Pears Building - Monitoring Review 13-07-2018 1415-01

Contents

1. Introduction

2. Site Works

3. Inclinometers

4. Piezometers

5. Optical Prisms

6. Vibration Monitoring

7. Conclusion

Appendices

A – Inclinometer resultant direction plots.

B - Vibration Monitoring graphs

Date Issue By July 2017 NS

4 Pear Tree Court, London EC1R 0DS 020 7870 8050 hts.uk.com

Heyne Tillett Steel Ltd is a Private Limited Company registered in England and Wales No. 7155581 Registered Office 4 Pear Tree Court, London EC1R 0DS

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1. Introduction

In response to our obligations under the Section 106 Agreement and in particular the requirement to implement the Monitoring Action Plan (ie Appendix N of the Detailed Basement Construction Plan (DBCP)) and the need to communicate the monitoring data to interested parties, we hereby issue our initial Structural Monitoring Report.

This report should be read in conjunction with the Monitoring Action Plan and the Monitoring Review Report Contained within (ie Appendix D) produced by A-squared which provides trigger levels for each parameter monitored.

In addition; further reference should also be made to the DBCP namely; Appendix B – Ground Investigation and Appendix D – Geotechnical Design Report.

This report aims to address concerns arising from a number of surrounding properties that the construction works will influence their property. These are:

• St Stephens Church • Hampstead Hill School • 7-21 Pond Street • The George Pub • The Wharrie Cabman Shelter

Continuous monitoring is being undertaken using a combination of high accuracy sensors linked to a bespoke data acquisition software and/or high accuracy manual read devices as detailed within the Monitoring Action Plan referred to above.

2. Site Works

To date the principle works on site have been the partial demolition of the existing car park and installation of the new contiguous pile retaining wall. These works, in particular the piling, have allowed further knowledge of the ground to be obtained.

In principle this has confirmed the findings from the site investigation but has also shown the ground conditions to be better than expected in terms of both the made ground and affected clay layer being thinner than thought.

It should also be noted that no ground water has been encountered during the works to date, although the piling has been completed during a prolonged dry period.

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3. Inclinometers (measuring ground movement) The inclinometers have been logging results since October 2017. In that time they have generally shown nominal movement, with typically less than 1mm movement of movement being observed, which could be attributed to the accuracy of the equipment.

Plan showing inclinometer locations

3.1 Inclinometer Anomalies There are anomalies within some boreholes, for example borehole 214 has an anomaly to at 6m depth, where 0.8mm movement is recorded and borehole 210 shows 1.5mm between 6m and 17m. Both of these are the resultant value. Looking purely at the resultant, there is a trend with this increasing in magnitude over time. However, looking at the direction of the resultant, there is no trend or pattern. For example, one week the resultant is south east, the next it is north-west, when the same depth is looked at. Similarly, when looking at the borehole as a whole, there is no consistent direction, or pattern with the direction. Furthermore, when the movements are then looked at across the array of inclinometers installed, there is again no pattern or consistency, see Appendix A for a summary of the direction of movement of the inclinometers showing no pattern.

As part of the monitoring action team, A-Squared have reviewed these anomalies and confirmed:

• The anomalies do not contradict or change the assumptions made within the Ground Movement Analysis (GMA) within

the DBCP • The random nature of the movement is not indicative of a slip plane, • Noting the random nature of the movement, and both borehole 210 and 214 are within areas where trees have been

removed, the movement is potentially the result of localised heave / local desiccation.

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Inclinometer 210:

Inclinometer 214:

3.2 Inclinometer summary. We will continue to keep a close eye on these boreholes. Once the capping beam to the continuous piled wall is installed we will have a further line of monitoring to help verify the actual movements and what in some instances could be the accuracy of the equipment.

Based on this we are happy to conclude that the ground is ‘stable,’ as would be expected noting no excavation works on site has begun and the movement is simply the natural movement of the ground ‘breathing.’

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4. Piezometers (measuring ground water levels) The vibrating wire piezometers (VWP) have been logging results since March 2018 with several hand dip standpipes being monitored since Jun 2017.

A detailed review of the findings of the VWP’s and manual readings has been undertaken which has uncovered the presence of a steady water level since monitoring began, with a small rise during the wet period of December.

During the last 6 weeks the ground water level has slowly lowered across all the piezometer’s, as one would expect noting the sustained hot / dry period we are experiencing.

Noting there has been no real change to the ground, this water level change is a natural occurrence. Extract from OGI Report.

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5. Optical Prisms (measuring structural movement) 5.1. Pond St The movement monitoring to the Pond St properties has been installed since February 2017. Reviewing the movement monitoring, this shows the Pond St properties are consistently moving on all three axis, with 13mm vertical, 12mm horizontal and 13.4mm translational movements being measured.

The movement appears to be ground breathing. ie while there is movement, vertical, horizontal and translational, over a period the properties return to a near constant baseline. The base unit and back sights have been checked to ensure this is not instrumental error. The graphs below indicate the typical movement being measure.

