Differential Settlement of Foundations on Loess

Transcription

1 Missouri Uniersity of Science and Tecnology Scolars' Mine International Conference on Case Histories in Geotecnical Engineering (013) - Seent International Conference on Case Histories in Geotecnical Engineering Apr 9t - May 4t Differential Settlement of Foundations on Loess Dušan Miloić Uniersity of Noi Sad, Serbia Mitar Djogo Uniersity of Noi Sad, Serbia Follow tis and additional works at: ttp://scolarsmine.mst.edu/iccge Part of te Geotecnical Engineering Commons Recommended Citation Miloić, Dušan and Djogo, Mitar, "Differential Settlement of Foundations on Loess" (013). International Conference on Case Histories in Geotecnical Engineering. 10. ttp://scolarsmine.mst.edu/iccge/7iccge/session0/10 Tis Article - Conference proceedings is brougt to you for free and open access by Scolars' Mine. It as been accepted for inclusion in International Conference on Case Histories in Geotecnical Engineering by an autorized administrator of Scolars' Mine. Tis work is protected by U. S. Copyrigt Law. Unautorized use including reproduction for redistribution requires te permission of te copyrigt older. For more information, please contact scolarsmine@mst.edu.

2 DIFFERENTIAL SETTLEMENT OF FOUNDATIONS ON LOESS Dušan Miloić Uniersity of Noi Sad Noi Sad, SERBIA Mitar Djogo Uniersity of Noi Sad Noi Sad, SERBIA ABSTRACT Experience gained during seeral decades sows tat te loess soil in some cases undergoes structural collapse and subsidence due to inundation and tat in some oter cases te sensitiity of loess to te collapse is considerably less pronounced. In tis paper te beaiour of tree 1 story buildings A, B and C, of te same static system and te identical sapes ae been analyzed. Te measurement of settlements of building A oer a period of 10 years indicate tat te alues were situated between te limits 9 cm to 13 cm, and tat tey are larger tan te calculated alues, but tere was no damage reported in tis case. Howeer, te measured settlements of building B oer te same period of time were considerably larger, reacing 46 to 51 cm, causing seere damages of te building. In order to find te explanation for suc beaiour of loess subsoil, te additional field and laboratory testing of loess ae been carried out. Some of te obtained results are presented in tis paper. INTRODUCTION Seeral years ago 0 residential buildings were built on te location near Belgrade (Serbia). All of tem were statically identical and 1 stories ig, wit te dept of foundation D =.0 m, wit te widt of strip foundations B = m and wit te contact pressure σ = 170 kn/m. By field and laboratory inestigation it was found tat te soil profile on eac microlocation was composed of loose, open structured macroporous landloess, and tat its tickness was 0 m or more. On te basis of te laboratory inestigations of soil properties te firm responsible for te project found tat te calculated settlement, arying between 6 and 10 cm, could be accepted for all buildings. Howeer, te measured settlements oer te period of more tan 10 years, were considerably greater tan te calculated alues, reacing in some cases cm. Te differential settlements were also unexpectedly large, producing seere damages. It is also important to notice tat te settlement of some buildings was sligtly greater tan calculated, and tat in suc cases te damage was not reported. Difficulties wit building on loess and te results of numerous studies indicate tat tis soil is of an unusual kind (Moretto and Bolognesi, 1957; Lariono, 1965; Miloic, 1969; Nortey, 1969; Miloic, 1978; Lutenegger et al., 1979; Miloic, 1980). In order to better understand te beaiour of loess subsoil under te applied load, in te frame of one Researc Project dealing wit te foundation problems on loess soils, te enlarged program of inestigations was realized. LABORATORY TEST RESULTS Unconfined compression test results Experience gained in te past years confirm tat te metod of sampling as a ery significant influence on te quality of loess samples and of sensitie clays samples. Te mecanical disturbance of loess samples taken by tin walled piston was registered (Miloic, 1971). For tat reason a comparatie laboratory testing was made on piston samples and on samples obtained from and cared blocks, remoed from test pits. Typical unconfined compression test results are sown in Figs Te cure in Fig. 1 sows te relationsip between te ertical stress σ 1 and ertical deformation ε 1, for block and piston samples. Te results presented in Fig. 1 were obtained on block samples wit γ d = 1.9 kn/m 3 and water content w = 1 %. Te unconfined compression strengt was q u = 7 kn/m and modulus of deformation E =.5 MN/m. From te same dept piston samples ad γ d = 15.1 kn/m 3, w = 13 %, q u = 74 kn/m and E = 4.1 MN/m. Paper No..65 1

