PURPOSES: The objective of this study is to evaluate road subsidence based on model chamber tests. METHODS : A theoretical review of road subsidence mechanisms was carried out, and a series of soil chamber tests with initial cavities were conducted under various conditions. Road subsidence risk was analyzed based on these results. RESULTS: The cavity collapse risk was affected by multiple factors, including cavity location, traffic loading, and asphalt layer thickness. The Nf number of loading required to reach cavity collapse increased as the cavity width increased, cavity depth decreased, and asphalt layer thickness increased. CONCLUSIONS: The effects of asphalt thickness on the risk of road subsidence was assessed to have an additional 1.5-fold effect on the subgrade thickness. This study proposed an effective cavity depth (Deff), considering the strength of the asphalt layer. Based on the results of the model chamber test, a four-class road subsidence risk model was proposed with effective cavity depths and widths. It was found that the risk of road subsidence increased as the cavity width increased and the effective cavity depth decreased. This trend is also well matched to the road subsidence risk models of Japan and Seoul.

PURPOSES: The purpose of this study is to identify the mechanism of road subsidence caused by damaged water and sewer pipes. METHODS: A series of soil chamber test using damaged water and sewer pipe models were conducted under various conditions. RESULTS : Characteristics of cavity expansion and collapse caused by damaged pipes were affected by the damaged location in the sewer pipe, the head on the water pipe, the distance between the damaged water pipe and outlet, and relative soil density. CONCLUSIONS: Sewer-pipe damage was considered a direct cause of road subsidence, and the cavity expanded discontinuously. When the outlet was located under the damaged water pipe, the cavity expanded in the water pipe’s direction, and collapse occurred above the pipe. However, when the outlet was located atop the damaged water pipe, the cavity expanded toward the outlet direction and resulted in a subsidence. Cavity expansion speed was affected by various conditions, such as the pipe’s water head, outlet position, distance between the damaged water pipe and outlet, and relative soil density. However, the cavity expansion shape did not affect factors, except for outlet position.

PURPOSES: The purpose of this study is to evaluate and improve the potential risk of road cave-ins due to subsurface cavities based on the deflection ratio measured with light falling weight deflectometer (LFWD) tests. METHODS : A cavity database for Seoul was developed and sorted. LFWD tests based on the database were conducted on pavement sections with and without road cavities detected by ground-penetrating radar (GPR) tests; after excavating the area, the cavity sizes were measured. The deflection ratio was applied and analyzed by cavity management grade methods of Japan and Seoul. RESULTS : The results of comparative analysis show that the deflection method can detect road cavities in areas of the narrow road (or in narrow areas of the road). The average deflection ratio of the cavity sections to the robust sections were 2.48 for high-risk cavities, 1.85 for medium-risk level cavities, and 1.49 for low-risk cavities. Risk levels in Japan and Seoul were reclassified according to the deflection ratios. CONCLUSIONS : LFWD test results can be applied to verify and improve the subsurface cavity risk level by comparing maximum deflection and deflection ratio between cavity area and non-cavity area at the loading center. LFWD devices also have more advantages compared with larger NDT(Nondestructive test) because FWD and GPR encounter difficulties in traffic control and they could not get in a narrow roads.

PURPOSES: This paper develops a new stochastic approach to analyze the pavement-vehicle interaction model with a certain roughness and elasticity for the pavement foundation, thereby accommodating the deflection of the pavement, and to identify the road subsidence zone represented with a sudden changes in the elasticity of the foundation. METHODS: In the proposed model, a quarter-car model was combined with a filtered white noise model of road roughness and a two-layer foundation (Euler-Bernoulli beam for the top surface and Winkler foundation to represent the sub-structure soil). An augmented state-space model for the subsystems was formulated. Then, because the input is White noise and the system is represented as a single system, the Lyapunov equation governing the covariance of the system’s response was solved to obtain a structurally weak zone index (WZI). RESULTS: The results showed that the WZI from the pavement-vehicle interaction model is sensitive enough to identify road subsidence. In particular, the WZI rapidly changed with a small change in foundation elasticity, indicating that the model has the potential to detect road subsidence in the early stage. CONCLUSIONS: Beacause of the simplicity of the calculation, the proposed approach has potential applications in managing road conditions while a vehicle travels along the road and detecting road subsidence using a device with an on-board computational capability, such as a smart phone.

