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GROUND VIBRATIONS FROM PILE AND SHEET PILE DRIVING Part 1 — BUILDING DAMAGE K. Rainer Massarsch, Geo Risk & Vibration AB, Bromma, Sweden, <rainer.massarsch@georisk.se> Bengt H. Fellenius, 2475 Rothesay Ave., Sidney, BC, Canada, <Bengt@Fellenius.net> Abstract Vibrations generated during the driving of piles or sheet piles can affect buildings or installations in the ground. However, observed building damage is not always caused directly by dynamic effects, but can also be related to foundation conditions. The importance of geotechnical conditions is usually not taken into consideration in most vibration standards. This paper describes how building foundations, and in particular mixed foundation types, are especially sensitive to ground vibrations. A method is presented for assessing the risk of settlement due to vibrations in granular soils. Also, a simple concept is proposed for estimating settlements in the vicinity of driven piles. 1. INTRODUCTION 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. In many countries, national or international standards are used to assess the risk of vibration damage to buildings. As such standards are often prepared by engineers with little or no geotechnical knowledge, the effects of ground conditions on vibration propagation and damage to building foundations are generally not recognized, and 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 these are founded on loose granular soils. When seeking guidance in the literature, little or no information can be found regarding methods to assess permissible levels of ground vibrations with respect to risk for settlement. This paper focuses 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 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, frequently located in urbanized areas. Sheet piles are commonly used as support for deep excavations. While impact driving is most commonly used to install piles, vibrators are frequently used for driving (and extraction) of 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, also the vibration frequency is important. When the dominant frequencies of the generated vibrations harmonize with the 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, which can be chosen and modified by the operator. The operating 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

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generated. This effect can be used to increase the efficiency of deep vibratory compaction systems, such as “resonance compaction” (Massarsch and Fellenius, 2005). 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 depend thus 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 that the location is known of stiff soil layers, through which the pile shall be driven and which can give rise to strong ground vibrations. An in-depth discussion of factors influencing ground vibrations due to pile driving was given by Massarsch and Fellenius (2008). 2. RISK OF BUILDING DAMAGE DUE TO VIBRATIONS In assessing the risk for damage to a building due to pile-driving induced vibrations, it is important to make clear—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. 2.1 Damage Categories Damage can occur to buildings in connection with construction activities. Although noise and vibrations caused by pile and sheet pile installation can be experienced by humans as annoying or disturbing, these do not necessarily cause damage. Damage to buildings and their foundations can be related to different mechanisms. In Figure 1, four different damage categories are shown. Damage Category (I) comprises static ground movements, which can occur for instance in the vicinity of deep excavations. The installation of displacement piles can also give rise to heave and lateral displacements, which can damage buildings. Another possible source of static soil movement can be from the vibration-triggered movement of adjacent slopes or excavations with low factor of safety. Damage to buildings caused by ground distortion belongs to Damage Category (II). When the waves propagate along the ground surface, foundations of adjacent buildings can be subjected to a large number of upward (hogging) and downward (sagging) movements. The risk of damage is then largest when the length of the building corresponds to approximately half the wave length. Massarsch and Broms (1991) described how damage to buildings or installations can be analyzed when subjected to ground distortion. Damage to buildings due to settlement caused by ground vibrations belong to Damage Category (III). Settlement due to vibrations is largest in loose, granular soils, such as sand and silt. In the case of pile driving, differential settlements are more critical than total settlement. This topic is the main subject of this paper. Damage Category (IV) comprises building damage caused by dynamic effects in the building itself and is the only damage category, which is normally considered in vibration standards. 2.2 Influence of Building Foundation Ground conditions play an important role when assessing the risk of damage due to pile-driving induce vibrations. Figure 2 illustrates three examples with larger than normal risk for damage to buildings. The first example (a) is a building on loose sand or silt. If the building is founded on uncompacted granular soil and the area has not previously been exposed to strong ground vibrations, the building is more prone to suffer vibration damage. When piles or sheet piles are driven at a distance of approximately less than one pile length, the risk of differential settlement damage can be significant, especially when piles are driven through a stiff surface layer. This is due to the fact that vibrations will be largest close to the source but attenuate quickly, thereby increasing the risk of differential settlement. However, when the vibration source is located at larger depth, vibrations at the ground surface will vary less at the ground surface and the risk of differential settlement is lower.

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Figure 1 Possible damage mechanisms due to driving of piles or sheet piles, (Massarsch, 2000). The second case (b) shows a building, which is founded in a slope, where material had been excavated from the slope and placed to create a level foundation. Unless the placed fill is well-compacted, the risk of differential settlement between the fill and the excavated area below the building will be high. The third example (c) shows a building, which is partly founded, on natural or filled soil, and partly on piles. Differences in stiffness of the foundation can lead to differential movements between different parts of the building. 3. SETTLEMENT IN GRANULAR SOIL DUE TO GROUND VIBRATION While most vibration standards address the effects of ground vibrations on buildings, few recognize the potential risk of building damage that can be caused by settlement in the ground below a building foundation. A major improvement of the understanding of vibration-induced settlement resulted from the knowledge gained within the area of earthquake engineering, e.g., Seed and Silver (1972), Youd (1972), and Seed (1976). Recent work by Stewart et al. (2004) is of particular relevance when assessing the effect of ground vibrations on settlement in sand. This knowledge can—with some modifications—also be applied to other types of vibration problems, such as pile driving. The risk of settlement due to ground vibrations exists primarily in loose sand and silt. In other soils, such as soft clays, vibrations can contribute to but are rarely the main source of settlement. The topic of settlement due to pile-driving induced ground vibrations has been addressed only in a limited number of publications (Brumund and Leonards, 1972; Clough and Chameau 1980; Mohamed and Dobry 1987; Massarsch 2000; Massarsch 2002; and Massarsch 2004a). There is a need for guidance documents that can be used by the practicing engineer to assess the risk of settlements.

