a case study the construction work of a shopping centre Setubal s Alegro. 1

2 In the study of the said solutions, FE modelling was used as a tool to assess the adequacy of the initial solution, that came to be executed, and to devise another one that optimize the first one. For the construction of both models, the geological and geotechnical conditions of the site had to be ascertained. For that purpose, a back analysis was performed, using the instrumentation measurements. This analysis aims to approximate to the utmost the modelling results to the actual displacements that occur in structures of the work, in order to calibrate the geotechnical parameters of those soils. The alternative solution was also followed by an economic and a risk analysis, both for the building and adjoining one. Foundations The foundations are structural elements whose purpose is to transmit loads to the ground, that play a key role in the structures stability and durability. They ensure and control the soilstructure connection and interaction under the influence of internal loads and variable actions, such as wind, earthquakes, etc (Brito, 1999b). The study of foundations, which is an integral part of geotechnical projects, must be based on onsite geotechnical investigations to ascertain the geological e geotechnical conditions of the construction. When the grounds are appropriate to ensure support the loads at the surface, shallow foundations can be used. Otherwise, another type of foundation must be adopted, which explore better conditions at a deeper level, named deep foundations (Guerra, 2008). There are several different types of shallow and deep foundations, according to its component materials, dimensions and structural configuration and to the constructive process thereof. Isolated Footings Isolated Footing is the most common type of shallow foundations. It is used when the ground has adequate characteristics and the loads are small to medium load level with no special requirements or sensitivity to differential settlement (Brito, 1999b). The bearing capacity of shallow foundations depends on a study of the bearing capacity to the vertical loads and to horizontal loads (slip), although the first one is often more condition. From a derivation of limit-equilibrium, proposed by Therzaghi, it could be adopted a pre-design method to calculate the bearing capacity of the soil to support the footing and its loads. The safety verification regarding the bearing capacity condition to vertical loads is given by: V d R d Where, V d is the design vertical load and R d is the design vertical load capacity of the soil. The latter may be determined to drained and undrained conditions. The Service Limit States must also be considered in the design of foundations of this kind. These structure states correspond to the mobilization of movements at the building foundations, such as settlements, differential displacements and relative rotations. Such movements differ, depending on the foundation soil and the presence or absence of groundwater table level (Coelho, 1996). Figure 1 - Definition of foundations movements of the building (Santos, 2002) When controlling the displacements associated with the building foundations, it is crucial to take into account the structure type, the use the construction work is designed for and its expected service life. According to these criteria, allowable settlements are defined, compatible with the proper serviceability of the structures, allowing safe and economic solutions. This study is named risk damage analysis. Bored Piles Foundations Bored piles foundations are the most widely used, among all the solutions for deep foundations. These piles reach great depths and are executed insitu, by casting the concrete against the ground (Brito, 1999a). 2

3 In particular, the bored pile of continuous flight auger technology is the only method where rebar is introduced after concreting. Its construction process is as follows. Figure 2 - Construction process of bored piles, using continuous flight auger (Santos, 2008) 1- Drilling with auger; 2- The auger is extracted as it proceeds to cast concrete through the auger, taking the place of the extracted soil; 3- Placement of the rebar (Santos, 2008) The design of this type of foundations relies, essentially, on the bearing capacity of the soil for vertical actions. The latter is then compared with the vertical load of the pile (F c or F t ). In the case of piles which are compressed, the soil can provide resistance either at the base, along the shaft or both. In the case of tensioned piles, the soil only provides lateral resistance, along the shaft. The bearing capacity of the soil depends on the stiffness of the foundation soil. Compressed Pile Tensioned Pile R c = R b + R s F c R c R t = R s F t R s Where R b and R s are the bearing capacity of soil at the base and along the pile shaft, respectively (Santos, 2008). Flexible Retaining Walls According to Peck (1972), flexible earth retaining structures are all retaining structures, whose deformations induced by the pressure of the soil have a significant effect on the distribution of pressures as well as on the magnitude of the bending moments and the shear effects. There are several different types of earth retaining structures, according to its adopted constructive elements, the materials and the construction process (Matos Fernandes, 1983). The earth retaining structures may be combined with bearing by means of props or pre-stressed anchors. Multi-anchor curtains are frequently preferred to multi-shored ones, however, for its superior costeffectiveness and for the reduced construction times and increased flexibility of the work schedule, provided by the free space inside the excavation, they allow (Guerra, 2007)(Matos Fernandes, 1983). Figure 3 - Multi-shored curtain (a) and Multi-anchor curtain (b) Multi-anchored Retaining Walls In contrast to props which produce a passive action at the supported soils, in the anchor solutions case the displacement doesn t depend, primarily, on its supporting elements, where the axial stiffness is much smaller. Such solutions depend on the changing stress state of the soil and the stiffness of the wall. Changing the stress state corresponds to the installation of a large part of the design load on the anchors, performing an active character. The question here rests in the values of the prestressing load applied to the anchors. It is not proper to revise the maximum stress that will be submitted to the anchors, it is before adopting the pre-stressing to impose in each one (Matos Fernandes, 1983). Excavation s influence in the nearby structures The movements of earth retaining systems derive from vertical and horizontal initial stress relief caused by excavation, leading to changes of the stress state in the soil. Therefore, excavation works in urban areas require special care in the project design stage, in order to control the displacements at the earth retaining structure as well as at the 3

