Deep stabilisation of slopes using lime piles

A comprehensive review of the literature provided much evidence of the success of lime piles in treating both soft ground and slopes. The mechanisms of stabilisation postulated by researchers is often contradictory or misleading. The use of the literature for the basis of a definitive experimental programme was not possible. An iterative approach was adopted for the laboratory programme whereby the results from one series of tests were used in the design of the next. This resulted in a range of tests including full-scale box loading tests in which lime piles were installed in clay samples, model scale lime pile tests and soil element tests. The stabilising mechanisms that have been established by the laboratory study are: generation of negative pore water pressure, overconsolidation of the shear zone, clay dehydration, pile strength and increased strength of stabilised clay due to lime migration. These mechanisms combine to improve any particular clay slope containing one or more shear zones. Three field trials were conducted. A small-scale trial was carried out on a canal cutting and provided useful data regarding pore water pressure changes and installation processes. Quantitative data produced by the laboratory study, were used to design two further trials. One trial treated a 30 m stretch of failing slope using a single 'Minuteman' rig (small and lightweight plant). Quicklime was 'poured' into open holes and compacted by the drill operators. Work was complete within two weeks. The third trial, again sited on a canal cutting, was carried out using a much larger rig. One hundred and fifty 200 mm diameter piles were constructed to a depth of 3 metres within a two week period. Monitoring of pore water pressures on both sites is still occurring on a regular basis. Excavation of sections of both trials at some future date will provide additional data on stabilising mechanisms. The research has considerably extended the understanding of the mechanisms controlling lime pile stabilisation, particularly when applied to failing slopes in British soils. Areas where further research would improve this understanding have been highlighted and in some cases work is already underway

Challenges in monitoring and managing engineered slopes in a changing climate

© 2016 The Authors. Geotechnical asset owners need to know which parts of their asset network are vulnerable to climate change induced failure in order to optimise future investment. Protecting these vulnerable slopes requires monitoring systems capable of identifying and alerting to asset operators changes in the internal conditions that precede failure. Current monitoring systems are heavily reliant on point sensors which can be difficult to interpret across slope scale. This paper presents challenges to producing such a system and research being carried out to address some of these using electrical resistance tomography (ERT). Experimental results show that whilst it is possible to measure soil water content indirectly via resistivity the relationship between resistivity and water content will change over time for a given slope. If geotechnical parameters such as pore water pressure are to be estimated using this method then ERT systems will require integrating with more conventional geotechnical instrumentation to ensure correct representative information is provided. The paper also presents examples of how such data can be processed and communicated to asset owners for the purposes of asset management

Development of a multi-phase numerical modeling approach for hydromechanical behavior of clay embankments subject to weather-driven deterioration

Clay embankments used for road, rail and, flood defense infrastructure experience a suite of weather-driven deterioration processes that lead to a progressive loss of hydromechanical performance: micro-scale deformation (e.g., aggregation and desiccation), changes in soil-water retention, loss of strength, and macro-scale deformation. The objective of this study was to develop a numerical modeling approach to simulate the construction and long-term, weather-driven hydromechanical behavior of clay embankments. Subroutines within a numerical modeling package were developed to capture deterioration processes: (1) strength reduction due to wet-dry cycles; (2) bimodality of the near-surface hydraulic behavior; (3) soil-water and soil-gas retentivity functions considering void ratio dependency; and (4) hydraulic and gas conductivity functions considering void ratio dependency. Uniquely, the modeling approach was comprehensively validated using laboratory tests and nine years of field measurements from a full-scale embankment. The modeling approach captured the variation of near-surface soil moisture and matric suction over the monitored period in response to weather cycles. Further, the developed model approach could successfully simulate weather-driven deterioration processes in clay embankments. The model predictions manifested the ability of the modeling approach in capturing deterioration features such as irrecoverable increase in void ratio and hydraulic permeability near surface. The developed and validated numerical modeling approach enables forecasting of the long-term performance of clay embankments under a range of projected climate conditions.</p

The seasonal ratcheting of clay cut slopes in response to seasonal weather cycles

Many cut slopes in the UK are in the later stages of their operational life but continue to support road and rail transportation networks. Some of these slopes experienced delayed, deep-seated, first-time failures between 10 and 50 years after construction. However, some continue to seasonally deform and then fail at shallow depth due to the process of seasonal, downslope ratcheting. This paper reviews the evidence for seasonally-induced, downslope ratcheting movements in clay cut slopes, gathered from physical model tests, in-situ monitoring and numerical simulations. The evidence shows that seasonal ratcheting is an increasingly dominant mechanism of slope deformation and ultimate failure for some high-plasticity clay cut slopes as they are exposed to many seasonal weather cycles. The rate of downslope ratcheting depends on the slope age (i.e., number of seasonal weather cycles since construction), the slope geometry (i.e., slope height and angle) and the strain-softening behaviour of the slope material (e.g., as observed in stiff, high-plasticity clays). This rate, when measured, can be used to inform monitoring and management strategies for old, clay cut slopes (e.g., ageing railway and highway cuttings) by identifying the slopes that are prone to seasonally-induced, downslope ratcheting towards the end of their operational life.</p

Evidence for the weather-driven deterioration of ageing transportation earthworks in the UK

Seasonal, weather-driven pore pressure cycles alter and degrade the hydro-mechanical engineering properties of earthworks as they age. The accumulating effects of deterioration over many years can lead to the excessive deformation or failure of earthworks; requiring interventions to ensure their reliable performance. This paper reviews the evidence for the weather-driven deterioration of ageing transportation earthworks, with a focus on clay earthworks in the UK. These include earthworks of various ages (up to ∼200 years old), formed from a range of clay-rich strata and at various stages of deterioration. Evidence is considered for both past behaviour and projected behaviour in response to continued ageing and a changing climate. There is clear evidence that some clay earthworks are influenced by the cumulative effect of seasonal weather cycles over many decades. Simulations show that seasonal slope ratcheting will become an increasingly dominant driver of shallow failures in high-plasticity cut slopes as they age and in response to projected climate change. The evidence can inform performance curves describing the deterioration of individual earthworks in response weather-driven ageing. This can help identify earthworks with the highest likelihood of failure and inform decisions made by earthwork asset managers.</p