• Tunnel and Underground Space
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• v.24 no.6
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• pp.440-448
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• 2014

Hydraulic fracturing technique has been applied in various fields in order to improve the recovery of energy resources such as gas, oil and geothermal energy and research about finding out hydraulic fracturing mechanism and application has been steadily proceeded. In this study, for effective hydraulic fracturing, a scale modeling was progressed to simulate similarly with the actual site. In order to analyze the development aspect of surface crack initiation pressure during hydraulic fracturing followed by different conditions, the number of guide holes hydraulic fracturing test was carried out by setting up a hydraulic fracturing test equipment. Also, through the result, we tried to derive reliable results by comparing and analyzing the value of numerical modeling which is obtained based on the physical properties and mechanical properties with 3DEC, a three-dimensional discrete element method program. As a result, it is considered possible to generate effective crack using the guide hole.

• Journal of the Korean Geotechnical Society
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• v.17 no.4
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• pp.93-105
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• 2001

해성토층(풍화토 및 모래질 충적토가 암반 위에 쌓인) 위에 준설매립된 수도권 해안매립지역에서 원형의 대심도 굴착공사로 인하여 발생하는 지중연속벽의 수평변위에 대한 현장계측을 중심으로 연구를 수행하였다. 현장측정으로는 지중연속벽의 8방향에서 벽체수평변위와 철근응력, 토압, 간극수압 등이 측정되었고, 정확한 측정결과를 얻기 위하여 합리적인 해석 및 보정방법이 연구되었다. 현장측정결과 굴착시공단계에 따라서 벽체수평변위가 증가하였으며, 일정깊이에 존재하는 점토층을 굴착함으로써 간극수압의 급격한 변화가 측정되었다. 한편, 굴착전후의 구속압감소에 따른 지반물성치의 변화를 정리하였다. 굴착전후의 탄성계수는 일정깊이까지 상당한 정도로 감소됨을 알 수 있었고 굴착전후 여러 가지 시험방법에 따른 탄성계수의 차이들을 비교하였다.

• Geotechnical Engineering
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• v.14 no.6
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• pp.45-56
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• 1998

In order to select a proper ground improvement technology and to assess the quality and rate of improvement in the soft deposits. it is essential to characterize in-situ properties of the soft marine clay layer that may have many thin silt or sand seams. In this paper, both piezocone and flat dilatometer tests were performed to characterize in situ properties of a marine clay. Both tests provided quite similar site classifications, and in both tests the penetration pore water pressure was the better indicator for the classification of marine clay layer, especially in which sand or silt seams are frequently interbedded. Undrained strengths determined by both the cone tip resistance and the excess pore water pressure measured from piezocone were very similar in clayey soil layers. And the untrained strength determined by dilatometer had an approximately average value of undiained strengths obtained from piezocone. In addition, the theoretical time factor that can consider pore pressure dissipation effect during cone penetration may provide a reliable estimation of the coefficient of consolidation, especially for a coastal site which includes many silt or sand fractions or seams.

• Tunnel and Underground Space
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• v.30 no.3
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• pp.242-255
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• 2020

The numerical simulations of Heater Experiment-D (HE-D) at the Mont Terri rock laboratory in Switzerland were performed to investigate an applicability of FLAC3D to reproduce the coupled thermo-hydro-mechanical (THM) behaviour in Opalinus Clay, as part of the DECOVLEX-2015 project Task B. To investigate the reliability of numerical simulations of the coupled behaviour using FLAC3D code, the simulation results were compared with the observations from the in-situ experiment, such as temperature at 16 sensors, pore pressure at 6 sensors, and strain at 22 measurement points. An anisotropic heat conduction model, fluid flow model, and transversely isotropic elastic model in FLAC3D successfully represented the coupled thermo-hydraulic behaviour in terms of evolution for temperature and pore pressure, however, performance of the models for mechanical behavior is not satisfactory compared with the measured strain.

• Journal of the Korean Geotechnical Society
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• v.15 no.4
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• pp.85-97
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• 1999

For a deltaic clay in the mouth of the Nakdong river, OCR was investigated through methods using the results of field measurement, laboratory and field soil tests. As a result, OCRs were obtained around the range of 0.95 to 1.20 by analysis of field measurements, although they were estimated around the values of 0.4 to 0.7 by the results of conventional consolidation tests for the clay. From the dissipation test it was found out that the excess pore pressures scarcely existed in the clay deposit and then the soil was not in the underconsolidated condition. And the OCRs obtained through methods of Mayne(1991) and Cao et al(1996) using the piezocone test and of Mayne & Kemper(1988) using the cone penetration test were in good agreement with those of field measurement.

