Analytical Method for Evaluating the Ground Surface Settlement Caused by Tail Void Grouting Pressure in Shield Tunnel Construction

The tail void grouting is a key step in shield tunnel construction and has an important influence on the loading on the surrounding soil and on the resulting settlement. In order to estimate the ground surface settlement caused by tail void grouting pressure in tunnel construction, the loading on the surrounding soil is simplified as an expansion problem of the cylindrical cavity in semi-infinite elastic space. A simple analytical formula is deduced by using the virtual image technique and Fourier transform solutions. The effectiveness of the proposed method is verified by case studies. The effects of elastic modulus, tail void grouting pressure, tunnel radius, and tunnel depth on the ground surface heave are conducted. The results indicate that the computed results are in accordance with Ye’s solution and it is more rational to consider the ground surface heave induced by tail void grouting pressure in the prediction of ground settlement due to shield excavation. Moreover, the ground surface heave owing to tail void grouting pressure resembled a Gaussian distributed curve. Thus, no matter the ground surface subsidence or ground surface heave can be predicted by means of adding the presented empirical formula to the Peck formula which cannot predict the ground surface heave. The ground surface heave decreases with an increase in elastic modulus. On the contrary, as the tail void grouting pressure and tunnel radius increase, the ground surface heave increases, respectively. The ground surface heave first steadily increases and then declines gradually with the tunnel depth increase.

Poromechanical behaviour of a surficial geological barrier during fluid injection into an underlying poroelastic storage formation

A competent low permeability and chemically inert geological barrier is an essential component of any strategy for the deep geological disposal of fluidized hazardous material and greenhouse gases. While the processes of injection are important to the assessment of the sequestration potential of the storage formation, the performance of the caprock is important to the containment potential, which can be compromised by the development of cracks and other defects that might be activated during and after injection. This paper presents a mathematical modelling approach that can be used to assess the state of stress in a surficial caprock during injection of a fluid to the interior of a poroelastic storage formation. Important information related to time-dependent evolution of the stress state and displacements of the surficial caprock with injection rates, and the stress state in the storage formation can be obtained from the theoretical developments. Most importantly, numerical results illustrate the influence of poromechanics on the development of adverse stress states in the geological barrier. The results obtained from the mathematical analysis illustrate that the surface heave increases as the hydraulic conductivity of the caprock decreases, whereas the surface heave decreases as the shear modulus of the caprock increases. The results also illustrate the influence of poromechanics on the development of adverse stress states in the caprock.

Migration of sodium chloride in dry porous materials

Groundwater can saturate soil above the water table within the capillary fringe associated with the pore size of the parent soil. External evaporation has been viewed as a mechanism for enhancing upward flow, potentially creating problems of salt heave beneath roads and foundations if the groundwater is saline, analogous to concerns with efflorescence in masonry. The role of internal evaporation in promoting crystallization, and especially in altering the transport process of the pore fluid, has been recognized but is only partially understood. The purpose of this paper is to examine evidence for the upward percolation of brine accompanying salt crystallization inside a porous granular material. A series of experiments are described using vertical flow columns packed with dry sand above a reservoir of saline fluid, to explore whether salt transport could take place by autogenous wicking above the initial capillary fringe. The conditions inside the columns were monitored at specific elevations with sensors measuring bulk electrical conductivity, dielectric constant and relative humidity. Dendritic salt crystallization was observed inside the sand, accompanying surface heave. Ultimately, efflorescence on the surface led to the formation of a salt crust. Some implications for the potential damage to roads and foundations in arid regions, and to masonry subject to rising damp, are discussed.

Pile groups under deep expansion: a case history

A viaduct in a high-speed railway line experienced severe heave of its central pillars as a result of deep expansion of an anhydrite rock. Bridge pillars were founded on pile groups that experienced vertical heave displacements as well as lateral displacements and rotations. A semi-analytical solution for the response of a pile group under loading and arbitrary located soil expansion was developed, integrating fundamental solutions for the elastic half-space. The procedure was first validated and then applied to explain the recorded behaviour of the pile groups. The deep expansion was identified from independent surface heave and continuous extensometer readings. Group rotations were well predicted. Observed tensile fissures at the cap–pile contact were explained by the calculated forces and moments on the piles.