Guojun Cai

Predictions of coefficient of consolidation from CPTU dissipation tests in Quaternary clays

The coefficient of consolidation (cv or ch) is an important parameter in both geotechnical and geo-environmental engineering, measuring the flow characteristics of soils. In China, conventionally it is obtained from the oedometer test, which is time consuming and of limited accuracy due to inevitable sample disturbance. These difficulties can be overcome by in situ pizeocone dissipation tests which provide continuous measurements of pore water pressure with time. In this paper, existing methods for interpreting coefficient of consolidation in clays from piezocone dissipation tests are briefly reviewed. Piezocone tests were undertaken at different sites in the Jiangsu province of China and piezocone dissipation tests conducted at different penetration depths. Based on the reference cv values determined from the laboratory oedometer tests, comparisons with existing interpretation methods were undertaken. It is shown that the Teh and Houlsby’s interpretation method has a much higher accuracy for determining the coefficient of consolidation of Quaternary clay deposits while the values of ch obtained by the CPTU method are in the range of back-analyzed field values.RésuméLe coefficient de consolidation (Cv ou Ch) est un paramètre important en géotechnique et génie géologique. Il est obtenu conventionnellement en Chine par l’essai oedométrique qui demande beaucoup de temps et dont la précision est limitée du fait du remaniement inévitable des échantillons. Ces difficultés peuvent être surmontées par l’essai in situ au piézocône qui donne des mesures continues de la dissipation des pressions interstitielles en fonction du temps. Dans cet article, les méthodes permettant la détermination du coefficient de consolidation dans des argiles à partir d’essais au piézocône sont brièvement passées en revue. Les essais au piézocône ont été réalisés sur différents sites de la province de Jiangsu en Chine et pour différentes profondeurs dans le terrain. Considérant des valeurs de référence de Cv déterminées au laboratoire à partir d’essais oedométriques, des comparaisons avec différentes méthodes d’interprétation ont été réalisées. On montre que la méthode de Teh et Houlsby permet d’obtenir le coefficient de consolidation avec une très grande précision pour des dépôts argileux du Quaternaire tandis que les valeurs de Ch obtenues avec la méthode CPTU se présentent dans la plage de valeurs de terrain obtenues par rétro-analyse.

Assessment of the Coefficient of Lateral Earth Pressure at Rest (Ko) from In Situ Seismic Tests

The coefficient of the earth pressure at rest (Ko) is an important soil parameter that influences shear strength, stress-strain behavior, and compressibility characteristics of both cohesive and cohesionless soils. This paper presents a research study in which the Ko values of natural soil deposits are interpreted on the basis of seismic shear wave velocity measurements made at two research sites in Jiangsu province of China. Seismic piezocone penetration tests and cross-hole seismic logging tests were performed at both sites and these results have been used in two existing Ko-shear wave velocity models for prediction of Ko. The predicted Ko at both sites are compared with values determined utilizing Jaky’s formula via laboratory measured strength parameters. Certain variations between predictions and measurements are noted for soils at shallow depths, which are attributed to stress history and desiccation in these layers. Further analysis developed correction factors that accounted for both soil types and overconsolidation ratio effects. These factors can be used with original models for better interpretation of Ko for overconsolidated clays. This approach showed a reasonable match between interpreted Ko properties from both models and those that utilize laboratory test results.

Retesting of Liquefaction and Nonliquefaction Case Histories from the 1976 Tangshan Earthquake

A field investigation was performed to retest liquefaction and nonliquefaction sites from the 1976 Tangshan earthquake in China. These sites were carefully investigated in 1978 and 1979 by using standard penetration test (SPT) and cone penetration test (CPT) equipment; however, the CPT measurements are obsolete because of the now nonstandard cone that was used at the time. In 2007, a modern cone was mobilized to retest 18 selected sites that are particularly important because of the intense ground shaking they sustained despite their high fines content and/or because the site did not liquefy. Of the sites reinvestigated and carefully reprocessed, 13 were considered accurate representative case histories. Two of the sites that were originally investigated for liquefaction have been reinvestigated for cyclic failure of fine-grained soil and removed from consideration for liquefaction triggering. The most important outcome of these field investigations was the collection of more accurate data for three nonliquefaction sites that experienced intense ground shaking. Data for these three case histories is now included in an area of the liquefaction triggering database that was poorly populated and will help constrain the upper bound of future liquefaction triggering curves.

Evaluation of Pile Bearing Capacity from Piezocone Penetration Test Data in Soft Jiangsu Quaternary Clay Deposits

This article mainly focuses on the piezocone test results and the related interpretation procedures to predict ultimate pile load capacities of driven and jacked piles for the Jiangsu soft soil deposits. First, the available literature on the evaluations of CPTU methods was collected and reviewed. The new CPTU-based method has been proposed to predict the ultimate capacity from effective cone tip resistance to excess pore water pressure. Second, this article presents an analysis of these pile load tests and correlations to neighboring piezocone data. Four CPTU-based methods are evaluated using the ultimate pile load capacities, shaft friction, and end bearing capacity. The results of a comparison of the calculated pile capacities to the measured pile capacities are very favorable to the new CPTU method, which shows better agreement with the capacity determined in a static loading test and less scatter than other CPTU methods.

Review of ground improvement using microbial induced carbonate precipitation (MICP)

ABSTRACT Mechanical compaction and chemical grouting are the two most widely used methods in geotechnical engineering for ground improvement. However, the disadvantages of the methods including high cost, high energy consumption, and potential environmental pollution are obvious. A new ground treatment technique, i.e., microbial induced carbonate precipitation, was developed recently. Review of ground improvement using microbial induced carbonate precipitation technique was performed in this study. The mechanism of microbial induced carbonate precipitation-treated soils was first introduced followed by the review of many aspects of the microbial induced carbonate precipitation technique. Thereafter, the related engineering applications were presented and summarized. Moreover, some problems existing in current theoretical and experimental studies as well as the possible solutions for future development were also discussed. Some recommendations were proposed for a wider application of this technique.

