Parwich Hall: Report on Impact of Constructing Proposed Extension for Mr Robert Shields Ref: 7656A Date: 20 th June, 2018 CONTENTS 1. Introduction 2. Proposed Works 3. Existing Building 4. Proposed Method Statement 5. Impact on Existing Building Fabric 6. Conclusions and Recommendations Appendix A – Plan Appendix B – Statement of Experience 2 7656 – Parwich Hall – Report on impact of constructing proposed extension GCA (UK) Ltd, Babington Lodge, 128 Green Lane, Derby, DE1 1RY Proposed Extension to Parwich Hall 1. INTRODUCTION 101. GCA Consulting have been requested by Mr Robert Shields to undertake a review of the methods of construction for an underground extension of the ground floor with a tunnel leading from the existing basement under it to give access, including wheelchair access, to the basement, ground level of the Hall, proposed new extension and the garden fishpond level. 102. Our brief was to: 1. Review the proposed building drawings, and visit the site. 2. Set out a proposed method statement for the construction works. 3. Determine the potential impact of the construction method on the existing building which is Grade II* listed. 103. The property and site have not been tested for any form of contamination, pollution or any other environmental impairment (methane, radon, etc.) and we are unable to make any comment in this regard. 105. Whilst we have used all reasonable skill and care in preparing this report, it should be appreciated that we cannot offer any guarantee that the property will be free from future defects or that existing ones will not suffer from further deterioration. 4 7656 – Parwich Hall – Report on impact of constructing proposed extension GCA (UK) Ltd, Babington Lodge, 128 Green Lane, Derby, DE1 1RY 2. PROPOSED WORKS The following image shows the proposed extension as viewed from the rear gardens. The following images shows a section, illustrating the relationship of new and existing construction. 5 7656 – Parwich Hall – Report on impact of constructing proposed extension GCA (UK) Ltd, Babington Lodge, 128 Green Lane, Derby, DE1 1RY 3. EXISTING BUILDING 301. The existing building is a Grade II* listed stone, brick and timber building, with the main extended part of the building at the same level as the existing ground floor. 302. The building is thought likely to be founded on rock, based on the visible rock head in the rear garden, and anecdotal knowledge of geology around Parwich. 303. Excavation for the new extension will not undermine the foundations of the original building. 304. The new extension may undermine the right wall of the modern rear extension (‘E’ in Appendix A), although it is likely to be founded on rock. The condition of the bearing strata will be confirmed during excavation and allowance for this is made in the method statement. 305. The ground conditions near to the proposed excavation are thought likely to comprise top- soil and soft ground overlying the rock head at between 0m and 1.5m below existing ground levels. 6 7656 – Parwich Hall – Report on impact of constructing proposed extension GCA (UK) Ltd, Babington Lodge, 128 Green Lane, Derby, DE1 1RY 4. PROPOSED METHOD STATEMENT The following sequence is proposed, to be read in conjunction with Appendix A. It will require further developing with the eventual contractor to agree a safe method of works. 401. Carefully scrape off top 350mm of top-soil from area of proposed excavation and set aside on polythene sheet. 402. Undertake trial excavations to confirm the founding strata to Outbuilding A, Tank store D and Extension E. The following method statement assumes that these are founded on stable rock and will not require underpinning. 403. Carefully take down walls and steps to region B, retaining stone. 404. Carefully remove stone paving to immediate rear of Parwich Hall and set aside if required. 405. Remove vegetation from area C, and reduce levels to form stable surface in preparation for adjacent excavation to form basement. 406. Install conveyor on route as shown. 407. Commence excavation of ground down to rock head with mechanical excavators. 408. Remove rock down to required depth with low-vibration methods as noted in section 4 of this report; shoring to be installed as required, subject to ground conditions. 409. Consider temporary storage of crushed material for later backfilling if suitable. 410. Install rock anchors to support proposed retaining walls. 411. Place formwork, place reinforcement and pour concrete to form basement excavation walls. 412. Construct superstructure. 413. Backfill all excavations in 150mm layers, with granular material. 414. Replace topsoil and undertake landscaping works. 7 7656 – Parwich Hall – Report on impact of constructing proposed extension GCA (UK) Ltd, Babington Lodge, 128 Green Lane, Derby, DE1 1RY 5. IMPACT ON EXISTING BUILDING FABRIC Risk of undermining Parwich Hall 501. Undermining, if present, is a significant risk to a listed building. 502. The geometry of the proposed extension is such that its construction will not undermine the historic parts of the building. 