• Geotechnical Engineering
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• v.14 no.4
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• pp.141-150
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• 1998

Based on the authors' previous research, we have conducted more researches on finding out the coefficient of consolidation from Piezocone dissipation test results, especially measured behind the cone tip which is mostly used in Korea, by adopting the optimization technique. By analyzing numerical and real field examples, it can be found that the adopted optimum technique that minimizes the differences between the predicted dissipation curve and the measured one gives consistent and convergent results, irrespective of initial values. Such technique also provides horizontal coefficient of consolidation which is able to simulate real field consolidation behavior more effectively.

• Tunnel and Underground Space
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• v.11 no.3
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• pp.237-247
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• 2001

In this study, the fracture mechanics model as well as the elastic model was reviewed theoretically and four field case studies were conducted to investigate the feasibility of fracture mechanics model for hydraulic fracturing test. There was a difference between the result by fracture mechanics model and the one by elastic model. And the smaller initial crack length is, the larger the difference is. It is considered that the fracture mechanics model can be applied to the specific case of which the crack length is known. In this study, the rock tensile strength is measured using fracture mechanics model, brazilian test and elastic model. The measured tensile strength by the fracture mechanics model is the largest and the elastic model is the smallest. This result is due to the size effect of the each test. And the tensile strength from the elastic model for hydraulic fracturing test can be used to estimate the in-situ rock tensile strength.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.28 no.6C
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• pp.339-347
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• 2008

The penetration of the probe produces the excess pore pressure due to the disturbance. The objective of this study is to evaluate the disturbance zone by using the dissipation of the excess pore water pressure, which was generated due to the penetration of the penetrometer with different size. The CPT, DMT and FVP (Field Velocity Probe) are adopted for in-situ tests. The tests are carried out in the construction site of north container pier of Busan new port, Korea where is accelerating the consolidation settlement using plastic board drains (PBD) and surcharges by crushed gravels. The coefficient of consolidation $(C_h)$ and soil properties are deduced by the laboratory test. The in-site tests are performed after the predrilling the surcharge zone at the point of 90% degree of consolidation. To minimize the penetration effect, the horizontal distance between penetration tests is 3m, the change of the pore pressure is monitored at the fixed depth of 24m. The coefficient of consolidation $(C_h)$ and the $t_{50}s$ are calculated based on the laboratory test and the in-situ data, respectively. The equvalent radi based on the $t_{50}$ shows that the FVP and the DMT produce the smallest and the greatest equivalent radi, respectively.

• Tunnel and Underground Space
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• v.18 no.1
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• pp.80-89
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• 2008

The influence of in-situ rock stress on the stability of an underground rock structure increases as the construction depth become deeper and the scale of a rock structure become larger. In general, hydraulic fracturing stress measurement has been performed in the surface boreholes of the target area at the design stage of an underground structure. However, for some areas where the high horizontal stresses were observed or where the overstressed conditions caused by topographical and geological factors are expected, it is desirable to conduct additional in-situ stress measurement in the underground construction site to obtain more detailed stress information for ensuring the stability of a rock structure and the propriety of current design. The study area was a construction site for the additional underground oil storage facility located in the south-east part of OO city, Jeollanam-do. Previous detailed site investigation prior to the design of underground structures revealed that the excessive horizontal stress field with the horizontal stress ratio(K) greater than 3.0 was observed in the construction area. In this study, a total of 13 hydraulic fracturing stress measurements was conducted in two boreholes drill from the two water tunnel sites in the study area. The investigation zone was from 180 m to 300 m in depth from the surface and all of the fracture tracing works were carried out by acoustic televiewer scanning. For some testing intervals at more than 200 m ind depth from surface, the high horizontal stress components the horizontal stress ratio(K) greater than 2.50 were observed. And the overall investigation results showed a good agreement with the previously performed test.

• Journal of Korean Tunnelling and Underground Space Association
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• v.4 no.1
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• pp.57-70
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• 2002

Since early in the 90's, the need for construction of underground rock structures such as long and large section traffic tunnel, energy storage cavern, industrial facility, etc. has been largely increased because the Korean territory is not wide and about 65 % of the land consists of mountainous region. The initial rock stress measurement has been widely conducted to provide the quantitative information on the stress state of engineering site at the design stage of underground rock structures. Among the diverse methods developed for measuring rock stress, hydraulic fracturing test is most popularly used because it is applicable at pre-construction stage and has no limit in testing depth. In this paper, the characteristics of initial rock stress state in mountainous region were studied on the basis of the in-situ hydraulic fracturing stress measurement results from the 60 test boreholes in various parts of Korea.