Time-varying deformations of locally compacted basement soil under the piles with broadening

Материалы первого заседания российско-американской рабочей группы на конгрессе в Гео-Институте США "The proceedings of the first U.S.A. - Russia geotechnical engineering workshop" 25-28 марта 2012.
ISBN 978-5-93093-957-6 Москва, издательство АСВ, 2014


В.С. Глухов
Генеральный директор ООО «Новотех»
Заведующий кафедрой Геотехники и дорожного строительства,
Пензенский государственный университет архитектуры и строительства,
к.т.н., профессор
М.В. Глухова
Инженер-проектировщик ООО «Новотех»
Ассистент кафедры Геотехники и дорожного строительства,
Пензенский государственный университет архитектуры и строительства

     
      In the building of construction projects with loading on foundations of more than 5000 kN or with pressure within the site of a structure projection of not less than 500 kPa in most cases pile foundations are used. In this case piles in pierced holes with widening placed by the use of the technology of extended foundations in rammed foundation pits may be considered as one of the perspective variants [1].
      The placement characteristic feature of the mentioned piles is the formation of compacted ground zones along the lateral surface of piles and under widening. This effect is caused by the process of release (displacement) of ground aside and downwards. Within compacted zones the compactness, strength and deformation characteristics increase.
      The results of a number of researches on studying of compacted zones deformation process and improvement of ground base construction properties testify to the increase of the ground moisture degree up to maximum values (Sr  ≥ 0,9…0,95), the increase of ground compactness around the pile column in the horizontal plane up to (2,0…2,5)d (d – pile diameter) and in the vertical direction lower than the widening footing in the limits of (0,7 …1,2)dy (dy – pile diameter) [2, 3]. Taking into account the compaction process is mostly important while planning the considered piles in well-compacting soils. The latter include loessial slump, made ground and other covering clayey soils with fluidity index IL       The above-mentioned process of the change of the base properties because of ground compacting is taken into account in the present day recommendations on calculation and planning of piles in pierced holes with widening to a certain extent [4].
      The important factor of the foundation reliability alongside with providing ground strength is also the proper estimate of foundation deformations. Taking into account the effect of the compaction of a large ground area with piles placing in pierced holes and possible diversity of stratification, prognostication of deformations (structure settlings) for such foundations is one of the difficult tasks in view of soil mechanics. In the present paper the authours study not only stabilized dislocations, but consider the process of deformation developments in the course of time duration.
      While planning the task of possible providing equal final settlings of foundations of different construction project areas is being solved. However, in cases of significant difference of the active zone of ground base compression under foundation footings the difference in soil stratification with differentiated module values of deformation, with the foundation footing placing at different levels and other similar factors quite great divergence of the process of non-stabilized dislocations of ground base in the course of time duration can occur. In this case in the construction process including the final stage of the latter the difference of settlings in various areas of a building can reach inadmissible values. The mentioned fact will cause an accident, until the moment when the settlement of a structure stabilizes. Therefore, the development of proper prognosis methods of settling progress in the course of time duration is considered to be an important factor of providing the foundation reliability. The theory of filtration consolidation is assumed the basis of such calculation methods [5].
      In the course of study of the deformation process of compacted and heterogeneous ground base of piles in pierced holes with widening (PPHW) the method of the equivalent layer proposed by N. A. Tsytovich which makes it possible for multi-layer bases to significantly simplify the calculation of final settlings and their progress in the course of time duration [6]. The mentioned method makes a complex spatial task equivalent one-dimensional. According to theoretical requisites it takes a progress position between strict analytical solutions and the method of layer-specific summation.
      A long-term development of settlings in the course of time duration is caused by the low speed of water filtration characteristic for clayey soils and slow processes of ground creep especially in clayey soils of hard-plastic and firm consistence. The calculation of the two types of foundation settlings in the course of time duration applied to the averaged loam characteristics has been done by the authours as an example. Traditional type of shallow columned foundation on the natural base under loading of NII  = 2000 kN and the foundation in the form of a pile in a pierced hole with widening placed with the use of the foundation technology in rammed foundation pits have been considered (fig.1).

