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CONTENTS
Volume 24, Number 2, January25 2021
 

Abstract
Deformations in soils induced by dynamic loads cause damage to the structures above the soil layers. It is important for geotechnical engineering practice that how the soil behaves due to repeated loads and the necessary precautions to be taken accordingly. Turkey is one of the most important seismic regions in Europe and earthquake studies to be conducted in this area are intended to reduce the damage as a result of taking the necessary measures. To determine the properties of soils under dynamic loads, stress-controlled dynamic triaxial and resonant column tests can be performed. In this study, these experiments were implemented in the laboratory on the clayey sand soil samples obtained from Bilecik Sogut. To evaluate the effects of the confining pressure and rate of loading on the dynamic behavior of soils, samples were dynamically loaded by different rates at varying confining pressures. As a result, the changes in stress-strain properties of soils under dynamic loads were investigated. The alteration in behavior in terms of modulus reduction and damping ratios was obtained to vary a lot with the change of the lateral pressure on soil along with the frequency of the load.

Key Words
dynamic properties; dynamic triaxial; resonant column; stress-strain behavior

Address
Ersin Guler: Sivrihisar Vocational School, Eskisehir Osmangazi University, 26480, Turkey

Kamil B. Afacan: Department of Civil Engineering, Eskisehir Osmangazi University, 26480, Turkey

Abstract
This paper investigates the effects of soaking on a lime stabilized high plasticity clay and evaluates the implications for pavement design. In this context, the soil was stabilized by 4%, 6% and 9% hydrated lime. The soil was pulverized in two different gradations so that representative field gradations could be simulated. Both soil pulverization levels passed the relevant field gradation criteria. Curing durations were chosen as 7 days, 28 days and 56 days. Two groups of samples were prepared and were tested in unconfined compression test apparatus to measure the strength and secant modulus at failure values. One of the groups was tested immediately after curing. The other group of samples were first cured and then subjected to soaking for ten days before testing. Visual observations were made on the samples during the soaking period. The results showed the superiority of fine soil pulverization over coarse soil pulverization for unsoaked conditions in terms of strength and modulus values. Soaking of the samples affected the unconfined compressive strength and modulus values based on lime content, curing duration and soil pulverization level. In soaked samples, fine soil pulverization resulted in higher strength and modulus values compared to coarse soil pulverization. However, even with fine soil pulverization, effects of soaking on modulus values were more significant. A new term named as "Soaking Influence Factor (SIF)" was defined to compare the reduction in strength and modulus due to soaking. The data was compared with the relevant design guidelines and an attempt was made to include Soaking Influence Factors for strength and modulus (SIFS and SIFM) into pavement design processes. Two equations which correlated secant modulus at failure to unconfined compressive strength were proposed based on the samples subjected to soaking. The results of this study showed that in order to decrease the diverse effects of soaking for lime stabilized soils, soil pulverization level should be kept as fine as possible in the field. Importance of proper drainage precautions in the pavements is highlighted for better performance of the pavements.

Key Words
high plasticity clay; lime stabilization; soil pulverization level; soaking; unconfined compressive strength; modulus

Address
Ilknur Bozbey, M. Kubilay Kelesoglu and Sadik Oztoprak: Department of Civil Engineering, Istanbul University-Cerrahpasa, Istanbul, Turkey

Muhammet Komut, Senol Comez, Tugba Ozturk, Aykan Mert and Kivilcim Ocal: Department of Research and Development, Turkish General Directorate of Highways, Ankara, Turkey



Abstract
The vacuum preloading method has been used in many countries for soil improvement and land reclamation. However, the treatment time is long and the improvement effect is poor for the straight-line vacuum preloading method. To alleviate such problems, a novel combined air booster and straight-line vacuum preloading method for shallow ground treatment is proposed in this study. Two types of traditional vacuum preloading and combined air booster and straight-line vacuum preloading tests were conducted and monitored in the field. In both tests, the depth of prefabricated vertical drains (PVDs) is 4.5m, the distance between PVDs is 0.8m, and the vacuum preloading time is 60 days. The prominent difference between the two methods is when the preloading time is 45 days, the injection pressure of 250 kPa is adopted for combined air booster and straight-line vacuum preloading test to inject air into the ground. Based on the monitoring data, this paper systematically studied the mechanical parameters, hydraulic conductivity, pore water pressure, settlement and subsoil bearing capacity, as determined by the vane shear strength, to demonstrate that the air-pressurizing system can improve the consolidation. The consolidation time decreased by 15 days, the pore water pressure decreased to 60.49%, and the settlement and vane shear strengths increased by 45.31% and 6.29%, respectively, at the surface. These results demonstrate the validity of the combined air booster and straight-line vacuum preloading method. Compared with the traditional vacuum preloading, the combined air booster and straight-line vacuum preloading method has better reinforcement effect. In addition, an estimation method for evaluating the average degree of consolidation and an empirical formula for evaluating the subsoil bearing capacity are proposed to assist in engineering decision making.

