Disentangling Shallow and Deep Processes Causing Surface Movement

Understanding and predicting surface movement is important both technically and for social reasons. The shallow processes contributing to subsidence include construction works, peat oxidation, clay compaction, and groundwater withdrawal; deep causes are hydrocarbon and salt production. We describe an inversion procedure we have devised to disentangle the deep and shallow causes of surface movement. It employs a Bayesian inversion scheme, using forward models and other ‘a priori’ information about shallow and deep compaction. Parameter estimation thus takes place at two different depths, thereby disentangling the deep and shallow compaction processes responsible for surface movement. The uncertainty in the surface measurements and ‘a priori’ estimates is naturally incorporated. Furthermore, spatial and temporal correlations can be taken into account through inclusion of the covariance matrix. The inversion scheme is demonstrated for two synthetic cases. The first combines a compacting gas field and a compacting shallow peat layer. We demonstrate that assumptions on the shape of the subsidence bowl are not necessary. We also show how neglecting either deep or shallow causes of subsidence can produce spurious results. The advantage of using the ‘a priori’ estimates of the compaction and the covariance matrix obtained by Monte Carlo simulations is demonstrated with a second synthetic example involving two polders and different depths of their water table. A robust solution is obtained for each polder unit, while a simpler (and faster) ‘a priori’ estimate based on the expected average clay thickness fails to reproduce the actual compaction. Monte Carlo simulations can also be applied to compaction in depleting gas reservoirs. Information on spatial correlations is often available, even when the absolute values of the ‘a priori’ compaction data are quite uncertain. Explicitly incorporating such ‘a priori’ known spatial correlations improves the result significantly.     

塑料套管混凝土桩挤土效应现场试验

结合申嘉湖杭高速公路练杭段加固软土地基工程开展了塑料套管混凝土桩挤土效应的现场试验研究。设地表观测点观测桩打设过程中桩周土体隆起情况;采用绑定仪器的方形木桩等效模拟塑料套管混凝土桩研究不同时刻桩侧挤土压力和孔隙水压力的变化。结果表明打桩过程中在距桩中心约2.2倍沉管外径处地面隆起量最大,约为沉管外径的17.2%;打设完成第一根相邻桩(距离1.6 m)桩侧土压力及孔隙水压力增大10%～20%,其后随着打设桩距木桩距离的渐远,两次打桩后的土压曲线和孔压曲线变化不大,仅在下部桩端处变化较大;场地内桩打设完毕后,沉桩引起的超孔隙水压力逐步消散。根据实测数据建议塑料套管环刚度控制在8～16级。     

砂岩压实作用的计算机模拟及对储层物性的预测研究

通过分析学者Buryakovsky等所建立的比较成熟的沉积岩压实过程的数学模型,结合国内学者提出的砂岩压实过程的各种影响因素,应用计算机编程、根据实例数据,最终得到了比较满意的模拟结果,绘制出砂岩孔隙度、渗透率、密度随埋深的变化曲线。利用多项式拟合,建立了砂岩孔隙度、渗透率、密度与埋深的多项式方程关系式,对砂岩储层物性的预测起到了良好的指导作用。     

Compaction Rate of an Early Permian Volcanic Tuff from Wuda Coalfield, Inner Mongolia

Compaction rates of sediments or volcaniclastic material are needed to reconstruct original thickness of a bed,which in turn is required to reconstruct subsidence rates,sea-level rise,or in the case of volcaniclastic,the location or direction of the eruption site.The knowledge of compaction rates can also aid in the reconstruction of deformed fossils.The known shape of deformed fossils can allow the determination of the compaction they experienced.Here we report the compaction rate in an early Permian volcanic tuff from Wuda,Inner Mongolia,determined from the deformation of standing tree fern stems of known anatomy.The compaction rate has been found to be 0.56 in this case,indicating that 44% of original thickness remains.     

Volume changes during diagenesis

The literature pertaining to volume change during diagenesis of clastic sediments is reviewed with respect to the problem of calculating pre- and syn-compaction thicknesses of sediments for basin reconstruction and stragraphic correlation studies. Four major mechanisms for volume change are identified: mechanical compaction, mechanical dissolution, chemical dissolution and phase change. The first two of these are found to be strongly dependent on the effective stress whilst the latter two show at least a pressure dependence. Quantification of the relationships between porosity and depth of burial of a sediment seems to be possible only for specific examples of the first of these processes at present. This quantification is dealt with in the accompanying related publication.     

