Arash Alimardani Lavasan

Behavior of Shallow Strip Footing on Twin Voids

This paper numerically examines the bearing capacity and failure mechanism of a shallow strip foundation constructed above twin voids. The voids may refer to caves, caverns, underground aqueduct or tunnels due to water seepage, chemical reaction or deliberately excavated in soil deposit. The ability of numerical model to accurately predict the system behavior is evaluated by performing verification analyses on existing researches. Subsequently, a parametric study carried out to reveal the influence of size of footing/voids and their location (i.e. depth, spacing, eccentricity) on the bearing capacity of footing. To clarify the failure mechanism, the distribution of shear strain in the soil for different scenarios is assessed. The parametric study provided a new framework to determine the bearing capacity and the mode of failure for footings on voids. Based on the results, a criterion can be issued to avoid collapse of footing/voids regarding the shape, location and size of voids. The results can also be used to design construction of a footing on existing voids while the acquired failure mechanisms can be appointed to develop analytical solutions for this problem. Results demonstrated that a critical depth for voids and a critical distance between them exist where the influence on the ultimate bearing capacity of footing disappears.

Numerical study on backfilling the tail void using a two-component grout

AbstractIn shielded mechanized tunneling, the annular gap caused by the tunnel boring machine (TBM) driving must be backfilled instantaneously using a suitable grout. A two-component grout composed...

Constitutive Parameter Adjustment for Mechanized Tunneling with Reference to Sub-system Effects

In this research, the effect of sub-system on model response for mechanized tunneling process has been taken into consideration. The main aim of this study is to modify the constitutive parameters in a way that the best agreement between numerical results and measurements is obtained. The sub-system includes supporting pressure at the face of the TBM, contraction along the TBM-shield and grouting pressure in the annular gap. The commercially available finite element code, PLAXIS is adopted to simulate the construction process. The soil behavior during the excavation is numerically reproduced by utilizing Hardening Soil model with small strain stiffness (HSsmall). The constitutive parameters are obtained via sensitivity and back analyses while they have been calibrated based on the real measurement of Western Scheldt tunnel in the Netherlands. Both local and global sensitivity analyses are used to distinguish which parameters are most influencing the soil deformation. Thereafter, the model validation is accomplished by applying different scenarios for face pressure distributions with respect to the slope of the tunnel. In addition, the effect of contraction factor is modified individually or coupled with the variation of grouting pressure. Evaluating the influence of the sub-system is conducted to assess its effects on the model responses and to seek the possibility to decrease the disagreement between the calculated displacement and real measured data.

Adequate Numerical Simulation of Tail Void Grouting for Tunneling in Saturated Soil

In this research, a 3D numerical simulations of tunneling in saturated soil are conducted. The tunnel shield and lining segments with different diameters are explicitly modeled during the progressive excavation. Special attention is paid to accurate simulation of the grout hydration in the tail void where a constitutive model that accounts for the time dependent stiffness is developed to describe the hardening behavior of grout mortar in the coupled hydro-mechanical analysis. Additionally, the effect of grouting pressure distribution along the lining segments is investigated as well. The results reveal that neglecting the evolution of stiffness may lead to underestimation of surface settlements. The results also indicated the insignificant impact of the shape of the annular gap on the soil deformations and lining forces.

Bearing Capacity of Interfering Strip Footings

AbstractIn this paper, the ultimate bearing capacity of two closely spaced rigid strip footings with rough base on granular soil is examined based on enhanced limit equilibrium, plastic limit analy...

Closure to “Behavior of Shallow Strip Footing on Twin Voids” Published in Geotech and Geol Eng. (2016), 34(6): 1791–1805

The writers appreciate the discusser’s comments (Jamali 2017) and interest in their original subject paper (Lavasan et al. 2016). However, for the reasons and clarifications briefly stated below, we disagree with the discusser’s comments on the paper. This reply helps to clarify the fundamental aspects in terms of the definition of the technical terms used in the published paper. Two comments raised by the discusser that deal with (1) the (B/D)cr for the soils considered in the original paper and (2) the extensibility of the bearing pressure-embedment depth ratio curve to estimate the (Z/D)cr. The discusser has first figured out two different values mentioned in the original article for the ratio (B/ D) where no further ‘‘significant influence’’ on the bearing pressure ratio (qv/qnv) can be observed. Secondly, a larger critical footing width (B/D)cr = 1 has been suggested through an extrapolation of the numerical results. The writers would like to emphasize that determination of the critical geometrical parameters (e.g. B/D, Z/D, L/D and D/L) is not an absolute and straightforward procedure in terms of defining the ‘‘significance of influence’’. Besides this rather relative approach to determine the critical parameters, different phenomena such as type of the soil have impacts on the variation of the bearing pressure and therefore the critical geometrical parameters. To clarify the influence of the soil type, Fig. 16 in the published article is re-plotted separately for soils 1 and 2 that demonstrates a slight difference in the (B/D)cr for these two types of the soils. As seen in Fig. 1, variation of the bearing pressure ratio becomes negligible when B/D is equal to 0.75 and 0.5 for soils 1 and 2, respectively. In addition, to evaluate the variation of bearing pressure ratio above B/D = 0.75, a number of complementary analyses have been conducted for B/D[ 0.75 and the results are represented with dashed line in Fig. 1. The trend of variation of the curves indicates no significant change in the bearing pressure ratio for B/D[ 0.75. Despite the slight difference for soils 1 and 2, the overall critical parameter is still equal to (B/D)cr = 0.75. The discusser extrapolated the curve in Fig. 16 (in the original paper) and suggested (B/D)cr = 1. However, the writers would like to draw the attention of the A. A. Lavasan (&) T. Schanz Chair of Foundation Engineering, Soil and Rock Mechanics, Faculty of Civil and Environmental Engineering, Ruhr University Bochum, Bochum, Germany e-mail: arash.alimardanilavasan@rub.de