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Figure 1. Bearing walls constitution Rehabilitation and underpinning of a building classified as a national patrimony Sabrime BOUBAKER Terrasol Tunisie, Tunis, Tunisia, s.boubaker@terrasol.com SUMMARY: Located in the center of a dense urban area of the Tunisian capital, the building to be modified is a construction that dates from the early twentieth century and classified as a national heritage. The purpose of this project is to preserve intact the original building facade by underpinning the internal structure and foundation system. The site on which is based the building is characterized by the presence of a highly compressible soil over several tens of meters. As there is no information about the initial foundation system of the building, a specific campaign of surveys was conducted to analyze the structural elements present in the basement. The purpose of this study will be to create a new level of basement and to insure the stability of the building for the next decades. In this paper will be detailed the project elements and the results of all structural and geotechnical investigations. Then will be exposed the different undertaken stages for the operation of underpinning of the building. KEYWORDS: underpinning, basement, inclusion, settlement. 1 INTRODUCTION The building to be rehabilitated is compound of a first floor and three levels built on a 580 m² corner lot. The construction is situated in a dense urban area at the most important street of Tunis: the Habib Bourguiba Str (geographic coordinates: 36°47’58’’N, 10°11’1’’E). It is delimited on the North and West sides by two roads. The East side is adjacent to a similar old building and in the south side is next to a lot where has been constructed another similar building been demolished since several years. The project consists in removing and changing all the internal existing structure of the building to create a new hotel compound of a first floor and four levels with an additional level of basement. The creation of such a new basement will require an excavation of about 3 m depth. All this work will have to be undertaking while keeping intact the initial facades considered as a national heritage. 2 PROJECT CONTEXT. 2.1 Building structure and fondation prospection The construction is established on external bearing walls of big thicknesses built in freestones. The central core, including the stairwell, is constituted by big supporting walls in masonry rubble stones and inserted bricks (cf. Figure 1). Floors are shaped in full brick-built jack arches and metallic profiles. The whole is bond by a complex system of floors – metallic stringers which insures the equilibrium of the structure by conferring it a monolithic behavior. The building dates from the early XXth century; no documentation on the foundation details was preserved since the time. So, and to make a clearer idea about the existing system of foundation, we proceeded to prospecting works in various areas of the construction. Therefore, an excavation was opened up against a bearing wall on a surface of 1.35 x 1.37 m² on 2.1 m depth (cf. Figure 2). The auscultation of the underground structure let take the following hypotheses on the existing foundation mode: - Bearing walls rest on stone bases of a trapezoid shape until the depth of 2.1 m; - This pad lets the transfer of loads to a cyclopean raft foundation (which thickness is about 1.6 m according to a borehole). This raft would be generalized on all the surface of the building. - The whole parts constitute a tight system (flow rate not exceeding 2.10-5 l/day and the initial water level not restored even after a week). Besides, according to a nearby similar old building where basement extension works and excavations were made, it was discovered a similar foundation system made of trapezoid shaped stone bases laying on a cyclopean raft foundation. The subgrade was improved by a regular mesh of rigid inclusions. These elements are made of eucalyptus trunks of 120 to 150 mm diameter. Furthermore, the topographic surveys have shown that the building would have swing to the South-East side according to maximal amplitude of 46 cm. This tipping would be due to the location of the building next to the nearby constructions. Indeed, the building had been, according to its history, bounded on two sides by roads and on other sides (South and East) by constructions. Besides, the system of foundation such as it was explored supposes that all the adjoining constructions rest on a structure equivalent to a common raft foundation whose settlement is more important in the center (cf. Figure 3). On the other hand, and according to excavation works inside the building, it was noticed the presence of an old flooring at 1 m under the ground level, buried on all the surface of the building. This give evidence of a long- term generalized settlement that the construction would have sudden: it is about soil consolidation. Figure 2. Foundation system prospecting Figure 3. Settlement of the group of buildings Figure 4. Principal of the existing foundation system Therefore, and according to these diagnostics, we had concluded that the foundation system layout would be as shown in the Figure 4. 