New Concepts for Sustainable Application of Deep Vibratory Compaction

Deep vibratory compaction of mainly granular soils can be performed by different methods and has been successfully performed on many projects worldwide. More powerful and sophisticated vibrator driving systems have become available, which have resulted in increased production rates and treatment quality. However, the potential of deep vibratory compaction is not yet fully realized mainly due to the following reasons: a) compaction specifications are generally chosen based on past experience without taking into account the project-specific requirements, b) the execution is usually performed on a routine basis without optimization of the compaction process and c) a lack of understanding how vibratory excitation affects the geotechnical conditions after treatment. Vibratory compaction is usually chosen to reduce total and differential settlement or to mitigate the liquefaction hazard. Settlements can be estimated based on information from in situ tests, but these are rarely performed due to a lack of experience by the designer. For instance, the tangent modulus method is a reliable concept to predict settlements of the untreated and compacted soil. An essential task for the designer is determining whether – or to which extent – vibratory treatment is necessary at all. New concepts have been introduced, making it possible to assess geotechnical parameters such as the soil modulus based on the cone penetration or dilatometer test. However, these methods are rarely applied in design due to a lack of experience in predicting settlements after compaction. Based on such analyses it is possible to prescribe the required treatment effect in terms of penetration resistance and to prescribe project-specific requirements. Modern vibrators are powerful, and their operation can be monitored and adjusted electronically to the project-specific requirements. The optimal compaction process depends on several parameters, such as vibrator operating frequency, movement amplitude, and dynamic ground response. These and other parameters should be adjusted during compaction to achieve optimal compaction effect. It has been shown when the vibration frequency approaches system resonance (vibrator-probe-soil system), the compaction effect increases. At the same time, the power consumption of the vibrator decreases markedly, resulting in a reduction in energy consumption and consequently making compaction more sustainable. It is generally appreciated that vibratory compaction compresses voids in granular soils (and consequently increases relative density). However, another equally important effect is the permanent increase in horizontal stresses, which results in the preloading of the soil deposit (increase in OCR). This horizontal prestressing effect is important as it affects OCR, reduces settlement, and increases the resistance to cyclic loading (liquefaction). Experience from many projects suggests that compaction design criteria are frequently chosen too conservative and cause unnecessary costs. Considering the above-described factors, more appropriate, project-specific compaction requirements could be applied, making deep vibratory compaction more competitive compared to alternative foundation solutions. Another important aspect is monitoring and optimizing the compaction process - by adjusting different vibrator operating parameters – to achieve more reliable compaction results. Finally, the improved understanding of the effects of cyclic loading on the treated soil, which leads to permanent horizontal prestressing, can be used to incorporate the preloading effect in the design. The paper describes and illustrates the three main effects of vibratory compaction, which can lead to more sustainable application of ground improvement. Data from case histories are used to demonstrate the potential for improving deep vibratory compaction.