3 Fig. 1. Unconfined compression test results Figure sows te results on block and piston samples wit considerably greater dry density. Fig. 3. Bisop s strain indicator Te results of tese tests are sown in Figs. 4 and 5. In Fig. 4 te alues of te ertical stresses wic initiate orizontal deformations are sown by points on cures. Te samples were wit γ d = kn/m 3 and w = %. Fig.. Unconfined compression test results Figure sows te results obtained on block samples wit γ d = 17.3 kn/m 3, w = 0.6 %, q u = 150 kn/m and E = 13.3 MN/m. From te same dept piston samples ad γ d = 16.5 kn/m 3, w = 3 %, q u = 86 kn/m and E =.3 MN/m. Te obtained results clearly sow tat te metod of sampling is of primary importance in determining geotecnical properties of loess soils. Seeral groups of block loess samples were tested wit lateral strain measurements in unconfined compression tests. Bisop s strain indicator was used to register te stresses wic initiated te lateral deformations. Bisop s ring and loess sample are sown in Fig. 3. Fig. 4. Stresses wic initiated te lateral deformations Paper No..65

4 Te cures sown in Fig. 4 indicate ow te alue of te unconfined compression strengt depends on te water content, wen te alue of te density is te same. In Fig. 5 are sown te results for block samples wit γ d = kn/m 3, wit te same water content w =.5 %. Full lines indicate te cures obtained on ertical samples (V) and das dot lines te cures on orizontal samples (H). Te tested samples were wit γ d = kn/m 3 and w = 1. ± 1 %, E V = 16 ± 5 MN/m and E H = 13 ± 1 MN/m. Te degree of transerse anisotropy n ranged between 1.30 and Consolidation test results In order to indicate te influence of te initial dry density of loess samples on te alues of modulus of compressibility E oe, a relatiely great number of te consolidation tests was carried out by one dimensional compression of te loess samples, wic were laterally restrained. Undisturbed samples, cut from blocks, were saturated at ery beginning of te testing. Te alues of te modulus of compressibility were determined for te range of stresses σ = kn/m on loess samples wit γ d = kn/m 3. Fig. 5. Stresses wic initiated te lateral deformations Te obtained results sow tat for te samples wit te relatiely ig dry density te lateral deformations started at low ertical pressures. Howeer, for te samples of low density te lateral deformations were registered under te stresses wic were close to te ultimate stress q u. A number of unconfined tests were performed on block loess samples, cut in ertical and in orizontal direction, in order to determine te degree of anisotropy. Typical results are sown in Fig. 6. Fig. 7. Modulus of compressibility E oe for saturated loess samples It is of interest to notice tat for te alues of dry density greater tan ~ 14 kn/m 3 te alues of te modulus of compressibility remarkably increase. Te cange of te oid ratio and te degree of subsidence were also determined by one dimensional compression tests and ten te coefficient of subsidence was obtained by te following expression: i e e` Δe n n n m = = (1) 1+ en 1+ e n were e n is te oid ratio before saturation at te ertical stress σ n, and e n is te oid ratio at te end of subsidence under te same ertical stress σ n, as sown in Fig. 8. Fig. 6. Unconfined compression test results for block samples Paper No..65 3

5 In Fig. 10 are presented cures of te coefficients i m for te samples wit γ d = kn/m 3. Cure 1 is related to te samples wit w = 13 % and cure indicates te alues of i m for te saturated loess samples at a gien stress leel. Fig. 8. Typical subsidence consolidation test result Coefficient of subsidence i m can be used for te prediction of te additional settlement of loess soil due to an increase of water content. Terefore, for te relatiely large number of undisturbed less samples te standard test procedure for ealuation of te ydroconsolidation potential was used. A sample was loaded to some alue of σ n and after te consolidation of te specimen was reaced, te water was added. Tese tests ae been performed on loess samples, coering te large range of dry density and initial water content. In Figs are presented te cures of te coefficients of subsidence i m for te undisturbed loess samples wit arious alues of dry density, coering te range from 1.5 to 15.5 kn/m 3. Cure 1 in Fig. 9 is related to te samples wit γ d = kn/m 3 and w = 13. %. Cure indicates te alues of i m for te samples wic were saturated at a gien stress leel. Fig. 10. Coefficients i m for γ d = kn/m 3 In Figs are sown te cures of i m for te loess samples wit γ d = kn/m 3, γ d = kn/m 3 and γ d = kn/m 3, respectiely. Te water content before saturation was w = 1.6%, w = 13.% and w = 13.8%. Fig. 11. Coefficients i m for γ d = kn/m 3 Fig. 9. Coefficients i m for γ d = kn/m 3 Paper No..65 4

