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VIBRATION PREDICTION AND MONITORING -

DYNAMIC COMPACTION AT ...


Scope of Report

To predict vibration levels that could be expected in residential properties near a development site, for which dynamic compaction is proposed.

1. INTRODUCTION

1.1 The site of an existing ... has been purchased by ... for re-development as a supermarket.

1.2 The ground conditions are such that the building contractor are using dynamic compaction to improve the ground bearing capacities.

1.3 The dynamic compaction procedure involves dropping a heavy weight on the ground concerned. Such a procedure inevitably produces vibration signals that are unusual, in terms of their magnitude or frequency (or both), for typical urban areas. ... anticipate the works taking three weeks to complete, if there are no delays.

1.4 This report presents a prediction of vibration that could be expected at nearby residential properties during the proposed works. The predictions are evaluated as to the possibility of cosmetic or structural damage to the buildings. The possible response of the residents is also considered.

1.5 The prediction is supplemented by on-site trials.

1.6 Measurements were made in specific properties in response to comments from occupants. These results are presented in an appendix.

2. THE SITE

2.1 The site in question is to the north of Station Road in. Figure 1 shows a plan of the site.

2.2 The land is essentially flat, rising slightly in the northeast corner.

2.3 A number of photographs were taken during the surveys. The locations of those referred to in this report are shown in Figure 1.

3. PREDICTION PROCEDURE

3.1 Vibration is predicted using the following conservative relationship from Attewell et al. (1991):

Where: v = simulated resultant PPV (Peak Particle Velocity) at base of foundation of a structure (mm/s)

E = pile energy input/cycle (J)

r = surface distance (m)

3.2 The Attewell relationship is for predicting vibration levels due to impact piling rigs. In the absence of any specific relationships for dynamic compaction rigs, this is considered to be a reasonable method of prediction, as the physical mechanisms involved (vertical transient force) are similar.

3.3 The dynamic compaction rig used on site drops a weight of 4 tonnes from a maximum height of 5m. (The rig can be seen in photograph Ph1.) It would be unreasonable to expect all of the potential energy to be transferred to the ground. Initial site measurements indicate that 50% of the energy may be a reasonable value to use.

3.4 For the purposes of prediction, the area over which dynamic compaction is to occur is divided into a grid with nine divisions in the north-south direction (0 - 8), and five divisions in the east-west direction (A - E).

3.5 Values of resultant Peak Particle Velocity (PPV) possible at the foundation of a number of surrounding buildings are shown in Table A. (Due to its' size, Table A is presented in this report with the figures).

3.6 Ground vibration is very site specific. The vibration experienced by a structure will depend on soil properties, foundation type, local geological features (etc) that are not included in the Attewell relationship. Therefore, the values in Table A should be treated as conservative values that may occur in the structure concerned, and not as levels that necessarily will occur.

3.7 The Attewell relationship is inaccurate for locations close to the source. Where a value in Table A is replaced with a '!' this denotes a surface distance less than 10m, and considered to be too small for a reliable prediction. This is not to infer that values generated for distances just greater than 10m are considered to be as reliable as the values predicted for far-field distances, but may serve as an indication to vibration levels that may occur.

3.8 It is acknowledged that the Attewell relationship does not predict the vibration magnitude in any one direction separately, as used by BS7385 Part 2. However, we are unaware of any more suitable expressions. It could be assumed that the simulated resultant PPV is acting all in one direction, but this is likely to be an over-estimation. PPV in any one direction can be determined following monitoring.

4. MEASUREMENT PROCEDURE

4.1 All measurements were undertaken using the same equipment.

4.2 Vibration samples were taken using a Di2200 Portable FFT analyser with two Bruel & Kjaer 4378 accelerometers and 2646 line drivers. The sampling rate used was 1280 Hz. The length of each sample was 3.2 seconds. One accelerometer was orientated vertically, the other was orientated horizontally, radial to the source.

