П. С. Пойта, Н. Н. Шалобыта, Т. П. Шалобыта, Е. Н. Шалобыта. Осадки плитных фундаментов многоэтажных зданий.The purpose of this article is to consider slab (raft) foundation settlement of multi-storey buildings, to analyse their occurrence and development, and to propose methods and recommendations for their prevention or minimisation. The article considers engineering-geological conditions typical for the Brest south-west microdistrict-1, and proposes a method of levelling uneven deformations of the slab (raft) foundation base

Poita, P. S.

Shalobyta, E. N.

Shalobyta, N. N.

Shalobyta, T. P.

Пойта П. С., Шалобыта Н. Н., Шалобыта Т. П., Шалобыта Е. Н. Осадки плитных фундаментов многоэтажных зданийThe purpose of this article is to consider slab (raft) foundation settlement of multi-storey buildings, to analyse their occurrence and development, and to propose methods and recommendations for their prevention or minimisation. The article considers engineering-geological conditions typical for the Brest south-west microdistrict-1, and proposes a method of levelling uneven deformations of the slab (raft) foundation base

Poita, Petr Stepanovich

Shalobyta, Nikolay Nikolaevich

Shalobyta, Tatiana Petrovna

Shalobyta, Elena Nikolaevna

Repository of Brest State Technical University

Vestnik of Brest State Technical University. 2023. No. 3(132) Civil and Environmental Engineering  https://doi.org/10.36773/1818-1112-2023-132-3-11-14 11 UDC 624.15:692.115 MULTI-STOREYED BUILDING SLAB FOUNDATION SETTLEMENT  P. S. Poita1, N. N. Shalobyta2, T. P. Shalobyta3, E. N. Shalobyta4 1 Doctor of Technical Sciences, Professor, Professor of the Department of Geotechnics and Transport Communications,  Brest State Technical University, Brest, Belarus, e-mail: ppsbrest@mail.ru 2 Ph.D in Engineering, Associate Professor, Vice-Rector for Scientific Work, Brest State Technical University,  Brest, Belarus, e-mail: nnshalobyta@mail.ru 3 Ph.D in Engineering, Associate Professor of the Department of Concrete Technology and Building Materials,  Brest State Technical University, Brest, Belarus, e-mail: t_shalobyta@mail.ru 4 Master's student, Saint Petersburg State Electrotechnical University "LETI", St. Petersburg, Russia, e-mail: enshalobyta@stud.etu.ru   Abstract The purpose of this article is to consider slab (raft) foundation settlement of multi-storey buildings, to analyse their occurrence and development, and to propose methods and recommendations for their prevention or minimisation. The article considers engineering-geological conditions typical for the Brest south-west microdistrict-1, and proposes a method of levelling uneven deformations of the slab (raft) foundation base.  Keywords: foundation base, foundation settlement, stress-strain state, relative settlement difference, deformation modulus, central and peripheral zones.   ОСАДКИ ПЛИТНЫХ ФУНДАМЕНТОВ МНОГОЭТАЖНЫХ ЗДАНИЙ П. С. Пойта, Н. Н. Шалобыта, Т. П. Шалобыта, Е. Н. Шалобыта Реферат Целью данной статьи является рассмотрение осадок плитных фундаментов многоэтажных зданий, анализ их возникновения и развития, а также предложение методов и рекомендаций по их предотвращению или минимизации. В статье рассмотрены инженерно-геологические условия, характерные для ЮЗМР-1 г. Бреста, а также предложен способ выравнивания неравномерных деформаций основания плитного фундамента.  Ключевые слова: основание фундаментов, осадки фундаментов, напряженно-деформированное состояние, относительная разность оса-док, модуль деформации, центральная и периферийная зоны.   Introduction  Currently there is an intensive construction of residential multi-storey buildings in the Brest south-west microdistrict-1. The territory of the micro-district is a wasteland overgrown with shrubs. The landform of the area is flat, the absolute level of which varies from 133.28 m to 133.98 m. The geological structure of the territory includes soil formations represented by a vegetation layer with a thickness from 0.1 to 0.4 m; alluvial and lake-marsh deposits represented by sands of different coarseness: from dusty to medium; sandy loam and loam – weak and medium strength, including weakly peat-covered, with a thickness of up to 1.4 m; peat and medium peat-covered soils with a thickness of up to 1.0 m. Beneath alluvial and lake-marsh sediments, lake-alluvial sediments were uncovered, repre-sented by interlaced sands of various sizes from dusty to coarse, sandy loam and loam with plastic, soft-plastic and flowing.  