Xueyou Li

Characterization of dual-structure pore-size distribution of soil

The microporosity structure of soil provides important information in understanding the shear strength, compressibility, water-retention ability, and hydraulic conductivity of soils. It is a soil characteristic that depends on sample preparation method and wetting–drying history. A comprehensive study of the microporosity structure of a lean clay with sand was conducted in this research to investigate variations of the microporosity structure during compaction, saturation, and drying processes. Scanning electron microscopy was used to observe the microporosity structure of soil sample surfaces. Mercury intrusion porosimetry was used to measure the microporosity structure quantitatively by showing the relationship between cumulative pore volumes and pore radius. The experimental results show that a dual-porosity structure (i.e., interaggregate pores and intra-aggregate pores) forms during the compaction process. The interaggregate pores are compressible and the associated volume is closely related to the f...

Microporosity Structure of Coarse Granular Soils

To date the microporosity structures of coarse soils with various coarse/fines contents are still not fully understood. In this study, the pore-size distributions (PSDs) of five types of soil varying from gravel to clay were characterized using mercury intrusion porosimetry. The soil with a coarse content below 70% (i.e., fines content above 30%) is found to have a fines-controlled microstructure, which is sensitive to water content changes. Such soil forms a dual-porosity structure due to compaction, in which both intraaggregate pores and interaggregate pores are dominant. After saturation, the dual-porosity structure evolves into a unimodal porosity structure dominated by the intraaggregate pores. During drying, such soil exhibits a significant reduction of total volume. The soil with a coarse content above 70% instead has a coarse-controlled microstructure, which is stable upon water content changes. Such soil maintains dual-porosity structures no matter if the soil is compacted, saturated, or dried. As an example of application, the measured PSDs are used to predict the soil water characteristic curves (SWCCs) for the test soils and the predictions are consistent with the SWCCs measured in the laboratory.

Using Conditioned Random Field to Characterize the Variability of Geologic Profiles

AbstractGeologic profiles cannot be identified with certainty due to inherent spatial variability and limited site-investigation data. Random field theory is a powerful tool for evaluating the spatial characteristics of soil and rock properties, but may overestimate the site variability if the site investigation data at borehole locations within the site are not considered. This paper aims to propose a method to generate a conditioned geologic random field based on available measurement information. It incorporates both direct and indirect information within the site to constrain the random field. The conditioned random field matches the measured data at the measurement locations, and has much-reduced uncertainty adjacent to the measurement locations due to spatial correlation. The proposed method is applied to evaluate the depth of Grade III rock surface at a construction site in Hong Kong using borehole data. With the proposed method, a more accurate description of the geologic profile can be obtained, ...

Development of a Modified Axis Translation Technique for Measuring SWCCs for Gravel Soils at Very Low Suctions

For gravel soils, water content changes at very low suctions (e.g., smaller than 1 kPa) are significant, and such changes affect the permeability and shear strength of the soils significantly. Difficulties in studying the soil-water characteristic curves (SWCCs) at very low suctions include the lack of a proper experimental device and the suction difference induced by the gravitational hydraulic gradient along the sample height. In this study, a large size modified axis translation device was developed. The device can control the soil suction accurately (i.e., a precision of 0.005 kPa) using a water-head control method. A new interpretation procedure was also proposed to consider the suction difference along the sample height and to extend the minimum measurable suction from 0.1 to 0.01 kPa. The device was used to measure the SWCCs for two gravel soils. The experimental results demonstrate that the SWCCs for gravel soils in the low-suction range show bimodal features, where the water content decreases sharply at suctions smaller than 1 kPa. Such bimodal behavior may not be revealed using conventional SWCC devices. SWCCs at low suctions are sensitive to soil density. Significant hydraulic hysteresis is also present in the SWCCs at low suctions.

Assessment of Slope Stability in the Monitoring Parameter Space

AbstractSlope monitoring is routinely conducted, and observational information such as surface/underground displacements, groundwater levels, and rock bolt forces at multiple locations is collected. How to make use of the monitoring information to reveal failure mechanisms and assess the slope stability is a key issue in slope engineering. This paper presents a method for assessing the slope stability by integrating monitoring parameters with physical analysis. The observed information first was used to back analyze the strength and loading parameters, and then the updated basic parameters were used to calculate the factor of safety or failure probability of the slope. The dominant basic parameters whose uncertainties influence the observed results the most were identified from the probabilistic back analysis. Alert levels were defined in the monitoring parameter space on the basis of a factor of safety or failure probability criterion. A rock slope example was worked out to illustrate the application of ...

Probabilistic Stratification Modeling in Geotechnical Site Characterization

AbstractStratification in geologic profiles is one of the most significant uncertainties in geotechnical site characterization. In this paper, a three-level probabilistic framework is proposed for ...

Reply to the discussion by Chen et al. on “Wetting front advancing column test for measuring unsaturated hydraulic conductivity”Appears in Canadian Geotechnical Journal, 47(10): 1159–1161.