6 FIELD TEST RESULTS Static penetration tests in te field were carried out in loess deposits wit natural water content and in saturated loess. Some typical results are sown in Figs. 14 and 15. Fig. 1. Coefficients i m for γ d = kn/m 3 Fig. 14. Cone penetration test results Fig. 13. Coefficients i m for γ d = kn/m 3 Te obtained results clearly sow tat te coefficient of subsidence i m depends to a large extend on te initial alue of γ d, on te natural water content and on te applied load σ during te saturation. Beside of tis, it is also sown tat te structural collapse due to saturation is well pronounced on te samples wit low dry density and low initial water content. Howeer, te samples wit low dry density and ig water content exibit large settlements before saturation. On te oter and, te samples wit ig dry density, approximately iger tan 15 kn/m 3, undergo relatiely small settlements due to saturation. Fig. 15. Cone penetration test results Paper No..65 5

7 Full line in Fig. 14 sows te cone resistance of te loess deposit wit natural water content, and dotted line represents te cone resistance of te saturated loess soil. In Fig. 15 similar results of te static penetration tests are presented. From seeral test pits te undisturbed block samples of loess were obtained and laboratory tests were carried out. Te dry density of tese samples was γ d = kn/m 3 natural water content w = 15 1 % and modulus of compressibility E oe =.5 MN/m. Howeer, after saturation tese alues were E oe = MN/m. te soil profile was te elastic and isotropic or, alternatiely, anisotropic alf space. In some cases te compressible layer of limited tickness was also considered. Geometry of te problem is sown in Fig. 17. Te obtained results of te penetration tests illustrate te effect of water content on te stress deformation relationsip and confirm te alidity of te consolidation test results. On seeral locations field load tests were performed in order to establis te relation between stresses and displacements. Typical results are presented in Fig. 16. Fig. 17. Geometry of te problem Relatie rigidity K of te foundation is gien by: 3 ( 1 μs ) E b 3 ( 1 μ ) E B K = () 6 b were: E b = modulus of elasticity of concrete, E s = modulus of elasticity of soil, μ b = Poisson s ratio of concrete, μ s = Poisson s ratio of soil, = tickness of strip foundation and B = widt of foundation. In te finite element analysis, in te case of te elastic and isotropic soil mass, te stiffness matrix is gien by: s Fig. 16. Field load test results Cures A are related to te loess deposits wit γ d = kn/m 3 and natural water content w = 0.5 %. Cures B were obtained for loess deposits wit γ d = kn/m 3 and natural water content w = %. Cure C indicates load settlement relationsip for loess wit γ d = kn/m 3 and water content w = %. One may say tat in all cases water content is practically almost te same, wereas te initial dry density aries between te ery low and ery ig alues. Te presented cures indicate te influence of te initial dry density on te amount of subsidence, wat is in a good agreement wit te results sown erein. THEORETICAL STUDY Te problem of calculating te stresses and displacements was studied by te finite element metod and by Fourier s series. In te teoretical consideration it was assumed tat te strip foundation was perfectly rigid, perfectly flexible, including also te foundation of any rigidity. It was also assumed tat 1 μ μ 0 E (3) D e = μ 1 μ 0 ( 1+ μ)( 1 μ) 1 μ 0 0 and in te case of an anisotropic soil te stiffness matrix is gien by te following expression: D e = ( 1+ μ ) ( 1 μ kμ ) ( kμ ) kμ ( 1+ μ ) k 1 kμ 0 ( 1+ μ ) E 1 μ 0 ( )( ) + μ 1 μ kμ m 1 were: E = modulus of elasticity in te ertical direction, μ = Poisson s ratio in te ertical direction, E = modulus of elasticity in te orizontal direction, μ = Poisson s ratio in te orizontal direction, k = E /E, m = G /E and G = sear modulus in te ertical direction. 0 0 (4) Paper No..65 6

