Atividade - Mecânica dos Solos 2

PROBLEMS 271 The clay layer shown in Fig P271 has drainage at both its top and bottom surfaces The coefficient of consolidation cv is 5 ft²yr A surface loading of 1000 lbft² is applied to this site by placing a fill a Make a neat drawing showing 1 The distribution of pore pressure before the fill is placed 2 The distribution of pore pressure immediately after the fill is placed 3 The distribution of pore pressure 6 yrs after the fill is placed b How long will it take for the average consolidation of the layer to exceed 90 272 After consolidation under the load in Problem 271 is complete the piezometric level in the lower sand stratum is lowered 15 ft by pumping while the piezometric level in the upper sand strata remains unchanged Answer all questions asked in Problem 271 Does the clay layer compress or expand as a result of this change 273 The soil of Fig P271 is subjected instantaneously to a loading of 1000 psf Six years later this load is removed instantaneously Assuming cv is the same for both loading and unloading construct plots of the following quantities versus time a Vertical total stress at middepth of the clay b Pore pressure at middepth of the clay c Vertical effective stress at middepth of the clay d Average excess pore pressure within the clay 274 Considering the difference in the behavior of soil during loading and unloading answer the following questions relative to Problem 273 a Will the excess pore pressures after unloading dissipate more or less quickly than dissipation after the initial loading 1 10ft Sand 10ft 20ft Clay Sand Fig P271 422 PART V SOIL WITH WATERTRANSIENT FLOW b Will the final thickness of the clay be the same as or more or less than the initial thickness 275 The soil in Fig P271 is subjected to a load which increases linearly with time for 1 year until the load reaches 1000 psf and thereafter the load remains constant Construct the curve of average consolidation ratio versus time 276 Assume that the clay used for the test results shown in Fig 274 came from a stratum 60 ft thick with double drainage How long will it take for the entire stratum to consolidate T 1 277 A clay with cv 103 cm²sec is formed into a triaxial specimen 5 cm in diameter and 15 cm long Full drainage is provided at both ends and around the cylindrical surface Neglecting the effect of changes in total stress during consolidation how long will be required to achieve 95 consolidation following an increment in chamber pressure Solucoes Comentadas dos Exercıcios 271 a 277 Mecˆanica dos Solos Adensamento Exercıcio 271 a A pressao neutra antes da aplicacao da carga e dada pela profundidade e o peso especıfico da agua Apos a aplicacao de uma carga superficial de 100 kPa o acrescimo de pressao neutra ocorre de forma instantˆanea e uniforme pois a argila esta saturada e o comportamento inicial e nao drenado O grafico a seguir mostra essas duas situacoes b Para calcular o tempo necessario para que a pressao de poros no centro da camada caia para 52 kPa usamos o conceito de grau de adensamento U que indica a fracao do excesso de pressao de poros ja dissipada Usando a solucao de Terzaghi para adensamento unidimensional temos u0 100 kPa pressao de poros inicial u 52 kPa valor desejado U 1 u u0 1 52 100 048 Tv 0069 obtido por tabela para U 048 Hdr 4 m 400 cm drenagem dupla Cv 15 103 cm2s t Tv H2 dr Cv 0069 4002 15 103 736 106 s 85 dias Conclusao O tempo necessario e de aproximadamente 85 dias 1 Exercıcio 272 Neste caso desejase encontrar o tempo necessario para que a pressao de poros caia para 60 de seu valor original Isso implica um grau de adensamento de 40 U 1 060 040 Tv 0031 por tabela Hdr 25 m 250 cm Cv 002 cm2s t 0031 2502 002 96875 s 269 horas Conclusao Aproximadamente 27 horas para que a pressao de poros caia a 60 de seu valor inicial Exercıcio 273 Sabendose que 90 do adensamento ocorre em 15 anos queremos determinar o coefi ciente de adensamento Cv Tv 0848 para U 90 t 15 365 24 3600 473 106 s Hdr 4 m 400 cm Cv Tv H2 dr t 0848 4002 473 106 114 103 cm2s Conclusao Cv 114 103 cm2s Exercıcio 274 Desejase calcular o tempo para que ocorra 50 de adensamento U 050 Tv 0197 Hdr 25 m 250 cm Cv 002 cm2s t 0197 2502 002 615625 s 171 horas 71 dias Exercıcio 275 Com U 75 Tv 0405 Cv 00035 e Hdr 300 cm t 0405 3002 00035 104 106 s 120 dias 2 Exercıcio 276 A partir do tempo e grau de adensamento calculamos Cv U 60 Tv 0287 t 10 365 24 3600 3154 106 s Hdr 300 cm Cv 0287 3002 3154 106 261 104 cm2s Exercıcio 277 Com Tv 020 Hdr 200 cm e Cv 00025 t 020 2002 00025 32 106 s 37 dias Conclusao O tempo estimado para 20 de adensamento e de 37 dias 3