2.2 Soil investigation results The analysis of the results of the geotechnical investigations made on the site, allows highlighting the following geological levels: - Level 1: (from 0 to 3.7 m) sandy fill put on a layer of calcareous blocks with joints of sand and gravel (the existing raft foundation); - Level 2: (from 3.7 to 18 m) gray muddy clay with shell debris the first ten meters; - Level 3: (from 18 to 20 m) sandy greenish-yellow with shell debris; - Level 4: (from 20 to 29 m) grayish brown clay with traces of silt and shell debris; - Level 5: (from 29 to 32 m) yellowish gray laminated clay with traces of sand; - Level 6: (from 32 to 45 m) grayish brown clay with silty crossings and cemented concretions; - Level 7: (from 45 to 48 m) silty fine sand with traces of cemented concretions on the first meter; - Level 8: (from 48 m) compact brownish clay with traces of silt. The following table summarizes the average pressuremeter values measured. Table 1. Soil mechanical properties Level Z (m) Em (MPa) Pl* (MPa) Em/pl* 1 0 – 3.7 1.9 0.21 9 2 3.7 – 18 3.4 0.41 8 3 18 – 20 16.1 1.6 10 4 20 - 29 6.2 0.7 9 5 29 - 32 11.3 1.2 9 6 32 - 45 9.3 0.97 10 7 45 - 48 30 2.7 11 8 > 48 26.7 2.2 12 The water level is almost situated at 0,8 m depth. Several laboratory tests were made on some samples of level 1 to 6, it consists on oedometer and triaxial tests with Atterberg limit determination. However, in level 4 and 5 no eodometer tests were done. These parameters are very important to estimate the consolidation settlement of the new structure. Therefore, and while having an idea about the initial project loads and its long term settlement, we have tried to make a model calibration of the soil based on the following hypothesis: - Project loads are composed of a general stress of 50 kPa and a linear load due to the walls weight estimated at 300 kN/m. - The similar neighboring buildings would have the same load distribution. - The building settlement consists on a general settlement of about0,5 m with a differential settlement of 0,4 m to the South-East side; - All soil layers are supposed to be normally consolidated; - The ratio between the compression and the recompression index is considered equal to 10 (Cc/Cs = 10) ; These entire hypotheses, coupled with the laboratory tests, let clayey soil oedometric calibration as following. Table 2. Soil oedometric parameters Level γh (kN/m3) OCR Cc/(1+e0) Cs/(1+e0) 1 17 1 0.18 0.018 3 19 0.12 0.012 4 20 0.05 0.005 5 20 0.07 0.007 3 UNDERPINNING SOLUTION DESIGN 3.1 Principal of the underpinning solution Given the diagnostic results of the building, previously presented, and to ensure a new redevelopment of the building on a basement level, the foundation system will be inspired from the existing system and will be directed to a mixed foundation that consists of a generalized slab resting on a quasi-regular mesh of micropiles. Underpinning works of the current system will begin by creating an internal rigid portal frame (main framing structure of the future building) provisionally based groups of 30 m long micropiles. This structure will retain the original structure of the building (including the front walls) before demolition of interior walls and floors. Once all elements of internal structure demolished, earthworks to create the basement will be started gradually and in parallel with the works of final raft construction. This raft will be sealed to the foundation members newly created in previous step (micropiles groups) and will ensure “rigid box” behavior in the basement. The principal of this underpinning system is shown in the Figure 5 below. 3.2 Design of the temporary retaining system In order to begin earthworks without disturbing the behavior of the original facade structure, it will be realized at the future columns positions, behind the outside bearing walls, a retaining system of reinforced concrete walls, based on micropiles. Foundation micropiles will be 30 m long equipped with tubular steel frames of 114 mm diameter and 7.5 mm thick. They will be sealed by injection into boreholes of 250 mm diameter. 