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a) Variation of vibration amplitude with distance from source

b) Variable ground conditions below building

c) Mixed foundation types

Figure 2 Importance of foundation conditions on total and differential settlement caused by ground vibrations.

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3.1. Vibration Limits The first step is to determine vibration limits below which the risk of settlement is negligible. It is possible to determine critical vibration levels, which are based on the shear strain level generated by ground vibrations. When vibrations pass through material, strain is induced. Strain, ε, caused by propagation of a compression wave (P-wave) can be determined from Eq. 1 if the particle velocity, vP, measured in the direction of wave propagation, and the wave speed, cP are known.

% % % % % % % % % % % % % % % % % % % %(1)%

Similarly, as shown in Eq. 2, the shear strain, γ, can be calculated by dividing the particle velocity measured perpendicularly to the direction of wave propagation with the shear wave speed, cS.

% % % % % % % % % % % % % % % % % % % %(2)%

As will be shown, shear strain is an important parameter when assessing the risk of settlement in granular soils or disturbance of cohesive soils. A threshold strain level, γt, exists below which it is unlikely that any rearrangement of soil particles can occur and, therefore, the vibrations will not generate an increase of pore water pressure in water-saturated sands. Drnevich and Massarsch (1979) and Mohamed and Dobry (1987) have shown that soil disturbance will not occur if shear strain are below a threshold value of γt ≈0.001%. When this level is exceeded, the risk of particle rearrangement and thus settlement increases. At a shear strain level of 0.01 %, vibrations can start to cause settlement and this value should not be exceeded. Significant risk of settlements exists when the shear strain level exceeds 0.1 %. It is important to note that shear modulus and shear wave speed are affected by shear strain. Massarsch (2004a) has shown that the shear wave speed decreases with increasing shear strain and that this reduction depends on the fines content (plasticity index) of the soil. The reduction of shear wave speed is more pronounced in gravel and sand than in silt and is even smaller in clay. The effect must be appraised when determining the shear wave speed at a given strain level. Based on Eq. 2, and taking into account the reduction in shear wave speed with shear strain level, Massarsch (2002) proposed a simple chart (Figure 3) showing the relationship between vibration velocity (particle velocity) and shear wave speed due to ground vibrations for three different levels of shear strain in relation to the risk for settlement in sand.%%%

Figure 3 Assessment of settlement risk in sand as function of shear wave

speed for different vibration velocity amplitudes.

ε =vPcP

γ =vScS

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In the following example, piles are driven in the vicinity of a building founded on medium dense sand. It is further assumed, that the sand has an average shear wave speed of 200 m/s. It should be noted that the peak particle velocity, which is relevant for settlement, should be measured perpendicular to the direction of vibration propagation. From Figure 3 it is now possible to determine the three damage threshold levels; no risk (0.001 % shear strain): 2 mm/s; low risk (0.01 % shear strain): 20 mm/s; and high risk (0.1 % shear strain): 75 mm/s. In practice, a planning engineer can require pile driving tests, where the expected vibration levels can be determined as a function of pile penetration depth and at different distances. This information can be used to assess the risk level with respect to settlement in the sand. If the predicted vibration level exceeds the “low risk” level, a detailed monitoring program should be implemented. This simple example illustrates that it is possible to assess the risk of settlement when sandy soil is subjected to ground vibrations. Of course, a more detailed analysis can be performed which also takes into account other important factors, such as number of vibration cycles etc. However, for many practical purposes, a simple assessment of the settlement risk in combination with field monitoring will suffice. 3.2 Estimation of Settlement Adjacent to Pile Driven in Sand Vibrations, which are caused by driving piles into dry or permeable soils, can cause settlements. The magnitude of settlement depends on several factors, such as soil type and stratification, groundwater conditions (degree of saturation), pile type, and method of pile installation (driving energy). For estimating settlements in a homogeneous sand deposit adjacent to a single pile, Massarsch (2004b) proposed the basic procedure, illustrated in Figure 4.

Figure 4 Basic method of estimating settlements adjacent to a single pile in homogeneous sand.

It is assumed that the most significant densification due to pile driving occurs within a zone corresponding to three pile diameters around the pile being driven. The volume reduction resulting from ground vibrations will cause significant settlements in a cone with an inclination 2(V):1(H), with its apex at a depth of 6 pile diameters below the pile toe. Thus, the settlement trough will extend a distance of 3D + L/2 from the centre of the pile, with maximum settlement at the centre of the pile. Maximum settlements, and average settlements, can be estimated using the Eq. 3 relationship, for an appropriate value of the soil compression factor, α %

% % % % % % % % % % (3)

smax =α(L+6D); sav =α(L+6D)

3

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Table 1 shows compression factors, based on experience from soil compaction projects, which are applicable to driving in very loose to very dense sand. The intensity of ground vibrations can be assessed based on vibration levels indicated in Fig. 3.

TABLE 1 Compression factor, α, for sand based on soil density and level of driving energy.