503. Where the new access from the cellar is to be constructed, the base of the excavation necessary to construct the access floor may be slightly lower than the existing cellar floor but this can be dealt with by providing a transition area adjacent to the building to restrict undermining. 504. The modern, rear left extension may require underpinning to construct the left wall of the access corridor from the existing cellar to the new basemen (although anecdotally this was constructed on bedrock) but this is not thought to impact the historic fabric of the building. 505. Overall, there is an acceptably low risk of destabilising the existing foundations to Parwich Hall. Careful monitoring will be require when excavation commences, which should be undertaken by a suitably experienced and qualified Structural Engineer. Vibration in historic buildings: 506. ANC Guidelines (Measurement & Assessment of Groundborne Noise and Vibration 2 nd Edition 2012) note that, for vibration-induced cracking in plaster to occur, the vibration-induced stress, added to the existing stress, must exceed the critical stress for the material. 507. It should be appreciated that the dynamic stresses corresponding to a peak particle velocity of 10mms-1 range typically from only 0.4 % to 2.3 % of the allowable design stress for typical building materials. 508. Vibration-induced damage is therefore comparatively rare in buildings, and the relative risk of damage arising from vibration during the works is low. 509. ‘Vibration Management and Listed Buildings (a Crossrail Case-Study - 2016)’ noted that vibration values of 10mms -1 were accepted for both the Paxton Canopy at Paddington Station, and Macmillan House basement, adjacent. Furthermore, the following values were measured during Crossrail works: • 3t excavator + breaker: 3mms -1 at 3m from building • 8t excavator + breaker: 12.5mms -1 at 3m from building 510. The finishes in the cellar of the existing hall (the area at greatest risk of cracking) are comparatively modern, and during a site inspection of this area of the building we did not any features thought to be at particular risk of damage during the works so long as a vibration limit of 10mms -1 is maintained. 511. In line with BS7385, there are three categories of vibration induced damage: a) Cosmetic – formation of hairline cracks in plaster, growth of existing cracks or opening of old cracks; formation of hairline cracks in mortar joints, dislodging of loose objects; 8 7656 – Parwich Hall – Report on impact of constructing proposed extension GCA (UK) Ltd, Babington Lodge, 128 Green Lane, Derby, DE1 1RY b) Minor – formation of large cracks or loosening or falling of plaster, through to cracks in bricks or concrete blocks; c) Major – damage to structural elements resulting in serious weakening of the structure, cracks in support columns, loosening of joints, shifting of foundations or masonry walls, major settlement. 512. There is thought to be a low risk of minor or major cracking, but a low-moderate risk of Cosmetic cracking which should be mitigated by appropriate use of plant. Options appraisal of rock breaking options: 513. The following plant was considered: a) 3t excavator + breaker: Potential generation of up to circa 3mms-1 at 3m from building, which would be acceptable. b) 8t excavator + breaker: Potential generation of up to circa 12.5mms-1 at 3m from building, which is greater than considered acceptable for works unless they are further away from the building. c) Hydro demolition: Significant volumes of water are produced by hydro-jetting which may have a deleterious effect on the building fabric and ground. d) Diamond wire cutting: This is low vibration method of removing stone and considered suitable. e) Saw cutting: This is low vibration method of removing stone and considered suitable. f) Hydraulic breaking (drill and burst): This is a low vibration method, but comparatively slow, and when working with open excavations it is considered beneficial to minimise the time for which the excavation is active to minimise the risk of collapse. 514. Overall, a combination of a 3t excavator with a breaker attachment (with use of a larger breaker at around 6m from the building) and diamond wire\saw cutting is thought to be the best approach. Hydraulic breaking may be used close to the existing building foundations. Alterations to the existing building: 515. There are two openings to be created in the rear wall of the existing building, (shown on the planning drawings). We understand that the wall in the approved locations is in poor condition and therefore deemed the most appropriate place to form these openings, and that this has been agreed with English Heritage and the Peak Park Conservation officer. 516. Further investigation is required once works commence to carefully open-up the relevant areas and determine the provision of adequate structural support to the new openings, and the existing building fabric above.