Centrifuge modelling of capillary rise

This paper reports results from centrifuge tests designed to investigate capillary rise in soils subjected to different gravitational fields. The experimental programme is part of the EU-funded NECER project (Network of European Centrifuges for Environmental Geotechnic Research), whose objective is to investigate the appropriateness of geotechnical centrifuge modelling for the investigation of geoenvironmental problems, particularly with reference to partially saturated soils. The tests were performed at the geotechnical centrifuge laboratories of Cardiff, Bochum, Manchester, and LCPC in Nantes. The aim was to determine the scaling laws of capillary rise under both equilibrium and transient conditions.

In all laboratories, column wetting tests in fine poorly graded sands (Congleton Sand, Bochum Normsand, HPF5 Sand, and Fontaineblau Sand) were performed. Capillary rise above the phreatic surface of the sand model was distinguished in a continuous capillary zone (completely saturated) and a discontinuous capillary zone (partially saturated).

The Cardiff Geotechnical Centrifuge Laboratory used matrix potential probes to follow the capillary rise of the continuous zone and, therefore, determine the suction above the phreatic zone during centrifuge testing. At Bochum, two cameras were used for optical and volumetric measurements, in order to follow the rise of the visible wetting front (upper limit of discontinuous zone) in the sand within the sample column. At Manchester, the movement of the wetting front was observed by video cameras over periods up to 8 h, whereas in LCPC pore pressure transducers recorded the changes in pressure caused by capillarity.

A simple centrifuge similitude law for capillary rise in these sands has been established and the kinetic phenomena have been measured as a function of the gravitational field. The results from these experiments verify that both the continuous and discontinuous capillary zones are scaled at a factor 1/N whereas the time for rise seems to be scaled at a factor 1/N². This research suggests that capillary phenomena can be modelled using a geotechnical centrifuge. Therefore, centrifuge testing can be a useful tool for future modelling of boundary value problems involving complex transport phenomena.     

Shonkin Sag laccolith,Montana

The Shonkin Sag differentiation sequence displays systematic variations in major and trace elements with evolution. The accumulative more mafic shonkinites are enriched in Co, Ni, Cr; La, Ce, Zr, and Nb are concentrated in the most evolved differentiate, chemically a nepheline syenite. Maximum abundances of K, Rb, Ba, Pb, and Sr occur in the intermediate syenitic differentiates (differentiation index=64.0–68.8). A change in the variation trends for K, Rb, Pb, Sr, Na, Ce, and Zr coincided with a change in the rate of decrease of oxygen fugacity with falling temperature.     

Centrifuge modelling of twin-tunnelling induced ground movements in loess strata

A great concern for the safety of large cross-section tunnels, which are being or to be built in the loess strata of China, is attracted. Generally, loess is a multi-phase porous medium and develops complex stress and strain variation while executing a tunnel project. Another problem is that the soil surrounding both tunnel arches is subjected to a complex loading due to the double excavation. To obtain an in-depth knowledge of the mechanism of tunnel deformation induced by the twin-tunnelling, we conducted comprehensive centrifuge tests, which can simulate and reproduce strictly the action process of twin-tunnelling. Through the model tests, the response of twin-tunnelling on loess stratum deformation was obtained. The investigations showed that with the increase of tunnel spacing, the stratum deformation distribution near the vault changes from a single-peak V shape to a double-peak W shape. Additionally, the height of the stratum pressure arch effect increases significantly. The settlement of the preceding tunnel is slightly larger than that of the latter tunnel, and the twin-tunnelling effect gradually decreases with the increase of tunnel spacing. Through comparative analysis of the different combinations of tunnel spacing and tunnel interval, the interaction between two tunnels with different spacing and interval during tunnelling was investigated, further optimizing the reasonable tunnel spacing and construction steps, as well as providing reference for tunnel route selection in the loess strata.     

Centrifuge modelling of rainfall-induced slope failure in variably saturated soil

Acta Geotechnica - This paper presents the results of centrifuge tests on rainfall-induced instabilities in variably saturated slopes. The roles of rainfall intensity and initial conditions, such...     

Centrifuge modelling of slope stabilisation using a discrete pile row

Discrete pile rows are an established technique for improving the stability of infrastructure slopes. However, the impact of pile spacing along the row is not clearly understood from a current design perspective in terms of the ultimate interaction capacity. A series of geotechnical centrifuge model tests were undertaken with the upper layer of a slope tending to fail and interact with a discrete pile row. The results were interpreted to clarify the effects of pile row interaction with the slope, and verify results from other recent numerical studies.     

