XV Danube - European Conference on Geotechnical Engineering (DECGE 2014) H. Brandl & D. Adam (eds.) 9-11 September 2014, Vienna, Austria Paper No. 176 Testing of ground anchorages for a deep excavation retaining system in Bucharest A. Ene 1, D. Marcu 1, H. Popa 2 1 SC Popp & Associates Geotechnical Engineering SRL, Bucharest, Romania 2 Technical University of Civil Engineering, Geotechnical and Foundations Department, Bucharest, Romania Abstract. The demands for designing more economical and practical solutions for deep excavations lead to the necessity of better understanding the behaviour of the supporting systems. Thereof, this paper presents the design of the ground anchorages for a deep excavation retaining system based initially on prescriptive methods and further on investigation tests performed prior to the execution of the supporting system. Moreover, the results of the suitability and acceptance tests performed on working anchorages are presented and discussed within this paper. Keywords: Deep excavation, Retaining system, Ground anchors, Site tests, Design 1 INTRODUCTION Real estate developments office, residential or commercial buildings especially the ones performed in urban built-up areas, need more basement levels and, therefore, deep excavations. The Romanian normative NP 120-2006 for deep excavations in urban areas requires particular care to minimize the influence of such works on the existing neighboring structures. The present paper refers to a development of three office buildings, under construction in Bucharest, having 10-12 floors and a common basement on three levels. The basement occupies almost the entire property (circa 10 000 m 2 ), while the three structures above the ground occupy about 60% of the site. A preliminary design phase consisted of analysing the optimum structural solutions. For the temporary retaining system, it resulted that supporting with prestressed ground anchorages leads to the most economical solution both from financial and execution time and technological flow point of view. The approach of the project was as follows: - Preliminary design of the vertical excavation retaining system, in compliance with one of the design approaches in SR EN 1997-1; - Preliminary prescription based design of the ground anchors, according to Romanian normative NP 114-04 (see paragraph 3); - Dimensioning and design of the retaining system; - Performance of the investigation tests on 4 ground anchors on site loaded to a force higher than the bearing capacity estimated in the preliminary design; - Execution of working ground anchors following the interpretation of the investigation tests; - Selection of 8 ground anchors for suitability tests based on individual drilling and injection logs; - Tensioning and performing of acceptance tests to a load 10% higher than the lock-off load, followed by eventual interventions and final lock-off; - Excavating to the final level, performing of the raft foundation and the structure of the third basement (including the slab above) with the role of final support of the excavation; - Release of the ground anchors and even extraction of the reinforcement where it was required.
2 GROUND CONDITIONS AND RETAINING SYSTEM 2.1 Ground lithology and design parameters The typical lithology for Bucharest was encountered on site during the performance of the geotechnical investigations: filling, followed by silty clay at the surface (almost the entire excavation depth), then Colentina sands with rare gravel - very variable in level and depth, intermediate clay and then Mostistea fine sands. The above mentioned variability conducted to the impossibility of adapting the length and angle of the ground anchors in order to reach the higher capacity uncohesive layer. In Table 1, the schematic lithology on site can be followed as well as the characteristic values of the main geotechnical parameters - as resulted from the geotechnical report. The design geotechnical model can also be observed in Fig. 1. Table 1. Lithology and main geotechnical parameters Layer description Absolute level [m ASL] Characteristic values γ [kn/m 3 ] Ic [-] E eod [kpa] Filling +89.00... +88.00 18-10 000 20 0 Silty clay +88.00... +79.00 20 0.75 15 000 22 35 Sand with gravel +79.00... +74.00 20-35 000 30 0 Clay +74.00... +68.00 20 0.80 18 000 20 40 Fine sand +68.00... +64.00 20-35 000 30 0 Clay < +64.00 20 0.80 20 000 20 40 Φ' [ ] c' [kpa] The groundwater level as indicated in the geotechnical report was around +77.00...+79.00 m ASL. 2.2 Excavation pit retaining system For performing the excavation works from a working platform at +86.00 m ASL, a diaphragm wall of 60 cm thickness and 18 m length was realized through the slurry wall technology. The slope for performing the working platform (almost vertical in some areas) was retained by Berlinese wall or stabilized by shotcrete. The calculations of the retaining system were performed by 2D plain strain Finite Element Method considering Hardening Soil with small stiffness behavior and linear elastic behavior for the diaphragm wall and ground anchors. 2.3 Temporary supporting After performing a feasibility study, ground anchors performed on one level were chosen for the temporary support of the diaphragm wall. This meant that after performing the diaphragm walls together with the capping beam and excavation down to +81.70 m ASL, 190 ground anchors were drilled, installed, injected and finally tensioned. The geometrical characteristics are indicated in Fig. 1. The platform from which the ground anchorages were performed, was in reality a 10 m width bench with a slope of 3:2. Hence, the great dimension of the excavation pit permitted working at the same time at the structure (pouring of the raft foundation) and performing the supporting system without affecting the excavation stability.
