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2014ABSTRACT : Korea has four distinct seasons, showing hot and humidity in summer and cold weather lasted in winter. Domestic research on earth work has been developed according to the seasonal characteristics, and most of research topics have focused on the effect of freezing-thawing on the performance of geo-materials. However, the previous research was performed on the ground compacted at room temperature and therefore, the effect of the sub-zero temperature at the time of construction was not fully investigated. The ground characteristics compacted at freezing temperature can be different from those at room temperature and show different characteristics of strength and deformation caused by freezing and thawing. Therefore, the compaction tests on sandy materials were conducted under various temperature at -3℃ and -8℃ with various fine contents of 0%, 5%, 10% and 15% in weight fraction. The effectiveness of soil compaction at below-freezing temperatures were compared with the compaction at room temperature at 18℃ in terms of the maximum dry unit weight and optimum water contents. Based on the test results, the maximum dry unit weight tends to decrease with the freezing temperature and the relative compaction at -8℃ can not be satisfied with general specification standard.Keywords : Below-freezing, Soil compaction, Maximum dry unit weight요지 : 우리나라는 사계절이 뚜렷하며 여름에는 고온다습한 무더운 날씨를 보이고 겨울에는 혹한의 날씨가 지속된다 . 이러한 계절적 특성을 고려하여 국내 토공 분야에서는 많은 연구들이 있었으나 대부분의 연구가 지반의 동결 ･융해에 초점을 맞추어 이루어졌다. 이러한 연구들은 시공 당시의 기온에 따른 지반특성을 반영하지 못하였으며 상온에서 다져진 지반을 대상으로 실시되었다 . 그러나 동절기 다짐의 경우 상온에서의 다짐과는 다른 지반상태를 나타낼 것으로 예상되며 , 그에 따른 동결･융해로 인한 강도 및 변형 특성 또한 다르게 나타날 것으로 판단되나 이를 고려한 연구는 아직까지 전무한 상황이다 . 따라서 본 연구에서는 -3℃와 -8℃에서 주문진 표준사에 카올리나이트를 0%, 5%, 10%, 15%의 중량비로 혼합하여 다짐시험을 실시하였으며 , 상온(18℃)에서의 다짐곡선과 비교하여 온도 및 세립분 함량에 따른 다짐 효과를 살펴보았다. 시험 결과, 동결 온도에서 다짐 시 최대건조단위중량이 감소하는 경향을 보였으며, -8℃ 이하에서 다짐 시공 시 시방서의 다짐기준을 만족하지 못하는 것으로 나타났다 .주요어 : 동결 온도, 다짐, 최대건조단위중량

