Сборник трудов конференции СПбГАСУ 2014 ч
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Раздел 2. Проектирование и строительство оснований и фундаментов с применением…
Изучено влияние сильных движений грунта в разжиженных грунтах, при которых происходит подъем подземных сооружений и исследуется влияние разлома на поведение подземных трубопроводов и свай. Показано влияние глубины грунтовых вод. Результаты показали, что наличие грунтовых вод имеет эффект затухания по амплитуде. Было установлено, что на подъем подземного сооружения повлияли: длина шпунтовых свай, сила разжижения песчаного грунта и время продолжения землетрясения. Определяется оценка потенциала сжижения. Таким образом, исследуется вероятность степени повреждения. Однако, в целом, ожидаемая степень повреждения от сжижения не оценивается, потому что ее трудно оценить. В качестве контрмеры рекомендуют цемент смешивать с заполненным грунтом, в результате чего могут сократиться подъемы колодцев и труб. В статическом и динамическом анализах соотношение напряжение-деформация, коэффициент фильтрации и коэффициент объемной сжимаемости разжиженного грунта должны определяться почвенно-геологическими изысканиями и лабораторными экспериментами [2, 5].
Для визуализации динамического поведения разжиженного песка вокруг сваи было проведёно несколько модельных экспериментов на виброплатформе [2]. Относительное движение между сваей и окружающим грунтом довольно большое, и грунт ведет себя как жидкость. Получается, что вязкая жидкость основной модели дает примерно приемлемый результат, когда вязкий коэффициент меняется от 1000 до 10000 раз больше, чем в простой чистой воде. Это согласуется с разными экспериментами в литературе. В эксперименте наблюдается некоторое скачкообразное поведение, что означает, что разжиженный грунт все еще обладает свойством трения какв твердомгрунте.
Проведены эксперименты на центрифуге на поведение подъема подземного сооружения в разжиженном грунте и песке. Получены следующие выводы [2]:
–В динамических экспериментах на центрифуге, скорость подъема модели подземного сооружения зависит от амплитуды и частоты начального ускорения. Из результатов видно, что коэффициент вязкости разжиженного грунта зависит от величины относительного сдвига.
–Скорость подъема сооружения после почти полного разжижения остается примерно постоянной при различных амплитудах и частотах начального ускорения. Кроме того, скорость подъема регулируется жесткостью пропорциональная коэффициенту затухания.
–Разжижения грунта и остаточные деформации, часто приводят к значительному ущербу.
–Длительность разжижения окружающих грунтов значительно влияет на поведение подъема. Следовательно, разжижение вызывает деформацию
восновном из-за дисбаланса собственного веса.
Разработаны контрмеры для канализационных колодцев и труб [2]. Уровень грунтовых вод понижается за короткий срок, затем увеличивается
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снова. Во время этого процесса происходит полное разжижение песка, модуль сдвига которого увеличивается за счет консолидации. Получены следующие выводы: 1) уплотненный песок более чем на 90 % уменьшает поднятие колодцев и труб; 2) заполнять траншеи гравием, а не песком; 3) смешивать песок с цементом и 4) шпунтовая свая из стальной трубы, используемая для основной части корпуса и подпорной стенки эффективна, как контрмера подъему.
Эксперименты взаимодействия разлом – трубопровод проводились на глубине 25 м сухого слоя песка, разлом под углом падения 60 . В результате, измеренная реакция трубопроводов, расположенных перпендикулярно по отношению к нормальному разлому, подчеркнула особенность взаимодействия трубопровод – грунт [2,5].
Итак, исходя из изученных материалов, ныне актуально рассматривать поведения систем «подземное сооружение – водонасыщенный грунт» и «грунт – фундамент – сооружение», что является продолжением разработанной сейсмодинамической теории подземных сооружений. Параметры составных моделей связаны со свойствами грунта, так как грунты, окружающие подземные трубопроводы, являются не только источником сейсмического воздействия, но и участвуют в колебательном процессе совместно с самим трубопроводом. В зависимости от однородности и плотности окружающей трубопровод грунтовой среды и степени обводненности грунтов будут различными интенсивность проявления землетрясения и механизм взаимодействия сооружения с грунтом. Возникают вопросы [2]: 1) какие свойства грунта являются более актуальными для решения конкретной геотехнической проблемы? 2) какие параметры модели являются наиболее подходящими и как их можно оценить и/или корректировать со свойствами грунта? 3) какова роль неопределенности свойств грунта? В этом вопросе необходимо проводить широкие как теоретические, так и экспериментальные исследования. В связи с этим нами разработаны модели взаимодействия трубопровода с окружающим водонасыщенным грунтом при поперечных и продольных, крутильных движениях, исследуется эффект выпучивания подземных трубопроводов, расположенных в водонасыщенных грунтах [3].