Ground waters are found at a depth of 0.1 – 1.0 m from the day surface, or at absolute levels of 132.28 – 133.27 m, their maximum predicted level (accord-ing to JSC "Brestproekt") should be taken at the level of 134.5  m [1]. Taking into account the geological structure of construction sites, when designing the foundations of multi-storey buildings, the characteris-tics of foundation soils are assumed to be equal to those given in Table 1. Shallow foundations and driven pile foundations were considered in the option design. The main variant of foundations for large-panel buildings of different number of storeys (from 5 to 9 storeys) is a solid reinforced concrete slab for the whole building with a height of 500 mm. The main specificity of this variant is that the soil formations, peats, marshy soils, as well as weak sandy and clayey soils, which cannot be used as foundations without their removal, com-paction or consolidation, are embedded from the surface of the site up to the 131.640 m mark. Taking these factors into account, as well as the adopted depth of the foundation footings, it was necessary to install the sand and gravel pad over the entire thickness of these soils, up to the foundation foot-ing mark. The sand and gravel cushion was poured in layers of 0.2 – 0.3 m thickness with subsequent compaction of each layer by rollers up to the com-paction coefficient comK  = 0.95. Sand cushion thickness was 3.0...4.0 m. Table 1 – Characteristics of subgrade soils Name of soil Specific weight of soil, kN/m3 Strength parameters Deformation modulus E,  MPa ϕ, deg C, kPa Sand and gravel mix 16,0 37 2,0 20,0 Dusty, medium strength sand 10,2 30 4,0 13,0 Dusty, firm sand 10,7 34 5,5 26,0 Fine sand, medium strength 9,8 31 13,0 17,0 Medium sand, medium strength 10,6 37 2,0 43,0  The slab foundation bottom mark is assumed to be 135.400 m (Project Work Directorate of the Communal Unitary Utilities Enterprise "Brestzhil-stroy"). Generally, the foundation slab made of concrete of class C25/30 was built under the house, with separation by expansion joints, within the section.  Research methodology and results We have performed calculations of the foundation slab in the engineer-ing-geological conditions specified in Table 1, with the only difference that the layer of flowing loam located at a depth of 5.9...6.9 m, with a thickness of 1.1 to 1.6 m in the left section of the house and 1.1...0.2 m in the right section was taken into account. The calculations were performed using the software packages LIRA-SAPR and PLAXIS 3D, which allow to perform a joint calculation of the system "base-foundation-building" [2, 3, 4]. Isopoles of vertical displacements of the foundation and the character of their development in different sections of the slab are presented in Fig-ures 1 and 2. Vestnik of Brest State Technical University. 2023. No. 3(132) Civil and Environmental Engineering  https://doi.org/10.36773/1818-1112-2023-132-3-11-14 12 a)   b)   Figure 1 – Isopoles of vertical displacements of the foundations determined by LIRA-SAPR (a) and PLAXIS 3D (b)   Figure 2 – Character of slab foundations settlement development  1, 2 - foundation slab settlement at the end and in the middle  (PC LIRA-SAPR); 3, 4 - the same, respectively, calculated using the PC PLAXIS 3D  The analysis of vertical displacements shows their significant differ-ence in magnitude and trajectories of propagation both in plan and in cross-sections of the slab. The settlement obtained with PLAXIS 3D at the same points is slightly greater than that calculated with LIRA-SAPR. The settlement at points along the perimeter of the foundation is almost always less than the settle-ment at points along the line passing through the centre of the slab along the long side. It is known that when a load is applied to a flexible slab, its centre gives a settlement 1.24...1.60 times greater than the edges [5]. This is explained by the lawful operation of the foundation and its non-uniform stiffness in the central and peripheral zones of the foundation [6]. Taking this into account, as well as the fact that a significant thickness of the foundation footing is prepared artificially, it is obvious that it is possible to prepare the footing in such a way that its stiffness is different within the foundation spot. The implementation of such a solution contributes to the reduction of settlement differences, i.e. their equalisation: an increase of settlement in the edge zones and its reduction in the centre. Thus, the task is to prepare an artificial foundation with different deformability character-istics. The performed calculations have shown that at a considerable thick-ness of the sand cushion (2.0 m...7.0 m), from 18 % to 54 % of the total foundation settlement is formed at the expense of the artificial foundation. Therefore, cushion thickness and the degree of soil compaction [7], i.e. the coefficient comK , will have a great influence on the relative settlement dif-ference or on the slab deflection. Using the recommendations of [8] the slab foundation settlement under the centre and midpoints of the sides of a rectangular slab can be determined by the formula  B mredSbPkm Е= ,                   (1) where P is the average pressure under the foundation footings; dmreЕ  – average reduced deformation modulus of the base; k = 0k – coefficient determined from the tables for the basement point under the foundation centre, depending on the ratio of the founda-tion sides n=l/b and the ratio of the thickness of the removed layer to the foundation half-width 2/ /m H b= ; k = 1k  – same under the centre of the larger side of the foundation; k = 2k  – same under the centre of the smaller side of the foundation; Bm  – coefficient of working conditions of the base, taken for bases with average reduced deformation modulus dmreЕ 10≥ MPa equal to Bm  = 1,35 at 10,0 m,  b <≤  15,0 m. Vestnik of Brest State Technical University. 