Key Words
land reclamation; ground treatment; vacuum preloading; air booster; consolidation

Address
Shuangxi Feng: Department of Civil Engineering, Tianjin University, Tianjin, 300072, China

Huayang Lei and Gang Zheng: 1.) Department of Civil Engineering, Tianjin University, Tianjin, 300072, China
2.) Key Laboratory of Coast Civil Structure Safety of Education Ministry, Tianjin University, Tianjin, 300072, China

Xiaodong Ding: China Zhonghua Geotechnical Engineering Co., Ltd., Beijing, 102600, China

Yawei Jin: Jiangsu Xintai Geotechnical Technology Co. Ltd., Jiangsu, 214213, China


Abstract
Rail transit lines usually pass through many complicated topographies in mountain areas. The influence of inclined bedrock on the train-induced soil vibration response was investigated. Model tests were conducted to comparatively analyze the vibration attenuation under inclined bedrock and horizontal bedrock conditions. A three-dimension numerical model was built to make parameter analysis. The results show that under the horizontal bedrock condition, the peak velocity in different directions was almost the same, while it obviously changed under the inclined bedrock condition. Further, the peak velocity under inclined bedrock condition had a larger value. The peak velocity first increased and then decreased with depth, and the trend of the curve of vibration attenuation with depth presented as a quadratic parabola. The terrain conditions had a significant influence on the vibration responses, and the inclined soil surface mainly affected the shallow soil. The influence of the dip angle of bedrock on the peak velocity and vibration attenuation was related to the directions of the ground surface. As the soil thickness increased, the peak velocity decreased, and as it reached 173% of the embedded pile length, the influence of the inclined bedrock could be neglected.

Key Words
single pile; vibration response; inclined bedrock; rail traffic; model tests; numerical simulation

Address
Xuanming Ding, Jinchuan Yang and Chenglong Wang: 1.) College of Civil Engineering, Chongqing University, Chongqing, 400045, China
2.) Key Laboratory of New Technology for Construction of Cities in Mountain Area, Ministry of Education, Chongqing, 400045, China

Liming Qu: College of Civil Engineering, Southwest Jiaotong University, Chengdu, 610031, China


Abstract
This paper aims at the temperature and slenderness ratio effects on physical and mechanical properties of Beishan granite. A series of uniaxial compression tests with various slenderness ratios and temperatures were carried out, and the acoustic emission signal was also collected. As the temperature increases, the fracture aperture of intercrystalline cracks gradually increases, and obvious transcrystalline cracks occurs when T > 600°C. The failure patterns change from tensile failure mode to ductile failure mode with the increasing temperature. The elastic modulus decreases with the temperature and increases with slenderness ratio, then tends to be a constant value when T = 1000°C. However, the peak strain has the opposite evolution as the elastic modulus under the effects of temperature and slenderness ratio. The uniaxial compression strength (UCS) changes a little for the low-temperature specimens of T < 400°C, but a significant decrease happens when T = 400°C and 800°C due to phase transitions of mineral. The evolution denotes that the critical brittle-ductile transition temperature increases with slenderness ratio, and the critical slenderness ratio corresponding to the characteristic mechanical behavior tends to be smaller with the increasing temperature. Additionally, the AE quantity also increases with temperature in an exponential function.

Key Words
rock; mechanical behavior; acoustic emission; high temperature; slenderness ratio

Address
Qiang Zhang, Yanjing Li and Binsong Jiang: School of Mechanics and Civil Engineering, State Key Laboratory for Geomechanics and Deep Underground Engineering, China University of Mining and Technology, Xuzhou 221116, China

Ming Min: School of Civil and Hydraulic Engineering, Huazhong University of Science and Technology, Wuhan 430074, China

Abstract
Spatial variability is an inherent uncertainty of soil properties. Current reliability analyses generally incorporate random field theory and Monte Carlo simulation (MCS) when dealing with spatial variability, in which the computational efficiency is a significant challenge. This paper proposes a KL-FORM algorithm to improve the computational efficiency. In the proposed KL-FORM, Karhunen-Loeve (KL) expansion is used for discretizing random fields, and first-order reliability method (FORM) is employed for reliability analysis. The KL expansion and FORM can be used in conjunction, through adopting independent standard normal variables in the discretization of KL expansion as the basic variables in the FORM. To illustrate the effectiveness of this KL-FORM, it is applied to a case study of a strip footing in spatially variable unsaturated soil under rainfall, in which the bearing capacity of the footing is computed by numerical simulation. This case study shows that the KL-FORM is accurate and efficient. The parametric analyses suggest that ignoring the spatial variability of the soil may lead to an underestimation of the reliability index of the footing.