Ground%Vibrations:% Low% Medium% High%

Soil%Density% E%E%E%E%E%Compression%factor,%α%E%E%E%E%E%Very%loose% 0.02% 0.03% 0.04%Loose% 0.01% 0.02 0.03

Medium 0.005 0.01 0.02

Dense 0.00 0.005 0.01

Very%dense 0.00 0.00 0.005

Assume that a concrete pile with diameter D = 0.3 m and an effective pile length L = 10 m is installed in a deposit of medium dense sand. The pile is driven using an impact hammer, and pile penetration is normal (stiff layers requiring high driving energy are assumed not to be present). The compression value for medium dense sand and average driving energy according to Table 1 is α = 0.010. The maximum settlement adjacent to the pile, and the average surface settlement of the cone are 118 mm and 39 mm, respectively. The radius of the settlement cone of the ground surface footprint is estimated to 5.9 m, resulting in an average surface inclination of 1:50 (0.118/5.90). 4. CONCLUSIONS AND SUMMARY Driving of piles or sheet piles can cause damage to building foundations or installations in the ground, such as sewage pipes and tanks. Some of the observed damage may not directly be related to vibrations but to static ground movement. Vibrations can cause settlement in loose granular soils, a fact which is not appreciated in building vibration standards. Differential settlements of the ground below a building are often the main reason for damage in building foundations, from where damage can propagate up the building structure and be interpreted as vibration damage. Therefore, it is important that the risk of settlement in granular soils due to ground vibrations is included in a risk analysis. An important aspect, which frequently is overlooked in a vibration risk analysis, is the type of building foundation. Of particular importance is the vulnerability of buildings with mixed foundations where one part of the building is founded on stiff ground or piles, and the other part on soft or loose soil. A basic method is proposed to estimate the risk of settlement due to ground vibration in granular soils. Even in very loose sand and silt with a shear wave speed of about 100 m/s, settlements are unlikely to occur when the peak particle velocity is below 1 mm/s. However, the risk of settlement increases when the peak particle velocity exceeds about 10 mm/s. In the case of important projects, pile driving tests and vibration monitoring will provide valuable information. Settlement around a pile driven into sand can be estimated based on a simplified concept. Average and maximum settlements can be estimated for loose to dense, granular soils by selecting a compression factor. Also the intensity of driving can be taken into account, selecting the level of pile driving energy. 5. ACKNOWLEDGEMENT The authors wish to acknowledge the constructive comments by the reviewer, which helped to clarify some aspects of the paper.

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REFERENCES Brumund, W.F. and Leonards, G.A. 1972. Subsidence of sand due to surface vibration. Journal of the Soil Mechanics and Foundation Division, Proceedings ASCE, 98(1) 27–42. Clough, G.W. and Chameau, J. 1980. Measured effects of vibratory sheet pile driving. Journal of the Geotechnical Engineering Division, ASCE, 106(GT10) 1080-1099. Chameau, J-L, Rix, G.J., and Empie, L. (1998). Measurement and analysis of civil engineering vibrations. Fourth International Conference on Case Histories in Geotechnical Engineering. St. Louis, Missouri, March 8-15, 1998, pp. 96–107. Drnevich, V.P. and Massarsch, K.R. 1979. Sample Disturbance and Stress - Strain Behaviour , ASCE Journal of the Geotechnical Engineering Division, 105(GT 9) 1001-1016. Massarsch, K.R., 2000. Settlements and damage caused by construction-induced vibrations. Proc., Int. Workshop, Wave 2000, Bochum, Germany, December 13–15, 2000, pp. 299 315. Massarsch, K.R. 2002. Effects of vibratory compaction. TransVib 2002 – International Conference on Vibratory Pile Driving and Deep Soil Compaction. Louvain-la-Neuve. Keynote Lecture, pp. 33–42. Massarsch, K.R. 2004a. Deformation properties of fine-grained soils from seismic tests. Keynote Lecture, International Conf. on Site Characterization, ISC’2 Sept. 19-22, 2004, Porto, pp. 133-146. Massarsch, K.R. 2004b. Vibrations caused by pile driving. Deep Foundations Institute, Magazine, Part 1: Summer Edition, pp. 41–44, Part 2: Fall Edition, pp. 39–42. Massarsch, K.R. and Broms, B.B. 1991. Damage criteria for small amplitude ground vibrations, Second International Conference on Recent Advances in Geotechnical Earthquake Engineering and Soil Dynamics, St. Louis, Missouri, March 11-15, 1991, Vol. 2, pp. 1451-1459. Massarsch, K.R. and Fellenius, B.H. 2005. Deep vibratory compaction of granular soils. In Ground Improvement Case Histories, Geo-Engineering Series Vol.3, Chapter 19, Elsevier Publishers (UK), Edited by Buddhima Indraratna and Jian Chu, pp. 633 - 658. Massarsch, K.R., Fellenius, B.H. 2008. Ground vibrations induced by pile driving. 6th International Conference on Case Histories in Geotechnical Engineering, Arlington, VA, August 11-16, 2008. Keynote lecture. 38 p. Mohamed, R. and Dobry, R. 1987. Settlements of cohesionless soils due to pile driving. Proceedings, 9th Southeast Asian Geotechnical Conference, Bangkok, Thailand, pp. 7-23. Seed, H.B. and Silver, M.L. 1972. Settlements of dry sand during earthquakes. Journal of the Soil Mechanics and Foundation Division, Proceedings ASCE, 98 (2) 381-396. Seed, H.B. 1976. Evaluation of soil liquefaction effects on level ground during earthquakes. ASCE Annual Convention and Exposition, Liquefaction Problems in Geotechnical Engineering. Philadelphia. pp. 1–104. Youd, T.L. 1972. Compaction of sands by repeated shear straining. Journal of the Soil Mechanics and Foundation Division, Proceedings ASCE, Vol. 98, pp. 709– 25.

GROUND VIBRATIONS FROM PILE AND SHEET PILE DRIVING Part 2 — REVIEW OF VIBRATION STANDARDS K. Rainer Massarsch, Geo Risk & Vibration AB, Bromma, Sweden, S-161 51, <rainer.massarsch@georisk.se> Bengt H. Fellenius, 2475 Rothesay Ave., Sidney, BC, V8L 2B9, Canada, <Bengt@Fellenius.net> Abstract Vibration limits for buildings with respect to ground vibrations from pile driving published in international standards are reviewed. At first, relevant vibration parameters are defined and the evaluation of vibration records is described. The sensitivity of structures to different vibration parameters (displacement, velocity and acceleration) is discussed. Most vibration standards relate to blasting activities, the dynamic characteristics of which can be significantly different to those from construction activities and in particular, pile driving. Several vibration standards are reviewed. The Swedish vibration standard is described in detail, as it is the only one accounting for several important factors, such as ground conditions, building material, and type of building foundation. Many standards limit the guidance to values based on the dominant vibration frequency and a wide scatter of frequency-dependent vibration limits exists. It is difficult to compare limiting values of ground vibrations proposed in different standards.

