Study on strength properties and soil behaviour type classification of Huanghe River Delta silts based on variable rate piezocone penetration test

Fine-grained silt is widely distributed in the Huanghe River Delta(HRD)in China,and the sedimentary structure is complex,meaning that the clay content in the silt is variable.The piezocone penetration test(CPTu)is the most widely approved in situ test method.It can be used to invert soil properties and interpret soil behavior.To analyse the strength properties of surface sediments in the HRD,this paper evaluated the friction angle and its inversion formula through the CPTu penetration test and monotonic simple shear test and other soil unit experiments.The evaluation showed that the empirical formula proposed by Kulhawy and Mayne had better prediction and inversion effect.The HRD silts with clay contents of 9.2%,21.4%and 30.3%were selected as samples for the CPTu variable rate penetration test.The results show as follows.(1)The effects of the clay content on the tip resistance and the pore pressure of silt under different penetration rates were summarized.The tip resistance Q_t is strongly dependent on the clay content of the silt,the B_(q)value of the silt tends to 0 and is not significantly affected by the change of the CPTu penetration rate.(2)Five soil behavior type classification charts and three soil behavior type indexes based on CPTu data were evaluated.The results show that the soil behavior type classification chart based on soil behavior type index ISBT,the Robertson 2010 behavior type classification chart are more suitable for the silty soil in the HRD.

Using Piezocone Tests for Analysis of Phreatic Water Conditions and Prediction of Soil Behavior in Tailing Dams

In this work the possibility of identifying two important aspects in the process of adopting soil parameters for calculating stability analysis models in tailing dams is discussed. The use of commercial computer programs for stability calculations allows obtaining numerically exact results. Its representativeness, however, will be linked to the correct definition of the phreatic regime and to the prediction of volumetric soil behavior during shearing (contractile vs. dilating materials). The theoretical principles for the selection of soils parameters for different failure models are briefly presented. Also, how the incorrect assumptions regarding material behavior can significantly affect the estimation of tailing dams’ stability. The results of CPTu tests for the diagnosis of the phreatic and mechanical condition of the materials are discussed and two examples are presented to remark on the care that should be taken to avoid incorrect soils parameters adoption.

A case study on the soil classification of the Yellow River Delta based on piezocone penetration test

Piezocone penetration test(CPTu),the preferred in-situ tool for submarine investigation,is significant for soil classification and soil depth profile prediction,which can be used to predict soil types and states.However,the accuracy of these methods needs to be validated for local conditions.To distinguish and evaluate the properties of the shallow surface sediments in Chengdao area of the Yellow River Delta,seabed CPTu tests were carried out at ten stations in this area.Nine soil classification methods based on CPTu data are applied for soil classification.The results of classification are compared with the in-situ sampling to determine whether the method can provide sufficient resolution.The methods presented by Robertson(based on soil behavior type index Ic),Olsen and Mitchell are the more consistent and compatible ones compared with other methods.Considering that silt soils have potential to liquefy under storm tide or other adverse conditions,this paper is able to screen soil classification methods suitable for the Chengdao area and help identify the areas where liquefaction or submarine landslide may occur through CPTu investigation.

Using Piezocone Dissipation Test to Estimate the Undrained Shear Strength (<i>s_u</i>), <i>N_kt</i>and <i>N_Δu</i>Factors in Cohesive Soils

The current practice of geotechnical engineering commonly uses a combination of theoretical and empirical correlations to estimate the soil undrained shear strength in clays from the piezocone test. In order to complement the use of such correlations, the application of a method to estimate the soil undrained shear strength, using measures of the excess pore pressure in dissipation tests of piezocone is presented. In cohesive soils, excess pore pressure and undrained shear strength are dependent on the same variables (stress state, stress history, soil stiffness), which allows them to be related by the theoretical cavity expansion-critical state framework. This paper mentions the mathematical formulation that supports the theoretical framework used, its relationship with the Nkt and NΔu factors and their estimation in a case studied. The results obtained are consistent within the dispersion found in the international literature and encourage the use of the method in engineering practice.