PURPOSES: The purpose of this study is to identify the road-subsidence mechanism in unsaturated sandy soils. METHODS: A series of soil chamber tests were conducted under various conditions. RESULTS: The cavity-expansion characteristics in unsaturated sandy soils due to seepage were affected by the outlet size, seepage intensity, relative density, and fine content. CONCLUSIONS: In unsaturated sandy soils, the cavity-expansion speed was affected by the outlet size, relative density, seepage intensity, and clay content; however, the cavity-expansion shape was very similar. As the outlet size and seepage intensity increased, the cavityexpansion speed increased. As the relative density increased, the cavity-expansion speed increased because of a sudden decrease in shear strength, resulting from the increased saturation (reduction of matric suction). The cavity expanded faster with the increasing clay content, up to a certain threshold. It expanded at a slower rate once it passed the threshold. Finally, it reached a stable state where the cavity did not expand due to seepage.

PURPOSES : The objective of this research is establishing system components and optimizing operational procedures in order to systematically manage road cave-ins in urban areas.METHODS: Based on the literature review and alternative comparison, optimization methods is suggested.RESULTS : Throughout the study, location referencing system, database structure, and operation strategy(procedure) were clarified, and the optimization methods for each item were suggested.CONCLUSIONS: Road cave-in management should be focused on user safety rather than focusing on economic aspects. The occurrence of road cave-in should be addressed thoroughly by road management system(location referencing system, database structure, and operation strategy(procedure), and the optimization methods), since they are closely related to road users' safety.

PURPOSES : The objective of this study is to evaluate the potential risk level of road cave-ins due to subsurface cavities based on the deflection basin measured with falling weight deflectometer (FWD) tests. METHODS: Ground penetrating radar (GPR) tests were conducted to detect road cavities. Then FWD tests were conducted on 13 pavement test sections with and without a cavity. FWD deflections and a deflection ratio was used to evaluate the effect of geometry of the cavity and pavement for road cave-in potentials. RESULTS: FWD deflection of cavity sections measured at 60 cm or a closer offset distance to a loading center were 50% greater than more robust sections. The average deflection ratio of the cavity sections to robust sections were 1.78 for high risk level cavities, 1.51 for medium risk level cavities, and 1.16 for low risk level cavities. The relative remaining service life of pavement with a cavity evaluated with an surface curvature index (SCI) was 8.1% for the high level, 21.8% for the medium level, and 89.8% compared to pavement without a cavity. CONCLUSIONS : FWD tests can be applied to detect a subsurface cavity by comparing FWD deflections with and without a cavity measured at 60 cm or a closer offset distance to loading center. In addition, the relative remaining service life of cavity sections based on the SCI can used to evaluate road cave-in potentials.

본 연구는 도로함몰과 관련된 연구 중 위험도 등급에 따른 소성변형 특성을 파악하기 위해 수행되었다. 도로함몰 위험도 등급 A, B에 해당하는 지표면으로부터 60cm, 90cm 하부에 모의 동공(얼음)을 설치하였으며, 동공의 크기는 그림 1과 같이 너비와 높이는 50cm, 길이는 100cm, 200cm로 각각 설계하였다. 또한, 배수층을 20cm 설치하여 배수가 가능하도록 설계하였다. 포장가속시험은 4ton의 축하중으로 10km/h의 속도로 운영하였으며, 1,000회, 2,000회, 6,000회, 10,000회 운행 시 마다 소성변형량 변화를 측정하였다. 그 결과 동공의 크기보다는 동공의 깊이가 소성변형에 영향을 크게 미치는 것으로 파악되었다. 그림 1의 1,3번 동공은 폭과 높이는 유사하지만 길이가 각각 1m, 2m로 위험도 등급에서 B, A이다. 하지만, 소성변형량을 비교했을 때 각 횟수마다 비슷한 경향을 보이며 증가하는 추세를 볼 수 있었다. 또한, 2번 동공의 경우 포장층에 가깝게 동공이 형성되어 1,3번 동공과 달리 소성변형량 증가 추세가 다른 동공들에 비해 월등히 빨랐으며, 6,000회 이상 포장가속시험을 수행하기 어려울 정도로 소성변형이 심하게 나타났다. 포장가속시험 수행 결과 동공 생성 초기에 소성변형량 변화폭이 가장 크며, 동공 길이보다는 심도에 포장층이 영향을 크게 받는 것으로 나타났다.