1. INTRODUCTION For poor ground conditions, placing structures on piles is the most common foundation solution, while sheet piles are widely used to support deep excavations. Piles and sheet piles must often be installed in the close vicinity to buildings, structures or installations in the ground, which can be affected by ground movements and vibrations, (Massarsch and Fellenius, 2014). In many areas, environmental concerns with respect to noise and ground vibrations can restrict or even prohibit driving of piles or sheet piles or reduce the efficiency of installation. The design engineer must prescribe – often at an early stage of a project — the permissible levels of ground vibrations in order to minimize the potential risk of building damage. When seeking guidance from the literature, little or practically no applicable information can be found on how to assess permissible levels of ground vibrations. Most vibration standards and guidance documents have been developed for rock blasting but are widely used also to asses damage risk due to vibrations caused by other types of construction activities, such as pile driving and soil compaction. However, such standards cannot be applied without taking into account the significance of different vibration parameters. Another limitation of existing standards is that these usually have been prepared by vibration specialists with little or no understanding of geotechnical problems. For instance, ground conditions, location of the groundwater table, and type of building foundation are important when assessing the risk for vibration damage. Moreover, ground vibrations can cause settlement in loose and medium dense soils below a building and give rise to differential movements in the building, which are not directly related to dynamic effects. Massarsch and Fellenius (2014) have discussed the importance of geotechnical conditions and the influence of foundation types on building damage. National vibration standards are not generally applicable as these were developed empirically and are based on regional experience, taking into account geological and geotechnical

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conditions, building types and construction methods, and building materials. Therefore, such standards need to be interpreted with judgment when applied to other geographic regions. Another important aspect is the fact that different standards use different parameters to define limiting values, which need to be taken into account when planning vibration measurements and interpreting the results. Also, equipment and measurement procedures vary in different standards and can have significant consequences when applied incorrectly. This paper deals with damage to buildings caused by ground vibrations from driving of piles and sheet piles. It does not address the mechanism of how vibrations are generated during the driving process. For a detailed description of how vibrations are generated during impact pile driving, reference is made to Massarsch and Fellenius (2008). Vibrations caused by vibratory driving and soil compaction has been discussed by Massarsch (2002). As an introduction to this paper, different vibration parameters are discussed and defined. Thereafter, an overview of some national standards and vibration guidance documents is given.

2. DEFINITION OF VIBRATION PARAMETERS The understanding of which parameters can be used to describe vibrations is an important requirement when assessing building damage. The following sections describe the most important parameters required for evaluating the effect of ground vibrations on buildings and building foundations. A comprehensive discussion of these parameters and their interpretation is given by Chameau et al. (1998).

2.1 Vibration Amplitude Vibration amplitude can be defined as the departure of a point on a vibrating body from its equilibrium position. It is equal to one-half the length of the vibration path. A typical vibration record from pile driving is shown in Figure 1. The following relationship exists between different expressions of vibration amplitude.

(1) where a = acceleration, v = particle velocity, d = displacement and f = vibration frequency. It is thus possible to derive the corresponding amplitude values if one of value of vibration amplitude (displacement, velocity or acceleration) and the vibration frequency are known. The maximum value of vibration velocity (peak value) occurring during the measuring period is in many standards defined as peak particle velocity (PPV). If particle motions are measured in three orthogonal directions (x, y, and z) simultaneously, it is possible to calculate the vector sum, |vi| of the three components. !!!! ! ! !"!! ! !"!! ! !"!! (2)

In the case of sinusoidal vibrations, the average vibration amplitude can be expressed by the ratio of root-mean-square (RMS)

(3) where x

n etc. are the set of n vibration values. The RMS value, which is frequently used to

describe the average vibration intensity, corresponds to the area under the half wavelength. In case of sinusoidal vibrations - and only then - it is related to the peak amplitude, vpeak

a = 2! f v = (2! f )2 d

xrms =1n x1

2 + x22 + x3

2 + x42 +....+ xn2( )

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(4) The relevance of the RMS value depends strongly on the duration of the signal. The so-called CREST factor (peak to average) is the ratio between the peak value and the rms value. In the case of transient vibrations, which are typically generated by impact pile driving, the duration of the largest motions is small compared to the total length of the signal. For such vibrations, it is possible to choose the minimum amplitude of interest (i.e. minimum value which is of relevance) and calculate the RMS amplitude from the time that the minimum amplitude is exceeded for the first time to the time that the minimum amplitude is exceeded for the last time in the record. The peak value of the wave is the highest value the wave reaches above a reference, normally zero. This definition is used in the above equations. Note that in engineering applications, frequently the peak-to-trough value (vertical distance between the top and bottom of the amplitude) is used to express vibration intensity. Ambiguity has been the reason for numerous interpretation errors.

2.2 Strain Vibrations passing through material impose strain, which can be calculated from the particle velocity and the wave speed. Strain, ", caused by propagation of a compressional wave (P-wave) can be determined from Eq. (5), if the particle velocity, vP measured in the direction of wave propagation, and the wave speed, cP are known.

(5)

Similarly, shear strain, # can be calculated from the particle velocity measured perpendicular to the direction of wave propagation and the shear wave speed, cS (Eq. 6)

(6) As will be shown, shear strain is an important parameter when assessing the settlement in granular soils or disturbance of cohesive soils.

