Why Were the Soil Tunnels of Cu Chi and Iron Triangle in Vietnam So Resilient?

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.

Review and Analysis: Successful Use of Soil Tunnels in Medieval and Modern Warfare and Smuggling

For more than 2500 years, soil tunnels have been used in warfare and smuggling. Initially tunnels were utilized to attack fortresses that were underlain by unconsolidated (non-bedrock) soil materials. Later tunnels provided housing and served as smuggling corridors. The medieval warfare undermining technique involved digging soil tunnels with wooden or beam props to hold up the soil ceilings. Then flammable material, such as hay or straw, was put in the tunnel and set on fire. The fire burnt the support beams which collapsed the soil tunnel ceilings and undermined the overlying perimeter wall. Later gunpowder and dynamite replaced fire when attempting to collapse a tunnel, fortress or perimeter defense. Modern warfare soil tunnels were the pathways used to move troops, weapons and supplies to the other side of a border or wall for surprise attacks. Most of the soil tunnels were placed in easy-to-dig unconsolidated soil materials that had a low water table and were not subject to flooding. Eventually, machinery was used to drill through bedrock permitting deeper and longer tunnels for troop movement or smuggling. However, when drilling through bedrock under international borders, the process creates both noise and vibrations which were often detected by the enemy. Once discovered the tunnels were often collapsed by blowing up the tunnel, injection of gas, filling with water or wastewater, or inserting barriers. A series of case studies will be examined with the goal of determining soil and site criteria required to permit successful tunneling. The most restrictive soil and geologic conditions will be identified as well as potential mitigation methods used to overcome the site restrictions will be documented. Countries with warfare or smuggling issues along their borders, such as Israel and United States, need to identify the sections of the border most likely to be undermined by soil tunnels. In the case of Israel their entire border is susceptible as a result of the favorable arid climate, soils and geology. The US border with Mexico can become vulnerable wherever a new wall is created. Without a wall there is usually no need for soil tunnels. The US Department of Homeland Security and border patrol will need to monitor the noise and vibrations, just like the Israel does, to identify future soil tunnel locations. Eventually most of 3200 km border will have a wall that will become the target of more soil tunnels for smuggling goods and people from Mexico into the United States.

Dynamic soil-tunnel interaction in layered half-space for incident plane SH waves

The dynamic soil-tunnel interaction is studied by indirect boundary element method （IBEM）, using the model of a rigid tunnel in layered half-space, which is simplified to a single soil layer on elastic bedrock, subjected to incident plane SH waves. The accuracy of the results is verified through comparison with the analytical solution. It is shown that soil-tunnel interaction in layered half-space is larger than that in homogeneous half-space and this interaction mechanism is essentially different from that of soil-foundation-superstructure interaction.

Soil pressure and pore pressure for seismic design of tunnels revisited: considering water-saturated, poroelastic half-space

This paper describes a systematic study on the fundamental features of seismic soil pressure on underground tunnels, in terms of its magnitude and distribution, and further identifi es the dominant factors that signifi cantly infl uence the seismic soil pressure. A tunnel embedded in water-saturated poroelastic half-space is considered, with a large variety of model and excitation parameters. The primary features of both the total soil pressure and the pore pressure are investigated. Taking a circular tunnel as an example, the results are presented using a fi nite element-indirect boundary element(FE-IBE) method, which can account for dynamic soil-tunnel interaction and solid frame-pore water coupling. The effects of tunnel stiffness, tunnel buried depth and input motions on the seismic soil pressure and pore pressure are also examined. It is shown that the most crucial factors that dominate the magnitude and distribution of the soil pressure are the tunnel stiffness and dynamic soil-tunnel interaction. Moreover, the solid frame-pore water coupling has a prominent infl uence on the magnitude of the pore pressure. The fi ndings are benefi cial to obtain insight into the seismic soil pressure on underground tunnels, thus facilitating more accurate estimation of the seismic soil pressure.