Таким образом, материалы всемирных исследований показали острую ситуацию на сегодняшний день и перспективы этой проблемы, дали возможность сформировать новые направления исследований. Еще раз подтвердилась актуальность и необходимость более глубокого изучения проблем сейсмостойкости подземных систем жизнеобеспечения.
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Литература
1.14th World Conference on Earthquake Engineering. – Beijing. 2008.
2.Proceeding of International conference on performance-based design in earthquake geotechnical engineering: Performance-based design in earthquake geotechnical engineering. – Tokyo, 2009.
3.X Всероссийский съезд по теоретической и прикладной механике. – Нижний Новгород, 2011.
4.The 15th World Conference on Earthquake Engineering. – Lisbon, 2012.
A.Zhussupbekov (Kazakhstan, ENU, Astana), Y. Ashkey (Kazakhstan, KGS, Ltd., Astana), J. Frankovská, J. Stacho (Slovakia, STU in Bratislava)
GEOTECHNICAL DESIGN OF CONTINUOUS FLIGHT AUGER PILES
Abstract
The Continuous Flight Auger (CFA) technology has evolved especially in recent decades. Using of CFA technology has expanded all over the world. The article deals with geotechnical design of pile foundations, especially by static load tests and by calculation using numerical models. Results of static load tests which were performed in Kazakhstan and in Slovakia are analysed and compared with numerical modelling. The technology of CFA has been used due to the advantages of this technology in comparison with driven and bored piles in local geological conditions. Software Plaxis 2011 was used for CFA piles analyses by FEM. Advanced constitutive hardening soil model was used for soil layers. Comparison of load-settlement curves, ratio of pile base and pile shaft resistance and distribution of unit shaft friction are presented in the paper. Using FEM are achieved adequate results to static load tests with precision equal to 95 %.
1. Introduction
Piles are columnar elements which have the function of transferring load from an upper structure through weak stratum down to a suitable bearing stratum (Tomlinson, Woodward, 2007). Piles foundations are frequently used especially to reduce settlement of upper structure. Piles may be crudely classified as displacement and replacement according to the method of construction (Fleming et al., 2008). CFA piles are classified by many authors as a replacement piles (e. g. Masopust, 2004; Fleming et al. 2008), but this technology also causing partial displacement which have to be taken into account for their design. Advantages and disadvantages of CFA piles were well described by Brown (2005).
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Replacement piles, both rotary bored piles and CFA piles are the most used technologies in Slovakia. It is caused due to variable geological conditions where layers of fine-grained soils are combined with coarse-grained soils, occasionally with rock bed. Fine (silty and clayey soils) and coarse grained sediments (sandy and gravely soils) are cyclically alternating layers not only in vertical but also in horizontal direction. Replacement piles are therefore more suitable in geological conditions of Slovakia than displacement piles.
Due to large area of Kazakhstan, many replacement and displacement technologies are used. The most used replacement piles are rotary bored piles and CFA piles. Drilled displacement piles, driven H-steel and driven concrete piles are the most used displacement piles. Selection of displacement and replacement technologies are preferred in view of advantages for local geological conditions.
CFA piles are often calculated as bored piles with higher value of shaft friction (Viggiani, 2012). At present, numerical modelling, FEM is increasingly popular for pile design. Verification of geotechnical design of CFA piles by FEM, and by three static load tests from different countries, Slovakia and Kazakhstan, are analysed.
2. Technology of CFA piles
Continuous Flight Auger piles are classified as a replacement piles, because soil is removed from the borehole. It is important to say, that due to the installation process, there is also partial displacement which is caused by the auger and pressured concreting. CFA pile installation process includes the following: the auger is drilled to a required (design) depth in the initial (first) phase. The auger is pulled out in the second phase. At the same time as the auger is withdrawn, concrete is placed by pumping through the hollow centre pipe of the auger. Drilling must be as fast as possible with minimum speed, thereby reducing the negative impact on the surrounding soil (Masopust, 1994). The typical length of CFA piles is 30 m with diameter of 0.3 – 0.9 m (Fleming et al., 2008; Brown et al., 2005; Tomlinson, Woodward, 2007). At present, piles length of 34 m with diameter of 1.4 m is possible to construct in suitable geological conditions (www.soilmec.it/CFA).
