Protective Engineering

Pathak, S., & Ramana, G. V. (2017). Air-blast induced ground displacement. Procedia Engineering: 11th International Symposium on Plasticity and Impact Mechanics–2016 , 173 , 555–562.

Outburst of nuclear explosion produces moving air-overpressure above ground surface. These overpressure fronts move at super-seismic speeds near ground zero and cause ground motion. Hence, determination of air-blast induced ground displacement is the first essential step in design of underground protective structures in super-seismic zone. For simplified analysis, air-blast load is modelled as linearly decaying pressure with time and using one-dimensional elastic wave propagation model, free-field vertical displacement response of elastic half-space can be expressed as a closed-form solution. However, such a simplified solution seems to be of very little practical utility in addressing problems in geotechnical environment. Hence, a new closed-form solution is proposed for displacement time-history which accounts for linear-inelasticity, plastic wave propagation velocity, and geometric stress attenuation. A conceptual pseudo-static procedure which computes vertical displacement as a static problem at each time instant based on dynamic properties of geomaterials is utilized to achieve the proposed solution. Predictions of the proposed model are compared with results of one of the atmospheric nuclear tests conducted at Nevada Proving Grounds and a reasonable agreement is obtained.

Pathak, S., & Ramana, G. V. (2018). A designer’s approach for estimation of nuclear-air-blast-induced ground motion. Advances in Civil Engineering, 2018,1–12.

A reliable estimate of free-field ground displacement induced by nuclear-air-blast is required for design of underground strategic structures. A generalized pseudostatic formulation is proposed to estimate nuclear-air-blast-induced ground displacement that takes into account nonlinear stress-strain behaviour of geomaterials, stress-dependent wave propagation velocity, and stress wave attenuation. This proposed formulation is utilized to develop a closed-form solution for linearly decaying blast load applied on a layered ground medium with bilinear hysteretic behaviour. Parametric studies of closed-form solution indicated that selection of appropriate constrained modulus consistent with the overpressure is necessary for an accurate estimation of peak ground displacement. Stress wave attenuation affects the computed displacement under low overpressure, and stress-dependent wave velocity affects mainly the occurrence time of peak displacement and not its magnitude. Further, peak displacements are estimated using the proposed model as well as the UFC manual and compared against the field data of atmospheric nuclear test carried out at Nevada test site. It is found that the proposed model is in good agreement with field data, whereas the UFC manual significantly underestimates the peak ground displacements under higher overpressures.

Pathak, S., & Ramana, G. V. (2018). A stress-strain model for geomaterials subjected to air-blast. In A. Zhou, J. Tao, X. Gu, & L. Hu (Eds.), Geoshanghai 2018 International Conference: Fundamentals of soil behaviours (pp. 388–396). Springer, Singapore.

The engineering design of underground protective structures subjected to blast loading requires an appropriate stress-strain relationship for surrounding geomaterials. The behaviour of geomaterials under blast loading depends upon strain rate, stress level and interaction among the three phases. A few advanced constitutive models are proposed in the literature to model stress-strain behavior. However, a less accurate but simple alternative is to use functional forms for capturing the experimental stress-strain curves. In this paper, the functional form of stress-strain curve of geomaterials subjected to air-blast (uniaxial high strain-rate loading) is proposed based on the deformation mechanism of geomaterials. The proposed model consists of two different expressions for loading and unloading and requires only three parameters. The physical meaning of the three model parameters is discussed and the procedure for their evaluation is outlined. It is found that the proposed functional form captures the experimental stress-strain curves very well.

Pathak, S., & Ramana, G. V. (2021). On stress-strain function of geomaterials subjected to blast-loads. Indian Geotechnical Journal, 51 , 520–538.

An appropriate stress–strain relationship of geomaterials subjected to blast loading is essential for the design of underground protective structures. Previous experimental and theoretical research efforts indicate that the constitutive behavior of geomaterials under blast loading depends upon strain rate, stress level, and interaction among the three phases (solid, liquid, and gases). In current state-of-the-art, various advanced constitutive models are available to model the stress–strain behavior of geomaterials under blast-loads. However, considering the cost of computation associated with such models, a functional form is discussed to model the loading and unloading branches of stress–strain curve of geomaterials subjected to blast load based on the three parameters: weight factor, initial modulus ratio, and strain recovery ratio. It is observed that the new functional form reasonably captures the mean trend of the experimentally obtained or simulated stress–strain data. This paper further investigates the applicability of this functional form and provides a catalog of the model parameters for direct use by practicing engineers. The dependence of function parameters on strain rate, lateral confinement, degree of saturation, initial compaction, and locking initiation stress is investigated, and some simple rules are proposed for reasonable estimation of the three parameters. The proposed functional form would be quite useful in practical design problems specially where cost of computation associated with advanced constitutive models is too high. In addition to this, the proposed simple parametrization of complex nonlinear stress–strain behavior also provides an opportunity to investigate the effect of uncertainties in soil parameters in a computationally efficient manner.