본 연구진은 도로함몰 동공평가를 위한 선행연구를 바탕으로 LFWD 장비를 이용한 건전구간과 비건전구간의 처짐값을 비교하여 동공 유무 예측식을 개선하였다. 기존 연구의 경우, 도로함몰이 발생한 구간에서의 건전 구간과 비전건 구간의 도로의 처짐을 비교한 결과로 처짐비가 1.5∼2.5에 수렴하는 것으로 나타났으며. 대부분의 깊이비가 1.5이내의 제한을 가지고 있음을 알 수 있었다 또한 기존의 DB구간의 데이터가 아닌 타 구간의 데이터를 기존에 제시한 동공규모 관계식에 적용한 결과 동공규모 예측 시에 오차가 큼을 알 수 있었다. 본 연구진은 이러한 오차의 이유로 크게 두 가지의 이유로 구분하여 개선하고자 하였다. 첫 번째로, 기존 제안식에서 고려하지 않은 함몰평가 인자가 존재 할 수 있다는 점이었다. 따라서 본 연구진은 제안식 안의 여러 인자 분석을 진행하였으며, 분석결과 현재 처짐비의 제곱값과 동공깊이를 추가 고려한 인자로 채택하였을 때, 동공규모 예측을 개선할 수 있었다. 두 번째로는 깊이비가 1.5이상인 심도가 깊은 곳일 경우 건전구간과 비건전구간의 LFWD(Light Falling Weight Deflectometer) 처짐 값의 차이가 상이하지 않았다. 이에 처짐값의 차이를 비교 분석하여 깊이비가 1.5이상인 도로함몰에서의 LFWD 처짐값을 이용한 동공 존재 예측이 가능한지 여부에 대한 지속적인 연구가 필요함을 알 수 있었다.

최근 도심지의 돌발적인 도로함몰로 인명해가 발생하고, 시민들에게 불안감을 야기하는 사례가 빈번히 발생하여 사회적 이슈로 부각되고 있다. 이에, 2015년 세종대학교 산학협력단을 주관으로 국토교통부, 국토교통과학진흥원이 지원하는「도로함몰 위험도 평가 및 분석기술 개발」에 대한 연구가 시작되었다. 본 연구는 장기적이고 근본적으로 도로함몰을 예방하기 위한 도로함몰 위험도 탐지시스템 및 분석 프로그램 개발, 도로함몰 위험도 평가기술 개발, 재난 예방형 도로관리 시스템 구축을 목표로 현재 3차년도 연구가 진행 중이다. 현재, 지표투과레이더와 노면영상 수집장치가 포함된 도로함몰 복합탐지 시스템을 구축하여 운용 중에 있으며 (그림 1), 해당 장비로부터 수집된 데이터를 이용하여 도로 지하시설물 및 동공탐사를 할 수 있도록 분석 자동화 알고리즘을 개발하고, 현장적용을 위한 GUI를 구축하여 그 성능을 검증하였다.

최근 전 세계적으로 SNS, 매스컴 등을 통해 싱크홀(Sinkhole)이 사회적 이슈가 됐다. 특히, 포장도로 하부 동공으로 인해 발생하는 도로함몰은 도로이용자에게 직접적인 안전상의 위험이 되기 때문에 시민들의 불안이 확산되고 있다. 서울시(2014)는 도로함몰에 대한 시민들의 불안을 조기에 해소하고 보다 안전하게 통행할 수 있는 도로환경을 조성하고자 ‘도로함몰 특별관리 대책’을 수립하여 발표했다. 환경부(2015)는 노후관로 약 40,000km 구간을 대상으로 정밀조사를 실시했으며 ‘하수관로 정밀조사 매뉴얼’을 발간했다. 국토부는 2018년부터 예방형 재난피해 지하시설물 관리를 위한 ‘지하시설물의 안전관리에 대한 특별법’을 공포했다. 이는 도로함몰의 사후 대책, 하수관 및 지하시설물의 관리 법률 등의 개선이다. 그러나 각 기관의 이러한 노력에도 불구하고 도로함몰을 체계적으로 관리하고 운영하는 시스템은 미흡한 상태이다. 도로함몰은 도심에 대규모로 발생하는 경우 도시의 이미지를 실추하고 도로이용자에게 안전상의 직접적 위협과 물적 피해를 야기하기 때문에 도로함몰에 집중된 관리전략이 필요하다. 전략에는 도로함몰의 조사, 분석, 예측, 평가, 보수, 모니터링, feedback의 체계적인 운영 절차와 구성요소의 최적화가 필요하다. 따라서 본 연구의 목적은 도로함몰관리시스템을 구성하는 위치정보, DB, 운영전략, 운영절차의 정의와 최적화 방안을 정리하고 분석하는 것이다. 그림 1.은 본 연구에서 제시하는 도로함몰 관리 시스템의 Tracking Tool과 Analysis Tool 사이의 System Framework의 구성이다

