REGARDING SOME DYNAMIC CHARACTERISTICS OF THE SOILS AND THE LIQUIFACTION POTENTIAL OF SANDS IN SOFIA Chief Ass. Prof. Ivailo Ivanov University of Architecture, Civil Engineering and Geodesy Department of Geotechnics ABSTRACT The contemporary construction on the territory of Sofia requires more and more responsible execution of the geological researches. The drilling of deep construction pits, as well as the construction of high-rise buildings (with more than 100 m height), are connected with serious problems, especially in the anti security insurance. One of the important questions which were raised in the construction of Metrostation 7 in the Sofia s centre, as well as in the designing of Europa Center, having 180 m office tower, is the obtaining of the dynamic characteristics of the soils and the possibility of of the shallow situated water saturated sands. Many test, in which the also author participated, were conducted to help solving this questions. The results from those tests, as well as the conclusions drawn from them, are presented in the report. There are also some recommendations about the requirements for the geological researches and safety in the designing of buildings in similar conditions. Key words: sand, liquefaction, problems, Sofia INTRODUCTION In the last few years the construction on the territory of Bulgaria is developing in very high rate. This process is ever increasing especially after the acceptance of Bulgaria in the European Union. A great deal of the investments is directed to the capital Sofia. Currently in the city are designed and in the process of construction more than 8 office towers 140 to 200 m high and over 10 Malls with 4 underground levels. Besides the high-rise building in the moment a modern city infrastructure is developing in very high rate and a special attention is given to the expanding of the Sofia s subway (Sofia Metro) with the biggest excavations in Sofia history (more as 25 m of depth). All these constructions need serious research and reinforcement measures. Having in mind the high level of seismicity of the Sofia region, caused by the crossing of several big faults, great attention is paid to the dynamic parameter of the soil. The big saturated sand deposits create a risk of in dynamic loading and in big s. In this report several examples of the research of the dynamic parameters of the soil in the engeneer-geological researches for the construction of Europe Center, with office tower 180 m high and in the reinforcement of the construction pit of Metrostation 7, situated in the center of Sofia city, are examined. These researches are connected with the possible danger, which can occur from of sand in the base of the construction.
The main issue that had to be answered is determining the possibility of sand in the base in dynamic loading. It has been known from some time that there is quicksand in Sofia (inclining to ). There are scientific researches made on those issues [4], but in reality the of the sands had not created any significant problems in construction. SHORT GEOLOGICAL CHARACTERIZATION OF THE SOFIA S KETTLE The Sofia s kettle represents a graben, shaped primarily in Cretaceous aged rocks. The kettle is filled mainly with Neogene and Quaternary sediments, with a total thickness more as 300 m [1]. In the larger part those are clay sand and gravel deposits, which in places are highly saturated. The depths of the water levels vary from 0,00 in the northern to over 40,00 m in the southern part of the kettle. According to the seismic zoning of Bulgaria the territory of Sofia is in a zone with activity danger with intensity VІІІ state in the MSK scale in a period of 1000 years. This high seismicity is due to the presence of many faults in the rock bed, some of which are presently active. SHORT CHARACTERIZATION OF THE PROJECTS FOR THE CONSTRUCTION OF EUROPE CENTER AND METROSTATION 7 Europe Center is one of the biggest construction projects on the territory of Sofia, realized by the German investment company ECE. The project consists of office tower app. 180 m high and a shopping center. The depth of the office tower foundation is app. 25 m and a mixed type of founding flat and with piles (Pic. 1). Metrostation 7 is situated in the center of Sofia city. It is the connecting station of the existing branch of the Sofia s subway. Besides the metro station a rotating shaft for the tunnel machine (TBM) is in the process of construction and the depth of the pits is over 10 m. Pic. 1. Europe Tower Sofia - project (www.ece.com)
MAJOR ISSUES CONNECTED TO THE GROUND DYNAMIC, RAISED IN THE DESIGNING AND CONSTRUCTION OF THE OFFICE TOWER AND THE METRO STATION The geological researches for the construction of Europe Center were carried out in the period between 2006 2008. There have been executed more than 50 research boreholes with depth from 10 to 90 m, as well as in situ testing pressiometric researches, DPH and SPT testing. More as Soil samples have been examined in laboratories in Bulgaria and Germany. Special attention is given to the seismic danger and the soil dynamic parameters. The main geological types, subject to special attention, are: Soil type 5 sand medium coarse, grey-yellow to rusty The depth of these sands varies from 1 to 4 m. Soil type 6 sand fine to medium coarse, with clayey interlayers, beige to grey-yellow These sands form layers 1 to 9 m thick. In the area they have been established at depth from 3 to around 17 m. In them pressure underground waters from the Neogene aquifer are accumulated. In the DPH tests, carried out in the upper layers of the construction site, it has been established that the sands from type 5 and 6 have a potential. This raises a number of questions and issues for the designers, and those issues had to be solved with additional researches. In the construction of a part of the concrete piles for the reinforcement of the construction pit of Metrostation 7 some of them collapsed with app. 0,5 m few days after their completion. This is probably due to the of the sands from the dynamic activity of the pile machine. An emergency research of the potential of the liquefied sands from the geological profile had to be done. The fine and silty sands create this danger, also small highly saturated gravels (soil types 2 and 3). RESEARCH METHOD For determining the danger of of the sands standard penetration tests SPT have been carried out in the research boreholes on the two sites (Pic. 2). Pic. 2. Moment of SPT tests in Metro station 7 subway