4.3 Where measurements were undertaken on a floor or a hard surface next to a structure (pavement etc.) the accelerometers were attached to a triangular metal bracket glued to a spiked plate. Where measurements are made on a carpeted floor, and if permitted, the spikes are driven through the carpet to increase coupling with the floor.

4.4 Where measurements are made on soil, the accelerometers are attached to a heavy triangular metal bracket with a six-inch spike on the base that is firmly driven into the soil.

4.5 Initial Measurements

4.5.1 Initial measurements were made when worked commenced on 4th June 01.

4.5.2 Measurements were taken on the pavement outside 112/114 Station Road while the rig operated in the south-west corner of the site.

4.5.3 Measurements were undertaken on the soil immediately adjacent to 123 Station Road at the south-eastern extremity of the site. Measurements were taken while the rig operated along the south-western boundary of the site, and then as it operated along the southern boundary progressing east towards the measurement location.

4.5.1 Work on site was prematurely stopped due to suspected gas leak. Therefore, there is not a measurement of the rig at it's closest distance to 123 Station Road.

4.6 Local Surveys

4.6.1 Additional measurements were made on the foundation and floors of properties from which there had been comments throughout the course of the works.

4.6.2 Details of these measurements can be found in Appendix A.

5. RESULTS

5.1 PPV Prediction

5.1.1 Table A shows the results of the prediction procedure. Highest values are predicted for 123 Station Road when the rig is in the south east corner of the site.

5.1.2 As a very rough guide, most people can feel a PPV of 1 mm/s. However, the response of humans to vibration is related closer to the acceleration of the vibration, and is also frequency dependant.

5.1.3 Figure 2(a) and (b) show what is said to be the threshold of human perception to vertical and horizontal vibration. Figure 2 shows that humans are generally more sensitive to low frequency vibrations, and are generally more sensitive to vertical vibrations than horizontal vibrations.

5.1.4 Note that the orientation is based on the attitude of the human subject. Therefore, a reclining (horizontal) person subjected to vertical vibration in the ground should be assessed by the response curve for horizontal vibration.

5.2 Initial Measurements

5.2.1 The results of the measurements made on 4th June 01 are shown in Table B. Also included are the corresponding values from Table A showing the predicted PPV for the operation at this location.

5.2.2 Figure 3(a) and (b) shows a sample time history and spectrum of the measurements made outside 112/114 Station Road.

5.2.3 Figure 4 - 5 (a) and (b) show sample time histories and spectra of the measurements made beside 123 Station Road.

5.2.4 PPV's at the measurement location are calculated using a sine wave relationship. Review of Figures 3 to 5 show that this is not unreasonable.

5.2.5 The time histories are post-processed using HV-Lab vibration data analysis software to generate Vibration Dose Values (VDV’s) for the event measured. The VDV parameter is used to assess the human response to building vibration, and hence the possibility of adverse comment.

TABLE B: Results of Initial Measurements 4th June 01

Measurement Location
Approximate Location of Rig
Peak Acceleration (m/s2)
Dominant Low Frequency (Hz)
PPV (mm/s)
Predicted PPV (mm/s)
VDV (m/s1.75) for 3.2s event
VERTICAL MEASUREMENTS
Pavement Outside 112/114 Station Road
A - B, 7 - 8
0.098
13
1.2
4 - 7
0.022
0.128
13
1.6
0.032
0.083
13
1.0
0.020
0.101
13
1.2
0.025
0.091
13
1.1
0.023
Soil Adjacent to 123 Station Road
A - B, 5 - 7
0.099
11
1.4
2 - 3
0.020
0.132
13
1.6
0.033
0.191
13
2.3
0.041
0.242
11
3.5
0.052
0.228
13
2.8
0.052
B7
0.124
10
2.0
3
0.031
0.120
11
2.0
0.031
0.203
13
2.5
0.043
D7
0.267
10
4.3
12
0.047
0.312
10
5.0
0.063
HORIZONTAL MEASUREMENTS
Pavement Outside 112/114 Station Road
A - B, 7 - 8
0.081
13
1.0
4 - 7
0.004
0.087
13
1.1
0.006
0.071
10
1.1
0.004
0.125
14
1.4
0.005
0.089
10
1.4
0.005
Soil Adjacent to 123 Station Road
A - B, 5 - 7
0.101
13
1.2
2 - 3
0.007
0.105
13
1.3
0.006
0.086
-
-
0.006
0.162
-
-
0.007
0.114
-
-
0.005
B7
0.121
27
0.7
3
0.004
0.081
10
1.3
0.003
0.144
20
1.1
0.004
D7
0.313
25
2.0
12
0.012
0.482
13
5.9
0.013

