Renpeng Chen

Field Tests on Pile-Supported Embankments over Soft Ground

When designing embankments over soft soils, geotechnical engineers face many challenges. These include potential bearing failure, intolerable settlement, and global or local instability. Pile-supported embankments have been emerged as an effective alternative successfully adopted worldwide to solve these problems. This paper focuses on three cases in which pile-supported embankments were used for constructing highways in the eastern coastal region of China. Each case provides a description of the soil profile, construction procedure, and field monitoring of the settlements, earth pressures, and pore-water pressures. Field monitored data from contact pressures acting on the piles and the soils, to the settlements of the piles and the soils are reported and discussed. The development of the earth pressures both on the piles and the soils shows that there was a significant soil arching in the embankment, and the measured earth pressures acting on the piles are much higher than that acting on the soils betwee...

A theoretical solution for pile-supported embankments on soft soils under one-dimensional compression

Pile-supported embankments are increasingly being used for highways, railways, storage tanks, etc. over soft soil because of their effectiveness in accelerating construction and minimizing deformation. The stress transfer mechanisms among all of the components in a piled embankment, including the embankment fill, the piles and (or) caps, and the foundation soils, are complicated. In this study, a closed-form solution for one-dimensional loading was obtained taking into consideration the soil arching in the embankment fill, the negative skin friction along the pile shaft, and the settlement of the foundation soil. In the derivations, the piles, the embankment fill, and the foundation soil were assumed to deform one-dimensionally. This study investigated the stress concentration on top of the pile, the axial load and skin friction distributions along the pile, and the settlement of the embankment. Comparisons demonstrate that the results from this solution are in good agreement with those obtained using a f...

Failure Investigation at a Collapsed Deep Excavation in Very Sensitive Organic Soft Clay

AbstractCollapse of a 15.7-m-deep excavation in very sensitive organic soft clay in Hangzhou, China, is presented in this paper. The collapse of the excavation caused the break and/or inclination of the retaining walls, the wracking of the supporting system, significant sinking of a major road adjacent to the excavation site, and also the break in a water main beneath the road. After the collapse, the ground disturbance due to the collapse was investigated by cone penetration tests (CPTs) and field vane shear tests (VSTs). A simplified method, based on the CPT and VST test results, was used to evaluate the strength reduction ratio (SRR) of the in situ soils. According to the SRR values of the soils at different depths, the lower boundaries of the severely disturbed soils were determined and a possible slip failure surface passing through the bottom of the severely disturbed soils was justified. The factors of safety (FOSs) against basal heave of the excavation calculated from various methods were 1.05, 0....

Large-Scale Modeling and Theoretical Investigation of Lateral Collisions on Elevated Piles

AbstractLarge deflection is mobilized in elevated piles of nonnavigable piers and some flexible protective systems subjected to lateral ship collisions. Because the current bridge design specifications are only suitable for the design of rigid foundations, new analysis and design methods are required for these flexible elevated piles. A large-scale lateral static load test and an additional three impact tests on piles in low liquid limit silt (ML) soil were carried out in a large soil tank. Both static and dynamic soil pressures on the pile shaft were measured, and then the soil-pile interaction was studied in detail. Based on the results, static hyperbolic p-y curves of the piles were derived for dynamic p-y curve models. Through investigations of three linear and nonlinear soil-pile dynamic interaction models, the dynamic p-y curve models with nonlinear static stiffness coefficients and constant damping coefficients are recommended for the analysis of piles subjected to lateral impact loading. Verified ...

Analysis of diffusion-adsorption equivalency of landfill liner systems for organic contaminants

The equivalence between multilayered barriers regarding diffusion and adsorption was studied. The bottom boundary of the liner system is defined by assuming concentration continuous and flux continuous conditions of the contaminant between the bottom liner layer and the underlying soil. Five different liner systems were compared in terms of solute breakthrough time. The results of the analysis showed that breakthrough time of the hydrophobic organic compounds for a 2-meter-thick compacted clay liner (CCL) could be 3-4 orders of magnitude is greater than the breakthrough time for a geosynthetic clay liner (GCL) composite liner. The GM/GCL and GM/CCL composite liner systems provide a better diffusion barrier for the hydrophilic organic compounds than that for the hydrophobic compounds due to their different Henrys coefficient. The calculated breakthrough times of the organic contaminants for the Chinese standard liner systems were found to be generally greater than those for the GCL alternatives, for the specific conditions examined. If the distribution coefficient increases to 2.8 for the hydrophobic compounds or 1.0 for the hydrophilic compounds, the thickness of the attenuation layer needed to achieve the same breakthrough time as the standard liner systems can be reduced by a factor of about 1.9-2.4. As far as diffusive and adsorption contaminant transport are concerned, GM or GCL is less effective than CCL.