PURPOSES : The objective of this study is to evaluate the effect of size and depth of cavities on the pavement failure using the full-scale accelerated pavement testing. METHODS: A full-scale testbed was constructed by installing the artificial cavities at a depth of 0.3 m and 0.7 m from the pavement surface for accelerated pavement testing. The cavities were made of ice with a dimension of 0.5 m*0.5 m*0.3 m, and the thickness of asphalt and base layer were 0.2 m and 0.3 m, respectively. The ground penetrating radar and endoscope testing were conducted to determine the shape and location of cavities. The falling weight deflectometer testing was also performed on the cavity and intact sections to estimate the difference of structural capacity between the two sections. A wheel loading of 80 kN was applied on the pavement section with a speed of 10 km/h in accelerated pavement testing. The permanent deformation was measured periodically at a given number of repetitions. The correlation between the depth and size of cavities and pavement failure was investigated using the accelerated pavement testing results. RESULTS : It is found from FWD testing that the center deflection of cavity section is 10% greater than that of the intact section, indicating the 25% reduction of modulus in subbase layer due to the occurrence of the cavity. The measured permanent deformation of the intact section is approximately 10 mm at 90,000 load repetitions. However, for a cavity section of 0.7 m depth, a permanent deformation of 30 mm was measured at 90,000 load repetitions, which is three times greater than that of the intact section. At cavity section of 0.3 m, the permanent deformation reached up to approximately 90 mm and an elliptical hole occurred at pavement surface after testing. CONCLUSIONS : This study is aimed at determining the pavement failure mechanism due to the occurrence of cavities under the pavement using accelerated pavement testing. In the future, the accelerated pavement testing will be conducted at a pavement section with different depths and sizes of cavities. Test results will be utilized to establish the criteria of risk in road collapse based on the various conditions.

PURPOSES: This study identifies the causes and the mechanism of the occurrence of underground cavities. METHODS: A case study on cave-in and a series of model tests with a small soil chamber were conducted. RESULTS: A hypothesis about the mechanism of the cave-in in road was established, and the basic influencing factors on underground cavity expansion were identified. CONCLUSIONS: It was found that the characteristics of shear strength of soil and direction of water flow had a larger influence on cavity formation and expansion than the characteristics of internal erosion. In addition, large cavities suddenly expanded when cavities were caused owing to breakage of buried sewer pipe.

최근 들어 도심지를 중심으로 도로함몰이 증가하고 있다. 도로함몰은 때로는 인명피해를 수반하는 심 각한 사고로 이어지기도 하고 교통흐름 방해를 초래하는 등 시민안전의 위협 요인이 되고 있다. 본 연구 에서는 도로함몰의 기초적인 매커니즘 규명을 위해 사례분석과 2차원 모형토조 시험을 수행하였다. 사례 분석을 통해 도로함몰의 공학적 특성과 매커니즘에 대한 가설을 수립하고 도로하부의 공동 발생･확장과 관련된 기본적인 영향요소를 도출하였다. 모형토조 시험은 다양한 조건에서의 도로함몰의 정성적인 특성 규명에 초점을 두고 진행하였고, 시험결과로부터 도로함몰의 기본적인 매커니즘을 제시하였다. 모형토조시험 결과, 동공의 발생 및 확장은 내부침식 특성보다는 토사의 전단강도 및 물의 흐름 방향이 중요한 영향요소로 나타났다. 하수관거의 손상에 의한 동공의 확장은 연속적으로 발생하는 것이 아니라 갑작스러운 확장 및 일시적 안정을 반복하는 형태로 나타나고, 손상된 하수관거 상부에 전단강도를 급격 하게 떨어뜨리면서 전체적인 동공의 발생 규모는 매우 큰 형태로 나타났다. 상수관과 하수관의 손상이 복 합적으로 발생한 경우, 동공은 상수관거 손상부위로 동공이 확장되었다.