Inversion Of Subsoil Bearing Capacity From Borehole Microtremor Testings

The characteristic value of subsoil bearing capacity is an important parameter reflecting the structure and properties of soil layers,and it is also an indispensable index in the site investigation,foundation design,foundation construction and other projects.The in-situ testing method,the theoretical formula method,the code table method and the local empirical method are the traditional methods to determine the characteristic value of subsoil bearing capacity.The in-situ testing method such the plate testing can determine the characteristic values of subsoil bearing capacity accurately,however,the processes of testing are characterized by large amount of work,long testing time,high cost,and the depth is generally small.The borehole micro-tremor testing is a new in-situ testing method,which can directly pick up the borehole micro-tremor signal and analyze the dynamic parameters of the deep soil layers.In this paper,the micro-tremor of soil layers at difference depths in the borehole was tested by self-developed equipment — BMT-I borehole micro-tremor testing equipment,studied the spectral characteristics of the deep soil layers,and the approximate relationship between the spectral characteristics and the characteristic values of subsoil bearing capacity was established.Compared with the plate test,the method has the advantages of less amount of work,higher testing efficiency,less disturbance to soil,and the depth is greatly improved,especially suitable for testing the subsoil bearing capacity of deep foundation bearing stratum in large-scale engineering such as high-rise and super high-rise,large bridges,high-speed railway,harbor,nuclear power and so on.The main achievements of this paper are as follows:(1)The Fourier spectrum amplitudes and predominant periods of soil layers with different depths were calculated by the Fourier spectrum synthesis and the H/V spectral ratio method,and the variation of spectrum characteristics with the type and the embedded depth of the soil is analyzed.The results show that: The spectral characteristics of each soil layer are closely related to its mechanical properties.The Fourier spectrum amplitudes and the predominant periods vary with the soil types.For the same soil,the Fourier spectrum amplitudes and predominant periods decrease with the increase of soil embedded depth.(2)Combined with the standard penetration test,the approximate relationship among the spectral characteristics and the characteristic values of bearing capacity is established based on the statistical method.The relationship shows that: The main influencing factor of the characteristic values of bearing capacity is the type of soil,and which is also related to the spectral characteristics of micro-tremor.For the same soil,the characteristic values of bearing capacity are in proportion to the square of the Fourier spectrum amplitudes,and they are in inverse proportion to the square of the predominant periods.(3)The universality of the approximate relationship was verified by the engineering site testing.The results show that: The inversion results of the characteristic values of bearing capacity are close to the measured results.In addition to the individual layers,the f relative errors are not more than ±6%,the spectral characteristics of the borehole micro-tremor are related to the characteristic values of bearing capacity.And the characteristic values of subgrade bearing capacity of the deep soil layers can be estimated simply and quickly by the borehole micro-tremor testing.