FEATURE ARTICLE Pile Driving Vibrations and Building Damage Under unfavorable conditions, the installation of piles or sheet piles can cause damage to buildings or other structures on the ground. Frequently, such damage is attributed to vibrations of the building itself. Many countries have national or international standards for assessing the risk of vibration damage to buildings. Since these standards are often prepared by engineers with little to no geotechnical knowledge, the effects of ground conditions on vibration propagation and damage to building foundations are generally not recognized. In addition damage criteria based solely on the dynamic response of buildings neglect the importance of damage mechanisms governed by geotechnical conditions. One potentially important damage mechanism is differential settlement below buildings, especially when they are founded on loose granular soils. When seek- ing guidance in geotechni- cal literature, little to no information can be found regarding methods to assess permissible levels of ground vibrations with respect to risk for settlement. This article summarizes a paper presented at the DFI/EFFC International Conference on Piling and Deep Foundations in Sweden in May titled: “Ground Vibrations from Pile and Sheet Pile Driving Part 1 – Building Damage.” Here the authors focus on building damage that can be attributed to foundation conditions. In the case of ground vibrations due to pile and sheet pile driving, differential settlements are of particular importance as the ground vibrations attenuate relatively rapidly from the source. Consequently, settlement below part of the building, which is located When generated vibrations coincide with the resonance frequency of building elements, the risk of damage increases. close to the vibration source, can be significantly larger than at a distance further away. Piles are often a cost-effective foundation solution for buildings on loose and compressible soil, prevalent in urbanized areas. Sheet piles are commonly used as support for deep excavations. While piles are commonly installed by impact driving, vibrators are frequently used for driving (and extracting) sheet piles. It is important to recognize the fundamental differences between impact and vibratory driving. During impact driving, the pile is subjected to stress waves of short duration. The driving process creates vibrations, which radiate from the shaft and/or the toe of the pile into the soil. The larger the intensity of the stress wave, the larger the dynamic force and the intensity of ground vibrations. In addition to the vibration intensity, which often is expressed in terms of particle velocity, the vibration frequency is also important. When the dominant frequencies of the generated vibrations c o i n c i d e wi t h t h e resonance frequency of buildings or building elements, the risk of building damage increases. In the case of impact pile driving, the frequency content of ground vibrations cannot be controlled by changing the pile driving process. In contrast, during vibratory driving, the pile or sheet pile is rigidly attached to the vibrator, which oscillates vertically at a frequency that can be chosen and modified by the operator. The frequency and amplitude of modern vibrators can be adjusted in order to achieve optimal driving while minimizing environmental impact. However, if a vibrator is operated at or near the resonance frequency of buildings or building elements, strong vibrations can be generated. This amplification effect can be used to increase the efficiency of deep vibratory compaction systems, by means of “resonance compaction.” When a pile penetrates easily into the ground, the intensity of transmitted vibrations will be low. However, vibrations increase when denser soil layers are encountered and pile penetration speed decreases. Ground vibrations thus depend on the geotechnical conditions which need to be considered in the risk assessment. During the initial phase of pile penetration, the source of vibrations will be located close to the ground surface. However, when the pile penetrates deeper into the ground, the source of vibrations becomes more complex. Vibrations can be emitted from the toe of the pile but also along the pile shaft. Therefore, geotechnical conditions are of great importance when trying to predict the intensity of ground vibrations. It is important to know the location of hard soil layers through which the pile will be driven since they may give rise to strong ground vibrations. Building Damage Due to Vibrations In assessing the risk for damage to a building due to pile-driving induced vibrations, it is important to define the type of building damage that is considered. Moreover, one must also realize that the damage can occur as a secondary effect of the vibration, that is, it can result from settlement of the soil on which the building is resting. Damage to buildings and their foundations can be related to four different damage categories (see Figure 1). Damage Category I comprises static ground movements, which can occur in the vicinity of deep excavations. The installation of Completed excavation AUTHORS K. Rainer Massarsch,Dr. Tech, Geo Risk & Vibration and Bengt H. Fellenius, Dr. Tech, P.Eng., Consulting Engineer DEEP FOUNDATIONS • SEPT/OCT 2014 • 79