2.3 Vibration Frequency The time history of the vibration record shown in Figure 1 can be transferred into the frequency domain, Figure 2. The frequency content of a signal is important when assessing the effect of vibrations on structures. The simplest method of estimating the dominant frequency is by examining "zero crossings" of the time history. This method works reasonably well for simple, periodic signals, but is less reliable for complex, multiple-frequency signals. A common method of estimating the frequency content (spectrum) of a signal is to perform a Fast Fourier Transformation (FFT). The resulting values are usually presented as amplitude and phase, both plotted versus frequency. A related quantity, which is also widely used to estimate the power of a signal, is the power spectrum, which in the frequency domain is the square of FFT´s magnitude. Despite its widespread use, there are several limitations associated with the use of Fourier methods for estimating spectra. The Fourier method implicitly assumes that the signal is stationary. For transient signals such as impact pile driving and blasting and for many dis-continuous signals, this assumption is not strictly valid.

vrms = 0.7 vpeak

" = vPcP

# = vScS

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Figure 1. Vertical vibration velocity as function of time, measured at 10 m distance, with

the pile toe 3 m below the ground surface during driving of precast concrete pile into sandy soil. The value of the peak particle velocity (PPV) is indicated.

Figure 2. Frequency spectrum of the time history shown in Fig. 1. The dominant frequency

range is indicated. A valuable method for estimating the response of a structure to an incoming ground motion is the response spectrum. The response spectrum is expressed as a pseudo-velocity or pseudo-acceleration versus the natural period (or frequency) of the structure, which is modeled by a single-degree-of freedom (SDOF) system.

2.4 Wave Length The wave length is an important parameter when assessing the risk of damage due to propagation of waves in the ground. The wave length, $ can be determined from the following relationship.

(7) where f is the vibration frequency.

$ = 2! f

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2.5 Cyclic Loading Another parameter, to consider in connection with pile-driving induced vibrations, is the number of vibration cycles. While structures usually are not very sensitive to fatigue (i.e. the number of vibration cycles), repeated vibration cycles can cause settlements in loose, granular soils (sand and silt) and/or lead to a reduction of the shear strength in fine-grained soils (clay, silt). In loose, water-saturated sand and silt, cyclic loading can lead to a build-up of pore water pressure and to a corresponding reduction in effective stress, which can result in a gradual or complete loss of shear strength (liquefaction). In the case of vibratory driving of piles and sheet piles, the number of significant vibration cycles during driving can be large and should not be neglected. Also, when a group of piles is driven, the accumulated effect of repeated vibrations can cause settlement or reduce the stability of slopes due to temporary increase in pore water pressure. A fundamental problem when analyzing the effect of ground vibrations on a soil deposit is the fact that the time history of vibrations usually consists of an erratic series of cycles of varying amplitude. It is therefore useful to convert the irregular series of vibration cycles into an equivalent number of uniform cycles. A concept was developed for liquefaction analysis to determine the number of equivalent vibration cycles, which has the same effect as the irregular vibration pattern, Seed (1976). This concept was developed for assessing the risk of liquefaction in medium dense to loose granular soils. Massarsch (2000) has adapted this concept to determine equivalent vibration cycles for settlement analysis in granular soils. Figure 3 shows a relationship to convert an irregular vibration record into equivalent, uniform vibration cycles. For example, the effect of one vibration cycle of maximum amplitude, vmax on soil compaction is assumed to have the same effect as for instance 10 cycles of 0.4xvmax. Experience has shown that the method of converting irregular vibration records in soils is robust and not too sensitive to variation of the assumed conversion relationship.

Figure 3. Conversion of irregular time history into equivalent number of uniform

(sinusoidal) vibration cycles, Massarsch (2000). In the example, the effect of one uniform cycle (1x vmax) corresponds to 4.3 x cycles of 0.5 vmax.

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Figure 4. Relationship for sinusoidal vibrations between frequency of vibration and

vibration amplitudes, cf. Eq. (1). At 10 Hz, the relationship between particle velocity of 30 mm/s the corresponding displacement is 0.5 mm and the acceleration is 0.2 g.

The black line in Figure 4 marks a vibration velocity of 30 mm/s. At a vibration frequency of 10 Hz, the corresponding displacement amplitude is 0.5 mm. If the vibration frequency decreases at constant vibration velocity, the displacement amplitude will increase. Correspondingly, if the vibration frequency increases at constant vibration velocity, the acceleration amplitude increases. It is difficult to give general recommendations regarding the damage effects of displacement, velocity and acceleration as the sensitivity of structures can depend on many factors. However, the following general observations can be made, which are not generally appreciated:

• Displacement: at low frequencies, displacement is the most relevant measure of structural damage as the failure mode is generally due to “static” stress caused by displacement (Hooke’s law), i.e. damage caused by exceeding material strength.

• Velocity: measures how often the displacement is applied in a given time period and

is thus related to the fatigue mode of failure (material degradation). As can be seen from Eq. (5) and (6), shear strain, which causes material distortion (settlement), depends on vibration velocity and is thus particularly important when assessing settlement in loose granular soils.

• Acceleration: at high frequencies, the failure mode is normally related to the applied

dynamic force caused by inertial forces (Newton’s law). It should be pointed out that the above failure modes (stress – fatigue – dynamic force) can overlap and the proper selection of vibration parameter must reflect the type of problem.

!DISPLACEMENT

!VELOCITY"

ACCELERATION"

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3. VIBRATION STANDARDS Environmental authorities in many countries are increasingly aware of the importance of environmental problems and apply standards more rigorously than in the past. Vibrations from construction activities are normally not likely to cause damage to buildings or building elements. Only in the case of very sensitive buildings with poor foundation conditions, damage may be initiated or existing cracking aggravated. However, in the case of vibration-sensitive foundation conditions, such as mixed foundations or foundations on loose, granular soils, damage can be caused by total and/or differential settlements. This aspect is not included in most vibration standards. In practice, vibration standards, which were primarily developed for blasting applications, are also often used to regulate vibrations from construction activities. The following review of existing standards deals with damage criteria for buildings.