4 adjacent buildings. These are a consequence of the ground movements induced by the changes of the stress state in the soil. Settlements which occur behind the earth retaining structures are induced by deflection, during excavation and construction of the retaining wall, or caused by the laying of anchors. It means that, in addition to the geotechnical scenario, the stiffness of the support system (props or anchors), its position and the contribution to the stiffness of the retaining wall leads as well to the relation between the horizontal and vertical displacements. As it is quite difficult to establish direct relations between the excavations /walls movements and the displacements at the building, the common practice consists of limiting the wall s displacement, as an indirect control method to prevent any damage to structures adjacent to excavations (Kempfert & Gebreselassie, 2006). A relation is thus established between the horizontal extension (ε h), based on the differential horizontal displacements, and the parameter of angular distortion (β) caused by bending distortion of the wall. Then, this relation is correlated with a certain level of damage risk to the structure (Boscardin & Walker, 1998). connected to each other through a capping beam at the top and several distribution beams in depth, at each ground anchors level. This is a widespread construction technology in earth retaining works in urban areas, for its economic competitiveness, its adaptability to different geological and geotechnical scenarios and space constraints in the implementation of the structure, and for the speed of execution as well. Case Study The case-study presented in this paper is the construction work of the expansion and remodelling of a shopping centre in Setúbal. This work involves the remodelling of the existing building, as well as the construction of a new facility (Figure 5). The construction of the latter is a two-stage process with Phase 1 and Phase 2 (as shown in Figure 6). This study concerned itself exclusively with Phase 2, the one which involved excavation, peripheral walls and foundation-laying work. The peripheral wall system was executed as a temporary retaining structure designed to provide support for the construction of a threestory underground car parking. Figure 4 Horizontal and vertical displacements: a) Curtain without support system; b) Anchored or shored curtain (Kempfert & Gebreselassie, 2006) adapted by (Oliveira, 2012) Where, Figure 5 - Building area of the project (Google) ε h = δ h L ; β = δ v L L is the extension of the building that is behind the wall. Bored Piles Walls Bored piles walls comprise a set of piles concreted against the ground, using the same construction process which is applied in the making of Bored Piles Foundations, using the continuous flight auger technology. The piles are distributed consecutively along the excavation perimeter and Figure 6 - Scheme of construction phases Geologic and Geotechnical Scenario To establish the geology of the site some auxiliary tasks were carried out: drilling of 12 mechanical 4