8 Te dimensionless coefficients I for te calculation of componental stresses and displacements ae been determined wit arious alues of Poisson s ratio, wit seeral alues of foundation rigidity and for two degrees of anisotropy. Some of te obtained results are presented grapically. More results can be found elsewere (Miloic and Djogo, 005; Miloic, 199; Miloic and Djogo, 005). In Fig. 18 are sown te coefficient I w for seeral alues of te ratio H/B, were H is te tickness of te compressible layer and B is te widt of te flexible strip foundation. Fig. 1 sows te coefficients I w for isotropic and anisotropic soil wit te Poisson s ratio μ = Fig. 1. Coefficients I w ; μ = 0.30 Te coefficients I w, determined for arious alues of rigidity K, are sown in Fig.. Fig. 18. Coefficients I w ; n = 1 In tis case it was assumed tat te soil was elastic and isotropic, wit n = E /E = 1. In Figs. 19 and 0 are presented te alues of coefficient I w for te anisotropic soil wit n = 0.50 and n =.0, respectiely. Fig.. Coefficients I w ; strip foundation for arious aluse of rigidity K Fig. 19. Coefficients I w ; n = 0.5 Using te presented cures it is possible to determine te differential settlements of te strip foundation of any rigidity K. Te problem related to te determination of stresses and displacements in te elastic and isotropic soil of finite tickness as also been studied by Fourier s series. Componental displacements u and w, produced by a ertical uniform load oer a flexible strip, are expressed by te following trigonometric series: Fig. 0. Coefficients I w ; n = u = U(z, m) sin( α x) (5) m= 0 w = W z,m m= 0 ( ) cos( α x) (6) Paper No..65 7

9 m π α = (7) L x By deeloping tese displacements functions in te Fourier s series te solution for componental stresses and displacements as been obtained. Using tese solutions dimensionless coefficients ae been determined for seeral alues of te ratio H / B and for four alues of te Poisson s ratio. Some of te obtained results are sown in Figs. 3 and 4. It is of interest to empasize tat te results obtained for stresses and displacements by finite element metod and by Fourier s series are in a ery good agreement. PREDICTED AND MEASURED SETTLEMENTS Te measured settlements of twenty 1 story buildings oer a relatiely long period of time sow tat in some cases tere is no acceptable agreement between te calculated and obsered settlements. On te basis of te aailable results of measurements it was concluded tat for fie buildings te measured settlements were considerably larger tan te predicted alues. Howeer, in oter cases te calculated settlements were in reasonable agreement wit te measured alues. On te location of eac building seeral boreoles were performed, reacing te dept of approximately 0 m, and te undisturbed samples of loess were taken from arious dept wit tinwall piston. Fig. 3. Coefficients I z for flexible strip foundation As can be seen, te ertical stresses σ z are bigger tan tose obtained by Boussinesq s alf space. In Fig. 4 are presented te alues of te coefficient I w for seeral alues of H/B and for tree alues of Poisson s ratio μ. Te laboratory test results ae sown tat te soil profile was uniform and tat te foundation subsoil consisted of approximately 0 m macroporous landloess underlain by stiff clay. Dry density γ d of piston samples aried from 15.8 to 16.6 kn/m 3 and natural water content w from 13. % to 17 %. Using tese data it was obtained tat te calculated settlements were ρ = 7 10 cm. On te basis of tese acceptable alues te firm responsible for te constructions assumed te same system on strip foundations for all buildings. Te Building A was performed witout serious problems, and te settlements stayed in te frame of te calculated alues. Howeer, it is important to note tat te excaation for foundation pit was not additionally wetted. Tanks to te uncanged water content in te foundation subsoil, te alues of te differential settlements were acceptable, and in tis case tere was no reported damage. In Fig. 5 are presented te alues of te measured settlements for eac corner of te building. Fig. 4. Coefficients I w for flexible strip foundation Fig. 5. Time settlement cures; Building A Paper No..65 8