Dynamic soil-tunnel interaction in layered half-space for incident P-and SV-waves

The dynamic soil-tunnel interaction is studied by the model of a rigid tunnel embedded in layered half-space, which is simplified as a single soil layer on elastic bedrock to the excitation of P- and SV-waves. The indirect boundary element method is used, combined with the Green＇ s function of distributed loads acting on inclined lines. It is shown that the dynamic characteristics of soil-tunnel interaction in layered half-space are different much from that in homoge- neous half-space, and that the mechanism of soil-tunnel interaction is also different much from that of soil-founda- tion-superstructure interaction. For oblique incidence, the tunnel response for in-plane incident SV-waves is com- pletely different from that for incident SH-waves, while the tunnel response for vertically incident SV-wave is very similar to that of vertically incident SH-wave.

New methods of safety evaluation for rock/soil mass surrounding tunnel under earthquake

The objective of this work is to obtain the seismic safety coefficient and fracture surface and proceed with the seismic safety evaluation for the rock mass or soil mass surrounding a tunnel,and the limitation of evaluating seismic stability is considered using the pseudo-static strength reduction.By using the finite element software ANSYS and the strength reduction method,new methods of seismic safety evaluation for the rock mass or soil mass surrounding a tunnel are put forward,such as the dynamic finite element static shear strength reduction method and dynamic finite element shear strength reduction method.In order to prove the feasibility of the proposed methods,the results of numerical examples are compared with that of the pseudo-static strength reduction method.The results show that 1) the two methods are both feasible,and the plastic zone first appears near the bottom corners; 2) the safety factor of new method Ⅱ is smaller than that of new method I but generally,and the difference is very small.Therefore,in order to ensure the safety of the structure,two new methods are proposed to evaluate the seismic stability of the rock mass or soil mass surrounding a tunnel.A theoretical basis is provided for the seismic stability of the rock mass or soil mass and the lining surrounding a tunnel and also provided for the engineering application.

Investigating Effect of Tunnel Gate Shapes with Similar Cross Section on Inserted Forces on Its Coverage and Soil Surface Settlement

According to technology development and relative facilitation in digging and underground structures, ways, highways, all types of tunnels, underground train network, and other underground settle, storage are number of structure built and developed in advanced countries. In most situation, tunnel digging operations are done years after its construction or are not recorded in new structures regulations;therefore, this research investigates soil settlement and inserting force to tunnel coverage by limiting studies about effects of tunnel shapes on soil settlement using Plaxis, Seismo Signal, and Seismo Aspect. This study shows that rectangular tunnel has the most settlement in soil surface and circular tunnel has the least settlement but horseshoe tunnel has similar behavior to circular tunnel;however, earth subsidence level by digging this tunnel is more than circular tunnel. In addition, sectional shape has direct effect on inserting forces on tunnel coverage.

Dynamic interaction of twin vertically overlapping lined tunnels in an elastic half space subjected to incident plane waves

The scattering of plane harmonic P and SV waves by a pair of vertically overlapping lined tunnels buried in an elastic half space is solved using a semi-analytic indirect boundary integration equation method. Then the effect of the distance between the two tunnels, the stiffness and density of the lining material, and the incident frequency on the seismic response of the tunnels is investigated. Numerical results demonstrate that the dynamic interaction between the twin tunnels cannot be ignored and the lower tunnel has a significant shielding effect on the upper tunnel for high-frequency incident waves, resulting in great decrease of the dynamic hoop stress in the upper tunnel; for the low-frequency incident waves, in contrast, the lower tunnel can lead to amplification effect on the upper tunnel. It also reveals that the frequency-spectrum characteristics of dynamic stress of the lower tunnel are significantly different from those of the upper tunnel. In addition, for incident P waves in low-frequency region, the soft lining tunnels have significant amplification effect on the surface displacement amplitude, which is slightly larger than that of the corresponding single tunnel.

A unified critical state model for geomaterials with an application to tunnelling

This paper is prepared in honour of Professor E.T.Brown for his outstanding contributions to rock mechanics and geotechnical engineering and also for his personal influence on the first author’s research career in geomechanics and geotechnical engineering.As a result,we have picked a topic that reflects two key research areas in which Professor E.T.Brown has made seminal contributions over a long and distinguished career.These two areas are concerned with the application of the critical state concept to modelling geomaterials and the analysis of underground excavation or tunnelling in geomaterials.Partially due to Professor Brown’s influence,the first author has also been conducting research in these two areas over many years.In particular,this paper aims to describe briefly the development of a unified critical state model for geomaterials together with an application to cavity contraction problems and tunnelling in soils.

