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Differential Settlement of Foundations on Loess

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Differential Settlement of Foundations on Loess Missouri University of Science and Technology Scholars' Mine International Conference on Case Histories in (2013) - Seventh International Conference on Geotechnical Engineering Case Histories in Geotechnical Engineering 02 May 2013, 2:00 pm - 3:30 pm Differential Settlement of Foundations on Loess Dušan Milović University of Novi Sad, Serbia Mitar Djogo University of Novi Sad, Serbia Follow this and additional works at: https://scholarsmine.mst.edu/icchge Part of the Geotechnical Engineering Commons Recommended Citation Milović, Dušan and Djogo, Mitar, "Differential Settlement of Foundations on Loess" (2013). International Conference on Case Histories in Geotechnical Engineering. 10. https://scholarsmine.mst.edu/icchge/7icchge/session02/10 This work is licensed under a Creative Commons Attribution-Noncommercial-No Derivative Works 4.0 License. This Article - Conference proceedings is brought to you for free and open access by Scholars' Mine. It has been accepted for inclusion in International Conference on Case Histories in Geotechnical Engineering by an authorized administrator of Scholars' Mine. This work is protected by U. S. Copyright Law. Unauthorized use including reproduction for redistribution requires the permission of the copyright holder. For more information, please contact [email protected]. DIFFERENTIAL SETTLEMENT OF FOUNDATIONS ON LOESS Dušan Milović Mitar Djogo University of Novi Sad University of Novi Sad Novi Sad, SERBIA Novi Sad, SERBIA ABSTRACT Experience gained during several decades shows that the loess soil in some cases undergoes structural collapse and subsidence due to inundation and that in some other cases the sensitivity of loess to the collapse is considerably less pronounced. In this paper the behaviour of three 12 story buildings A, B and C, of the same static system and the identical shapes have been analyzed. The measurement of settlements of building A over a period of 10 years indicate that the values were situated between the limits 9 cm to 13 cm, and that they are larger than the calculated values, but there was no damage reported in this case. However, the measured settlements of building B over the same period of time were considerably larger, reaching 46 to 51 cm, causing severe damages of the building. In order to find the explanation for such behaviour of loess subsoil, the additional field and laboratory testing of loess have been carried out. Some of the obtained results are presented in this paper. INTRODUCTION Several years ago 20 residential buildings were built on the under the applied load, in the frame of one Research Project location near Belgrade (Serbia). All of them were statically dealing with the foundation problems on loess soils, the identical and 12 stories high, with the depth of foundation D = enlarged program of investigations was realized. 2.0 m, with the width of strip foundations B = 1.80 – 2.40 m and with the contact pressure σ = 170 kN/m2. By field and laboratory investigation it was found that the soil profile on LABORATORY TEST RESULTS each microlocation was composed of loose, open structured macroporous landloess, and that its thickness was 20 m or Unconfined compression test results more. Experience gained in the past years confirm that the method of On the basis of the laboratory investigations of soil properties sampling has a very significant influence on the quality of the firm responsible for the project found that the calculated loess samples and of sensitive clays samples. The mechanical settlement, varying between 6 and 10 cm, could be accepted disturbance of loess samples taken by thin walled piston was for all buildings. However, the measured settlements over the registered (Milovic, 1971). For that reason a comparative period of more than 10 years, were considerably greater than laboratory testing was made on piston samples and on samples the calculated values, reaching in some cases 50 – 55 cm. The obtained from hand carved blocks, removed from test pits. differential settlements were also unexpectedly large, Typical unconfined compression test results are shown in Figs. producing severe damages. It is also important to notice that 1-5. The curve in Fig. 1 shows the relationship between the the settlement of some buildings was slightly greater than vertical stress σ1 and vertical deformation ε1, for block and calculated, and that in such cases the damage was not piston samples. reported. The results presented in Fig. 1 were obtained on block samples 3 Difficulties with building on loess and the results of numerous with γd = 12.9 kN/m and water content w = 12 %. The 2 studies indicate that this soil is of an unusual kind (Moretto