Shonkin Sag laccolith,Montana

A variety of rock types is developed in the Shonkin Sag laccolith, with extreme compositions represented by the porphyritic pseudoleucite-bearing chilled margin, shonkinitic in composition, and by the final differentiate, chemically a nepheline syenite. During differentiation the pyroxenes changed in composition from Ca-rich varieties through aegirine-augite to acmite; there is no evidence of an immiscibility gap between Ca-rich and Na-rich pyroxenes. Olivine compositions changed from approximately Fa₂₀ to Fa₄₀, but in marked contrast the coexisting biotites exhibit a more extensive compositional range, from annite₂₄ to annite₁₀₀; the crystal margins of annites in the most evolved rocks are manganophyllite-rich. Titanomagnetites are TiO₂-poor varieties. Arfvedsonite and melanite occur in the most evolved syenites.From mineralogical and thermodynamic data initial and final temperatures of crystallization of the various rock types have been calculated. At an estimated total vapor pressure of 310 bars, the temperature of intrusion was 985° C. Final crystallization of the laccolith took place below 700° C, and crystallization intervals for most rock types are of the order of 170° C. Over the total magmatic temperature range the activity of silica in the melt decreased from 0.13 to 0.09. Oxygen fugacity falls with temperature approximately parallel to the synthetic fayalite-magnetite-quartz oxygen buffer until olivine disappears. The crystallization of Na-rich pyroxenes does not demand an increase in the fugacity of oxygen, but rather requires that the oxygen fugacity fall less rapidly with temperature than would be the case if olivine and magnetite were present.     

Centrifuge modelling of inclined micro-piles for liquefaction remediation of existing buildings

This paper describes four centrifuge tests investigating the performance of non-structural inclined micro-piles as a liquefaction remediation method for existing buildings. Two soil profiles with the same superstructure founded on each were tested under earthquakes of different magnitudes and durations. The first profile consisted of a deep, homogeneous layer of loose, liquefiable sand. The second comprised a shallow layer of loose sand overlying dense sand. Centrifuge tests were carried out with and without inclined micro-piles in each soil profile. The superstructure was modelled as an idealised single degree of freedom (SDOF) system. It is found that the micro-piles have no detrimental effect on the performance of the structure during and after earthquakes. It is also possible that their presence may decrease structural settlements in earthquakes which cause liquefaction to a depth less than that of the improved zone. However, no conclusive evidence is obtained to show that the micro-piles significantly restrain lateral soil movement due to monotonic shearing from the structure or impede the migration of excess pore pressures from the free field to the foundation zone. Both these processes have critical effects on structural settlement. The use of inclined micro-piles for liquefaction remediation should therefore be considered with caution.     

Centrifuge modelling of landslides and landslide hazard mitigation: A review

Landslides are serious geohazards that occur under a variety of climatic conditions and can cause many casualties and significant economic losses. Centrifuge modelling, as a representative type of physical modelling, provides a realistic simulation of the stress level in a small-scale model and has been applied over the last 50 years to develop a better understanding of landslides. With recent developments in this technology, the application of centrifuge modelling in landslide science has significantly increased. Here, we present an overview of physical models that can capture landslide processes during centrifuge modelling. This review focuses on (i) the experimental principles and considerations, (ii) landslide models subjected to various triggering factors, including centrifugal acceleration, rainfall, earthquakes, water level changes, thawing permafrost, excavation, external loading and miscellaneous conditions, and (iii) different methods for mitigating landslides modelled in centrifuge, such as the application of nails, piles, geotextiles, vegetation, etc. The behaviors of all the centrifuge models are discussed, with emphasis on the deformation and failure mechanisms and experimental techniques. Based on this review, we provide a best-practice methodology for preparing a centrifuge landslide test and propose further efforts in terms of the seven aspects of model materials, testing design and equipment, measurement methods, scaling laws, full-scale test applications, landslide early warning, and 3D modelling to better understand the complex behaviour of landslides.     