3.3 Design of the temporary foundation system The temporarily foundation system will let maintaining the portal frame structure during demolition phases of interior walls and roofs before placing permanent foundation system. This system is compound of groups of four micropiles at each future column, realized from pre-excavation of 0.8 to 1 m depth. These micropiles will be sized according loads corresponding to the own weight of the beams and columns of the portal frame. Bearing capacity of micropiles is normally counted from 4 m depth. Moreover, and in order to add additional security to the foundation system, we have considered only 30% of the effect of lateral friction throughout the thickness of soft muddy clay (Level 2). Thus, as these micropiles will be included in the final foundation system, we have considered elements of 30 m in length, with a bearing capacity of isolated micropile given by the following table. Table 3. Bearing capacity of micropiles L (m) QSLS (kN) QSLS (kN) QULSfd (kN) QULSac (kN) 30 404.7 495.7 578.4 637.2 These results are supposed to be confirmed by at least one static axial load test (control test) on a micropile foundation. 3.4 Design of the final foundation system The final system will be a mixed type. It will consist of a foundation raft 40 cm thick resting on a semi-regular mesh of micropiles of 250 mm in diameter and 25 to 30 m depth. Micropiles used for temporary foundation will be an integral part of the final mesh. The settlement under the foundation raft will have two components: - Elastic settlements: immediate settlement consumed during the construction work progress; - Oedometer settlement: consolidation settlement consumed very long term after dissipation of excess pore pressures generated in the clay layers. Elastic settlements are estimated using a 3- dimensional simulation of the raft foundation. The soil is modeled as an elastic multilayer (horizontal layers characterized by Young's modulus and Poisson's ratio). Micropiles are introduced by their equivalent stiffness. The maximum elastic settlement at the foundation raft is about 1.7 cm. The calculation of oedometric settlement take into consideration the effect of over- consolidation of the soil due to the load of the initial embankment of 0.5 m and the existing slab 1.6 m thick, placed over the entire area of the building for several years. The impact of unloading will be modeled by OCR values based on a model initially normally consolidated as follows. Figure 5. Principal of the solution to be undertaken Table 4. OCR values retained for oedometer settlement calculation Level OCR I 1.63 III 1.25 IV 1.19 V 1.14 The maximum calculated settlements are about 3.5 cm. On the other hand, as the foundation system is floating (micropiles arrested at -25 and -30 m), it will be necessary to estimate the subsidence that may occur under micropiles. Thus, it was considered the load transfer of the project to the deep compressible layers by the foundation system. This led to an additional settlement of about 1.3 cm. The total long term settlements would then not exceed the 5 cm. 4 PHASING OF WORK In order to reduce the impact of earthworks on the elements of the original structure, including the foundation elements, following phasing was recommended (Figure 6): 1. Generalized excavation of 0.8 to 1 m depth; 2. Achievement of temporary foundation micropiles (groups of 2 to 3 under each column) and final micropiles (regular mesh under final raft); 3. Portal frame structure construction; 4. Demolition of interior walls and roofs; 5. Slabs construction for different levels All this phasing has to be undertaken after securing the exterior by a peripherally confinement system to avoid any disturbance of the existing structure 5 CONCLUSION AND PROSPECTS When editing this paper, underpinning works were not yet undertaken. Several additional investigations have to be made to fix most soil’s parameters and to confirm the study hypothesis. Moreover, micropiles’ loading tests will be conducted to estimate the real bearing capacity of these foundation elements. Figure 6. Underpining works phasing External side Embakement NG External side Embakement NG External side Embakement NG External side Embakement NG External side Embakement NG Another alternative of the solution was proposed providing more space in the basement. The main idea was to partially demolish the existing foundation pads so that basement’s peripheral structure can be executed just under the first floor’s columns. However, the excavation works could not be undertaking before performing trial tests in small areas of the existing building (not more than 2 m x 2 m surfaces). REFERENCES Fascicule 62 – Titre V : « Règles techniques de conception et de calcul des fondations des ouvrages de génie civil », LCPC-SETRA. « L’usage des modules de déformation en géotechnique », O. COMBARIEU. NF P94-262 « Justification des ouvrages géotechniques », national application of the standardEurocode7, July 2012.
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