3. Design methods for pile foundations
Currently, a number of design methods are known to determine the pile resistance or load-settlement diagram. The choice of design method depends often on the experiences, economy and technology availability in given countries. Bearing capacity of single pile can be determined by the following (Masopust, 2004):
the results of static load tests carried out on working piles in real scale (experimental resistance),
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the results of static load tests carried out on model (trial) piles (pile of smaller diameter),
the results of in-situ tests (e.g. CPT tests),
empirical or analytical calculations based on the results of laboratory and insitu soil testing to determine the geotechnical parameters of soils,
calculations which take into account damage of pile body.
Best results can be obtained by static load tests of instrumented piles, which are installed in the same geological conditions, by the same technology and the same dimensions as system piles (Masopust, 1994). Analytical and semi-empirical calculation methods based on the results of site investigation and ground testing are also often used for pile design, especially due to the cost saving. Analytical or empirical calculation methods are also the simplest methods which provide an adequate view of load distribution from pile to the surrounding soils. The using of numerical methods (e.g. FEM) for pile design has increasing in last years. FEM allows introducing more complex boundary ground conditions and also local inhomogeneity of subsoil, local effects of concentrated load at the pile base (Feda, 1977) and deeper understanding of behaviour of the soil-pile system as shown in many papers(e. g. Wehnert, Vermeer, 2004; Mohamedzein et al., 1999; Wakai et al., 1999).
The pile design approaches are different in our countries. In Kazakhstan, static load tests are used for pile design, while in Slovakia, piles are design only by analytical methods and static load test are used only in exceptional cases (large constructions and projects) for verification of design resistance and settlement. Results of three static load tests in Slovakia and in Kazakhstan (Figure 1) were compared with results of numerical analysis.
Figure 1. Static load test in Bratislava – Slovakia (left) and Karagandy – Kazakhstan (right)
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3.1 Numerical modelling of CFA piles
Numerical modelling often does not take into account the effect of pile installation, because piles are modelled as a "wished-in-place". Modelling of pile installation is needed especially for displacement piles where the influence of technology is more significant than for bored piles.
Using cavity expansion theory published by Mecsi (2013) have been estimated that a lateral earth pressure around pile shafts was only slightly increased. This finding has led to the fact that the technology in these cases could be taken into account in numerical models only by using interface elements between Piles and surrounding soils.
CFA piles presented in this article are modelled by using the software Plaxis 2011. Tasks were done as axisymmetric ones using 15 node elements. Linear elastic (LE) constitutive model was used for CFA pile body and advanced Hardening Soil (HS) constitutive model was used for soil layers. Hyperbolic relationship between the vertical stress and the deviatory stress in primary triaxial loading was considered for HS model. Soil shows a decreasing stiffness and simultaneously irreversible plastic strain development during the primary deviatory loading (Schanz et al., 1999). Yield surface form in the space a hexagonal pyramid given by MohrCoulomb criterion. Hardening parameter is not constant, but varies depending on accumulated plastic strain soil.
3.1.1 Numerical model of CFA pile in Bratislava
Static load test on CFA pile in Bratislava, Slovakia (Figure 1, left) has been made on system (working) pile of piled raft foundation below high-rise building. Depth of excavation pit is 6.3 m below the original surface. CFA piles are 15.0 m long with diameter of 630 mm. Geological conditions over the pile are very varied. Quaternary well graded gravels are located in upper part of pile environment up to 2.5 m. Free phreatic level has been taken into account in stratum of gravel. Sandy silts and silts with low plasticity are situated below layer of gravel from 2.5 to 12.5 m. These thin soil layers have similar geotechnical properties and the ground model of homogenous layer was therefore used. The silty sand with confined groundwater in the depth 12.5 – 13.9 m has been defined with pore pressure equal to 100 kPa. Silts with medium plasticity are located below sandy layer. Geotechnical model of CFA pile is shown in Figure 2.
3.1.2 FEM analyse of CFA piles in Karagandy
Two static load tests (Figure 1, right) have been done below a 10th floor residential building in Karagandy. Both test piles were 8.0 m long with diameter of 630 mm. Excavation pit for foundation of building was approximately 3.7 m below
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the surface. Geological conditions of Karagandy are represented by fine grained soils. This area is formed predominantly by silts to sandy silts with lenses of clay
to sandy clay. Geotechnical models of both piles are shown in figure 2. Input soil
parameters for HS model are taken into account as: Eoedref = E50ref; Eurref = 4 . Eoedref and m = 0.7 for fine-grained soils. Effect of excavation was considered using the
value of over consolidation ratio in both cases.