Deformation structures associated with the Trachyte Mesa intrusion,Henry Mountains,Utah: Implications for sill and laccolith emplacement mechanisms

Deformation structures in the wall rocks of igneous intrusions emplaced at shallow crustal depths preserve an important record of how space was created for magma in the host rocks. Trachyte Mesa, a small Oligocene age intrusion in the Henry Mountains, Utah, is composed of a series of stacked tabular, sheet-like intrusions emplaced at 3–3.5 km depth into sandstone-dominated sedimentary sequences of late Palaeozoic–Mesozoic age. New structural analysis of the spatial distribution, geometry, kinematics and relative timings of deformation structures in the host rocks of the intrusion has enabled the recognition of distinct pre-, syn-, and late-stage-emplacement deformation phases. Our observations suggest a two-stage growth mechanism for individual sheets where radial growth of a thin sheet was followed by vertical inflation. Dip-slip faults formed during vertical inflation; they are restricted to the tips of individual sheets due to strain localisation, with magma preferentially exploiting these faults, initiating sill (sheet) climbing. The order in which sheets are stacked impacts on the intrusion geometry and associated deformation of wall rocks. Our results offer new insights into the incremental intrusion geometries of shallow-level magmatic bodies and the potential impact of their emplacement on surrounding host rocks.     

地层错动引起的上覆饱和黏土层变形特性的离心试验研究

地震断层错动会引起上覆土层变形,从而造成断层附近的建筑结构、管线产生附加的变形和内力引起破坏。通过一个土工离心机试验分析上覆饱和黏土层在4步连续断层错动作用下的静力响应行为。着重分析断层错动引起的地层变形的范围、不均匀沉降区的分布特点、剪切裂缝在土层传播路径及地表开裂的位置等工程上重点关注的问题。得到以下几点认识：(1) 基岩断层错动引起的地层变形范围基本上不受基岩错动量大小的影响。(2) 断层错动引起地层的不均匀沉降区基本呈三角形分布,其地表宽度约为1倍左右的土层厚度。(3) 基岩错动引起的主剪切裂缝基本沿竖直方向向上传播,其传播距离取决于基岩错动量及土体的破坏应变。(4) 基岩断层错动在主剪区的下盘一侧边缘会产生张拉裂缝,且产生张拉裂缝所需基岩错动量远小于产生剪切裂缝所需的错动量。     

盾构隧道注浆对既有隧道影响的离心模拟研究

随着地铁网络不断完善,越来越多的新建盾构隧道近距离穿越既有隧道,然而对于盾构隧道近距离穿越既有隧道影响的研究尚不够完善。以上海典型软弱地层为背景,通过离心模型试验,研究了不同注浆率下的盾构上穿越施工对既有隧道以及周围地层的影响。选用排液法在离心场中模拟盾构施工,在不停机状态下成功模拟隧道开挖卸载、地层损失和注浆效应。分析了在不同的注浆率条件下,既有隧道在上穿越施工期和工后长期的位移、周围孔压和纵向应力的变化规律。试验结果表明,新建隧道近距离上穿越既有隧道时,隧道开挖的卸载效应等会导致既有隧道的隆起,但随着注浆率增大,既有隧道的隆起量减小。但过高注浆率对周围土体扰动较大,从而导致工后既有隧道的沉降也越大。     

小型土石坝加密抗液化离心机振动台试验研究

地震作用下土石坝液化易导致坝坡失稳滑移等严重后果,加密法是常用的抗液化手段之一。针对坝趾压重与坝壳翻压两种坝身加密加固方法,开展了离心机振动台试验,分析了不同加密型抗液化处理的小型土石坝坝坡地震响应规律。试验结果表明,由于高水头作用下坝坡底部土体软化,未处理坝坡加速度放大系数沿高程先减小后增大,而加密坝坡加速度放大系数沿高程逐渐增大,且坝坡表面处加速度存在表面放大现象。坝趾压重和坝壳翻压提高了坝身有效应力,降低地震产生的超静孔压比,有效防止土体液化。未处理坝坡在峰值加速度为0.24g地震作用下即发生坝趾液化现象,而加密坝坡在峰值加速度为0.24g和0.45g下均未发生液化。未处理坝坡整体侧向位移大,加密处理后,在峰值加速度为0.24g下坝坡整体表现为竖向位移。坝趾压重区坝趾水平位移明显减小,坝壳翻压区坡顶沉降减小了50%。试验结果验证了坝趾压重和坝壳翻压的抗液化效果,为小型土石坝抗震加固设计提供了参考。     