5 boreholes, followed by Dynamic Penetration tests (SPT). According to the results, from the surface it was intersected a landfill to a variable depth of 1.25 to 5.80 meters. Along the drilling work, the groundwater level was detected at a depth of 15.5 meters. According to test results obtained from dynamic penetration SPT, geotechnical zoning was defined into four distinct zones (GZ). This information is presented in Table 1. Table 1 - Geotechnical zoning (GEOTEST, 2012) GZ Description N SPT 1 Recent landfill deposits Uncompressed Pliocene formations - Medium to coarse sand and coarse clay and silty Medium to coarse sand and coarse clay and silty (Pliocene 4 formations) >49 Vicinity Constraints Alegro Shopping Center compound is located at one of Setúbal's main access ways and bounded by other streets. The construction site is surrounded by an elementary school and a AKI megastore to the North, and by residential buildings to the West. In phase 2 of the construction, the one in which the excavation took place and which is the object of this study, the hypermarket building, located to the East, imposed an important constraint. Considering that it was to continue in full operation, the integrity of its structures had to be assured, throughout the excavation and construction work. Monitoring and Survey Plan The Monitoring and Survey Plan is a crucial tool in the execution of geotechnical structures. In this study it was only considered the Monitoring and survey plan that regards earth retaining solutions. Since flexible earth retaining structures, like bored piles curtains, are extremely sensitive to the displacements caused by excavation works, it is essential to know and control such movements. Therefore, from the design stage, a risk management approach must be adopted in conjunction with the constructive solution, in order to control these movements and ensure safety in the work. In this case study, instrumentation was installed in the existing building, at the capping beam and at the distribution beam. It consisted of topographic rulers and targets to measure the horizontal and vertical displacements of the structure. In addition, it was installed an inclinometer, which measures the displacements of the wall, in depth and during the work. Monitoring renders design optimization possible as well, enabling corrective actions to be undertaken in response to unforeseen situations. The instrumentation plan should be viewed as an investment, instead an additional cost. In this study, monitoring allowed the calibration of the soils parameters, through a back analysis. Studied Solutions In the present paper are studied solutions based on bored piles, such as peripheral walls, on the one hand, and footings and bored piles as foundation techniques, on the other hand. Their location is represented at Figure 7. In order to study those solutions, a section of the blueprint was selected that presents the most adverse conditions for the two solutions (earth retaining structure and foundations). Figure 7 - Peripheral and foundation studied solutions The following scheme represents the geotechnical model applied in the study of structures and solutions behaviour (Figure 8). N 5

6 Figure 8 - Geotechnical study model Solution s Modelling The modelling was performed by means of finite element software (FEM). The existing building was represented by a distributed load behind the curtain. Figure 9 - Model Geometry Solution The results thus obtained constitute a good approximation to reality. This happens because it was possible to respect the model geometry, as well as the structure and the geologic settings. The program has also the ability to simulate the constructions processes. The easy handling of the most sensitive parameters that influence the structure behaviour, combined with a good approximation of the model to the reality, allowed to obtain economically and safe solution. Modelling was used to explore the real characteristics of the four geotechnical zones. This was made performed using through a retroanalysis study based on iterative comparison between the measurements from instrumentation and the ones obtained through the software. The goal was to approximate to the highest degree possible the software results to actual displacements, and thus to allow the assessment of real soil parameters. Through the back analysis results, it was possible to study other constructive solutions potentially more cost-effective. At the project it was defined gap was fixed for each geotechnical zone. Therefore, the back analysis consisted in geotechnical parameter calibration. The modelling work starts by testing the solution, using the Mohr-Coulomb constitutive soil model to simulate the soil response. However, this proved to be conservative and provided curtain deflexion data different from the inclinometer results. It was decided then to use another constitutive soil model to represent its behaviour. The Hardening soil model is based on a hyperbolic model, which is able to simulate with greater precision the soil behaviour, since it factors in the hardening phase of soils. After several iterations using the latter, it was noticed that the calibration based on shear strength provides better results than the one based on the Young modulus. It means that, despite being composed by sand, the soils proved to have more cohesion and more shear strength than it was initially predicted. Thus the back analysis was made up of four iterations (A to B). These iterations focused on changing, primarily, the soils parameters of GZ3 and GZ4, because they are the one with the most influence in the structures behaviour. In iteration A, a low shear strength of the soils (10 kpa) is assumed; in B this is increased to 25 kpa, all the other values remaining constant. Iteration C tested the effect of increasing Young modulus from to kn/m2, in the GZ3 soil. On the other hand, iteration D tested the effect of a modulus of deformability increase from kn/m2 to kn/m2, in GZ4. It was concluded that the angle of friction resistance didn t have influence in the soil behaviour. The results of the said iterations are shown at the Figure 10 alongside actual deflexion. Back Analysis As mentioned, it was made a back analysis to calibrate the model to the reality. 6

