At the peak of the Vietnam War, the network of tunnels in the Iron Triangle and Cu Chi linked Viet Cong (VC) support bases over a distance of some 250 km, from the Ho Chi Minh Trail and Cambodian border to the outskir...At the peak of the Vietnam War, the network of tunnels in the Iron Triangle and Cu Chi linked Viet Cong (VC) support bases over a distance of some 250 km, from the Ho Chi Minh Trail and Cambodian border to the outskirts Saigon. In the early 1960s, the United States escalated its military presence in Vietnam in support of a non-Communist regime in South Vietnam. The North Vietnamese and VC troops gradually expanded the tunnels. Tunnels frequently were dug by hand in Old Alluvium terraces, and only a short distance at a time. Four major efforts were made by the US Military to locate and destroy these tunnels. These included Operation Crimp, a search and destroy mission which began in 1966 and a geological and soil survey approach was used to detect VC tunnels. Later in 1967, General William Westmoreland tried launching a larger assault on Cu Chi and the Iron Triangle areas. The operation called Operation Cedar Falls was an expanded version of Operation Crimp. Finally in 1969, B-52s started carpet bombing the Cu Chi and Iron Triangle areas and destroyed many of the tunnels. However, not before the tunnels had proven very effective in 1960s at hiding and protecting the VC during US occupation of the area. The nature and properties of the Old Alluvium soils were key to the soil tunnels being so resilient. Soils located in Old Alluvium terraces had high levels of clay and iron. Iron (Fe) leached from the upper soil layers (0 to 1.5 m) and accumulated in the lower layers (1.5 to 20 m) and became a cement-like binding agent. When dried the soil layers took on properties close to concrete, and were resistant to ever becoming soft and moist again especially around the aerated tunnel walls. The tunnels were dug in the monsoon season when the upper layers of soil were soft and moist but not in dry season. The soils were highly stable without any lining or support. After drying out, the soil materials surrounding the tunnel turned into concrete like material that could withstand adjacent explosive blasts.展开更多
Ground movements due to tunneling are becoming increasingly critical as buildings are located around construction sites.This study proposes a new combined reinforcement method using a foundation grouting oblique pipe ...Ground movements due to tunneling are becoming increasingly critical as buildings are located around construction sites.This study proposes a new combined reinforcement method using a foundation grouting oblique pipe roof.The former improves the bearing capacity of the subsoil,and the latter blocks the transmission of soil deformation,which weakens the influence of construction during overlapped tunnel under-crossing.Based on this new method,a case study of the shield tunneling response to an old building in Line 6 of China’s Chengdu Metro is presented.Additionally,three-dimensional numerical models without reinforcement,traditional foundation grouting reinforcement,and the new combined reinforcement schemes were compared.The numerical simulation performance was verified using a set of field instrumentation data,which demonstrated that the old building response to the overlapped tunnels was under control,and the maximum deformation,angular distortion,and principal tensile strain of the building were 5.25 mm,5.1010–6 rad/m,and 0.0081%,respectively.Compared with the traditional reinforcement scheme,the deformation,angular distortion,and principal tensile strain in the combined reinforcement scheme were reduced by 54.78%,71.02%,and 70.22%,respectively.These results have important implications for the design and construction of shield tunnels and their response to old buildings.展开更多
文摘At the peak of the Vietnam War, the network of tunnels in the Iron Triangle and Cu Chi linked Viet Cong (VC) support bases over a distance of some 250 km, from the Ho Chi Minh Trail and Cambodian border to the outskirts Saigon. In the early 1960s, the United States escalated its military presence in Vietnam in support of a non-Communist regime in South Vietnam. The North Vietnamese and VC troops gradually expanded the tunnels. Tunnels frequently were dug by hand in Old Alluvium terraces, and only a short distance at a time. Four major efforts were made by the US Military to locate and destroy these tunnels. These included Operation Crimp, a search and destroy mission which began in 1966 and a geological and soil survey approach was used to detect VC tunnels. Later in 1967, General William Westmoreland tried launching a larger assault on Cu Chi and the Iron Triangle areas. The operation called Operation Cedar Falls was an expanded version of Operation Crimp. Finally in 1969, B-52s started carpet bombing the Cu Chi and Iron Triangle areas and destroyed many of the tunnels. However, not before the tunnels had proven very effective in 1960s at hiding and protecting the VC during US occupation of the area. The nature and properties of the Old Alluvium soils were key to the soil tunnels being so resilient. Soils located in Old Alluvium terraces had high levels of clay and iron. Iron (Fe) leached from the upper soil layers (0 to 1.5 m) and accumulated in the lower layers (1.5 to 20 m) and became a cement-like binding agent. When dried the soil layers took on properties close to concrete, and were resistant to ever becoming soft and moist again especially around the aerated tunnel walls. The tunnels were dug in the monsoon season when the upper layers of soil were soft and moist but not in dry season. The soils were highly stable without any lining or support. After drying out, the soil materials surrounding the tunnel turned into concrete like material that could withstand adjacent explosive blasts.
基金funding provided by the National Natural Science Foundation of China(Grant No.51808469)the Basic Applied Research Projects of the Sichuan Science and Technology Department(Grant No.2022NSFSC0442).
文摘Ground movements due to tunneling are becoming increasingly critical as buildings are located around construction sites.This study proposes a new combined reinforcement method using a foundation grouting oblique pipe roof.The former improves the bearing capacity of the subsoil,and the latter blocks the transmission of soil deformation,which weakens the influence of construction during overlapped tunnel under-crossing.Based on this new method,a case study of the shield tunneling response to an old building in Line 6 of China’s Chengdu Metro is presented.Additionally,three-dimensional numerical models without reinforcement,traditional foundation grouting reinforcement,and the new combined reinforcement schemes were compared.The numerical simulation performance was verified using a set of field instrumentation data,which demonstrated that the old building response to the overlapped tunnels was under control,and the maximum deformation,angular distortion,and principal tensile strain of the building were 5.25 mm,5.1010–6 rad/m,and 0.0081%,respectively.Compared with the traditional reinforcement scheme,the deformation,angular distortion,and principal tensile strain in the combined reinforcement scheme were reduced by 54.78%,71.02%,and 70.22%,respectively.These results have important implications for the design and construction of shield tunnels and their response to old buildings.