Figure 1. Characteristic section of the excavation pit retaining system 3 PRELIMINARY DESIGN OF THE GROUND ANCHORS Calculations using parameters derived from ground tests are considered to be for the preliminary estimation of the bond length only, and the design is then verified by load tests. The characteristic value of the bearing capacity of the bond length (ground-grout interface) is estimated by the following formula, according to Romanian normative NP 114-04: Σ (1) where: - the mean effective diameter of the injected bulb, given by the following formula: 1.8. [m] (2) - the total quantity of cement [in tonnes] introduced in the bulb area by injection and post-injection; it was estimated to inject a quantity of 1.25 tonnes of cement in order to obtain a mean diameter of 0.30 m. - the bond length within layer i; due to the level variability of the sand layer, it was safe to consider that the entire 10 m long bulb was situated within the silty clay layer. - characteristic resistance on the lateral surface of the bond area; for cohesive layers having the consistency I C =0.75... 1 and using post-injection, NP 114-04 gives =100 kpa. As a result of the prescription based design, the characteristic value of the anchor bearing capacity resulted of 940 kn. The design bearing capacity resulted by prescription methods is 530 kn according to the Romanian normative NP 114-04.
4 DESCRIPTION AND RESULTS OF THE INVESTIGATION TESTS After performing the diaphragm wall retaining system, prior to the execution of the working anchors, investigation tests were performed on four ground anchors on locations considered as representative for the soil conditions revealed by the geotechnical report. The test anchors were executed using the same technology and procedures and having the same inclination as the working anchors. The tests were performed by method 1 according to SR EN 1537-04, in which the axial load was applied in six cycles to a proof load P P =1300 kn, higher than the one estimated to reach failure of the ground-grout interface. At the maximum load of the first two cycles the load was maintained constant for 15 minutes and at the maximum load of the next four cycles the load was maintained constant for 60 minutes. The tests involved measurement of the tendon head displacement versus applied load and, for each load step, measurement of tendon head displacement versus time as shown in the diagrams below for two of the tests anchors. Figure 2. Measurements of the tendon head displacement versus applied load and versus time for the investigation tests on two of the test ground anchorages Two of the test anchors did not reach failure (defined as creep ratio of 2 mm) at the maximum applied load of 1300 kn and the other two test anchors reached failure at 85% of the proof load (1105 kn) and at the proof load respectively due to unstable deformations at the following load step. It was therefore considered the characteristic value of the anchor bearing capacity of 70% of the proof loaf (910 kn) as the minimum load that had reached stable deformations at constant load. The design bearing capacity resulted from load tests is 580 kn according to the Romanian normative NP 114-04. Figure 3. Creep ratio versus applied load
On an area where it felt necessary to ensure higher stability of the excavation and the neighboring structure, because of some heavy traffic, the result of the test anchor A1T performed in the referred area was considered which did not reach failure to the maximum test load P P =1300 kn. The geotechnical report also revealed that the lithology there permitted partial performance of the fixed length of the anchor in the non-cohesive layer with higher bearing capacity. These anchors were locked off at P 0 =560 kn - lower than the design bearing capacity of 850 kn according to NP 114-04 - load for which the reinforcement in the diaphragm walls would still comply with the resulted forces in the retaining system. The rest of the working ground anchorages were locked off at P 0 =500 kn. 5 DESCRIPTION AND RESULTS OF THE SUITABILITY AND ACCEPTANCE TESTS As the working ground anchorages were being performed, based on the drilling-injection individual logs, on the site lithology and on the neighboring of the excavation, the designer chose eight anchorages for suitability tests. The tests were performed by method 1 according to SR EN 1537-04, in which the axial load was applied incrementally in five cycles to the proof load P P =580 kn and P P =650 kn, respectively. At the maximum load of the first three cycles the load was maintained constant for 15 minutes and at the maximum load of the last two cycles the load was maintained constant for 60 minutes. The tests involved measurement of tendon head displacement versus time for each load step. The diagrams in Fig. 4 represent the measurements at the last loading cycle for six of the tests anchors, since the other two chosen for suitability