Evaluation of subsurface spatial variability in site characterization based on RCPTU data

The resistivity piezocone penetration test (RCPTU) does not only include a conventional piezocone penetration test, but also provides a continuous profile of electrical resistivity. Therefore, RCPTUs have the potential for wide application in environmental geotechnics. Electrical resistivity is a fundamental parameter characterizing soil conductivity, one of the inherent physical properties of soil. The objective of this study was to evaluate subsurface spatial variability using RCPTU data and laboratory tests. Normalized electrical resistivity was proposed for developing the relationships between electrical resistivity and soil behavior type index, relative density and cyclic resistance ratio. These relationships are used for evaluation of subsurface spatial variability. The soil behavior type index can be represented by the function of normalized electrical resistivity R and normalized cone tip resistance. Additionally, there is a positive correlation between relative density and normalized electrical resistivity. The evaluation of cyclic resistance ratio adopting the normalized electrical resistivity and normalized cone tip resistance was effective.

Stabilization Mechanism and Effect Evaluation of Stabilized Silt with Lignin Based on Laboratory Data

The potential of a lignin-based by-product to stabilize silt was evaluated. The physical and mechanical properties of silt in its natural state, as well as when treated with varying proportions of lignin, were analyzed. The parameters tested include the particle size distribution, Atterberg limits, compaction characteristics, unconfined compressed strength, pH value, and electrical resistivity. To understand the stabilization mechanism of lignin-treated silt at a microscopic level, scanning electron microscopy (SEM), X-ray diffraction (XRD) analysis, and Fourier transform infrared resonance (FTIR) spectroscopy were also carried out on lignin and representative samples after 28 days of curing. The results indicate that the engineering properties of silt are improved by the addition of lignin. Particle size distribution is changed and plastic index is reduced from 8.8 to 7.7. After improvement, the maximum dry density increases and the optimum moisture content decreases, while the change of dry density with moisture content is enhanced. The treated silt has greater strength performance than the natural silt in terms of unconfined compressed strength and all of the samples have a pH value lower than 10. Curing time and moisture content have a significant impact on unconfined compressed strength but almost no effect on pH. Micro-chemical analysis reveals that the improvement of performance exhibited by lignin-treated silt may be mainly attributed to the cation exchange and the formation of more stable soil structure by lignin cementing. The stabilization mechanism of lignin-treated silt was proposed according to the results of chemical analysis. It is shown that lignin-based stabilizers have potential to improve the engineering properties of silt.

Identification of Soil Strata Based on General Regression Neural Network Model From CPTU Data

In geotechnical engineering, assessment of the depth location of stratigraphic interfaces and the depth and thickness of thin layers can be critical in the design process. For example, stratigraphic interfaces can promote anisotropic soil strength response and potentially provide preferential slip planes that create slope instability. The piezocone penetration test (CPTU or PCPT) is an extension of the cone penetration test (CPT) and is able to measure cone tip resistance, sleeve friction, and generated pore-water pressures simultaneously. In this paper the relationship between CPTU parameters and soil types and strata is analyzed, and the structure of a general regression neural network (GRNN) is designed for soil classification and soil strata identification. This research discusses soil strata automation identification, supplies a new way to deal with the CPTU data, and has actual significance in promoting efficiency and precision of CPTU data processing. The example results are presented and verified using backup laboratory and in situ data for case study to identify soil strata by CPTU and have confirmed that GRNN can be used to carry out automatic soil strata identification. GRNN-based model was found to be correlating well for the 87% of the cases with the USCS classification system results.

Safety Assessment of Buried Pipeline during Pile Driving Vibration in Offshore Engineering

To assess the impact of pile driving on adjacent submarine pipelines during the reconstruction of a pier berth, the local damage model of submarine pipelines is established to explore the safety thresholds of the particle peak velocity and horizontal displacement. The results are analyzed and adjusted by the existing standards and the corresponding literatures. Then, a three-dimensional numerical model is presented to assess the feasibility of the construction of piles by the obtained safety limits, in which the nonlinear behavior of the soil and stress–seepage coupling analysis are considered. After the construction, the safety of submarine pipelines is rechecked by the measured value of the particle peak velocity and horizontal displacement. Meanwhile, the propagation law of vibration, the horizontal displacement of underground soil, and the pore pressure during pile driving are explored. The results indicate that the construction of piles of 2# mooring pier did not cause damage to adjacent submarine pipelines. However, the construction of piles of 1# mooring pier which is nearer may cause damage to submarine pipelines.

Consolidation Parameters Interpretation of CPTU Dissipation Data Based on Strain Path Theory for Soft Jiangsu Quaternary Clays

When a piezocone (CPTU) penetrates the ground, pore pressures are set up due to the stress changes. Pore pressure dissipation occurs in the subsequent consolidation process in a manner dependent on the initial stress distribution. The process of analysis can be used to interpret the consolidation parameters from CPTU data based on strain path approach. For the two approaches to the CPTU analysis, the Houlsby and Teh method used an elastic-plastic model, while the Levadoux and Baligh solution used parameters specifically appropriate for Boston Blue Clay. This article discusses a comparison between the two theoretical predictions and field CPTU measurements deployed in Jiangsu (China) Quaternary clays. The values of the coefficient of consolidation calculated from the two methods are then compared with high-quality laboratory tests. Both theoretical predictions provide good agreement for the shape of the dissipation curve, but the predicted c h values are different and thought should indeed be given to the use of an appropriate value of G/s u.