非饱和黄土地基降雨入渗离心模型试验及多物理量联合监测

对非饱和土降雨入渗过程及其致灾机制的深入认识有赖于室内外试验及多物理量联合监测。基于离心机超重力环境下土体响应监测技术,开展了非饱和黄土地基降雨入渗离心模型试验,测试研究了超重力环境对新研制的微型TDR探针及张力计测量的影响规律,并利用TDR、张力计及弯曲元对降雨入渗过程中土体响应进行多物理量联合监测。研究结果表明:在不同离心加速度下微型TDR探针与给定含水率土体实测原始波形重合,说明超重力环境对TDR测试没有影响,含水率的测试误差在2%以内。在离心机加速过程中,张力计所测吸力下降约2.0~2.9 kPa,当离心加速度稳定在40g时,所测吸力在10min内上升并接近常重力下土体初始吸力。在降雨入渗过程中,埋设在同一深度的TDR探针、张力计和弯曲元对湿润锋响应的时间点基本一致,降雨入渗导致土体含水率增加,基质吸力降低,剪切波速降低。这些多物理量监测数据有助于建立非饱和土含水率-吸力-剪切模量之间的关系。     

Centrifuge modelling of the effects of root geometry on transpiration-induced suction and stability of vegetated slopes

Shallow landslides (i.e. 1–2-m depth) on both man-made and natural slopes are of major concern worldwide that has led to huge amount of socio-economical losses. The use of vegetation has been considered as an environmentally friendly means of stabilising slopes. Existing studies have focused on the use of plant roots with different geometries to mechanically stabilise soil slopes, but there are little data available on the contribution of transpiration-induced suction to slope stability. This study was designed to quantify both the hydrological and mechanical effects of root geometry on the stability of shallow slopes. Centrifuge tests were conducted to measure soil suction in slope models supported by newly developed artificial roots. These artificial roots exhibit three different representative geometries (i.e. tap, heart and plate) and could simulate the effects of transpiration. The measured suction was then back-analysed through a series of finite element seepage-stability analyses to determine the factor of safety (FOS). It is revealed that after a rainfall event with a return period of 1000 years, the slope supported by heart-shaped roots retained the highest suction within the root depth, and thus, this type of root provided the greatest stabilisation effects. The FOS of the slope supported by the heart-shaped roots, through both mechanical reinforcement and transpiration-induced suction, is 16 and 28 % higher than that supported by the tap- and plate-shaped roots, respectively.     

Dynamic process analyses of saline intrusion with over-exploitation by coupled flow and dispersive modelling

Understanding of saline intrusion into coastal aquifer is an important issue in management and protection of groundwater resource, which can be well achieved by groundwater modelling. To explain some phenomena of correlation between groundwater level and salinity in observation wells in coastal area, the authors compare the migration velocity of saline particles and transmission velocity of water pressure and derive analytical equations of these two velocities for plane and radial flows. The driving force and resistance of saline intrusion were analysed based on the analytical modelling. The destruction and reconstruction of the equilibrium between fresh water seepage towards sea and saline dispersion to inland were considered as an essence of whole intrusion process. The dynamic process of seepage and dispersion at different stages of saline intrusion were analysed under groundwater over-exploitation. The basic equations of saline intrusion were derived and the mechanism of transitional zone movement was discussed. These constitute coupled seepage–dispersion theory of saline intrusion, which becomes an important supplement to existing theory of saline intrusion. As a case study on saline intrusion in Guangrao County in Eastern China, this theory was applied to predict the development of saline intrusion in the study area.     

Characterisation of sea-water intrusion in the Pioneer Valley, Australia using hydrochemistry and three-dimensional numerical modelling

Sea-water intrusion is actively contaminating fresh groundwater reserves in the coastal aquifers of the Pioneer Valley, north-eastern Australia. A three-dimensional sea-water intrusion model has been developed using the MODHMS code to explore regional-scale processes and to aid assessment of management strategies for the system. A sea-water intrusion potential map, produced through analyses of the hydrochemistry, hydrology and hydrogeology, offsets model limitations by providing an alternative appraisal of susceptibility. Sea-water intrusion in the Pioneer Valley is not in equilibrium, and a potential exists for further landward shifts in the extent of saline groundwater. The model required consideration of tidal over-height (the additional hydraulic head at the coast produced by the action of tides), with over-height values in the range 0.5–0.9 m giving improved water-table predictions. The effect of the initial water-table condition dominated the sensitivity of the model to changes in the coastal hydraulic boundary condition. Several salination processes are probably occurring in the Pioneer Valley, rather than just simple landward sea-water advancement from “modern” sources of marine salts. The method of vertical discretisation (i.e. model-layer subdivision) was shown to introduce some errors in the prediction of water-table behaviour.     