tests were performed and designed for a different technology (being extractible as required in the neighbor s approval) and are not relevant for the present study. Figure 4. Tendon head displacement versus time for suitability tests Test anchors A39C and A45C were performed in the area where it was intended to ensure better support of the excavation and the neighboring structure. As it can be noticed from the overlaid diagrams, the suitability tests confirmed once again the higher bearing capacity indicated previously by the favorable lithology and the results of the investigation tests, as the measured displacements are smaller, although of the same order. It was considered that all eight suitability tests confirmed the ability to sustain the proof load. All working ground anchorages were subjected to acceptance tests prior to locking off. The load was applied in three equal increments to the proof load P P =550 kn and P P =650 kn, respectively. For nearly 5% of the anchorages were re-injected since the registered values of the head displacement were not comparable with the ones measured during the control tests. Since it is quite difficult to assess the load loss after locking the neighboring anchorages and no monitoring of the force was implemented, the following loading and locking procedure was used after applying the proof load for the acceptance tests, unloading and fixing the locking device: i) every two
anchorages were locked off to half of the final locking load; ii) the intermediate anchorages were locked off to half of the final locking load; iii) the first series of anchorages were locked off to the final locking load iv) the intermediate anchorages were locked off to the final locking load. This procedure was also thought to minimize negative effects such as load loss, unbalance, load concentration etc. It should be noted that for all the performed ground anchorages (including the ones for investigation tests) the total quantity of cement introduced in the bulb area was less than 1 tonne. Hence, although the minimum bearing capacity determined by tests was very close to the characteristic value estimated by prescriptive measures, the latter was determined for a greater quantity of injected cement meaning a greater mean effective diameter of the injected bulb (see Eq. (2) above). 6 CONCLUSIONS Based on the results of the tests performed, the supporting system can be redesigned. However, since at least a part of the retaining system is already performed at the moment of performing the test anchorages, this redesign can only be allowed within some margins. It can, therefore, be advisable that this is taken into consideration early in the dimensioning of the retaining structure. Usually, the results of the tests are positive and the possible corrections of the supporting system can lead to savings (length of the bond area, distance between ground anchorages if possible, quantity of injected cement etc.). Taking this project as an example having almost 200 ground anchors performed, it saved 0.3 tonnes of cement and maybe one day for re-injection per ground anchor. As a guiding first remark for further analysis of the ground anchorages bearing capacity, a back calculation based on the actual quantity of cement introduced in the bulb area leads to characteristic resistance on the lateral surface of the bond area of 135 kpa; for cohesive layers having the consistency I C =0.75... 1 in Bucharest. A great importance for this type of works is represented by a complete, adequate monitoring of the retaining system and of the neighboring structures. Fortunately, great investors and designers are becoming more aware of the benefits brought by minimizing the risks and prevention of bad evolutions. For this reason, we are trying to include also the monitoring of the supporting system in terms of displacements and forces of the ground anchorages. For the scope of research, monitoring together with site tests would help correct and calibrate the geotechnical parameters and last, but not least, it would help improving the models used for design. REFERENCES Draft EN ISO 22477-5 (2009). Geotechnical investigation and testing - Testing of geotechnical structures Part 5: Testing of anchorages Merrifield, C., Moller, O., Simpson, B. and Farrell, E. (2013) European practice in ground anchor design related to the framework of EC7. In: Proceedings of the 18th International Conference on Soil Mechanics and Geotechnical Engineering, September 2013, Paris, France NP 114 (2004). Technical normative regarding the design and execution of ground anchorages NP 120 (2006). Norm regarding design and construction of deep excavations in urban areas SR EN 1537 (2004). Execution of special geotechnical works. Ground anchorages SR EN 1997-1 (2004)/NB (2007) Eurocode 7: Geotechnical design. Part 1: General Rules. National bulletin. SR EN 1997-1 (2006). Eurocode 7: Geotechnical design. Part 1: General Rules.