Explosive Compaction Inc. (ECI) has carried out research sponsored by the British Columbia Science Council and Sato Kogyo Ltd. to develop techniques based on the use of the controlled detonation of explosives to generate long duration, cyclic shaking of the ground and thereby test the in situ liquefaction potential of the ground. The advantages of such a test are:
The method can be applied to all soil types, especially problematic soils such as sands and gravels or low-plastic silts and clays where current liquefaction evaluation methods are subject to considerable interpretation and uncertainty.
Dynamic Testing of Earth Structures
The sequential detonation of below ground explosives can be used for purposes of dynamic testing of structures to assess in situ liquefaction potential of foundation soils, as well as to provide low to moderate shaking of civil engineering structures (e.g. buildings or dams) to simulate the effects of earthquake excitation.
In situ liquefaction testing within a test volume of soil is carried out by instrumenting the ground with pore pressure transducers and accelerometers. The test area is surrounded by an array of blast holes loaded with low charge weight explosives (typically 1 to 2 kg per charge) and the explosives sequentially detonated using relatively long delays to subject the test volume to displacements and shear strains representative of a design level earthquake. The accelerometer measurements combined with blast analysis are used to deduce the shear strain levels in the test volume of soil. Explosive charge weights are selected to mimic shear strain levels induced by the design earthquake. The total number of charges detonated (N) is selected to simulate the effective number of cycles of strong shaking desired to simulate the design magnitude earthquake. Residual pore water pressures developed as a function of strain amplitude and number of strain pulses are plotted, which typically show an increase with strain amplitude and number of strain cycles similar to that shown in cyclic laboratory testing. With such test data, the liquefaction susceptibility of the test volume of soil can be checked. This test method was developed to test the liquefaction susceptibility of soils whose liquefaction resistance is not well defined using more routine penetration test methods of assessment (for example, in highly silty or gravelly soils and sensitive, low plastic clays). The test provides a complement to other laboratory-based methods of liquefaction assessment or methods based on soil penetration testing (e.g. Standard Penetration Test, Electronic Cone Penetration Test or Becker Penetration Test). The advantage of the method is that it can be carried out at any depth under the same stress and drainage boundary conditions as exist in the actual soil mass. In addition, there are no problems associated with soil sample disturbance as exist when one recovers a soil sample for purposes of laboratory testing.
The above in situ liquefaction test is carried out typically within less than 15 m of a blast hole where shear strain, displacement and ground velocity levels are representative of moderate to strong earthquake shaking. To subject a dam or other earth structure to such strong shaking levels could be potentially dangerous, especially if the foundations have high liquefaction susceptibility. Therefore, for purposes of subjecting a structure to more moderate levels of earthquake-like excitation which would not result in damage to the structure, a test method has been developed where line arrays of blast holes are located typically 30 to 50 m away from the structure of interest. The charges in each blast hole are then detonated sequentially to generate long duration, low to moderate amplitude shaking of the structure. The distances of the blast holes, charge weights per delay and time delays are selected to achieve the desired ground velocity amplitudes and frequency contents of interest. At large distances from a blast hole it has been found that the higher frequency ground motion amplitudes attenuate and lower frequency ground motions predominate due to the presence of shear wave and surface wave energy. Single hole test blasts and site-specific attenuation relationships are developed to assist in the blast array design. During the detonation of multiple blast holes, the structure is instrumented with vibration sensors. The test is then numerically simulated using a 2 or 3-dimensional soil-structure interaction model which permits calibration of the dynamic properties (stiffness and damping characteristics in the low to moderate strain range) of the structure. This information can then be used for purposes of dynamic modelling of the structure to stronger levels of earthquake shaking for purposes of assessing earthquake vulnerability.
The free vibration response of the structure after the multiple blast hole loading has ceased is also used to assess the natural frequency and damping characteristics of the structure. This type of test can be carried out periodically over the lifetime of the structure to assess changes in dynamic properties of the structure. If significant increases in natural periods of vibration occur over time, this is often indicative of gradual deterioration (decreasing stiffness) of the structure. This type of testing can therefore be used as a diagnostic tool to assess the condition of a particular structure.
|Home | Contact Us|