7 Depth (m) Setúbal's Alegro Shopping Center Geotechnical Solutions for Peripheral Horizontal Displacements of the curtain Instrumentation 28 Iteration A Iteraion B Iteration C Figure 11 - Horizontal displacements (Max. 6.9 mm) Figure 12 - Vertical displacements (Max. 2.1 mm) 22 Iteration D Horizontal Displacement (mm) Figure 10 - Horizontal displacements obtained from back analysis Iteration D model being found to provide the best approximation to the actual displacements, it was the one elected to study the adequacy of the earth retaining walls and foundations solutions in this case-study. The Table 2 shows the values used in earth retaining walls and foundations solutions modelling. Upon simulation of the entire constructive process expectable displacements in the soil were calculated to be as shown in Figures 11 to 13. Table 2- Geotechnical parameters used in modelling Material GZ1 GZ2 GZ3 GZ4 Type of material γsat (kn/m 3 ) γunsat (kn/m 3 ) E50 ref (kn/m 2 ) Eoed ref (kn/m 2 ) Eur ref (kn/m 2 ) Drained Drained Drained Drained νur 0,3 0,3 0,35 0,35 Figure 13 - Foundation settlements (Max mm) The bored piles retaining wall deformations and stresses were also checked, and the results are compatible with the alert criterion established at the design stage. Thus, the good performance of the earth retaining walls and foundation solutions was proved. However, results show that structures might have been overdesigned. Alternative Solutions Alternative solutions were studied to ascertain whether the adopted solution could be optimized. Two new solutions, for earth retaining walls and foundations came subsequently to be proposed. The alternative containment solution consisted of the same bored piles, but with a different spacing between piles and anchors. Instead of 1m apart, the piles would now be distributed at 1.2m intervals and instead of 3m anchors would now stand 4.8 m apart from one another. The modelling results and the executed solution are shown side by side for comparison (Figure 14). c'ref ϕ' (ᵒ) ψ (ᵒ)

8 Depth (m) Setúbal's Alegro Shopping Center Geotechnical Solutions for Peripheral Horizontal Displacements of the Curtain Horizontal Displacements (mm) Figure 14 - Comparison between Executed and Proposal Solution The modelling process was exactly the same that had previously been used in the executed solution. However, new structural properties had to be calculated, considering the geometry changes. The alternative foundations solution consisted of replacing sets of five bored piles by footings. The dimensions of these were determined on the base of the allowable stress for the foundation soil in question. σ adm N Rare A B Executed Solution Proposal Solution Figure 15 shows the results of the new foundation solution. adverse condition of foundation soil (EGZ3=40000 kn/m2) was assumed, in order to test the resistance limit of the soil for the new solution. An analysis of the ultimate limit states and service limit states for both solutions was also carried out, which allowed the conclusion to be drawn that the displacements and stress results did not jeopardize the structure safety. Therefore, the alternative solution was proved as possible, appropriate and viable from the constructive and economic point of view. Economic analysis After the structure behaviour and the displacements assessment, there followed an economic analysis and a risk analysis. These are relevant for every structural proposal: no solution can be deemed appropriate that is not complemented with an economic viability and constructive analysis. The following tables present the economic viability of each solution (Table 3) and the total economic benefit of the optimizations (Table 4). The estimated costs are strictly for Phase 2. Table 3 - Economic viability of executed and proposal solution Estimated Total Costs ( ) Executed Solution Proposal Solution Peripheral Wall Foundations Total Table 4 Economic balance of the solutions Figure 15 - Settlements results from the proposal foundations solution (Max mm) As already studied, the new structural properties were calculated, considering the geometry changes. On the other hand, the soil characterization changes in one term. Since there was no instrumentation data available for the structure settlements, during the construction work, an Estimated Cost Difference Between the Solutions ( ) Earth Retaining Solution Foundations Solution Total The benefit to be had from the solutions optimization was valued at The gap between the cost differences in earth retaining solutions, on the one hand, and foundations ones is remarkable. The new earth retaining solution presents a much greater benefit. The gain brought 8

