Problems of building and designing on semi-rocky soils

Transcription

1 Problems of building and designing on semi-rocky soils D. Sursanov 1 and E. Sytchkina Construction industry Department, Perm Technical University, Perm, Russia ABSTRACT Building and designing on semi-rocky soils is problematic for many countries in the world. In particular, in Russia the problems of building on semi-rocky soils are not given clearly enough in national construction norms and standards. There is a necessity to analyse strength and deformation properties of semi-rocky soils, to work out the theory of interaction between a deep base and semi-rocky soils, and to develop a mathematical model of "deep base - semi-rocky ground" interaction. In that paper some results of static load soil test are presented. The possibility of the use of lower Permian soils as the end-bearing pile base within the bounds of a particular case was confirmed. Keywords: laboratory test, deformation, strength, semi-rock soil, pile, foundation 1 INTRODUCTION At present high-rise building is actively carried out in the central districts of Perm. It leads to the load increase on the soils and the foundation depth as well. Under these conditions pile foundations on geological Lower Permian deposits are the most effective and sometimes the only possible type of foundations. As a rule geological Lower Permian deposits are represented by jointed, argillites weathered in the top, siltstones and sandstones, which are lying at a depth of m. According to GOST «Soils. Classification» [1] (Russian standard document) these soils refer to the class of rocky soils with strong cementation ties, to the group of semirocky (Rc < 5МPа), and to the subgroup of sedimentary, e.g. aleurolit is characterized as strongly condensed and strong fine-grained soil, with dense slate structure similar to loam. It is known, that the properties of semi-rocky soils depend on the rate of water saturation and cementation. Therefore they lose their strength properties under damping. Mechanical properties of soils can be expressed as both the conventional values of deformation modulus (E oed ), and the parameters of angle of internal friction and specific cohesion. In Russia opinions on the use of geological Lower Permian deposits, as the bases for the pile foundations of buildings with considerable loadings are inconsistent enough. On the one hand, such soils are considered to stand the load of 50 MPa [2]. But on the other hand, laboratory tests show the deformation modulus less than 15 MPa. Results of these tests are summarized in Table 1. 1 Construction industry Dep., Perm State Technical University. Komsomolsky prospekt, Perm, Russian Federation. sursanov@mail.ru

2 Table 1. Deformation properties of semi-rocky soils of Lower Permian in Perm. Soil characteristics Semisolid clay, saturated with water (argillite) Solid clay, damp Fine graded, homogeneous, damp sand (sandstone) Fine graded, homogeneous, saturated with water sand (sandstone) Fine graded, homogeneous, damp sand (sandstone) Depth of occurrence, m Deformation modulus Е , MPa The difference in opinion of Russian geologists and geotechnicians, can be explained by the fact that direct investigation of these soils haven t been done so far. There was just no necessity investigating them as the upper covering layers of alluvial and dealluvial deposits were used as bearing bases. Furthermore, Russian technique of laboratory tests doesn`t allow to avoid some errors in the course of work. The most essential of them are: 1) structure failure during soils specimen preparation; 2) friction effects between soil specimen and the walls of laboratory instrument iron ring resulting in test distortion; 3) speed of loading that is important for clayey soil. Therefore, as a result of various laboratory investigations we obtain substantially different values of deformation modulus (E oed ). Soil strength and deformation characteristics are very often defined according to the Russian standard document «Design and construction of pile foundations» [3] - section 6. These characteristics presented in tables 113, 119, 123 of the document mentioned above are based on archive materials (aleurolites of Donbass, Central Asia, Zabaikalye, Eastern Siberia, Far East, etc.) and given for a preliminary estimation of alluvial soils. Therefore in the conditions of building in Russia it is important to know the real properties of Lower Permian semi-rocks for the forecast of the settlement and bearing capacity of the pile foundations based on them. 2 OBJECT OF RESEARCH The construction of 20-storey residential houses «Soldatskaya slobodka» is carried out now in one of the central districts of Perm. Direct laboratory tests of Lower Permian semi-rocky soils on the construction site soils aren't presented in engineering-geological reports. Therefore supervising organizations had some fears concerning the legitimacy of applying "end-bearing pile" in design schemes as well as the possibility of transferring design loads to Lower Permian semi-rocky soils [4]. 2.1 Geomorphology With respect to the geomorphology the site belongs to the fourth terrace of the river Kama, and is complicated by a broad valley of the river Stiks. The designed building is located on a quite flat part of a dealuvial slope. 2.2 Stratigraphy With respect to the geology the research site consists of the Quarternary and the Lower Permian deposits. The Quarternary deposits (Q4) are presented by alluvial loams, gravel soils and loams, eluvial deposits with gruss and crushed stone of aleurolite and argillite. The Lower Permian deposits are presented by jointed aleurolites and argillites weathered in the top to loams [5]. The surface of the site is covered in man-made ground partly with a soil-vegetative layer. Soil profile is presented in Figure 1.The detailed stratigraphy of the soil layers are given below: - Layer 1:brown loam, heavy, plastic, with areas of gravel up to 25% and organic impurities. - Layer 2: brown, light and heavy, pulverescent, plastic loam, with areas of gravel up to 25-35% and organic impurities.