Long-Term Fate of Agent Orange and Dioxin TCDD Contaminated Soils and Sediments in Vietnam Hotspots

The soils, tropical climate, and network of canals and rivers of southern Vietnam have created one of the most diverse tropical jungles and intensely cultivated landscapes of Southeast Asia. This paradise has a long history of numerous wars, foreign occupations, and most recently the Second Indochina War (aka the Vietnam War 1965-1972) which defoliated rain forests and ancient wetland mangroves and left behind contaminated soil and sediment hotspots. During this war, the United States (US) military sprayed 80 million liters of Agent Orange contaminated with the dioxin TCDD in a guerrilla war against communist insurgents. Agent Orange was a synthetic plant growth regulator comprised of equal amounts of two herbicides 2,4-dichloro phenoxyacetic acid C8H6Cl2O3 (2,4-D) and 2,4,5-trichlorophenoxyacetic acid C8H5Cl3O3 (2,4,5-T). TCDD, the dioxin, 2,3,7,8-tetrachlorodibenzodioxin (C12H4Cl4O2) was an unintended byproduct of the accelerated combustion process used in the manufacture of herbicides containing 2,4,5-T. Agent Orange has frequently been blamed for soil and sediment contamination and long-term human health problems;however, the true source of harm is the dioxin TCDD. Agent Orange has a short half-life of days and weeks after application to vegetation, and has not been found to persist, after 50 years, in the water or soils of southern Vietnam. However, the half-life of dioxin TCDD depends on where it is deposited and varies from 1 to 3 years on soil surfaces that have been fully exposed to sunlight, to as long as 20 to 50 years or more when buried in tropical subsoils, and more than 100 years in river and sea sediments. Dioxin TCDD was heavily concentrated in the US Air Force bases in Vietnam where the herbicides were stored, loaded on planes and helicopters for aerial spraying, and used extensively around military base perimeter fences as a security measure to prevent surprise attacks. Bien Hoa Air Force base, 40 km northeast of Ho Chi Minh City, continues to be one of the mega-hotspots where after 48 years the dioxin TCDD levels in fish and shrimp are still high and fishing is banned in ponds and lakes adjacent to the airbase. Although expensive, one of the most effective remediation to dioxin TCDD contaminated soils is incineration which is the recommended method of dioxin TCDD disposal.

Seismic isolation effect of lined circular tunnels with damping treatments

The Havriliak-Negami model for dynamic viscoelastic material behavior and Biot＇s theory of poroelasticity are employed to develop an exact solution for three-dimensional scattering effect of harmonic plane P-SV waves from a circular cavity lined with a multilayered fluid-filled shell of infinite length containing viscoelastic damping materials and embedded within a fluid-saturated permeable surrounding soil medium. The analytical results are illustrated with numerical examples where the effects of liner/coating structural arrangement, viscoelastic material properties, liner-soil interface bonding condition, seismic excitation frequency, and angle of incidence on the induced dynamic stress concentrations are evaluated and discussed to obtain representative values of the parameters that characterize the system. It is demonstrated that incorporating viscoelastic damping materials with a low shear modulus in the constrained layer configuration is an efficient means of enhancing the overall seismic isolation performance, especially for near-normally incident seismic shear waves where the amplitudes of induced dynamic stresses may be reduced by up to one-third of those without isolation in a relatively wide frequency range. Some additional cases are considered and good agreements with solutions available in the literature are obtained.