Fif.1. Diagram for the calculation of settling in the course of time duration
a) shallow columned foundation
b) pile in a pierced hole:1 – compacted ground zone


      While calculating base deformations in the course of time duration it is important to define the degree of consolidation U0(t). For the purpose of practical calculations at U0(t) ˃ 0,25 the following formula is offered [6]

                                                                                                                        (1)

where
      The values of N are given in the table for different degrees of consolidation in the range from 0,05 up to 1,00. Therefore, it is not difficult to define the time corresponding to consolidation degrees of a compressed ground layer using the formula:

                                                                                                                     (2)

where h – power of compressed ground layer;
Сv – coefficient of ground consolidation calculated with the formula
                                                                                                                                 (3)
where γw – relative density of water;
K – filtration coefficient;
mv – coefficient of relative compression of soil which is calculated with

.

E – module of soil deformation;
β – coefficient depending on Poisson’s ratio υ and lateral pressure coefficient ξ. Coefficient β is taken as equal to: for sands  – 0,8; sandy loams – 0,7; loams – 0,5; clays – 0,4.
      Since coefficient β for every kind of ground is taken unchangeable, coefficient of relative compression of ground is the less, the more deformation module of soil E of compacted zone under pile widening. The latter may change towards the increase from 3,0 to 5,0 times. As it is shown in formula (3) as the module of consolidation coefficient of compacted zone increases, consolidation coefficient of compacted ground increases linearly, and time t is reduced up to a certain degree of consolidation.
      With the placement of piles according to the technology of foundations in rammed foundation pits the ratio of compacted ground layer hy and ground of natural formation h0 within total compressed layer Hc of ground base under the widening footing significantly influences consolidation time t.
      According to the calculation results the diagrams of settling change in the course of time duration are drawn (fig. 2). The calculations testify to the existence the different speed of deformation process in clayey soils of natural formation and compacted base. Thus, at equal loading NII on the two types of foundation under analysis 95% of design final settling which is equal to 100 mm is reached for the foundation on natural base during 30 days , and in case of piles with widening on compacted clayey soils 185 days are required for completion of deformations till the mentioned stabilization.

      The above-mentioned difference of the speed of settling process in the course of time duration for ground bases of natural formation and compacted ground bases proved the significance of such calculations. The several times difference of the speed of settlings may cause the increase of permissible difference of settlings.
      Therefore, especially if the application of different in the technology of placement types of foundation in one construction project is required, there must be the calculation of settlings in the course of time duration.
      The conditioning reason of considerable duration of the stabilization process of pile settling in pierced holes with widening is a sharp decrease of filtration coefficient in a compacted area of ground base. Consequently, while developing the monitoring programme the period of observing the settlement of a structure should be fixed taking into account the prognoses of settling of the mentioned foundations in the course of time duration.
 

REFERENCES

 
1. Krutov V. I., Kogay V. K., Glukhov V. S. Pile foundations made of built-in-place piles in pierced holes. / “OFMG”. – 2010. №2, pp. 10-14.
2. Krutov V. I., Kogay V. K., Glukhov V.S. Consideration of ground contracting in the calculation of pile foundations. / Publications of international conference in geotechnics “Geotechnics problems of megalopolises”. – M., 2010, Vol. №4, pp. 1385-1388.
3. Glukhov V.S., Holodkov R.A., Mel’nik E. N., Vinokurov V. L. Application experience of foundations in rammed foundation pits. / Higher Education Institutions Proceedings “Construction and Architecture”, Novosibirsk, №4, 1987, pp. 131-134.
4. Planning and placement of pile foundation and compacted foundations made of built-in-place piles. / Standard of Organization. STO 36554501-018-2009. OJSC “Research Center “Construction”, M., 2010. – 27 p.
5. Ter-Martirosyan Z. G. Soil mechanics / Textbook. – M.: Publishing House “Higher Education Institutions of Construction”, 2005. – 488 p.
6. Tsytovich N. A., Ter-Martirosyan Z. G. Fundamentals of applied geomechanics in construction. / Textbook. – M.: Higher School, 1981. – 317 p.



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