Fatigue behaviour of geomaterials under cyclic environmental loading

循环荷载是工程应用中经常遇到的问题。风对高层建筑施加动荷载。波浪和冰荷载对北极海域的海上结构和交通基础设施构成威胁。降水和地下水位的季节变化给土方工程带来了反复的荷载。随着交通荷载的作用，地面的循环冻融影响着路面和埋地设施的长期性能。地震、滑坡等自然地质灾害本质上是周期性的、动态的。矿产开采和油气开采也涉及到对周围环境的循环加载。该研究计划的长期目标是开发量化不同岩土材料在循环载荷下的疲劳行为和预期寿命的技术，以便开发新的技术来预防危害，保护环境，优化材料寿命和自然资源回收。本研究首先研究的是加拿大主要的天然油气资源之一——蒙特尼地层。由于其致密的孔隙结构，水力压裂已被用于该资源的开发。该技术将经过化学处理的流体高压注入地层，形成裂缝网络，提高油气产量。水力压裂产生的压裂液和碳氢化合物污染对天然水源的潜在风险引起了公众的关注。水力压裂还会诱发地震事件，对地面设施造成不利影响。迫切需要改进现有的HF处理方法，以增强这种致密材料中的水力流动，并减少处理释放的地震能量。我们建议通过可控的“疲劳”方式来解锁紧绷的材料。众所周知，材料在循环外加应力低于单调模式下的循环疲劳模式下失效。我们假设在循环（脉动）荷载下，较低的破裂压力可以使致密材料破裂。在可控的脉动作用下，裂缝带可能更加均匀而非局部化。研究计划的第一阶段包括实验室规模的实验。在第二阶段，测试数据将用于小规模实地试验的可行性研究。第三阶段是开发Montney组新技术。这个研究项目显然具有科学和社会环境经济两方面的影响。这个研究项目的发现很容易被转移到上面列出的研究领域。技术转让和与工业伙伴及监管机构的合作，解决公众关注的问题，促进环境保护，促进经济和就业。

Cyclic loadings are commonly encountered in engineering applications. Wind exerts dynamic loads on tall buildings. Wave and ice load imposes threat to offshore structures and transportation infra-structures in arctic waters. Seasonal variation of precipitation and water table imposes repeated loads on earthworks. Cyclic freezing-thawing of ground along with traffic loads affect the long-term performance of pavements and buried utilities. Natural geohazards such as earthquakes and landslides are cyclic and dynamic in essence. Mineral mining and oil and gas extraction also involve cyclic loading to the surrounding environment. The long-term objective of this proposed research program is to develop techniques to quantify fatigue behaviour and expected life span of different geomaterials under cyclic loading encountered in the field so that new technologies can be developed for prevention of hazards, protection of the Environment, and optimization of material life span and natural resource recovery. The first type of geomaterials studied in this proposed research is Montney Formation which is one of the major natural oil and gas resources in Canada. Due to its tight pore structure, hydraulic fracturing (HF) has been used for exploitation of this resource. It involves high-pressure injection of chemically treated fluid into the formation to create a fracture network enhancing oil and gas production. Potential risks to natural water sources by contamination of fracture fluid and hydrocarbons resulting from hydraulic fracturing have raised public concerns. Hydraulic fracturing also induces seismic events which may impose detrimental effects on surface facilities. There is an urgent need to modify the current HF treatment to enhance the hydraulic flow in this tight material and reduce the amount of seismic energy released from the treatment. We propose to unlock the tight material by failing it in a controllable "fatigue" manner. It is well known that materials fail in a cyclic fatigue mode under a cyclic applied stress lower than that in a monotonic mode. We postulate that a lower fracturing pressure is required to disintegrate the tight material under cyclic (pulsation) loading. Under controlled pulsation process, the fractured zone may be more homogeneous rather than localized. The first stage of the research program involves experimentation in laboratory scale. In the second stage the test data will be used and implemented to feasibility studies of small-scale field experiments. The third stage is the development of novel technologies for exploitation of Montney Formation. This research program clearly carries both scientific and socio-environmental-economic impacts. Findings from this research program are readily transferred to research fields as listed above. Technology transfer and collaboration with industrial partners and regulators address public concerns, promote protection of the Environment, and enhance our economy and employment.