Wind Tunnel Study of Multiple Factors Affecting Wind Erosion from Cropland in Agro-pastoral Area of Inner Mongolia,China

In this paper,the process of wind erosion on two kinds of soil from the agro-pastoral area of Inner Mongolia are studied using wind tunnel experiments,considering the wind speed,blown angle of wind and soil moisture content.The results showed that the modulus of soil wind erosion increases with an increase of wind speed.When the wind speed exceeds a critical value,the soil wind erosion suddenly increases.The critical speed for both kinds of soil is within the range of 7-8m·s-1.For a constant wind speed,the rate of soil wind erosion changes from increasing to falling at a critical soil slope.The critical slope of loam soil and sandy loam soil is 20° and 10°,respectively.Soil moisture content has a significant effect on wind erosion.Soil wind erosion of both soils decreases with an increase of the soil water content in two treatments,however,for treatment two,the increasing trends of wind erosion for two soils with the falling of soil water content are no significant,especially for the loam soil,and in the same soil water content,the wind erosion of two soils in treatment one is significantly higher than treatment two,this indicates reducing the disturbance of soil surface can evidently control the soil wind erosion.

Feasibility study of tar sands conditioning for earth pressure balance tunnelling

This paper presents the results of laboratory test on the feasibility of soil conditioning for earth pressurebalance （EPB） excavation in a tar sand, which is a natural material never studied in this respect. Thelaboratory test performed is based on a procedure and methods used in previous studies with differenttypes of soils, but for this special complex material, additional tests are also conducted to verify particularproperties of the tar sands, such as the tilt test and vane shear test usually used in cohesive materials, anda direct shear test. The laboratory test proves that the test procedure is applicable also to this type of soiland the conditioned material can be considered suitable for EPB excavations, although it is necessary touse a certain percentage of fine elements （filler） to create a material suitable to be mixed with foam. Thetest results show that the conditioned material fulfils the required standard for an EPB application.

软土地层盾构掘进土体稳定性模型试验研究

针对软土地层盾构掘进周围土体稳定性问题,自主研制了TJ-TBM2015多功能微型隧道掘进试验平台,通过改变外壳直径以模拟地层损失,采用动力控制系统,微型隧道掘进机可以实现盾构隧道的连续动态机械开挖。基于试验平台进行了地表无超载、地表有局部超载和隧道临近穿越群桩基础3种工况的盾构隧道掘进试验,通过传感器监测不同工况掘进过程中地表沉降变形和隧道周围土体的应力变化,研究土体的稳定性特征,并进行横向对比分析。结果表明,隧道开挖引起的土体应力重分布主要发生在隧道中心1倍直径范围内;局部超载对土体稳定性影响有限,但超载会造成其所在位置附近地表沉降增大;群桩基础对地层起到了一定的加固和隔离作用。

A field investigation of wind erosion in the farming–pastoral ecotone of northern China using a portable wind tunnel: a case study in Yanchi County

The farming-pastoral ecotone in northern China is an extremely fr@e ecological zone where wind erosion of cropland and rangeland is easy to occur. In this study, using a portable wind tunnel as a wind simulator, we conducted field simulated wind erosion experiments combined with laboratory analysis to investigate wind erosion of soils in trampled rangeland, non-tilled cropland and tilled cropland in Yanchi County, China. The results showed that compared with rangeland, the cropland had a higher soil water holding capacity and lower soil bulk density. The wind erosion rate of trampled rangeland was much higher than those of non-tilled cropland and tilled cropland. For cropland, the wind erosion rate of the soil after tilling was surprisingly less than that of the soil before tilling. With increasing of wind speed, the volume mean diameter of the eroded sediment collected by the trough in the wind tunnel generally increased while the clay and silt content decreased for all soils. The temporal variation in wind erosion of the trampled rangeland indicated that particle entrainment and dust emission decreased exponentially with erosion time through the successive wind erosion events due to the exhaustion of erodible particles.

Experimental study on the sampling efficiency of the whirl type separation sand sampler in a wind tunnel

A performance test was conducted in a wind tunnel by changing the principal configuration parameters of a sampler such as the diameter of the container, inlet width and cone height. The results show that the average sand collection rate is from 80% to 90% when any one of the configuration parameter levels is changed. However, the variation of a parameter level results in different ef-fects on the sand collection rate for each soil sample within a certain size range of sand grains. The results show that for various sand grain sized soil sample at each wind speed, the sand collection rate decreases when the diameter of the container changes from 50 mm to 40 mm, the sand collection rate increases by about 2%-3% when the inlet width changes from 10 mm to 8 mm, and the sand collection rate increases by about 3%-4% when cone height is altered from 100 mm to 125 mm. The average sand collection rate is enhanced by 2%-4% for the soil sample of different sized sand grains when the diameter of the container is 50 mm, the inlet width is 8 mm, and cone height is 125 mm.