The authors appreciate R. Chen, Z.H. Li, and J.H. Li for their interest in and valuable thoughts about the paper. Our paper presents an efficient method for measuring unsaturated hydraulic conductivity of soil. The proposed method utilizes water contents recorded at one monitoring section to calculate the hydraulic conductivity with the help of instantaneous wetting front advancing velocity. In the new method, a wetting zone (i.e., the zone between the monitoring section and the wetting front) is assumed to advance a distance of Dh in a small time step of Dt. In the wetting zone, the water content contours are assumed to be stationary. In this small time step, the water content change in the wetting zone is assumed to be very small compared with the water content change due to the advancement of the wetting zone, namely QB calculated using eq. [8] in Li et al. (2009). As mentioned in the Discussion, the water content profile advances along the soil column in a way similar to wave propagation. Such propagation is under constant boundary conditions both at the front and back of the wetting zone. In our experiments and the simulations in the Discussion, the water content at the front of the wetting zone is lower than the residual water content and the hydraulic conductivity at the front of the wetting zone is extremely low (e.g., lower than 10–13 m/s). Hence, the boundary conditions (i.e., the water content, suction, and hydraulic conductivity along the advancing path) at the front of the wetting zone are considered to be unchanged in the small time step. However, the boundary conditions at the back of the wetting zone vary during the wetting process. Consider a wetting zone with a suction equivalent to the air-entry value (AEV) at the back. If the wetting front advances a distance of Dh in Dt, the change in suction at the back of the wetting zone will be Du = gwDh where gw is the unit weight of water. The relative change of suction will then be Du/AEV. If the testing material is a coarse sand or gravel, the AEV is usually small and Du/AEV could be large. Such a large change induced by gravity in the boundary condition at the back of the wetting front will change the wetting front advancing process. For example, in the capillary experiment conducted on GW-GM with sand, the observed wetting front could only reach 113 mm above the water table. This is because the dominant pores in the GW-GM with sand are large, i.e., above 1 mm (Zhang and Li 2010). Similar problems should be considered in the numerical simulation of flow in uniform sand. Referring to Fig. D1 in the Discussion, the AEV of the uniform sand is about 3 kPa (i.e., 300 mm water head). During the capillary rise process, the final water content in a wetted section of the sand column will decrease gradually when the height is above 300 mm, approaching the water content at the hydrostatic state. Hence, the boundary condition at the back of the wetting zone changes during the capillary process. For such a soil, the wetting front advancing experiment should be carried out in a horizontal soil column, in which the effect of gravity on the boundary condition is less significant. For clays, the AEV values are likely high (say, above 100 kPa or 10 000 mm head); the change of suction, Du, at the back of the wetting zone is negligible. In this case, the boundary condition at the back of the wetting zone can be considered to be constant. Referring to Fig. D2 in the Discussion, the wetting front advancing velocities are indeed different if different water contents are used to define the wetting front. Let us consider the wetting front advancing velocities in silty clay in Fig. D2 at any instant. The slopes of the three curves may differ by 2 times at >3 104 s. That is to say, the calculated hydraulic conductivity may differ within 2 times when different definitions of the wetting front are used. The error is in reality much smaller when a consistent set of definitions is followed. Considering the high variability of soil hydraulic conductivity, such an error may be considered acceptable in engineering practice. Received 31 May 2010. Accepted 16 August 2010. Published on the NRC Research Press Web site at cgj.nrc.ca on 1 October 2010.

Estimating soil resistance at unsampled locations based on limited CPT data

An assessment of soil properties at unsampled locations is a common requirement during the design of a geotechnical structure/system. Such an assessment is challenging due to inherent soil variability and the limited number of in situ tests currently available. Here we present a three-dimensional conditioned random field approach for the estimation of anisotropic soil resistance at unsampled locations based only on data from a limited number of cone penetration tests (CPT). The novelty of this work is that the measured CPT data at arbitrary locations can be combined with random field theory to update the estimated soil properties in three dimensions. The accuracy of the estimation can be improved significantly using the proposed method. A case study in Australia is conducted to illustrate the procedure and to assess the capability of the method. The results indicate that the mean values of the conditioned random fields are good estimates of the soil resistance at unsampled locations. The prediction error of the normalized cone penetration resistance is about 0.08 when there are only six CPT tests. A unique feature of this method is its ability to obtain high-resolution results of soil properties in three-dimensional space using a limited number of CPT tests.

Interactive Evaluation of the Reliability of Engineered Slopes Utilising Multi-source Monitoring Information

Important engineered slopes are often heavily instrumented and their performance routinely monitored through these instruments. The evaluation of the safety of the slopes based on the monitored information is however a challenge. A systematic method is presented in this paper for evaluating the slope safety by combining multi-source monitoring information with underlying physical mechanisms. First, a Bayesian network with continuously distributed variables for a slope involving the factor of safety, multiple monitoring indexes and their influencing soil or rock model parameters is constructed. Then the prior probabilities for the Bayesian network are quantified considering model and parameter uncertainties. After that, multi-source monitoring information is used to update the probability distributions of the soil or rock model parameters and the factor of safety or failure probability using Markov Chain Monte Carlo simulation. Two rock slope examples are worked out to illustrate the proposed methodology. A non-intrusive stochastic numerical method is used in the reliability analysis in the examples.