5.3 Local Surveys

5.3.1 The measurements of additional local survey's are processed in a similar manner to above. The results are presented in Appendix A.

6. VIBRATION LIMITS

6.1 Guidance regarding vibration limits is provided by the following British Standards:

  • BS 7385 - Measurement and evaluation of vibration in buildings. Part 2: Guide to damage levels from groundborne vibration.
  • BS 6472 - Guide to evaluation of human exposure to vibration in buildings (1 Hz to 80 Hz).

6.2 Cosmetic / Structural Damage

6.2.1 BS 7385 suggests vibration levels, below which damage is unlikely to occur in 95% of buildings. For cosmetic damage, the level is 15 mm/s at 4 Hz, increasing to 20 mm/s at 15 Hz, increasing to 50 mm/s at 40 Hz and above. Minor structural damage is possible at vibration levels twice those given above, major damage at four times the levels given.

6.2.2 The vibration magnitudes and frequencies refer to Peak Particle Velocities (PPV) occurring in any single direction, measured on the foundation of the building concerned.

6.2.3 The PPV's measured outside 112/114 Station Road and beside 123 Station Road are significantly below the levels at which cosmetic damage may be expected to occur for the frequencies measured.

6.2.4 By comparison with the predicted PPV's shown in Table A, similar levels could be expected at all the buildings to the south of the site - the parade of shops and flats on Station Road and ..... (see photographs Ph1 and Ph3), while dynamic compaction procedures are undertaken at the south end of the site. Similar levels may be expected to occur in some of the properties in .... when dynamic compaction is undertaken at the north end of the site.

6.2.5 Vibration levels may significantly increase when the dynamic compactor is immediately adjacent to 123 Station Road. (Measurements of this situation was not possible.) Variations may occur due to subterranean features, such as soil properties and foundation details.

6.3 Human Response

6.3.1 Human response to vibration in buildings is assessed using the Vibration Dose Value (VDV) parameter. BS 6472 suggests values of VDV above which various degrees of adverse comment may be expected in residential buildings. These values are reproduced in Table C.

6.3.2 BS 6472 makes the following point:

'Within residential areas people exhibit a wide variations of vibration tolerance. Specific values are dependent upon social and cultural factors, psychological attitudes and expected degree of intrusion.'

TABLE C: Vibration dose values (m/s1.75)

Place Low probability of adverse comment Adverse comment possible Adverse comment probable
Residential buildings

16 h day

0.2 to 0.4
0.4 to 0.8
0.8 to 1.6
Residential buildings

8 h night

0.13
0.26
0.51

6.3.3 In this case, the VDV assessment is based on the measurement of one event and an estimate of the occurrence of the event throughout a day. .... inform us that the rig will typically achieve 100 - 150 positions in a 10-hour day, each position requiring 3 - 4 drops.

6.3.4 Dynamic magnification of vibration signals will occur between the foundation and suspended floors of a structure. This can be expected to be around 2 times, but can be as much as 3 times.

