Estimating undrained strength of an alluvial soil using resedimented samples prepared from disturbed samples


Sünnetci M. O., Ersoy H.

2. International Conference on Advances and Innovations in Engineering, Elazığ, Turkey, 21 - 23 September 2023, pp.23-24

  • Publication Type: Conference Paper / Summary Text
  • City: Elazığ
  • Country: Turkey
  • Page Numbers: pp.23-24
  • Karadeniz Technical University Affiliated: Yes

Abstract

In this study, an estimation model was developed to determine the shear strength parameters of disturbed soil samples using resedimented samples. In this context, plastic and index property determinations and UU triaxial compressive strength tests were carried out on undisturbed alluvial soil samples obtained by core drillings from Bafra district of Samsun city, Türkiye. 5 boreholes, each 10 m deep, were drilled in the study area and undisturbed soil samples were collected using custom-made 9 cm diameter, 60 cm long (maximum sample length is about 50 cm), thin-walled UD samplers. To minimize sample disturbance, no drilling fluid was used, and the samplers were driven into the soil by hydraulic pressure without rotation. Groundwater was encountered at a depth of 2 to 3 meters, and since the resedimented samples would be fully saturated, undisturbed samples were collected between 4 and 10 meters to ensure maximum saturation. By collecting a single undisturbed sample every 50 cm, a total of 60 undisturbed samples were collected from all 5 boreholes (12 samples from each borehole). Since it would be practically difficult to obtain 3 triaxial test specimens (needed to determine cohesion and internal friction angle) from one UD sampler, a single triaxial test specimen was obtained from each sampler. In this way, 3 specimens obtained from 3 consecutive UD samplers were considered as a single set of samples. The remaining part of the specimens in each UD sampler were used to determine their plastic and index properties. Natural water content, natural unit weight, specific gravity, void ratio, saturation degree, Atterberg limits, and grain size distribution of all samples are determined by laboratory tests conducted conforming to the corresponding international standards, or by using phase relation equations among the parameters determined during the tests. Additionally, undrained shear strength (s↓u) and shear strength parameters (cohesion, c and internal friction angle, ϕ) of the samples are also determined by unconsolidated-undrained (UU) triaxial tests conducted using a servo-controlled semi-automatic testing system. The triaxial tests were carried out under 50, 100, and 200 KPa of confining pressures with a 0.5 mm/min shear rate until 20% strain is occurred. Ultimately, 20 sample sets were created with known index, plastic, and strength properties. These samples, which were disturbed after the initial experiments, were resedimented in the second stage of the study by reflecting their original characteristics in the field. Consolidometers with a diameter of 9 cm, designed and produced within the scope of this study, were used in the resedimentation process. During the resedimentation process, each sample was subjected to a consolidation stress equal to the burden stress at the depth from which it was taken from the boreholes. The disturbed soil sample was dried in an oven at 105°C for 24 hours. The dried sample was ground in a ring grinder to pass through a #100 sieve. Approximately 1 kg of material was ground for each resedimented sample to be prepared. The ground soil sample was mixed with deaired distilled water of 2 times its initial liquid limit in weight. To obtain a homogeneous mixture, an attempt was made to mix the slurry with a laboratory mixer for 10 minutes. However, due to the high degree of agglomeration in the samples prepared in this manner, the mixing process was performed manually. The prepared homogeneous slurry was slowly poured into a consolidometer with filter paper on the bottom to avoid air bubbles. The slurry was left to stand for 24 hours. At the end of 24 hours, a piston with filter paper on the bottom was placed on top of the sample and the top cover of the consolidometer was closed tightly. The loading arms were attached to the top cover and a load of 0,5 kg (approximately 2.5 KPa of stress) was initially applied to the sample. The load was then gradually increased by doubling the load on the specimen until the desired consolidation stress was reached. Each loading step was applied for 24 hours. The final loading step was applied for 48 hours. Once the loading process was complete, the load on the sample was removed while holding the piston stationary. The UU triaxial compressive strength tests were repeated on the resedimented samples. Multivariate regression (MVR) analyses were performed on the physical and mechanical properties of undisturbed and resedimented samples to predict the cohesion, internal friction angle, and undrained shear strength of undisturbed samples. Multivariate regression analyses were performed using SPSS® v23 software and the significance of each independent variable was also tested. Independent variables with low significance coefficients were removed from the analysis and the analyses were repeated to develop empirical equations with high correlation coefficients (R) and as few parameters as possible. Again, for ease of application, linear equations were preferred to logarithmic or exponential equations unless there was a large difference in R values. As a result of the MVR analyses, 6 equations were obtained for undrained strength, cohesion, and internal friction angle with highest coefficient of correlation values of 0.803, 0.817, and 0.850, respectively. When the statistical analyses are evaluated, it can be seen that the MVR analyses produced equations based on a maximum of 3 different parameters and with high correlation coefficients (0.795 and above). Using these equations, the shear strength parameters of the undisturbed samples can be estimated with high accuracy using the shear strength parameters of the resedimented samples. Keywords: Resedimentation, disturbed sample, undrained shear strength, cohesion, internal friction angle, multivariate regression