Numerical analyses were done in form of a parametric study. Analyses include 24 models for CFA pile in Bratislava and 2 x 18 models for CFA piles in Karagandy. Technology impact, interface, phreatic level and overconsolidation were observed. Load-settlement curves obtained from static load tests were compared with load-settlement curves calculated by numerical modelling, as shown in Figure 3 for Bratislava and in Figure 4 and Figure 5 for two tests in Karagandy.
Static load test in Bratislava has been fully instrumeted. Distribution of load over the pile has been measured by deformation reading recorders. These results were used for comparison of real pile base and pile shaft resistances with calculated ones. Pile base and pile shaft resistances of CFA piles in Karagandy are presented only from numerical modelling, comparison with real measurements includes load-settlement curve.
Figure 2. Geotechnical models for numerical modeling
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Figure 3. Comparison of load-settlement curves by static load test and by numerical modeling for CFA pile B-TP1 in Bratislava
Figure 4. Comparison of load-settlement curves by static load test and by numerical modeling for CFA pile K-TP1 in Karagandy
The impact of CFA technology has a significant influence on the pile resistance, especially on shaft friction. Distributions of unit shaft friction over the CFA pile are shown in Figure 6.
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Figure 5. Comparison of load-settlement curves by static load test and by numerical modeling for CFA pile K-TP2 in Karagandy
As it is shown in case of CFA pile in Bratislava, unit shaft friction is visibly small in quaternary gravels under the phreatic level. A small decreasing is also in layer of silty sand which is caused by the pore water pressure. Distribution of unit shaft friction in cases K-TP1 and K-TP2 is constant and proportional to the depth due to relatively homogeneous soil profiles.
Figure 6. Distributions of shaft friction over CFA piles
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4. Conclusion
The installation process of CFA piles is less time-consuming like traditional rotary bored piles. Stability of borehole is ensured by the soil in continuous auger and therefore no other stabilization elements are needed. Suitable geological conditions for CFA piles are fine-grained soils of stiff consistency, weathered limestone and sandstone, residual fine-grained soils and medium dense to dense well-grained sands. Length of pile is limited by the length of auger, because continuous concreting is necessary for this technology.
Results of three static load tests, one from Bratislava (Slovakia) and two from Karagandy (Kazakhstan) have been compared and analysed by FEM using software Plaxis 2011. Numerical analysis has been done in form of a parametric study. Using the numerical modelling have been achieved very good results in comparison with results of static load tests. Precision of calculations have been corresponding to 95 %, only in one case (K-TP2) it was 92 %.
Acknowledgement
The paper is one of the outcomes of the Grant VEGA agency No. 1/0241/13.
References
1.Brown, D. A. Practical considerations in the selection and use of continuous flight auger and drilled displacement piles. In: Advances in design and testing deep foundations. Austin, Texas, USA, 2005, pp. 251 – 261.
2.Feda, J., 1977. Interaction between pile and ground. Praha, Academia, 156 p. (in
Czech).
3.Fleming, K. – Weltman, A. – Randolph, M. – Elson, K. Piling Engineering – Third Edition. Taylor & Francis e-Library, 2008, 398 p.
4.Masopust, J., 1994. Bored Piles. Praha, Čenek a Ježek s.r.o., 263 p. (in Czech)
5.Masopust, J., 2004. Special geotechnical works. Part 1. Brno, CERN academic Publishing, 141 p. (in Czech)
6.Mecsi, J. Geotechnical Engineering examples and solutions using the cavity expanding theory. Hungarian Geotechnical Society, Budapest, 2013, 232 p., ISBN: 978-963-89854-1-5.
7.Mohamedzein, YEA., Mohamed, MG., El Sharief, AM. Finite element analysis of short piles in expansive soils. In: Computers and Geotechnics, Vol. 24, 1999, 231-243.
8.Schanz, T., Vermeer, P. A., Bonnier, P. G., 1999. The hardening soil model: Formulation and verification. Beyound 2000 in computational geotechnics – 10 Years of Plaxis. Balkema, Rotterdam
9.Tomlinson, M. – Woodward, J. Pile design and construction practice – Fifth edition. Taylor & Francis e-Library, 2007, 551 p.
10.Viggiani, C., Mandolini, A., Russo, G., 2012. Piles and Pile Foundations. London, Taylor & Francis, 278 p.
11.Wakai, A., Gose, S., Ugai, K. 3D elasto-plastic finite element analyses of pile foundations subjected to lateral loading. In: Soils and Foundations, Vol. 39, 1999, p. 97–111
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