Methods for evaluating overconsolidation ratio from piezocone sounding results

Methods for estimating the overconsolidation ratio(OCR)of soil deposits from piezocone sounding results have been investigated.Three existing methods have been briefly reviewed and a new method has been proposed.The proposed method can be applied to both normally consolidated/overconsolidated and underconsolidated deposits.Furthermore,existing methods have been modified so that they can be applied to underconsolidated deposits.Then,all the methods have been applied to 12 case histories collected from 6 different countries.The estimated values of OCR have been compared with the measured data.It has been shown that if the parameters(soil properties and empirical parameters)can be determined appropriately,then all the methods can achieve a reasonable prediction.It has also been shown that the proposed method exhibits a relatively better performance and results in less scattered data than the other methods.

Site observations of weathered granitic soils subjected to cementation and partial drainage using SCPTU

Because of the cementation inherited from the parent rock,weathered granitic soil is usually susceptible to disturbance,which poses considerable challenges for laboratory characterization.The cone penetration test with pore pressure measurements has long been known for its reliability in site investigations and stratigraphic profiling.However,although extensive piezocone test results and experience are available for sedimentary soil,similar advances are yet to be made for weathered granitic soil.Moreover,the experience from sedimentary soil may not be directly applicable to weathered profiles because of the essentially different natures of the two types of geomaterials.This study performs seismic piezocone tests in a weathered granitic profile comprising residual granitic soil,completely weathered granite,and highly weathered granite.Pore pressure is measured at both the cone mid-face and the shoulder,and the effects of penetrometer size and penetration rate are considered.A series of updated soil behavior type charts is proposed to interpret the test results,thereby allowing the effect of weathering to be evaluated.This paper offers an important extension to the sparse data on the in situ responses of weathered materials.

Undrained shear strength evaluation for hydrate-bearing sediment overlying strata in the Shenhu area, northern South China Sea

The undrained shear strength of shallow strata is a critical parameter for safety design in deep-water operations.In situ piezocone penetration tests(CPTU) and laboratory experiments are performed at Site W18-19 in the Shenhu area, northern South China Sea, where China's first marine hydrate exploitation operation is due to be located. The validation of the undrained shear strength prediction model based on CPTU parameters. Different laboratory tests, including pocket penetrometer, torvane, miniature vane and unconsolidated undrained triaxial tests, are employed to solve empirical cone coefficients by statistical and mathematical methods. Finally, an optimized model is proposed to describe the longitudinal distribution of undrained shear strength in calcareous clay strata in the Shenhu area. Research results reveal that average empirical cone coefficients based on total cone resistance, effective resistance, and excess-pore pressure are 13.8, 4.2 and 14.4, respectively. The undrained shear strength prediction model shows a good fit with the laboratory results only within specific intervals based on their compaction degree and gas-bearing conditions. The optimized prediction model in piecewise function format can be used to describe the longitudinal distribution of the undrained shear strength for calcareous clay within all depth intervals from the mud-line to the upper boundary of hydrate-bearing sediments(HBS). The optimized prediction result indicates that the effective cone resistance model is suitable for very soft to firm calcareous clays,the excess-pore pressure model can depict the undrained shear strength for firm to very stiff but gas-free clays,while the total cone resistance model is advantageous for evaluating the undrained shear strength for very stiff and gassy clays. The optimized model in piecewise function format can considerably improve the adaptability of empirical models for calcareous clay in the Shenhu area. These results are significant for safety evaluations of proposed hydrate exploitation projects.

Axial Load Capacity of Cast in Place Piles from SPT and CPTU Data

This paper aims to deal with the assessment of axial load capacity for cast in place pile foundations, which are made by the earth drill method, by using the data taken from Standard Penetration Tests (SPTs) and Piezocone Penetration Tests (CPTUs). These tests were carried out as part of the investigation program for P.N.G. Terminal-Power Plant, near Semani beach, in Hoxhara marsh, in the western part of Albania. The design of axial load capacity of piles is based on empirical formula using SPT and CPTU values. This study presents the results of axial load capacity analysis of cast in place piles by different analytical calculation methods, which are based on in situ tests results, and also referring to the Building Standard Law of Japan. In the end of our work, differences between calculations methods by using different in situ tests results are shown in tables and graphs.