unconfined compression strength was qu = 27 kN/m and and Bolognesi, 1957; Larionov, 1965; Milovic, 1969; Northey, modulus of deformation E = 2.5 MN/m2. From the same 3 1969; Milovic, 1978; Lutenegger et al., 1979; Milovic, 1980). depth piston samples had γd = 15.1 kN/m , w = 13 %, qu = 74 In order to better understand the behaviour of loess subsoil kN/m2 and E = 4.1 MN/m2. Paper No. 2.65 1 Fig. 1. Unconfined compression test results Figure 2 shows the results on block and piston samples with considerably greater dry density. Fig. 3. Bishop’s strain indicator The results of these tests are shown in Figs. 4 and 5. In Fig. 4 the values of the vertical stresses which initiate horizontal deformations are shown by points on curves. The samples 3 were with γd = 12.0 – 12.5 kN/m and w = 9 – 24.5 %. Fig. 2. Unconfined compression test results Figure 2 shows the results obtained on block samples with γd = 3 2 17.3 kN/m , w = 20.6 %, qu = 150 kN/m and E = 13.3 2 MN/m . From the same depth piston samples had γd = 16.5 3 2 2 kN/m , w = 23 %, qu = 86 kN/m and E = 2.3 MN/m . The obtained results clearly show that the method of sampling is of primary importance in determining geotechnical properties of loess soils. Several groups of block loess samples were tested with lateral strain measurements in unconfined compression tests. Bishop’s strain indicator was used to register the stresses which initiated the lateral deformations. Bishop’s ring and Fig. 4. Stresses which initiated the lateral deformations loess sample are shown in Fig. 3. Paper No. 2.65 2 The curves shown in Fig. 4 indicate how the value of the Full lines indicate the curves obtained on vertical samples (V) unconfined compression strength depends on the water and dash dot lines the curves on horizontal samples (H). The 3 content, when the value of the density is the same. In Fig. 5 tested samples were with γd = 16.5 – 17.5 kN/m and w = 21.2 2 2 are shown the results for block samples with γd = 15.1 – 16.2 ± 1 %, EV = 16 ± 5 MN/m and EH = 13 ± 1 MN/m . The kN/m3, with the same water content w = 22.5 %. degree of transverse anisotropy n ranged between 1.30 and 1.60. Consolidation test results In order to indicate the influence of the initial dry density of loess samples on the values of modulus of compressibility Eoe, a relatively great number of the consolidation tests was carried out by one dimensional compression of the loess samples, which were laterally restrained. Undisturbed samples, cut from blocks, were saturated at very beginning of the testing. The values of the modulus of compressibility were determined for the range of stresses σ = 80 – 120 kN/m2 on loess samples 3 with γd = 12 – 16.5 kN/m . Fig. 5. Stresses which initiated the lateral deformations The obtained results show that for the samples with the relatively high dry density the lateral deformations started at low vertical pressures. However, for the samples of low density the lateral deformations were registered under the stresses which were close to the ultimate stress qu. A number of unconfined tests were performed on block loess samples, cut in vertical and in horizontal direction, in order to determine the degree of anisotropy. Typical results are shown in Fig. 6. Fig. 7. Modulus of compressibility Eoe for saturated loess samples It is of interest to notice that for the values of dry density greater than ~ 14 kN/m3 the values of the modulus of compressibility remarkably increase. The change of the void ratio and the degree of subsidence were also determined by one dimensional compression tests and then the coefficient of subsidence was obtained by the following expression: en − e`n Δen (1) i m = = 1+ en 1+ e n where en is the void ratio before saturation at the vertical stress σn, and en′ is the void ratio at the end of subsidence under the same vertical stress σn, as shown in Fig. 8. Fig. 6. Unconfined compression test results for block samples Paper No. 2.65 3 In Fig. 10 are presented curves of the coefficients im for the 3 samples with γd = 13.5 - 14.0 kN/m . Curve 1 is related to the samples with w = 13 % and curve 2 indicates the values of im for the saturated loess samples at a given stress level. Fig. 8. Typical subsidence consolidation test result Coefficient of subsidence im can be used for the prediction of the additional settlement of loess soil due to an increase of water content. Therefore, for the relatively large number of undisturbed less samples the standard test procedure for evaluation of the hydroconsolidation potential was used. A sample was loaded to some value of σn and after the consolidation of the specimen was reached, the water was added. These tests have been performed on loess samples, Fig. 10. Coefficients i for γ = 13.5 - 14.0 kN/m3 covering the large range of dry density and initial water m d content.
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