A Simplified Calculation Method for Stress Concentration Ratio of Composite Foundation with Rigid Piles

Linchang Miao,Fei Wang,Weihua Lv 대한토목학회 2018 KSCE Journal of Civil Engineering Vol.22 No.9

Piles in a composite foundation share loads with the surrounding soil instead of carrying all loads alone as a pile foundation. Stress concentration ratio, defined as the ratio of the stress on the top of piles to the stress on the surrounding soil, is essential for the design of piles in the composite foundation. An equal strain condition is widely accepted and adopted in the existing methods of calculating the stress concentration ratio. In other words, the piles should have identical relative displacement with the surrounding soil in an equal strain condition. This assumption may work for flexible and semi-rigid piles; however, it may not be suitable for rigid piles considering the significant stiffness difference of pile and soil. The relative displacement of rigid piles and surrounding soil can significantly change the stress distribution on piles and soils due to the pile-soil interaction (i.e., pile side friction). Therefore, the pile-soil interaction should be considered in the design of the rigid piles in the composite foundation. In this study, a simplified theoretical solution of the stress concentration ratio was derived based on the deformation coordination between rigid pile and soil to consider the pile-soil relative displacement and thus the pile-soil interaction. The pile side friction was assumed as linearly increased with the pile-soil relative displacement and then the stress concentration ratio was derived based on the force and deformation equilibrium in the composite foundation. A neutral point (i.e., location with zero pile-soil relative displacement) was included in the derivation to take the negative skin friction along the pile into account. Lab-scale and full-scale field tests were conducted to verify the effectiveness of the proposed method. Comparison of measured stress concentration ratios with calculated ones using the proposed method demonstrates the effectiveness of the proposed method.

The Volume of Settlement Trough Change with Depth Caused by Tunneling in Sands

Fei Wang,Linchang Miao,Xiaoming Yang,Yan-Jun Du,Fayun Liang 대한토목학회 2016 KSCE JOURNAL OF CIVIL ENGINEERING Vol.20 No.7

The Volume of the settlement trough is essential for estimating the settlement caused by tunneling. Generally, the volume of the settlement trough was recognized as a constant which was equal to the volume of the ground loss in the tunnel buried in clay. According to observation data from both laboratory and field tests, the volume of the settlement trough caused by tunneling in sands changed with depth, although the mechanism behind the volume change has not yet to be understood. In the present study, a largescale laboratory model test was conducted to investigate the volume change of the settlement trough with depth caused by tunneling in sands. The results show that the volume of the settlement trough decreased significantly with a decrease in depth. Based on the data from both literature and the current study, a modified model was proposed to estimate the settlement caused by tunneling in sands considering variation of the volume of settlement trough with depth.

Experimental Investigation on the Bond Behavior of Deformed Bars in a Class of Structural Expanded Polystyrene (EPS) Concrete

Chao Li,Linchang Miao,Quan You,Lijian Lei,Xiaodong Liang,Junjie Wang 대한토목학회 2020 KSCE JOURNAL OF CIVIL ENGINEERING Vol.24 No.12

The structural expanded polystyrene (EPS) concrete (above 35 MPa) has the potential to be used as metro track bed to reduce and isolate vibration, but its bond behavior is still unclear. This inhibits its application because it is often used in combination with reinforcing bar. In this paper, a class of structural EPS concrete (above 35 MPa) was developed. Based on this, a series of pull-out tests were conducted and the bond behavior of deformed bars in these structural EPS concretes was investigated. In addition, effects of rebar bond length and rebar diameter were analyzed in detail. Experimental results indicate that the structural EPS concrete suffers splitting failure more easily than the conventional concrete (CC). Increase of EPS replacement, rebar bond length or rebar diameter tends to cause the splitting failure of structural EPS concrete. Moreover, with increase of EPS replacement, the failure process is less explosive and more gradual. The stress-slip relationships of the most specimens show three stages, namely linear ascending, non-linear progression, and descending stages. The bond strength of structural EPS concrete is found depended linearly on the power function of its compressive strength, relative cover thickness (c/d) and relative rebar bond length (L/d), respectively. Accordingly, a bond strength prediction model was formulated, and it can precisely predict the bond strength of deformed bars in structural EPS concrete, specimens failing in splitting or splitting-pull out modes, and no fiber added. The results can provide reference for the design of structural EPS concrete.

Locally Resonant Periodic Wave Barriers for Vibration Isolation in Subway Engineering

Lijian Lei,Linchang Miao,Chao Li,Xiaodong Liang,Junjie Wang 대한토목학회 2021 KSCE Journal of Civil Engineering Vol.25 No.4

Subway transportation is being promoted worldwide to effectively solve urban congestion. However, the vibration induced by subway traffic has caused a major adverse impact on building safety, precision instrument operation and human health. Wave barriers have been proven effective in mitigating ground vibration, whereas they have some limitations in achieving ideal attenuation zone and high efficiency to cover the low-frequency vibration in underground railway system. Based on locally resonant phononic crystals theory, this paper designs three-component locally resonant periodic wave barriers (LRPWBs), and investigates the effects of geometrical and material parameters on the bandgap features in detail. The band structures are calculated using improved plane wave expansion (IPWE), the transmission spectra and vibration modes are obtained by finite element method (FEM). The results indicate LRPWBs are able to give lower and wider bandgap to cover the main frequency of subway environment, which is proved by time and frequency domain analysis. For the bandgap mechanism, the local resonance features of LRPWBs result in the energy conversion between kinetic energy and elastic strain energy, thus the elastic wave energy is localized in resonance unit and then the locally resonant bandgap is created. In addition, the bandgap can be adjusted by carefully selecting proper geometrical and material parameters to actualize low-frequency broadband attenuation. Further studies about multi-oscillator system indicate that the appropriate combination of multiple LRPWBs are conductive to diverse and broad bandgaps. The investigations can provide inspiration for periodic wave barriers design in multi-frequency vibration attenuation field.

Effects of Reaction Conditions on EICP-Treated Desert Aeolian Sand

Linyu Wu,Linchang Miao,Satoru Kawasaki,Hengxing Wang 대한토목학회 2022 KSCE JOURNAL OF CIVIL ENGINEERING Vol.26 No.6

Aeolian sand is a fine, non-cohesive and homogeneous material widely distributed in desert areas, and therefore susceptible to wind erosion, causing serious environmental concerns. This study demonstrates the promise of enzyme-induced carbonate precipitation (EICP) as a means of solidifying aeolian sand to prevent wind erosion. Different cementation solution concentration, urease activity, temperature and number of treatments will have diverse influence on the mechanical property of solidified sand. The test-tube experiments were performed to evaluate the effect of these factors on the enzymatic calcium carbonate precipitation process. The method was then applied to solidify aeolian sand to assess the reinforcement effect by unconfined compressive strength (UCS), calcium carbonate content and mercury intrusion capillary pressure tests. The results demonstrated that the increase of urease activity from 2.95 U/mL to 5.39 U/mL and that of cementation solution from 0.25 M to 0.75 M resulted in an increase in UCS. The increase of number of treatments and that of temperature from 15oC to 45oC can also effectively enhance the mechanical property of aeolian sand. The mercury intrusion capillary pressure test revealed that the improvement of performance of solidified aeolian sand was mainly due to the reduction of porosity caused by the generated of calcium carbonate; and there is an exponential function relationship between the strength and porosity. Furthermore, the increase in urease activity and temperature significantly reduced the porosity and the proportion of larger pores in the solidified sand, thereby appreciably enhancing the strength of aeolian sand with a minor increase in calcium carbonate content.