4.1 Swedish Standard Due to difficult ground conditions in Sweden, pile driving is used frequently also in vibration-sensitive areas and extensive experience regarding the effects of driving preformed piles has been accumulated. The Swedish Standard SS 02 52 11, “Vibration and shock – Guidance levels and measuring of vibrations in buildings originating from piling, sheet piling, excavating and packing [sic] to estimate permitted vibration levels” was established in 1999. This standard – which is not widely known outside Scandinavia - was particularly developed to regulate construction activities and is probably the most elaborate standard currently available. It deals with vibrations caused by piling, sheet piling, excavation and soil compaction. Guidance levels of vibrations acceptable with respect to potential building damage have been established based on more than 30 years of practical experience in a wide range of soils. Under the Swedish standard, a risk analysis must be carried out for construction projects, involving the prediction of maximum ground vibration levels and statement of permissible vibration levels for different types of structures. The proposed vibration values do not take into account psychological effects (noise or comfort) on occupants of buildings. Neither do they consider the effects of vibrations on sensitive machinery or equipment in buildings. The vibration levels in the standard are based on experience from measured ground vibrations (vertical component of particle velocity) and observed damage to buildings, with comparable foundation conditions. The vibration level, v, is expressed as the peak value of the vertical vibration velocity. It is measured on bearing elements of the building foundation closest to the vibration source and is determined from the following relationship

(8) where: v0 = vertical component of the uncorrected vibration velocity in mm/s, Fb = building factor, Fm = material factor and Fg = foundation factor. Values for v0 are given in Table 1 for different ground conditions and construction activities, and are maximum allowable values at the base of the building. It should be noted that in the Swedish standard, the limiting vibration values are independent of vibration frequency. The main reason is that within the frequency range of vibrations generated by pile driving and soil compaction, the dominant frequency usually varies within a narrow range.

v = v0 Fb Fm Fg

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Table 1. Uncorrected vibration velocity, v0.

Foundation Condition Piling, Sheet piling or Excavation

Soil Compaction

Clay, silt, sand or gravel 9 mm/s 6 mm/s Moraine (till) 12 mm/s 9 mm/s Rock 15 mm/s 12 mm/s

Buildings are divided into five classes with respect to their vibration sensitivity cf. Table 2. Classes 1 – 4 apply to structures in good condition. If they are in a poor state, a lower building factor should be used.

Table 2. Building Factor, Fb.

Class Type of Structure Building Factor, Fb 1 Heavy structures such as bridges, quay walls, defence

structures etc. 1.70

2 Industrial or office buildings 1.20 3 Normal residential buildings 1.00 4 Especially sensitive buildings and buildings with high value

or structural elements with wide spans, e.g. church or museum buildings

0.65

5 Historic buildings in a sensitive state as well as certain sensitive historic buildings (ruins)

0.50

The structural material is divided into four classes with respect to their vibration sensitivity, cf. Table 3. The most sensitive material component of the structure determines the class to be applied.

Table 3. Material Factor, Fm.

Class Type of Building Material Material Factor, Fm 1 Reinforced concrete, steel or timber 1.20 2 Unreinforced concrete, bricks, concrete blocks with voids,

light-weight concrete elements 1.00

3 Light concrete blocks and plaster 0.75 4 Limestone, lime-sandstone 0.65

Table 4 defines a foundation factor. Lower factors are applied to buildings on shallow foundations, whereas buildings on piled foundations are accorded higher factors due to their reduced sensitivity to ground vibrations.

Table 4. Foundation Factor, Fg. Class Type of Building Material Foundation Factor, Fg 1 Slab, raft foundation 0.60 2 Buildings founded on friction piles 0.80 3 Buildings founded on end-bearing piles 1.00

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The following example illustrates the practical application of the standard: Piles are to be installed in the vicinity of a residential building with brick walls, which are supported by toe-bearing piles in clay. If the following factors are chosen according to Tables 1 to 4: v0 = 9 mm/s, Fb = 1.00, Fm = 1.00, Fg = 1.00, the maximum allowable vertical vibration velocity, v, measured at the base of the foundation is 9 mm/s.

4.2 US Bureau of Mines In North America, vibration codes are based on experience and statistical information, mainly from vibration damage caused by blasting and less from construction activities. One widely used vibration criterion for assessment of building damage is the frequency-dependent vibration limit, proposed by the U.S. Bureau of Mines, which is based on an extensive study of damage to residential structures from surface mine blasting. Siskind et al. (1980) found that horizontal peak particle velocity measured on the ground outside of a structure correlated well with "threshold damage", defined as cosmetic damage (e.g. cracking) within the structure. Although other descriptors of the ground motion were considered, peak particle velocity was chosen for its effectiveness and simplicity. These vibration criteria are presented in Table 5 and Figure 5, with transitions between frequency ranges. The lower limits at frequencies smaller than 40 Hz reflect the fact that the structural and mid-wall resonant frequencies are usually within this range for residential structures. Table 5. Simplified USBM Vibration Criteria for Peak Particle Velocity causing cosmetic

damage (from Siskind et al., 1980). Note: developed for blast-induced vibrations.

Type of Structure Ground Vibration

Low Frequency (< 40 Hz)

High Frequency (> 40 Hz)

Modern Homes (Drywall) 19.1 mm/s 50.8 mm/s

Older Homes (Plaster) 12.7 mm/s 50.8 mm/s

Figure 5. Comparison of frequency-dependent vibration parameters.