10 As can be seen, te alues of te measured settlements are sligtly bigger tan te calculated alues, because tey were determined wit te iger alues of γ d, obtained on piston samples. Taking into account tat te piston samples of loess were mecanically disturbed (Miloic, 1971), in te icinity of te Building B seeral pits were excaated and te undisturbed loess samples, cut from and cured blocks, were obtained. Te alues of dry density were γ d = kn/m 3 and water content w = %. It is wort noting tat tese alues are considerably lower tan γ d ~ 16 kn/m, obtained for piston samples. Te obsered settlements of Building B sow tat te total settlement and particularly te differential settlements were excessiely large. In Fig. 6 are sown te alues of te measured settlements. Fig. 7. Cone penetration test results Fig. 6. Time settlement cures; Building B Te Building C, founded on loess subsoil, represents one anoter type of beaiour, quiet different from te buildings A and B. In tis case te measured settlements are muc bigger tan te predicted alues. In Fig. 8 are sown te alues of tese settlements. As can be seen, in te first period of exploitation te settlements of te bencmarks 3 and 4 were larger tan tose wit bencmarks 1 and. Te amount of settlements was 0 to 30 cm, wit te differential settlement of about 10 cm. On te basis of te aailable information and tecnical documents it was concluded tat for some buildings te excaated foundation pit was open during te rainfall and te water penetrated in te loess subsoil and drastically canged te natural water content. Te results of te additional cone penetration tests, performed in loess soil wit natural water content and in te saturated loess are sown in Fig. 7. Te canalization pipes and water piping system were damaged by te differential settlements, prooking te additional infiltration of water into te loess subsoil. Tis cange of water content caused a considerable increase of settlements in te area of bencmarks 3 and 4 up to 5 cm, and also te increase of differential settlements up to te alue of approximately 7 cm. Tese settlements caused seere damages of building B. Te reparation of walls, stairs, eleators and oter damaged parts of building required a considerable amount of time and money. Fig. 8. Time settlement cures; Building C Te excaation of te foundation pit probably was uniformly wetted by intensie rainfall and te settlements were ery large and reacing 41 to 46 cm. Howeer, te differential settlements remained between te acceptable limits, wic did not cause seere damages. Paper No..65 9

11 CONCLUSIONS Loess deposits are ery sensitie to te mecanical disturbance, and te inadequate metod of sampling can lead to wrong laboratory results, wic are often on te unsafe side. Te sensitiity of loess to subsidence due to wetting or saturation depends to a large extent on te initial dry density, initial water content and stress leel during saturation. Te prediction of te beaiour of structures founded on loess deposits must be based on te settlement calculation in natural moisture conditions and also in wetted or saturated state. In settlement calculation te anisotropic properties of loess must be taken into account. Te selection of design geotecnical parameters is an essential component of design. Tis case study empasized once more tat te metod of calculation may play less significant role in settlement prediction tan te selection of te geotecnical parameters. Te use of sallow foundations as well for small or ig buildings founded on loess is always risky because, sooner or later, water can penetrate into foundation subsoil. In te extreme case saturation can cause inclination or een collapse of te structure, or seriously endanger its sericeability. REFERENCES Lariono, A.K. [1965]. Structural Caracteristics of Loess Soils for Ealuating teir Constructional Properties, Proc. Sixt Int. Conf. Soil Mec. Found. Engrg, Montreal, Vol. 1, pp Miloic, D. [1969]. Engineering Properties of Loess and Oter Collapsible Soils, Seent Int. Conf. on Soil Mec. and Found. Engrg., Mexico, Vol. 3, pp Miloic, D. [1971]. Effects of Sampling on Some Soil Caracteristics, Amer. Society for Testing and Materials. Special Tecn. Publ., Toronto, No 483, pp Miloic, D. [1978]. Loess Soils in Yugoslaia, Japanese Soil Mec. and Found. Engrg., Tokyo, Vol. 6, pp (in japanese). Miloic, D. [1980]. Some Laboratory and Field Test Results on Loess in Yugoslaia, Proc. Sixt Danube European Conf. on Soil Mec. Found. Engrg. Varna, Bulgaria, Vol. 1B, pp Miloic, D. and M. Djogo [1989]. Contact Stresses for Strip Foundation of any Rigidity, Proc, Second Int. Conf. of Found. and Tunnels, London, Vol. 1, pp Miloic, D. [199]. Stresses and Displacements for Sallow Foundations, Elseier Science Publisers, Amsterdam, Te Neterlands. Miloic, D. and M. Djogo [005]. Errors in Foundation Engineering, Faculty of Tecnical Sciences, Noi Sad, Serbia (in serbian). Moretto, D. and A. J. Bolognesi [1957]. Properties and Beaiour of Silty Soils Originated from Loess Formations, Proc. Fourt Int. Conf. Soil Mec. Found. Engrg., London, Vol. 1, pp Nortey, R. D. [1969]. Engineering Properties of Loess and Oter Collapsible Soils, Proc. Seent Int. Conf. Soil Mec. Found. Engrg., Mexico, pp Lutenegger, A., G. Hallberg and R. Handy [1979]. Reiew of Geotecnical Inestigations of Loess in Nort America, Int. Union for Quarternary Researc, Brussels. Paper No