2023. No. 3(132) Civil and Environmental Engineering  https://doi.org/10.36773/1818-1112-2023-132-3-11-14 13 The degree of variability of the base compressibility in plan is deter-mined by the formula  redmaxЕ redminЕЕα = ,                           (2) If Еα  ≤ 1.5, the foundation is considered to be homogeneous in terms of compressibility. Calculations have shown that Еα  determined for the ground conditions of Brest is 1.27. Therefore, the foundation is homo-geneous in terms of compressibility. Let's denote the foundation settlement under its centre by 0S . Then 0S  will be equal to   0remBd0bPkS =m Е,           (3) The settlement of the foundation under the centre of the larger side of the foundation is equal to 11 dB mrebPkSm Е= ,           (4) Designing a slab with a width of 15.0 m and taking into account the location of the internal longitudinal walls and the thickness of the slab, we assume the width of the central zone and the peripheral zones located on either side of its centre to be 5.0 m. This division into sections is justified by the layout of the buildings. For the large-panel buildings designed in the Brest south-west microdistrict-1, the distance between the axes of the in-ternal longitudinal walls is basically 2.1 m. The next axis of the longitudinal walls is set at a distance of 2.1 m to one side and 1.1 m to the other. Then the total distance between the axes of the outermost longitudinal internal walls is 5.3 m. In calculations, the distance between the longitudinal inter-nal walls furthest from the centre is assumed to be 5.0 m wide. Then the edge zones along the length of the slab will also be 5.0 m wide. Taking into account that the compaction of the sand cushion is carried out until 0E  = 20,0 MPa, let's assume that exactly this value of modulus should be pro-vided under the central zone of the slab, width 5,0 m. Then the edge zones should have a modulus of deformation less than 0E . The value of the de-formation modulus in the edge zones is denoted by 1E . Then the ratio of the reduced average deformation modulus will be as follows 10redredЕЕβ = ,                         (5) From here  1 0red redЕ Еβ= ⋅ . Then the difference in settlement between the points in the middle of the slab and on its edge is equal to 0 1 1 0 0 1011 0red redred red redB0Bk Е - k Е k kbP bPS - S = =m Е Е m Е   −      ββ,   (6) Obviously, the most acceptable slab conditions will be when the differ-ence in settlement is zero.  Then  0 1redB 0k kbP = 0m Е −   ββ.           (7) This expression is zero if  0 1k k 0− =ββ. From here 10kkβ = .  According to (5) – 1 10 0redredЕ kЕ k= . Consequently, the deformation modulus of the soil in the edge zones of the slab in the middle of the long side is equal to 11 0red red0k=kЕ Е ,   (8) Thus, by specifying the compaction factor of the sand and gravel mix-ture and the deformation modulus at a point in the centre of the slab, we obtain the required value of d1reЕ  at the edge zone of the slab in the cor-responding section. Determining 0k  and 1k  according to tables [4] it is possible to de-termine d1reЕ  for any initial conditions. For more convenient use in practi-cal calculations we have plotted the graphs of change of d1reЕ  = f ( 10kk) (Figure 3). From the graphs we can see that d1reЕ  depends on E0red and does not depend on the aspect ratio. At values d0reЕ  distinct from those shown in the graphs use interpolation.   Figure 3 – Variation of d1reЕ  depending on the 10kk  The values of d1reЕ  and d0reЕ  are taken at the points on the edge of the foundation slab and in its centre. Consequently, along the width of the edge zone (b = 5.0 m) the deformation modulus will vary from d1reЕ  to d0reЕ . This can be accounted by keeping in mind the technology of sand and gravel cushion preparation. If the width of the compacted zone for n passes of the roller is 1.7 m taking into account overlapping of its traces. Then in the area from the edge of the slab to 1.7 m the modulus of soil deformation should be d1reЕ .  The next roller trace is also equal in width to 1.7 m. Then the increase in deformation modulus at this section is 0.54( d0reЕ  – 1E ). The defor-mation modulus of the soil on the second trace of the roller excavation should be equal to d1reЕ  = 15,4 MPa. Similar calculations are performed for the third section. Modulus of ground deformation at the third track of the roller penetration is 0.9( d0reЕ  - 1E ). If the calculations are performed at d0reЕ  = 20,0 MPa, at the first section the deformation modulus d1reЕ  = 10,0 MPa; at the second section d1reЕ  = 15,4 MPa; at the third section – d1reЕ  = 19,0 MPa (Figure 4).  