无粘性土强夯夯点间加固效果的估算

利用前人的模型试验结果，提出了单点夯击时的侧向加固模式。以此模式为基础，提出了一种估算夯点外加固程度的计算方法，应用所提出的方法，得出了加固薄弱点的位置，建立了薄弱点处的加固程度与无量钢夯点间距间的关系式。     

Mechanical behavior and microstructural variation of loess under dynamic compaction

The laboratory dynamic compaction test was used to study the effects of dynamic impact loads on the shear strength characteristics of loess. Loess specimens were compacted by different numbers of blows and their shear strengths measured. The experimental results revealed that the shear strength varies in stages with the number of blows. During the initial stage, the shear strength increases with the number of blows. It reaches a peak value and then decreases with the number of blows. To reveal the relationship between this alternation in shear strength and microstructural variation, quantitative microstructural analyses were performed on some tested specimens at different stages of strength evolution. This microstructural study demonstrated that the variation of shear strength is directly related to the alteration of microstructure. It is concluded that the shear strength of loess is basically controlled by its microstructural state. Growing particle size heterogeneity, varying particle orientations and increasing microstructural damage are the main causes of the alternation of shear strength.     

Evaluation of shredded tyre chips as sorption media for passive treatment walls

The potential of shredded tyre chips, either on their own or mixed with gravel, to act as a permeable reactive barrier has been examined by carrying out tests for assessing sorption characteristics and establishing their mechanical properties. Sorption–desorption tests have been carried out using free product petrol and paraffin and aqueous phase phenol and cresols.

Compaction and compressibility tests have been carried out to establish optimum tyre–chip mixes in order to optimise the mix in terms of compressibility behaviour. The results demonstrate that tyre chips show significant promise for incorporation in a permeable reactive barrier. The evidence indicates that they would be particularly effective for the retardation of aqueous phase hydrophobic organic contaminants and residual free product. Optimum mixes of 2–6 mm tyre chips and 20 mm gravel, in terms of mechanical stability, will contain just under 10% of tyre chips.     

Ductile creep and compaction: A mechanism for transiently increasing fluid pressure in mostly sealed fault zones

A simple cyclic process is proposed to explain why major strike-slip fault zones, including the San Andreas, are weak. Field and laboratory studies suggest that the fluid within fault zones is often mostly sealed from that in the surrounding country rock. Ductile creep driven by the difference between fluid pressure and lithostatic pressure within a fault zone leads to compaction that increases fluid pressure. The increased fluid pressure allows frictional failure in earthquakes at shear tractions far below those required when fluid pressure is hydrostatic. The frictional slip associated with earthquakes creates porosity in the fault zone. The cycle adjusts so that no net porosity is created (if the fault zone remains constant width). The fluid pressure within the fault zone reaches long-term dynamic equilibrium with the (hydrostatic) pressure in the country rock. One-dimensional models of this process lead to repeatable and predictable earthquake cycles. However, even modest complexity, such as two parallel fault splays with different pressure histories, will lead to complicated earthquake cycles. Two-dimensional calculations allowed computation of stress and fluid pressure as a function of depth but had complicated behavior with the unacceptable feature that numerical nodes failed one at a time rather than in large earthquakes. A possible way to remove this unphysical feature from the models would be to include a failure law in which the coefficient of friction increases at first with frictional slip, stabilizing the fault, and then decreases with further slip, destabilizing it.     

Influence of time, temperature, confining pressure and fluid content on the experimental compaction of spherical grains

Theoretical models of compaction processes, such as for example intergranular pressure-solution (IPS), focus on deformation occurring at the contacts between spherical grains that constitute an aggregate. In order to investigate the applicability of such models, and to quantify the deformation of particles within an aggregate, isostatic experiments were performed in cold-sealed vessels on glass sphere aggregates at 200 MPa confining pressure and 350 °C with varying amounts of fluid. Several runs were performed in order to investigate the effects of time, fluid content, pressure and temperature, by varying one of these parameters and holding the others fixed. In order to compare the aggregates with natural materials, similar experiments were also performed using quartz sand instead of glass spheres. Experiments with quartz show evidence of IPS, but the strain could not be quantified. Experiments with glass spheres show evidence of several types of deformation processes: both brittle (fracturing) and ductile (plastic flow and fluid-enhanced deformation, such as IPS). In experiments with a large amount of water (≥ 5 vol.%), dissolution and recrystallization of the glass spheres also occurred, coupled with crystallization of new material filling the initial porosity. Experiments performed with a fluid content of less than 1 vol.% indicate creep behavior that is typical of glass deformation, following an exponential law. These experiments can also be made to fit a power law for creep, with a stress exponent of n = 10.5 ± 2.2 in both dry and wet experiments. However, the pre-factor of the power law creep increases 5 times with the addition of water, showing the strong effect of water on the deformation rate. These simple and low-cost experiments provide new insights on the rheology of soda-lime glass, which is used in analogue experiments, and of glass-bearing rocks under mid-crustal P–T conditions. They also highlight the strong enhancement of plasticity of natural rocks in presence of fluid or of a glassy phase.