Key Words
spatial variability; Karhunen-Loeve expansion; first-order reliability analysis; footing; rainfall

Address
Suozhu Fei, Xiaohui Tan, Xiaole Dong, Fusheng Zha and Long Xu: School of Resources and Environmental Engineering, Hefei University of Technology, Hefei 230009, China

Wenping Gong: Faculty of Engineering, China University of Geosciences, Wuhan, Hubei 430074, China

Abstract
In this study, the strength and failure mechanism of red sandstones with combined defects were investigated by uniaxial compression tests on red sandstones with different crack angles using two-dimensional particle flow code numerical software, and their mechanical parameters and failure process were studied and analyzed. The results showed that the mechanical characteristics such as peak strength, peak strain, and elastic modulus of the samples with prefabricated combined defects were significantly inferior than those of the intact samples. With increasing crack angle from 15° to 60°, the weakening area of cracks increased, elastic modulus, peak strength, and peak strain gradually reduced, the total number of cracks increased, and more strain energy was released. In addition, the samples underwent initial brittle failure to plastic failure stage, and the failure form was more significant, leading to peeling phenomenon. However, with increasing crack angle from 75° to 90°, the crack–hole combination shared the stress concentration at the tip of the crack–crack combination, resulted in a gradual increase in elastic modulus, peak strain and peak strength, but a decrease in the number of total cracks, the release of strain energy reduced, the plastic failure state weakened, and the spalling phenomenon slowed down. On this basis, the samples with 30° and 45°crack-crack combination were selected for further experimental investigation. Through comparative analysis between the experimental and simulation results, the failure strength and final failure mode with cracks propagation of samples were found to be relatively similar.

Key Words
numerical test; crack-crack combination; cracks-hole combination; acoustic emission count; crack propagation

Address
Bing Chen, Zhiguo Xia and Yadong Xu: State Key Laboratory of Mining Disaster Prevention and Control Co-founded by Shandong Province and the Ministry of Science and Technology, No.579, Qianwangang Road, Huangdao District, Qingdao, Shandong Province,266590, China

Shuai Liu: College of Earth Resource Sciences and Engineering, North China University of Water Resources and Electric Power,No.136, Jinshui Dong Road, Zhengzhou, Henan Province,450045, China

Xingzong Liu:College of Civil Engineering, Lu Dong University, No. 186, Hong qi Middle Road, Zhifu District, Yantai, Shandong Province, 264025, China


Abstract
The time-history finite element analysis is usually used to evaluate the seismic response of shallow foundations. However, the literature lacks studies on the influence of the soil constitutive model complexity on the seismic response of shallow foundations. This study, thus, aims to fill this gap by investigating the seismic response of shallow foundation resting on dry silica sand using the linear elastic (LE) model, elastic-perfectly-plastic (EPP) model, and hardening soil with small strain stiffness (HS small) model. These models have been used because it is intended to compare the results of a soil constitutive model that accurately captures the seismic response of the soil-structure interaction problems (which is the HS small model) with simpler models (the LE and EPP models) that are routinely used by practitioners in geotechnical designs. The results showed that the LE model produces a very small seismic settlement value which is approximately equal to zero. The EPP model predicts a seismic settlement higher than that produced using the HS small model for earthquakes with a peak ground acceleration (PGA) lower than 0.25 g for a relative density of 45% and 0.40 g for a relative density of 70%. However, the HS small model predicts a seismic settlement higher than the EPP model beyond the aforementioned PGA values with the difference between both models increases as the PGA rises. The results also showed that the LE and EPP models predict similar trend and magnitude of the acceleration-time relationship directly below the foundation, which was different than that predicted using the HS small model. The results reported in this paper provide a useful benchmark for future numerical studies on the response of shallow foundations subjected to seismic shake.

Key Words
soil constitutive model; finite element analysis; earthquake effect; shallow foundation

Address
Saif Alzabeebee: Department of Roads and Transport Engineering, College of Engineering, University of Al-Qadisiyah, Al-Qadisiyah, Iraq


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