Vestnik of Brest State Technical University. 2023. No. 3(132) Civil and Environmental Engineering  https://doi.org/10.36773/1818-1112-2023-132-3-11-14 14  Figure 4 – Scheme deformation modulus 1E  variation in the peripheral zones of a rectangular plate  Foundation slab settlement calculations performed in accordance with the values of 1E , which are recommended for edge zones, showed that the slab settlement at points in the middle of the long sides of the founda-tion is more uniform and varies from 4.82 cm to 4.87 cm for the left section. The settlement at the centre of the slab of the left section is 6.27 cm. Their magnitude is very important and is limited by the current norms [9]. The relative difference in settlement is 0.0019...0.00187. For the right section, where the load on the foundation is 22 % less compared to the left section, the settlement of the extreme points varies from 3.79 cm to 5.37 cm. The settlement at the centre of the slab is 5.87 cm. The relative difference in settlement is 0.0007 and 0.0021. The settlement of the same foundations without considering the change in deformation modulus for the left section ranged from 3.1 cm to 6.5 cm and at the centre 8.1 cm. The relative de-flection is respectively 0.0071 and 0.0025. For the right section, the settle-ment is more irregular at the extreme points. They vary from 2.3 cm to 5.9 cm. The draught at the centre is 6.8 cm. The relative difference is 0.006 and 0.0012. Thus, regulating the degree of compaction of the sand and gravel cushion over the width of the future foundation allows to obtain more uniform settlement and, most importantly, to reduce, and very significantly, the amount of deflections and the relative difference in settlement, which naturally affects the amount of internal forces in the foundation slab. The design of a slab foundation base to exclude non-uniform settle-ment is a very rational preparation of an artificial base with appropriate pa-rameters of its deformability, ensuring the presence of minimal non-uniform settlement or completely excluding it.  Conclusion When designing slab foundations of considerable thickness on artifi-cial foundations, their deformability should be regulated in order to mini-mise uneven settlement as much as possible. The proposed method of artificial base construction makes it possible to significantly equalise the foundation slab settlements, reduce their rela-tive difference by 5–8 times without changing the slab height, i.e. without changing its stiffness. The construction of foundations with an adjustable deformation mod-ulus is possible when using, as foundations, not only artificially laid em-bankments, but also in case of building foundations on naturally formed basis.  References 1. Mnogokvartirnyj zhiloj dom v mikrorajone «YuZMR-1» g. Bresta (KPD-9A) // Otchet po rezultatam inzhenerno-geologicheskih izyskanij. – Brest, 2022. – 17 s. 2. Nurieva, D. M. Raschet prostranstvennogo karkasa monolitnogo zhelezobetonnogo zdaniya s plitnym fundamentom na uprugom osnovanii s primeneniem raschetnogo kompleksa LIRA-SAPR / D. M. Nurieva. – Kazan : Izd-vo KGASU, 2017. – 71 s. 3. Modeli gruntov, realizovannye v programmnyh kompleksah SCAD OFFICE i PLAXIS 3D / E. S. Egorova [i dr.] // Stroitelstvo unikalnyh zdanij i sooruzhenij. – 2016. – S. 31–60.  4. Matveeva A. V. Raschet fundamentnyh plit s uchetom sovmestnoj raboty s konstrukciej i uprugim osnovaniem peremennoj zhestkosti / A. V. Matveeva. – M. : MGSU, 2016. – 101 s. 5. Gorbunov-Posadov, M. I. Raschet konstrukcii na uprugom osnovanii / M. I. Gorbunov-Posadov, T. A. Malikova, V. I. Solomin. – 3-e izd., pererab. i dop. – M. : Strojizdat, 1984. – 679 s. 6. Primenenie metoda B. N. Zhemochkina dlya sovmestnogo rascheta sistemy «osnovanie – fundament – zdanie» / N. Yu. Kiselev [i dr.]  // Internet-zhurnal «Naukovedenie». – 2017. – Tom 9, № 4.  7. Pojta, P. S. Napryazhenno-deformirovannoe sostoyanie v osnovanii shtampov pri peredache na grunt ravnomerno-raspredelennoj nagruzki razlichnoj intensivnosti / P. S. Pojta, N. N. Shalobyta, T. P. Shalobyta // Vestnik BrGTU. – 2023. – № 1 (130). – S. 56–59. 8. Rukovodstvo po proektirovaniyu fundamentnyh plit karkasnyh zdanij. – M. : Strojizdat, 1978. – 128 s. 9. Obshie polozheniya po proektirovaniyu osnovanij i fundamentov zdanij i sooruzhenij : SP 5.01.01-2023 / Stroitelnye pravila Respubliki Belarus. – Minsk : Minstrojarhitektury Respubliki Belarus, 2023. – 150 s.  Material received 12/11/2023, approved 30/11/2023,  accepted for publication 30/11/2023            

Multi-storeyed building slab foundation settlement

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Multi-storeyed building slab foundation settlement

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