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4.3 British Standard Vibration effects on buildings in the UK are covered by British Standard BS 7385. Part 2: (1993), "Evaluation and measurement for vibration in buildings - Part 2: Guide to damage levels from groundborne vibration". This part of BS 7385 provides guidance on the assessment of the possibility of vibration-induced damage in buildings due to a variety of sources and sets guide values for building vibration based on the lowest vibration levels above which damage has been credibly demonstrated. It also provides a standard procedure for measuring, recording and analyzing building vibration together with an accurate record of any damage occurring. The vibration criteria published in BS 5228-2:2009. “Code of practice for noise and vibration control on construction and open sites – Part 2: Vibration” are identical to those in BS 7385. Sources of vibration which are considered include blasting (carried out during mineral extraction or construction excavation), demolition, piling, ground treatments (e.g. compaction), construction equipment, tunnelling, road and rail traffic, and industrial machinery. Table 6 gives vibration limits for cosmetic damage expressed as the maximum value of any one of three orthogonal component particle velocities measured during a given time interval. In the standard, the resultant of the vibration levels in the three orthogonal axes is used. The values in Table 6 relate to transient vibrations, which do not give rise to resonant responses in structures, and to low!rise buildings. Where the dynamic loading caused by continuous vibration is such as to give rise to dynamic magnification due to resonance, especially at the lower frequencies where lower guide values apply, then, the guide values in Table 6 might need to be reduced by up to 50%. Also, for unreinforced or light-framed structures below 4 Hz, a maximum displacement of 0.6 mm (zero to peak) is not to be exceeded. Table 6. Transient vibration guide values for cosmetic damage, measured at base of the

building. BS 5228-2:2009. Type of building Peak component velocity in frequency range of

predominant pulse 4 to 15 Hz 15 Hz and above Reinforced or framed structures Industrial and heavy commercial buildings

50 mm/s at 4 Hz and above 50 mm/s at 4 Hz and above

Unreinforced or light framed structures Residential or light commercial type buildings

15 mm/s at 4 Hz increasing to 20 mm/s at 15 Hz

20 mm/s at 15 Hz increasing to 50 mm/s at 40 Hz and above

Vibration limits, which can cause structural damage, will be higher than those for cosmetic damage, but are not stated in the standard.

4.4 German Standard The German standard, DIN 4150, Part 3 (1999), “Vibration in buildings - Part 3: Effects on structures” addresses the effects of construction-induced vibrations on buildings for short-term and continuous vibrations. The code is applied to problems where vibrations affect buildings and structures, located on or below the ground surface. For short-term loading, the

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vibration velocity shall be measured at the building foundation level and is based on the largest maximum value of the three components. For continuous vibrations, the horizontal vibration components are measured at the top floor of the building and are set independent of vibration frequency. Table 7 gives frequency-dependent guidance values of peak particle velocity for different types of structures or buildings. Table 7. Guidance values of vibration velocity for the evaluation building damage for

short-term and long-term impact, DIN 4150, Part 3. Structure/Object Type

Frequency Peak Velocity Location of measurement

Hz mm/s mm/s Short-term: Long-term

Offices and industrial premises

1 20 -

Foundation

10 20 - 10 20 50 40 - 50 40 - 100 50 1 - 10 Top floor,

horizontal 100 - 10 Domestic houses and similar construction

1 5 -

Foundation

10 5 - 10 5 50 15 50 15 - 100 20 1 - 5 Top floor,

horizontal

100 - 5

Other buildings sensitive to vibrations

1 3 -

Foundation

10 3 - 10 3 50 8 50 8 - 100 10 1 - 2,5 Top floor,

horizontal 100 - 2,5 DIN 4150-3 also gives guidelines regarding the execution of vibration measurements in buildings. If resonance can be expected in the structure, vibration measurements shall be performed on several floor levels (not only on the top floor). In the case of floor vibrations, the vertical vibration amplitude shall be used. The code also points out the risk of cumulative damage effects from vibrations on buildings with existing (locked-in) stresses. Buildings, which are subjected to differential settlements, are considered particularly sensitive to ground vibrations.

4.5 Swiss Standard Swiss standard SN 640 312 has been introduced in 1979 and considers the effect of vibrations

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from transient (shock) and continuous vibrations. Buildings are divided into four categories, cf. Table 8. Table 8. Building categories, SN 640312. Building Category Building Type

I Reinforced concrete structures for industrial purposes, bridges, towers etc. Subsurface structures such as caverns, tunnels with or without concrete lining

II Buildings with concrete foundations and concrete floors, buildings made of stone and concrete masonry/blocks Subsurface structures, water mains, tubes and caverns in soft rock

III Buildings with concrete foundations and concrete basement, timber floors, masonry walls

IV Especially vibration-sensitive structures and buildings requiring protection Recommended vibration levels for continuous disturbance are presented in Table 9. Two different frequency ranges have been chosen, 10 – 30 and 30 – 60 Hz, respectively. Table 9. Swiss recommendation of maximum vibration levels for continuous vibrations,

SN 640312.

Building Category Frequency Range Recommended vibration velocity Hz mm/s I 10 – 30 12 I 30 - 60 12 – 18 II 10 – 30 8 II 30 - 60 8 – 12 III 10 – 30 5 III 30 - 60 5 – 8 IV 10 – 30 3 IV 30 - 60 3 - 5

4.6 Hong Kong Government Regulations The Hong Kong Buildings Department has issued a Practice Note, APP-137 “Ground-borne Vibrations and Ground Settlements Arising from Pile Driving and Similar Operations” which provides guidelines on the control of ground-borne vibrations and ground settlements generated from pile driving or similar operations with a view to minimizing possible damage to adjacent properties and streets. It is worth noting that of the standards reviewed in this document, this standard is the only one which suggests limiting values with regard to ground settlement and ground distortion. The effect of ground-borne vibration from piling works on adjacent structures is assessed by the maximum peak particle velocity (PPV). The maximum PPV shall be evaluated from the peak particle velocities at three orthogonal axes measured at ground levels of the structures in question. The guide values of maximum PPV presented in Table 10 are suggested to give minimal risks of vibration-induced damage.

