A. Mandal

Effect of Presence of Rigid Base within the Soil on the Dynamic Response of Rigid Surface Foundation

The present experimental investigations study the effect of presence of rigid base at any depth within the soil mass on the dynamic response of foundation under vertical mode of vibration. Model block vibration tests on rigid surface footing are conducted on a different soil layer of finite thicknesses underlain by a rigid base. A concrete block of size 400 × 400 × 100 mm is used as the model block, and a Lazan type mechanical oscillator is used for inducing vibration in vertical direction. The finite soil layers of different thicknesses are prepared in a pit at the bottom of which a massive concrete layer of 300 mm thick was cast to represent it as a rigid base. In the investigation two different soils, namely, local in situ soil and sand are used. In total 72 tests are conducted in different loading combinations and soil types, and several important observations are reported. Two different methods are proposed to analyze the dynamic response of a foundation resting on a finite layer underlain by a rigid layer. Finally, the experimental results are compared with the results that were obtained by the proposed method. A Mass-Spring-Dashpot (MSD) model with proper consideration of damping factor is found to provide reasonably accurate results. Elastic Half Space Theory (EHST) with equivalent soil properties is found to underestimate the displacement amplitude.

Interaction Mechanism between Two Large Rock Caverns

AbstractLong-term stability of geotechnical structures is critical to any engineering design. Design of underground structures is initially planned using empirical approaches based on the rock mass–classification system. This design is mainly based on limited data obtained from boreholes and surface geological mapping. However, this design needs to be continuously updated based on the information gathered as the excavation work progresses. Over the last few years, geotechnical monitoring has emerged as a major engineering tool for engineers to review and update their existing design configuration based on the actual behavior of excavated rock mass. This paper presents a case study of the interaction of two large unlined rock caverns that were excavated using a drill-and-blast technique with a heading-down benching approach. The mechanical response of the two caverns was numerically modeled and studied in terms of the stress-deformation behavior of the excavated caverns and rock support installed, taking i...

Poisson's Ratio of Layered Soils at Different Confining Stresses Using Bender/Extender Elements

There is a lack of guidelines or any particular methodology for the determination of Poissons ratio in layered soils. In this paper an attempt has been made to determine the Poissons ratio of double- and triple-layered soils by measuring the shear and compression wave velocities using bender-extender elements at different confining stresses. Details of the testing methodology are presented, and it has been demonstrated that application of bender-extender elements yields the Poissons ratio in layered soils quite easily.

Risk management by geotechnical monitoring for safe excavation of a large rock cavern: a case study

The present report highlights the methods used to monitor the rock mass behavior and performance of a cavern during excavation in order to verify and, if necessary, adjust the rock support to ensure safe construction at each stage of excavation. The present geotechnical monitoring practice for underground structures involves convergence monitoring with the help of optical targets and rock mass displacement with the help of borehole extensometers. More importantly, it involves detailed planning to finalize the position of each monitoring instrument, based on the location and orientation of geological features. It is equally important to ensure proper recording of monitoring data in order to analyze and take immediate action in case of any adverse situation. This requires dedicated high-end automated software, which can record, analyse and produce significant results from a large quantity of recorded data, to prevent what could otherwise be a catastrophic failure. Initially, geotechnical monitoring was carried out in the recently excavated zone of the cavern on a daily basis. Based on continuous monitoring data for at least 1 week, the frequency of subsequent monitoring can be decided. In most cases, the deformation of rock mass was considerably less than the alarming values which were calculated based on detailed design for different rock classes.

Analysis and Design Approach for Concrete Plugs for Underground Storage Caverns

AbstractIn underground crude oil storage facilities, concrete plugs are mainly required to contain crude oil inside caverns and are designed as gas-tight to prevent movement of any oil or vapor across the tunnel, either in a horizontal or vertical direction. Plugs are retained in a key excavated in the rock surface, which provides sufficient bearing to the concrete mass under sustained loading conditions. Construction of plugs involves mass concreting under the typically hot and humid conditions found in underground construction. Therefore, the construction of plugs requires an engineering design that takes into account not only design loads and reinforcement, but also the systematic cooling arrangement and efficient grouting mechanism so that the basic purpose of the plug design is not defeated. This paper discusses the design and construction aspects involved in the construction of large tunnel and shaft plugs for an underground crude oil storage project.

Analysis and Design of an Underground Portal in Lateritic Soils

This paper presents a case study of the analysis and design of slopes for the portal of an underground crude oil storage cavern site. The site selected for the slope study is characterized by residual soils and granitic rock formations, located in the southwestern part of India. It is observed that in tropical residual soils, most hillslope failures are caused by rainfall and thus it is important to consider hydrological conditions when attempting to analyze the stability of slopes in such material. Combinations of shallow slopes with lower overburden and high steep hillslope with large overburden were considered in the present study along with varying combinations of lateritic soils and weathered rock formations. The paper discusses the various investigations carried out to define the geotechnical properties of lateritic soil and weathered rock, followed by numerical modelling and remedial measures adopted to ensure the stability of slopes during design and construction phase. Since analysis and design procedures for such residual soils are not well established, comprehensive geological and geotechnical investigations were carried out prior to numerical model development for carrying out finite element studies in order to ascertain long term stability of slopes under differing ground conditions. The results of the stability analysis indicated that slope under existing condition were potentially unstable under rainy conditions and specific supporting measures were planned to ensure stability. Several alternatives were examined for improving the stability of slope taking into consideration existing facilities, space available for mobilization of equipments and environmental conditions in reference to specific project requirements. The convergence pattern obtained from geotechnical monitoring using optical targets along the slopes did not showed any alarming movement for over a year.

Experimental evaluation of vertical response of rigid surface footing on layered soil

Abstract The study of the dynamic response of foundations is of significant importance in the machine foundation design and soil-foundation interaction problems, in general. This paper presents experimental investigations in the field studying the effect of layering of soil mass on the dynamic response of foundation under vertical mode of vibration. Model block vibration tests on a rigid surface footing were conducted on different layered soil systems. A concrete block of size 400×400×100 mm was used as the model block and a Lazan type mechanical oscillator was used for inducing vibration in vertical direction. In the investigation two different soils, namely, gravel and fly ash were used. In total 132 tests were conducted on different layered soil beds with different loading combinations. Dynamic response of foundation resting on layered soil system were also analyzed by two simple approximate methods namely, mass spring dashpot model and equivalent elastic half-space theory. Results obtained from the two methods were compared with that obtained from the experimental investigation. It was found from the comparisons that the simple mass-spring dashpot model was capable of predicting the resonance frequency and the resonance amplitude satisfactorily. Elastic half space theory with equivalent soil properties was found to underestimate the displacement amplitude.