The estimation is made by the methodic of Seed [2], based on the results of the SPT tests. The method is also based on the comparison between the average cycling shear resistance τ AV, which can cause some dynamic loads (for example s, heavy machines works or other) and the reaction of the soil layer, expressed by the tangential stress τ. This comparison is expressed in the following relation: where или (1) is the coefficient of cycle tensions, which causes maximum pore pressure and the initial liquefaction of the sand in limited deformation. σ 0 initial effective tension in the layer of the geological load. σ 0 = γh + γ (z H) (2) where Н level of the underground waters; z the examined depth; γ bulk density of the soil; γ bulk density of the soil bellow water; where k s coefficient of seismicity; γ bulk density; h depth under the surface; r d coefficient of variation of the tangential stresses in depth (it is read from the graphics). The coefficient SR is taken from graphics and depends on the number of strokes in the SPT test. In the research the following values of the seismic parameters were accepted: - seismic coefficient k s = 0.27 accepted for the territory of Sofia - Maximal magnitude of a possible М = 6.5 RESULTS AND COMMENTS Results from the researches of the sands in Europe Center are presented in Table 1. From Table 1 it can be seen that from the five carried out tests two of them show that the sands have the potential of. For the area of Metrostation 7 the results are: For layer 2 fine to middle sand: τ AV /σ 0 = 0,124; SR(for magnitude 6,5) = 0,07., therefore there is a risk from liquefaction in s from magnitude 6.5 by Richter, coefficient of seismicity k s = 0,27 or similar dynamic loads. For layer 3 small gravels with sandy filling: ; SR(for magnitude 6.5) = 0.15., therefore there is a risk from liquefaction in s from magnitude 6,5 by Richter, K s = 0,27 or similar dynamic loads. (3)
Table 1. Results from the researches of the soil beneath the Office-tower in Europe Center Soil No. Layer 5 Layer 6 Layer 6 Layer 6 Layer 6 Depth m 5.60 7.50 15.00 7.00 6.80 0.15 14 7 8 16 7 SPT blows 0.30 25 19 10 15 9 0.45 26 20 15 20 12 N SPT (Last 30 cm) 51 39 25 35 21 γ - Unit volume weight kn 18.14 18.73 18.73 18.73 18.73 γ' - Volume weight under water kn 8.14 8.73 8.73 8.73 8.73 H - Depth of underground m 6 6 6 6 6 water level, m z - Depth of SPT = H m 6.05 7.95 15.45 7.45 7.25 k s - seismic coefficient 0.30 0.30 0.30 0.30 0.30 r d - Coefficient of change of tangential stress depend on depth (from graphics) 0.92 0.86 0.72 0.87 0.87 σ' 0 - Geological effective stress kpa 109.247 129.404 194.879 125.039 123.293 τ AV - ciclic shear stress kpa 19.689 24.971 40.629 23.673 23.037 τ AV / σ' 0 0.180 0.193 0.208 0.189 0.187 C N - correction coefficient 0.884 0.832 0.708 0.843 0.847 N 1 -corrected number of SPT blows 45 32 18 29 18 SR by M 6.5 (from graphics) 0.600 0.450 0.170 0.420 0.170 No No There is No There is As a whole the results confirm the presence of saturated sands, having the potential of on the territory of Sofia. The main danger in the two projects is the possibility that the sands can pass into flowing state in a powerful, because the geological load is bigger than the load of the buildings. This is the reason for the recommendation of the additional consolidation of the sand in the base of Europe Center with the creation of thick maze of piles. For the region of Metrostation 7 the main accepted measure is the lowering of the underground water levels in the construction process and the reinforcement measures, as well as increasing of the pile depth. Major issue in this case is created by the narrowed working conditions in the underpass near the Sheraton hotel, but in the moment this is solved by temporary removing the ceiling of the underpass. One of the reassuring news is that according to a number of researches [3] so far the registered s in the area of Sofia are with the magnitude of up to 6-th by the Richter scale.
CONCLUSION Pic. 3. Epicentres of s in Sofia`s area As a result of the carried out researches it has been established that some of the quaternary small grain sands in the area of Sofia have the potential to liquefy at s with magnitude М = 6.5 or bigger. For this issue we recommend when similar in physical parameters sands are encountered, researches for the potential of to be made, because those sand deposits are not consolidated enough. For the prevention of the it is recommended additional consolidation of the sands, their replacement with bigger-grain materials or the dewatering of the area. REFERENCES 1. Bonchev, Ing. Geology of Bulgaria, Sofia, Science and Art, 1955 2. Germanov, Т., Е. Balushev, D. Denev, Zh. Zhelev, G. Ilov, B. Kirov, Iv. Krastilov. Soil mechanic. С. UACEG, 1999. 3. Shanov S., Boykova A., 2007. Evaluation of the seismic characteristics of the site Balkan Centre - Sofia City, considering the geological and tectonic structure and the properties of the engineering geological layers. Report of BulGeoinvest Ltd. 4. Stoinev S. Liquifaction of water saturated sands assessment, prognosis, management and protection. PhD. UMG. 2004