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Table 10. Empirical guidelines according to HK Practice Note, APP-137, Appendix A. Type of building

Guide values of maximum PPV (mm/s) Transient vibration (e.g.

drop hammer) Continuous vibration (e.g.

vibratory hammer) Robust and stable buildings in general

15 7.5

Vibration-sensitive/dilapidated buildings

7.5 3.0

Due attention shall also be paid to sensitive buildings close to the piling site such as hospitals, academic institutes, declared monuments, old buildings with shallow foundations, old tunnels/caverns, buildings installed with sensitive equipment, masonry retaining walls or sites with history of instability, monuments or buildings with historical significance, etc. A more stringent control on the allowable limit of PPV for these buildings may be specified based on site and building conditions together with the duration and frequency of the exciting source. As different structures will have different tolerance in accommodating movements of their foundations, acceptance of estimated ground settlements should be considered on a case-by-case basis with respect to the integrity, stability and functionality of the supported structures. Provided that there are no particularly sensitive adjacent buildings, structures, and services, the guide values, Table 11, may be taken as the trigger values in accordance PNAP APP- 18. Table 11. Empirical guidelines according to HK Practice Note, APP-137. Instrument Criterion Alert Alarm Action Ground settlement marker

Total settlement 12 mm 18 mm 25 mm

Service settlement marker

Total settlement & angular distortion

12 mm or 1:600

18 mm or 1:450

25 mm or 1:300

Building tilting marker

Angular distortion

1:1000 1:750 1:500

4. CONCLUSIONS Driving of piles or sheet piles can cause ground vibrations, which under unfavorable conditions can adversely affect buildings or other structures. Therefore, it is important to perform a risk analysis and to define limiting values with respect to ground vibrations. Little guidance is found in the geotechnical literature with respect to the selection of appropriate vibration parameters. The paper defines different vibration parameters and offers guidance regarding the evaluation of vibration records. Damage to buildings can be caused by different mechanisms, such as deformation, velocity and acceleration, or a combination of these parameters. For pile-driving induced vibrations the most relevant parameter is vibration velocity. Vibration velocity is related to material strain and thus a measure of material deterioration (settlement). The Swedish standard SS 02 52 11was specifically developed for pile-driving induced ground vibrations. It is the only known standard specifically addressing vibrations due to pile-

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driving. It is based on the vertical vibration velocity and chosen independently of vibration frequency and takes into consideration ground conditions, building material, and type of building foundation. The frequency-dependent vibration parameters presented above are summarized in Figure 5. The blasting standard of the US Department of Mines (USBM) is also included as reference. The difference in particle velocity between different standards is large and cannot be explained on a rational basis. As the dominant vibration frequency of pile-driving induced vibrations usually is in the range of 10 to 30 Hz, there appears to be little justification to use frequency-adjusted velocity values. Based on the comparison in this paper, the Swedish standard appears to provide more relevant guidance values, as it takes into account important aspect, such as ground conditions and building foundation parameters.

5. REFERENCES British Standard BS 7385. 1993. Evaluation and measurement for vibration in buildings - Part 2: Guide to damage levels from groundborne vibration. 16 p. British Standards, 2009. Code of practice for noise and vibration control on construction and open sites – Part 2: Vibration BS 5228-2:2009. 98 p. Chameau, J-L., Rix, G.J. and Empie, L. (1998). Measurement and analysis of civil engineering vibrations. Fourth International Conference on Case Histories in Geotechnical Engineering. St. Louis, Missouri, March 8-15, 1998. pp. 96 – 107. DIN 4150. 1999. Vibration in buildings - Part 3: Effects on structures. Deutsches Institut für Normen, Beuth Verlag GmbH, Berlin. 12 p. Hong Kong Buildings Department. 2004. Ground-borne vibrations and ground settlements arising from pile driving and similar operations, Revision 2012 (AD/NB2), Practice Note PNAP APP-137, 7 p. Hong Kong Buildings Department. 2012. Pile Foundations. Practice Note PNAP APP-18, 7 p. Massarsch, K.R. 2000. Settlements and damage caused by construction-induced vibrations. Proc., Int. Workshop. Wave 2000, Bochum, Germany December 13-15, 2000, pp. 299–315. Massarsch, K. R. 2002. Ground Vibrations Caused by Soil Compaction. Wave 2002. Proc. Int. Workshop, Okayama, Japan, pp. 25 - 37. Massarsch, K.R. and Fellenius, B.H. 2008. Ground vibrations induced by pile driving. 6th International Conference on Case Histories in Geotechnical Engineering, Arlington, VA, August 11 -16, 2008. Keynote lecture. 38 p. Massarsch, K.R. and Fellenius, B.H. 2014. Ground vibrations from pile and sheet pile driving - Part 1 Damage. Submitted for publication to the DFI-EFFC International Conference on Piling and Deep Foundations, Stockholm, 21 – 23 May 2014. 10 p.

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Seed, H.B. 1976. Evaluation of soil liquefaction effects on level ground during earthquakes. ASCE Annual Convention and Exposition, Liquefaction Problems in Geotechnical Engineering. Philadelphia. pp. 1–104. Siskind, D.E., Stagg, M. S., Kopp, J. W. and Dowding, C. H. 1980. Structure response and damage produced by ground vibration from surface mine blasting. U. S. Bureau of Mines RI 8507, 74 pp. Swiss Standard. 1992. Erschütterungen – Erschütterungseinwirkungen auf Bauwerke (Swiss Standard on vibration effects on buildings), SN 640 312a, Vereinigung Schweizerischer Strassenfachleute (VSS). Kommission 272, Unterbau und Fundationsschichten. 9 p.

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