6.3.5 The vibration measurements were undertaken in free-field conditions. Free-field vibration levels will be higher than vibration levels at a foundation due to the mass loading of the building. Although it is very difficult to predict the reduction (anything from 20% to 60% can occur) we suggest that a 25% reduction is not an unreasonable assumption to make.

6.3.6 A hypothetical example is considered that assumes the dynamic compactor to be in the middle of the southern boundary of the site for an entire 10-hour day.

6.3.7 Using the time history measured outside 112/114 Station Road when the dynamic compactor was closest, VDV's of 0.20 - 0.36 m/s1.75 are calculated for the 10-hour day on a suspended floor in the buildings to the south of the site, such as the parade of shops and .... (see photographs Ph1 and Ph3).

6.3.8 Using the time history measured beside 123 Station Road when the dynamic compactor in the middle of the site, VDV's of 0.32 - 0.58 m/s1.75 are calculated for the 10-hour day on a suspended floor in the buildings to the south-east corner of the site, such as 123 Station Road and it's neighbours.

6.3.9 With reference to Table C, this would correspond to a low probability of adverse comment in the buildings to the south of the site, and a possibility of adverse comment from the buildings to the south-east corner of the site. However, it is noted that this hypothetical example is extreme. It assumes that all drops are on the same position, and that that position is closest to the buildings to the south of the site. The upper range assumes a dynamic magnification of 3 times the vibration levels measured at the foundation.

6.3.10 Using a time history taken adjacent to the western wall of 123 Station Road when the dynamic compactor was approximately 17m distant, a VDV on a suspended floor in 123 Station Road is calculated to be 0.39 - 0.70 m/s1.75, assuming that this position is maintained for a 10-hour day. This extreme condition would correspond to 'adverse comment possible' in Table C.

7. SUMMARY

7.1 Cosmetic / Structural Damage

7.1.1 Vibration levels at most locations are well below those that may be expected to cause cosmetic damage.

7.1.2 We suggest that 20 mm/s (measured at the foundation of the building concerned) is an appropriate limit to apply. This limit suggests that cosmetic damage is unlikely to occur in 95% of buildings. Minor structural damage may be expected at levels exceeding 40 mm/s, and major structural damage at 80 mm/s.

7.1.3 We have measured 6 mm/s in the ground adjacent to the 123 Station Road with the dynamic compactor 17m distant. There is a possibility that levels could exceed 20 mm/s in the building when the compactor is operating immediately adjacent. Therefore, we suggest the following:

7.1.4 Continuous monitoring at 123 Station Road (an interior location on the building foundation is preferred) during works in this location.

7.1.5 Interior and exterior photographic condition survey and possibly crack monitoring in 123 and 125 Station Road.

7.2 Human Response

7.2.1 Vibration is expected to be perceptible in all properties on Station Road to the south of the site. Where there is perceptibility there is a possibility of complaint. A VDV assessment based on site measurements suggest that there is a low probability of adverse comments if works were concentrated at the southern boundary of the site for an entire 10-hour day

7.2.2 When the dynamic compactor is at the south-east corner of the site, adjacent to 123 Station Road vibration levels in the immediately adjacent buildings (123 & 125 Station Road) are expected to be clearly perceptible. A VDV assessment based on site measurements suggest that adverse comments would be possible if works were concentrated here for an entire 10-hour day.

7.2.3 Considering that it is unlikely that the dynamic compaction procedure would be exclusively undertaken in one location intensively for an entire 10-hour day, the vibration levels in the surrounding buildings are expected to be perceptible but tolerable.

7.2.4 It may be interesting to note that because the dynamic compaction procedure is being used to improve the ground, piling is not being used in subsequent construction phases of the project. Piling operations, particularly impact piling, can be problematic in residential and commercial areas due to the high levels of noise that they can generate.

VIBRATION PREDICTION AND MONITORING -

DYNAMIC COMPACTION AT ...

APPENDIX A: Local Surveys

8th June 2001

Summary

Vibration levels at are well below those that may be expected to cause cosmetic damage.