Status of the Korba groundwater resources (Tunisia): observations and three-dimensional modelling of seawater intrusion

The Korba aquifer is located in the east of the Cape Bon peninsula in Tunisia. A large groundwater depression has been created in the central part of the aquifer since the 1980s, due to intense groundwater pumping for irrigation. The data collected show that the situation continues to deteriorate. Consequently, seawater is continuing to invade a large part of the aquifer. To better understand the situation and try to forecast its evolution, a three-dimensional (3D) transient density-dependent groundwater model has been developed. The model building process was difficult because of data required on groundwater discharge from thousands of unmonitored private wells. To circumvent that difficulty, indirect exhaustive information including remote sensing data and the physical parameters of the aquifer have been used in a multi-linear regression framework. The resulting 3D model shows that the aquifer is over-exploited. It also shows that after 50 years of exploitation, the time needed to turn back to the natural situation would be about 150 years if the authorities would ban all exploitation now. Such an asymmetry in the time scales required to contaminate or remediate an aquifer is an important characteristic of coastal aquifers that must be taken into account in their management.     

Three-dimensional hydrostratigraphical modelling to support evaluation of recharge and saltwater intrusion in a coastal groundwater system in Vietnam

Saltwater intrusion is generally related to seawater-level rise or induced intrusion due to excessive groundwater extraction in coastal aquifers. However, the hydrogeological heterogeneity of the subsurface plays an important role in (non-)intrusion as well. Local hydrogeological conditions for recharge and saltwater intrusion are studied in a coastal groundwater system in Vietnam where geological formations exhibit highly heterogeneous lithologies. A three-dimensional (3D) hydrostratigraphical solid model of the study area is constructed by way of a recursive classification procedure. The procedure includes a cluster analysis which uses as parameters geological formation, lithological composition, distribution depth and thickness of each lithologically distinctive drilling interval of 47 boreholes, to distinguish and map well-log intervals of similar lithological properties in different geological formations. A 3D hydrostratigraphical fence diagram is then generated from the constructed solid model and is used as a tool to evaluate recharge paths and saltwater intrusion to the groundwater system. Groundwater level and chemistry, and geophysical direct current (DC) resistivity measurements, are used to support the hydrostratigraphical model. Results of this research contribute to the explanation of why the aquifer system of the study area is almost uninfluenced by saltwater intrusion, which is otherwise relatively common in coastal aquifers of Vietnam.     

Centrifuge modelling of the influence of crustal fabrics on the development of transfer zones: insights into the mechanics of continental rifting architecture

Centrifuge analogue experiments are used to model the reactivation of pre-existing crustal fabrics during extension. The models reproduced a weakness zone in the lower crust whose geometry was varied in order to investigate its role in controlling the architecture of rift segments and related transfer zones. The typical rift system geometry was characterised by two offset rift segments connected by a major transfer zone in which boundary faults were oblique to the extension vector and displayed a significant transcurrent component of movement. The transfer zone was also characterised by cross-basin faults with both trend and strike-slip component of movement opposite to that displayed by the master faults. Typically, different structural patterns were obtained by changing the offset angle φ between the rift segments, supporting that the structural pattern at transfer zones is strongly influenced by the orientation of pre-existing discontinuities with respect to the stretching vector. In the models, the aspect ratio (ratio of length vs. width) of the transfer zone shows a positive correlation with the offset angle (i.e., the more the inherited fabric is parallel to the extension direction, the longer and narrower the transfer zones). In case of staircase offset of the rift segments (φ=90°), the structural pattern was characterised by two isolated rift depressions linked by a narrow transfer zone in which border faults with alternating polarity overlapped. Prominent rise of the ductile lower crust was also observed at the transfer zone. Many of these geometrical features display striking similarities with natural rift systems. The results of the current experiments provide useful insights into the mechanics of continental rift architecture, supporting that rift propagation, width and along-axis segmentation may be strongly controlled by the reactivation of pre-existing pervasive crustal fabrics.