9 by the alternative foundation solution was not bigger, because the replacing of deep foundation by shallow foundation requires an increase of the area of contact between the footing and the soil, in order to avoid settlements. This makes it necessary to increase the footings' dimension and, as a consequence, the concrete volume and rebar, which makes the solution more expensive. Risk analysis As it was mentioned above, the risk analysis complements the previous viability study. This is, mainly, directed to the performance of the structure in service conditions based on an analysis of the vertical and horizontal displacements of the structure. The study of the displacements induced by each solution (earth retaining and foundations) had different purposes. The former is intended to assess the risk in the existing building, while the latter aimed to determine the consequences in the structure. According to the terms mentioned above in Excavation s influence in nearby structures and Isolated Footings and considering the modelling results, the following parameters were reached. These correspond to maximum values. Movements of Earth Retaining Solution: δ h = 8, ε h = 0, δ v = 2, β = 0, L = 20 According to Boscardin & Walker (1998), the susceptibility to damage in the existing building can be classified on a scale from negligible to severe, as shown in Figure 16. Figure 16 Relationship between angular distortion, horizontal strain and damage category (Boscardin & Walker, 1998) adapted by (Boone, 2001). According to Figure 16 parameters, damages liable to occur in the existing building are rated as negligible. Movements of Foundations Solution: Figure 17 - Movements of foundations registered in the model of alternative solution (displacements magnified 100 times) δ max = 6, δ max = 10, β = 0, l H = 8 3 According to Boone, (2001), the existing building susceptibility to damage can be classified on a numerical scale, as shown in Figure 16 where 1 corresponds to negligible level and number 5 to severe. Figure 18 - Category of damage risk caused by differential settlements (Boone, 2001) Damage risk for the structure is also rated at 1, which is equivalent to negligible. Upon testing of the alternative solutions structural and service performance and economic and constructive viability was thus established the adequacy and suitability of the optimized solution. Conclusions The work of expansion and remodelling of Setúbal's Alegro Shopping Center stands out as an example of a project that takes an existing building as a base for enhancing goods and services supply and boosting the town economic development. 9

10 The expansion of the existing building involved the use of underground space, by means of earth retaining walls and foundations reliable solutions. As there were constraints due to the site's surrounding and geologic and geotechnical scenario, the instrumentation and its readings played a crucial role in the interpretation of the wall's performance. Recourse to 2D FEM software was crucial to support this study. It allows for fast, reliable and thorough analysis of the studied solutions, and makes it possible also further to adapt the construction process to any unforeseen setting and geotechnical conditions. The software in question made a major contribution for the calibration of the soil parameters. Without it, it would not be possible to study either the suitability of executed solutions or the viability of the proposed solutions. In addition, the back analysis allows to confirm that the soil has higher cohesion than expected, so a better performance to the lost of confinement (excavation). Thus, as it was found in back analysis that the change of shear strength parameter was effective, the results could be closer to reality if this parameter was further increased. Still, the soils being sandy, no higher values of shear strength were considered. It was concluded that both the earth retaining and foundation solutions could be optimized, respecting the security, constructive and economic viability requirements, and ensuring as well a good performance of structures in service, on the base of a risk analysis. This confirms the initial assumption that the structures could be optimized. References Brito, J. de. (1999a). Processos de Construção - Estacas Moldadas no Terreno. (IST, Ed.) (LEC.). Lisboa. Brito, J. de. (1999b). Tecnologia de Contenções e Fundações. (IST, Ed.) (MEC.). Lisboa. Coelho, S. (1996). Tecnologia de Fundações. (Escola Profissional Gustavo Eiffel, Ed.) (1 a Edição.). Amadora. GEOTEST. (2012). Estudo Geológico e Geotécnico - Relatório. Vialonga. Google. (n.d.). Google Maps. Retrieved July 19, 2014, from Guerra, N. M. da C. (2007). Estruturas de Contenção Flexíveis - Cortinas Multi-Ancoradas. Lisboa. Guerra, N. M. da C. (2008). Análise de Estruturas Geotécnicas. (IST, Ed.). Lisboa. Kempfert, H., & Gebreselassie, B. (2006). Excavations and Foundations in Soft Soils,. Springer-Verlag Berlin Heidelberg, Netherlands. Matos Fernandes, M. A. de. (1983). Estruturas Flexíveis para Suporte de Terras - Novos métodos de dimensionamento. (FEUP, Ed.). Porto. Oliveira, I. (2012). Soluções de Escavação e Contenção Periférica em Meio Urbano. Instituto Superior Técnico. Santos, J. (2002). Engenharia de Estruturas: Fundações Superficiais (Vol. Engenharia). Lisboa. Santos, J. (2008). Fundações por estacas - Acções Verticais. (IST, Ed.) (MEC.). Lisboa. Boone, S. J. (2001). Assessing construction and settlement-induced building damage : a return to fundamental principles, Boscardin, M., & Walker, M. (1998). Ground Movement, Building Response, and Protective Measures. American Society of Civil Engineers. 10