2014ABSTRACT : In winter season, the pore water inside the ground freezes and thaws repetitively due to the cold air temperature. When the freezing-thawing processes are repeated on the ground, the change in soil particle structure occurs and thus the damage of the infrastructure may be following. This study was performed in order to investigate the stiffness change of soils due to the freeze-thaw by using elastic waves. Sand-silt mixtures are prepared with in the silt fraction of 40 %, 60 % and 80 % in weight and in the degree of saturation of 40 %. The specimens are placed into the square freezing-thawing cell by the temping method. For the measurement of the elastic waves, a pair of the bender elements and a pair of piezo disk elements are installed on the cell, and a thermocouple is inserted into soils for the measurement of the temperature. The temperature of the mixtures is decreased from 20℃ to -10℃ during freezing, is maintained at -20℃ for 18 hours, is gradually increased up to the room temperature of 20℃ to thaw the specimens. The shear waves, the compressional waves and the temperature are measured during the freeze-thaw process. The experimental result indicates that the shear and the compressional wave velocities after thawing are smaller than those of before freezing. The velocity ratio of after thawing to before freezing of shear wave is smaller than that of the compressional wave. As silt fraction increases from 40 % to 80 %, the shear and compressional wave velocities are gradually increased. This study suggests that the freezing-thawing process in unsaturated soil loosens the soil particle structure, and the shear wave velocity reflects the effect of freezing-thawing more sensitively than the compressional wave velocity.Keywords : Elastic waves, Temperature, Frozen soil, Freeze-thaw, Sand-silt mixture요지 : 겨울철의 낮은 기온으로 인하여 지반 내부의 간극수는 동결과 융해를 반복한다 . 지반에 이러한 동결-융해작용이 반복되면 흙의 입자구조 변형이 발생하며 이는 지중 기반시설에 손상을 가져올 수 있다 . 본 연구는 흙의 동결-융해 과정에서의 탄성파 속도 변화를 통하여 흙의 강성 변화 양상을 알아보기 위하여 수행되었다 . 40 %, 60 %, 80 %의 3가지 모래-실트의 무게비를 가진 모래 -실트 혼합토를 포화도 40 %, 상대밀도 70 %로 동일하게 조성하였다. 각 시료를 동결-융해를 위해 제작된 사각형 형태의 셀에 다짐법으로 조성하였다. 탄성파를 측정하기 위하여 한 쌍의 벤더 엘리먼트와 피에조 디스크 엘리먼트를 시료 양편에 설치하였으며 , 시료의 온도 변화 양상을 관찰하기 위하여 탄성파 트랜스듀서와 같은 깊이의 중앙부에 열전대를 설치였다 . 조성한 시료에 대하여 시료를 20 ℃에서 -10 ℃까지 동결시킨 후 -20 ℃를 18시간 동안 유지하였으며, 다시 실험실 상온(20 ℃)까지 온도를 서서히 올려 융해시켰다 . 이 과정에서 온도, 전단파, 그리고 압축파를 측정하였다 . 연구결과, 융해 이후의 탄성파 속도는 같은 온도의 동결 이전보다 감소하였다. 이때 전단파의 속도가 압축파의 속도보다 더 큰 비율로 감소하는 모습을 보였다 . 실트의 함량이 40 %에서 80 %까지 증가함에 따라 탄성파의 속도는 증가하는 양상을 보였다. 본 연구를 통해 동결-융해가 불포화토의 입자구조를 느슨하게 만들며, 그 영향은 압축파에 비해 전단파 속도의 변화에서 잘 나타남을 알 수 있었다 .주요어 : 동결-융해, 동결토, 실트함량, 온도변화, 탄성파

Due to the latitudinal location of Korea, the seasonally frozen ground has been focused on as research topics such as the frost heaving under the asphalt road rather than the permafrost ground. However, the recent construction of the second Korean Antarctic research station, the Jangbogo station and the participation on the development of the natural gas pipeline in Russia arouse the research interests on the behavior of the permafrost ground. At the design process of the geotechnical structures on the permafrost ground, the evaluation of the creep characteristics of the frozen soil is very crucial. Since the domestic specification on the frozen soil testing does not exist currently, it is necessary to evaluate the creep characteristics of frozen soils systematically with regard to the affecting factors. Therefore, the creep characteristics of the frozen specimens of dense Jumoonjin sand were evaluated under various loads at -5 and . Based on the test results, as the load became close to the strength and the temperature became lower, the duration of the secondary creep became shorter and more distinct tertiary creep responses were observed.

Bearing capacity of pile is governed by only skin friction in frozen ground condition, while it is generally governed both by skin friction and end bearing capacity in typically unfrozen ground condition. Skin friction force, which arises from the interaction between pile and frozen soils, is defined as adfreeze bond strength, and adfreeze bond strength is one of the most important key parameters for design of pile in frozen soils. Many studies have been carried out in order to analyze adfreeze bond strength characteristics over the last fifty years. However, many studies for adfreeze bond strength have been conducted with limited circumstances, since adfreeze bond strength is sensitively affected by various influence factors such as intrinsic material properties, pile surface roughness, and externally imposed testing conditions. In this study, direct shear test is carried out inside of large-scaled freezing chamber in order to analyze the adfreeze bond strength characteristics with varying freezing temperature and normal stress. Also, the relationship between adfreeze bond strength and shear strength of the frozen soil obtained from previous study was analyzed. The coefficient of adfreeze bond strength was evaluated in order to predict adfreeze bond strength based on shear strength, and coefficients suggested from this and previous studies were compared.