3 2.3 Layer 3: brown, light and heavy, pulverescent and sandy, plastic loam, with areas of gravel up to 25-35% and organic impurities. Layer 4: loamy, sandy-loam and sandy gravel saturated with water. Layer 5: brown, sandy and gravel, plastic to hard loamy sand. Layer 6: plastic to hard, with crushed aleurolites and argillites up to 10-20% sandy loam Layer 7: brown, heavy weathered aleurolite. making final decision of the use of alluvial Lower Permian soils as a pile foundation base, it was decided to carry out static tests for three piles [6]. Hydrogeology Along the construction site there are ground waters of quarternary clay layers, sporadic gravel deposits and fractures of Sheshminsky waterbearing horizon. During engineering-geological research the established levels of ground waters on the site were registered at the depth of 2,9 6,0 m. 3 CONSTRUCTIVE DESIGN OF THE FOUNDATIONS Foundations of designed buildings are pile foundations with strip monolithic grillage. Piles are driven, sectional reinforced concrete of solid square section (300x300 mm) with untensioned reinforcement. With the total length of 21 m piles consist of two parts - upper and lower with the length of 12 and 9 m correspondingly, connected by welded joint. The design load per pile accepted in the project is 700 kn. Interaction scheme between soil and pile "end-bearing pile" was adopted. 4 PROCEDURE OF STATIC LOAD SOIL TEST In the initial stage dynamic pile tests were carried out on the construction site. According to the results the settlement range varied from 2 to 6 mm. It meant that the bearing capacity of the piles had the value of kn. Thereby, taking into consideration the value spread of pile bearing capacity and also the possibility of Figure 1. Soil profile. For that purpose three test beds were designed. The photo of one of them is given in Figure 2.

4 All structures of test beds were preliminarily rated at a load, which was 20% higher than the value specified by the test program and Russian normative documents [1, 2, 3]. Pile tests were carried out after pile driving with a 17-day interval. A hydraulic thruster with the loadcarrying capacity of 200 tons was used as a loading device. Each load step was registered by a manometer. Pile loading was done evenly by a pumping unit with load steps equal 96 kn each (that is about 10% of the maximum load value of kn). To measure pile settlement all meter readings were taken at each load step in the following succession: zero reading before pile loading; the first reading immediately after pile loading; next four readings with a 30-minute interval sequentially and further on every other hour until conditional settlement stabilization is reached. The rate of 0.1mm per hour was accepted as the criterion of conditional stabilization. So if the settlement rate was less than 0.1 mm per hour, the next load step was done. The final criterion of the static load test included two conditions: the general pile settlement could not be less than 20 mm and the maximum load did not have to exceed the value of kn. So, if the general settlement exceed 20 mm or load value attain the value of kn the static load test will finished. In that case the second condition was executed. The value of the unload step was 192 kn (double load step). To measure pile settlement all meter readings were taken twice at each unload step: the first reading immediately after pile unloading; the second reading every other 15 minutes. When pile unloading was completed, meter readings were being taken in the course of 60 minutes with a 15-minute interval. Displacements of the test beds and anchor piles were registered in each stage of loading and unloading. The results of the test are presented in Figure 3 in the form of a settlement-load diagram and a settlement-time diagram. Other 2 tests show similar results. Figure 3. Settlement-load diagram; settlement-time diagram (for loading steps only) Figure 2. Test bed of static load soil test. Pile unloading began after the maximum load value had been transferred to the pile. It was done by steps with a 15-minute interval each. 5 CONCLUSION According to the test data, pile settlement on loading equal to 1056kN totaled 4.58mm and

5 5.275 mm. It was less than 20 mm, so in that case the maximum test load (1056 kn) was accepted as a particular value of specific pile resistance, according to [2]. Then the assumed load on a pile according to the test data totaled N = 880kN. The pattern of the diagrams in Figure 3 allows to make a conclusion about the character of the interaction between soil and pile and to say that the piles in the load range act as end-bearing piles. As the result of static pile test, the interaction between soil and pile foundation was investigated. Also was proved the possibility of load transfer of design assumed load (N = 700kN) onto these pile foundations. The possibility of the use of lower Permian soils as the standing pile base within the bounds of a particular case was confirmed. A number of laboratory and field experiments are required in order to find an integrated solution of the problem under discussion. REFERENCES [1] GOST Soils. Field test methods by piles, Moscow, [2] SNiP Pile foundations, Moscow, [3] SP Design and construction of pile foundations. Moscow, [4] Manual to designing of the bases of buildings and constructions ( to SNIP ), Мoscow, 1986 [5] V.P.Ozhgibesov, Stratigraphy and geology of The Volgo-Ural oil-gas provinces. The general and regional stratigraphy divisions of the Perm System, Perm, [6] A.B. Ponomarev, A.V. Zaharov, D.N. Sursanov, Some results of field tests of piles static pressing loading, Articles of the International Scientific and Technical Conference, devoted Dalmatov 100-anniversary of the birth, St. Petersburg, 2010.

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