Seismic response of underground utility tunnels: shaking table testing and FEM analysis

Underground utility tunnels are widely used in urban areas throughout the world for lifeline networks due to their easy maintenance and environmental protection capabilities. However, knowledge about their seismic performance is still quite limited and seismic design procedures are not included in current design codes. This paper describes a series of shaking table tests the authors performed on a scaled utility tunnel model to explore its performance under earthquake excitation. Details of the experimental setup are first presented focusing on aspects such as the design of the soil container, scaled structural model, sensor array arrangement and test procedure. The main observations from the test program, including structural response, soil response, soil-structure interaction and earth pressure, are summarized and discussed. Further, a finite element model （FEM） of the test utility tunnel is established where the nonlinear soil properties are modeled by the Drucker- Prager constitutive model; the master-slave surface mechanism is employed to simulate the soil-structure dynamic interaction; and the confining effect of the laminar shear box to soil is considered by proper boundary modeling. The results from the numerical model are compared with experiment measurements in terms of displacement, acceleration and amplification factor of the structural model and the soil. The comparison shows that the numerical results match the experimental measurements quite well. The validated numerical model can be adopted for further analysis.

中国设施种植土壤可持续利用的难点与应对策略

中国设施种植大多采用中低端装备土壤栽培模式,具有高投入、高产出和高强度利用的特点。截至2021年,设施播种面积在267万hm2左右,占全球设施总面积80%以上。但长期设施种植导致土壤质量下降,诱发面源污染加剧、温室气体排放增加、农产品产量和质量下降等诸多问题。本文聚焦设施种植土壤,总结分析了中国设施土壤可持续利用的难点,主要表现为:土壤养分失衡、环境质量下降、农产品产量和质量降低、设施农用地管理制度不健全等。设施种植可持续利用难题主要归因于:装备水平落后导致设施微环境和土壤养分供应调控不精准、施肥不合理和作物种类单一诱发土壤连作障碍、设施农用地管理制度革新动力不足导致土壤管理技术集成创新缓慢等。本文围绕设施土壤可持续利用提出了7项应对策略:建设期耕作层构建、休闲期土壤改良、设施微环境调控、投入品管控、轮作模式构建、作物抗逆调控以及设施农用地管理制度优化,旨在为小农中低端装备条件下设施土壤可持续利用提供研究思路和科学依据。

不同埋深下类矩形与圆形隧道施工诱发土体变形透明土试验对比研究

类矩形隧道因其内部空间优势在工程中逐步得到推广,但其施工引发周边土层变化规律与常规圆形隧道的差异尚缺乏深入研究,且该差异与隧道埋深紧密相关.为此自主设计组装了类矩形与圆形隧道开挖装置,以透明土模拟周边土层,并采用粒子图像测速(PIV)技术精准测定土层变形,对不同埋深下等面积类矩形与圆形隧道动态开挖过程中诱发上覆土体变化规律进行了深入分析.试验对比结果表明,两类隧道引发上覆土层变形的滑裂面基本一致,大致符合太沙基理论的竖直滑动面假定,滑裂角θ约等于45°+φ/2;不同埋深情况下,圆形隧道上覆土层竖向变形始终保持“V”型的正态分布,即存在最大值,而类矩形隧道随着地层深度的增加,上覆土层变形由“V”型转变为“W”型,即存在多个沉降最值,表明该类隧道比圆形隧道地层变形更为均匀,且在相同埋深情况下类矩形隧道开挖引发的地表沉降最大沉降值小于圆形隧道,该现象在浅埋状态下尤为明显,二者差异随着埋深增大逐渐缩小,该结论可为施工方案设计选择提供参考.