Noting the magnitude of movement being measured, before any works began, it is nearly impossible to find a baseline to use, to apply trigger levels. Therefore, we have to revert to our plan A, where the inclinometers will be used to demonstrate that the ground has not moved and therefore the movement experienced at Pond St is not attributable to the works being undertaken on site.

Graph of Pond St properties – vertical movement.

5.2. St Stephens Church St Stephens Church has had manual readings of movement targets since August 2017. Reviewing the movement monitoring, this shows the Church is moving before any works have begun.

Noting the deep made ground and desiccation known to be at the site, the movement appears to be natural ground movement. Noting the magnitude of movement being measured, before any works began, it is nearly impossible to find a baseline to use, to apply trigger levels. Therefore, we will continue with our plan A; to use the array of inclinometers between the site and Church to demonstrate that the ground has not moved and therefore the movement measured are not attributable to the works being completed on site.

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Graph of Longitudinal movement for St Stephens south elevation.

5.3. Hampstead Hill School Hampstead Hill School has had manual readings of movement targets since August 2017. Reviewing the movement monitoring, this shows the school is moving before any works have begun. For example, vertical movements varying 6mm. Reviewing the data there is no pattern to the movements,

Noting the deep made ground and desiccation known to be at the site, the movement appears to be natural ground movement. Noting the magnitude of movement being measured, before any works began, it is nearly impossible to find a baseline to use, to apply trigger levels. Therefore, we will continue with our plan A; to use the array of inclinometers between the site and Church to demonstrate that the ground has not moved and therefore the movement measured are not attributable to the works being completed on site.

Graph of Longitudinal movement for Hampstead Hill School - north elevation.

5.4. The George Pub The George Pub has had manual monitoring since August 2017. Reviewing the movement monitoring, this shows the pub has had little movement over this period.

5.5. The Wharrie Cabman Shelter The Cabman Shelter has had manual monitoring since September 2017. Reviewing the movement monitoring, this shows the Cabman Shelter has had little movement over this period.

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5.6. Movement correlation Noting the movement being measured, we have tried to correlate the movement to several factors; temperature, rainfall time of day etc, but have been unable to find a correlation that works. Therefore, like the properties on Pond St, we will use the inclinometers to monitor any movement caused by the works on site.

Example of attempted correlation between vertical movement and temperature

6. Vibration Monitoring Vibration monitoring results show that the level of vibration recorded is generally very low with only a small number of exceedances only being for short periods, which are within the agreed limits. It should be noted that the limits agreed are well below those which are considered to cause structural damage.

See appendix B for recent vibration reports.

7. Conclusion Based on the above we are happy that the works on site to date have not affected any of the surrounding properties and therefore we will continue to monitor in accordance with the Monitoring Action Plan and no changes to the proposed sequence of works is needed.

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Appendix A - Inclinometer resultant direction plots.

Pears Building – Inclinometer Readings up to 11th March 2018 Preliminary Review

- direction of maximum horizontal ground movement to a depth of 2mBGL

1.4mm

<1mm

>5mm 1mm

0mm

>5mm

<1mm <1mm

<1mm

2mm

1.8mm

- direction of maximum horizontal ground movement at depth of 4mBGL

0mm

<1mm

0mm

4.8mm

0mm

<1mm

<1mm

- direction of maximum horizontal ground movement at depth of 6mBGL

1.2mm

1.2mm

0mm

<1mm

5mm

0mm

0mm

<1mm

- direction of maximum horizontal ground movement at depth of 8mBGL

<1mm

0mm >5mm

<1mm

0mm

<1mm

0mm

- direction of maximum horizontal ground movement at depth of 10mBGL

<1mm

<1mm

5mm 0mm

0mm

0mm

<1mm

<1mm

<1mm

8 68

:

Appendix B – Vibration Monitoring graphs

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Generated by LIVEsite (16 July 2018)

Royal Free Hospital – Pears Building Vibration Readings on Church Retaining Wall

from 09 July’18 to 14 July’18 (ie 5.5 days)

Action Level 7.5mm/s at 4Hz

Trigger Level 5.0mm/s at 4Hz

Church .1 Church .2 Church .3

Generated by LIVEsite (16 July 2018)

Royal Free Hospital – Pears Building Vibration Readings on Royal Free Hospital from 09 July’18 to 14 July’18 (ie 5.5 days)

Action Level Peak Particle Velocity (PPV) 7.5mm/s at 4Hz

Trigger Level 5.0mm/s at 4Hz

MRI Scanner Room 4 North-West External Concrete Stair

Generated by LIVEsite (16 July 2018)

7.5mm/s at 4Hz 5.0mm/s at 4Hz

Royal Free Hospital – Pears Building Vibration Readings on School Hall

from 09 July’18 to 15 July’18 (ie 7 days)

Action Level

Trigger Level

Saturday Sunday