Measuring dielectric constant in highly conductive soils based on surface reflection coefficients.

In soils with high electrical conductivities, time domain reflectometry (TDR) principles fail to measure the apparent dielectric constant by travel time analysis. This limits the application of water content measurement of existing TDR technologies on those materials. This paper describes a new approach for determining the dielectric constants in highly conductive soils from surface reflections of TDR signals. The multiple reflections at the interfaces of impedance mismatches in the ASTM standard probe were studied. Extension rods were used to avoid the overlap of the reflections along the probe. A relationship between the reflection coefficient at the soil surface and the apparent dielectric constant of the soil was established theoretically and validated by laboratory experiments. Results indicate that the dielectric constant can be determined with reasonable accuracy from the surface reflection coefficient even for soils with high electrical conductivities, where the conventional travel time analysis fails. Compared with alternative approaches, such as based on inversion model, this new method offers high time efficiency in extending TDR applications in highly conductive soils.

Cumulative settlement of track subgrade in high-speed railway under varying water levels

Water-level variation in the subgrade of high-speed railway influences the cumulative settlement of the subgrade. Full-scale model test on high-speed railway under varying water levels within the subgrade was conducted in this research. Dynamic soil pressures in different depths within the subgrade and cumulative settlements of subgrade were measured under different train speeds. The results demonstrated that the dynamic soil pressure increases significantly after the first wetting–drying cycles and then remains stable. The measured dynamic soil pressure was compared to the requirements in the current design method. Increasing train speed leads to higher cumulative settlement. Furthermore, a modified model is proposed in the article for the determination of cumulative settlement considering the influence of initial stress state, material properties of subgrade, and dynamic stresses of train loadings.

Measurement of electrical conductivity of pore water in saturated sandy soils using time domain reflectometry (TDR) measurements.

Studying solute transport in soils is hampered by a lack of technology for continuously monitoring ionic concentration of contaminants. The electrical conductivity of pore water is a strong indicator of ionic concentration of contamination in soil. Using the bulk electrical conductivity of a soil measured by time domain reflectrometry (TDR) to predict the soil pore-water electrical conductivity appears to be a promising technique. This study presents a new method for estimating the pore-water electrical conductivity of saturated sandy soils using a single TDR test. The effects of pore-water electrical conductivity, temperature, porosity, and ionic types on the electrical conductivity of soil were studied. An average value of the exponent in the Archie’s Law was found to be 1.457 for the saturated sandy soils used in this study. A laboratory model infiltration test was also conducted with continuous monitoring of the electrical conductivity of the pore water by TDR. The results showed that TDR is able to p...

Experimental and Analytical Models of Laterally Loaded Rigid Monopiles with Hardening p–y Curves

AbstractShort rigid large-diameter monopiles have been widely used for offshore wind turbines. The rotation of the piles is strictly controlled to render safe operation of the wind turbines. Consequently, the determination of the bearing capacity within an allowable angular rotation of the monopiles is an important issue. In this study, six lateral loading tests (at 1 g) were conducted on a rigid model pile in low liquid limit silt (ML) soil (with two relative densities). The tests reveal a rotational center at a depth of (0.75–0.85)L (L= embedded length of the pile) and work-hardening load–displacement curves (a continual increase in soil pressures even at a very large rotational angle). The test results were used to establish new p–y curves using a coefficient of subgrade reaction nh that is correlated to the local pile displacement. In particular, an analytical expression is developed to estimate the net lateral soil resistance p on a circular pile using peak-on-pile radial stress. The new p–y curves a...

A Newly Designed TDR Probe for Soils with High Electrical Conductivities

Time domain reflectometry (TDR) is a fast, accurate, and safe technology for field monitoring of soil moisture. Commonly used information in TDR signals includes the apparent dielectric constant and electrical conductivity. Because general TDR principles are not available for apparent dielectric constant measurements by travel time methods in soils with high electrical conductivities caused by the significant signal attenuation, the conventional commercial probes lose their purposes. For this reason, a new probe has been designed for measuring dielectric constants in highly conductive soils on the basis of the surface reflection coefficients method. This new probe can make the reflection at the soil surface more distinct. Experiments were conducted to verify the accuracy of measuring dielectric constants in different soils using this new probe. Finally, the probe was used to measure water content and dry density in the field. The results show that the probe has good integrity and high strength. This probe is capable of obtaining the dielectric constant in soils with high electrical conductivities using surface reflection coefficients methods with reasonable accuracy. In addition, it indicates that the dielectric constant measured by this approach matches well with that determined by travel time methods in the relative error range of 10 % in lowly conductive soils. Compared to oven-dry methods, the relative errors of water content and dry density determined using this new probe are less than 10 % and 3 %, respectively.