Vibration levels are expected to be perceptible but tolerable.

Foundation Measurement

Measured on hard standing immediately adjacent to front step of building.

Maximum PPV Measured at Foundation:
0.5
 mm/s
Lowest Dominant Frequency Measured at Foundation:
12
 Hz

BS 7385 suggests vibration levels, below which damage is unlikely to occur in 95% of buildings. For cosmetic damage, the level is 15 mm/s at 4 Hz, increasing to 20 mm/s at 15 Hz, increasing to 50 mm/s at 40 Hz and above. Minor structural damage is possible at vibration levels twice those given, major damage at four times the levels given.

Example (a) Time Histories and (b) Spectra of Measurements on Foundation

8th June 2001

Summary

Vibration levels at are well below those that may be expected to cause cosmetic damage.

Vibration levels are expected to be perceptible but tolerable.

Foundation Measurement

Measured on front door step.

Maximum PPV Measured at Foundation:
2.7
 mm/s
Lowest Dominant Frequency Measured at Foundation:
9
 Hz

BS 7385 suggests vibration levels, below which damage is unlikely to occur in 95% of buildings. For cosmetic damage, the level is 15 mm/s at 4 Hz, increasing to 20 mm/s at 15 Hz, increasing to 50 mm/s at 40 Hz and above. Minor structural damage is possible at vibration levels twice those given, major damage at four times the levels given.

Example (a) Time Histories and (b) Spectra of Measurements on Foundation

Suspended Floor Measurement

Measured on ground floor dental surgery. Floor assumed to be suspended concrete.

Maximum VDV of events measured:
0.038
 m/s1.75
Estimated VDV of 300 - 600 occurrences of above event: (extreme worst case)
0.16 - 0.19
 m/s1.75

BS 6472 suggests the following vibration dose values (m/s1.75) correspond to various levels of adverse comment:

Place Low probability of adverse comment Adverse comment possible Adverse comment probable
Residential buildings

16 h day

0.2 to 0.4
0.4 to 0.8
0.8 to 1.6
Residential buildings

8 h night

0.13
0.26
0.51

Note: BS 6472 makes the following point:

'Within residential areas people exhibit a wide variations of vibration tolerance. Specific values are dependent upon social and cultural factors, psychological attitudes and expected degree of intrusion.'

8th June 2001

Summary

Vibration levels at are well below those that may be expected to cause cosmetic damage.

Vibration levels are expected to be perceptible but tolerable.

Foundation Measurement

Measured on hard standing in front of building.

Maximum PPV Measured at Foundation:
0.6
 mm/s
Lowest Dominant Frequency Measured at Foundation:
10
 Hz

BS 7385 suggests vibration levels, below which damage is unlikely to occur in 95% of buildings. For cosmetic damage, the level is 15 mm/s at 4 Hz, increasing to 20 mm/s at 15 Hz, increasing to 50 mm/s at 40 Hz and above. Minor structural damage is possible at vibration levels twice those given, major damage at four times the levels given.

Example (a) Time Histories and (b) Spectra of Measurements on Foundation

Suspended Floor Measurement

Measured on first floor living room. Floor assumed to be suspended timber.

Maximum VDV of events measured:
0.026
 m/s1.75
Estimated VDV of 300 - 600 occurrences of above event: (extreme worst case)
0.11 - 0.13
 m/s1.75

BS 6472 suggests the following vibration dose values (m/s1.75) correspond to various levels of adverse comment:

Place Low probability of adverse comment Adverse comment possible Adverse comment probable
Residential buildings

16 h day

0.2 to 0.4
0.4 to 0.8
0.8 to 1.6
Residential buildings

8 h night

0.13
0.26
0.51

Note: BS 6472 makes the following point:

'Within residential areas people exhibit a wide variations of vibration tolerance. Specific values are dependent upon social and cultural factors, psychological attitudes and expected degree of intrusion.'