STRUCTURAL ENGINEERING REPORT on POST EARTHQUAKE DAMAGE

Transcription

1 STRUCTURAL ENGINEERING REPORT on POST EARTHQUAKE DAMAGE at 74A GLANDOVEY ROAD, FENDALTON for Stillwater Consultants STILLWATER REF: S15181 Project No Issue: B To: Stillwater 30 July 2015 Prepared by: Will Marshall BE Reviewed by: Athir Mansoor BScEngMEngst Approved by: Graham Rundle BE M.IPENZ EXECUTIVE SUMMARY House Garage Outbuilding Description Single Storey Separate Building Separate Building Exterior Cladding Concrete Blocks Solid Plaster over Ply Concrete Blocks Roof Materials Corrugated Metal Sheeting Corrugated Metal Sheeting Corrugated Metal Sheeting Foundation Type Mixture of Type B2 & C2 Type C1 Type C1 Ground Max. Floor Variance 48 mm 28 mm 20 mm Floor/Foundation Remedial Indication from Table 2.3 of MBIE Guidelines No Foundation Relevel Considered Necessary No Foundation Relevel Considered Necessary Foundation Relevel Required Assessed Foundation Remediation No Foundation or Floor with Partial Foundation Required Relevel Required Replacement Foundation Remedial Methodology Exterior Cladding Remedial Methodology Roof Remedial Methodology Foundation relevel using portable jacks with Partial Foundation Replacement External cladding will need to be repaired where required Refer separate Roof Inspection Report Floor slab relevel using portable jacks or mechanical lifting to correct reversed fall. External cladding will need to be repaired where required No Foundation Relevel Considered Necessary No Foundation or Floor Relevel Required No remedial work required No remedial work required Geotechnical Summary MBIE Technical Category The site conforms Ultimate Bearing Capacity Vertical land settlement SLS Index Value (Settlement in Upper 10m) Green Zone, Technical Category 3 (TC3) TC3 Ult. Bearing Capacity 200kPa at 0.2mbgl 15 mm < 100mm in the upper 10m of soil profile and so is in the "Minor to Moderate" category Retaining Walls No. Material Height (mm) Length (m) Remedial Work 1 Concrete blocks No work required CONTENTS: 1. Introduction General Description Geotechnical Investigation & Assessment Structural Observation Structural Assessment Structural Remediation... 9 Appendix A Geotechnical Investigation Report Consulting Professional Engineers adding 'enginuity' * to building projects Redco NZ Ltd Redco House 470 Otumoetai Road TAURANGA 3110 TELEPHONE: FACSIMILE: red@redco.co.nz

2 Consulting Professional Engineers Page 2 1. INTRODUCTION Redco carried out an inspection of the property at the above address on 11 th May 2015, at the request of Stillwater Consultants as part of their survey team. The visual inspection of the property and buildings was to assess the extent of damage sustained from the Canterbury event earthquakes and to ascertain the extent of remedial work required. There was no opportunity to remove cladding or linings to survey concealed or inaccessible structural elements. This report is based on the results of the inspection and on other documents supplied by Stillwater Consultants. This report has been prepared solely for Stillwater Consultants for use in reporting to their clients, their professional advisors and the Council in relation to the inspected property. No liability is accepted in respect to the use of this report by any other person or entity or for its use for any other purpose. Reference is made throughout this report to the MBIE publication Guidance on repairing and rebuilding houses affected by the Canterbury earthquakes dated December 2012, and will generally be referred to as the MBIE Guidance. 2. GENERAL DESCRIPTION 2.1 House This single storey house was built in the 1950 s. The roof cladding is corrugated metal sheeting. Internal linings are generally of plasterboard. The house is a concrete block building with a suspended timber floor structure supported on piles and with a perimeter concrete foundation. The exterior walls are plaster over concrete block. This foundation and floor is a Type B2, in terms of Table 2.1 of the MBIE Guidance. The kitchen has a Type C2 foundation and floor, in terms of Table 2.1 of the MBIE Guidance. 2.2 Chimneys There is one chimney in the house. 2.3 Garage The Garage is a separate light timber framed on a concrete floor (slab on grade). The roof cladding is corrugated metal sheeting. Internal linings are generally of plasterboard. The exterior cladding is solid plaster over ply. This foundation and floor is Type C1, in terms of Table 2.1 of the MBIE Guidance. 2.4 Outbuilding The Outbuilding is a separate concrete block building on a concrete floor (slab on grade). The concrete floor slab has uneven cut marble floor coverings. The roof cladding is corrugated metal sheeting. Internal linings are generally of exposed concrete block. The exterior cladding is plaster over concrete block. This foundation and floor is Type C2, in terms of Table 2.1 of the MBIE Guidance. 2.5 Retaining Walls There is one retaining wall in the property. The description and functions of the retaining walls are summarised in the table below: No. Material Height (mm) 1 Concrete blocks Length (m) Function The wall supports the garden and protects the paved courtyard. This wall is not a structural wall 2.6 Site Driveway There is a loose metal driveway entering on the north-west corner of the property. There is an asphalt driveway/parking area in front of the garage Landscaping Items There is a paved marble courtyard and footpath leading to the front entrance on the northern side of the property. There is a concrete tiled courtyard on the north-east corner of the property complete with a large water feature. There is another concrete tiled courtyard to the east of the house. There is a concrete footpath on the south and west elevations of the house.

3 Consulting Professional Engineers Page 3 3. GEOTECHNICAL INVESTIGATION & ASSESSMENT 3.1 Investigation A Geotechnical investigation was carried out by Coffey. Their Geotechnical report 5 th June 2015 is included in Appendix A. 3.2 Flood Hazard Flood Management Areas (FMAs) were identified by the Christchurch City Council (CCC) in a City Plan change before the Canterbury earthquakes. These areas are prone to flooding as a result of major tidal or rainfall events and/or are vulnerable to the effects of rising sea levels. The FMAs are currently being revised by CCC as part of their review of the City Plan and taking into consideration the impact of the earthquake sequence as well as flood control measures that have been installed since the last modelling took place. Interim floor levels were not available for this site and should be obtained from CCC. 3.3 Land Category This site has been classified as Green Zone, Technical Category 3 (TC3), blue on the CERA website. This means that moderate to significant land damage from liquefaction is possible in future large earthquakes. Site-specific geotechnical investigation and specific engineering foundation design is required. 3.4 Assessment The Geotechnical report confirms that the site conforms to the criteria for a TC3 foundation technical category. Ground water was observed in the bore holes at depth between 1.01 and 1.43m below ground level. The results of the investigation show that the ground is not "Good Ground" in terms of NZS 3604:2011. The geotechnical report recommends using the following Ultimate Bearing Capacity: Ultimate Bearing Capacity (kpa) Minimum depth below ground level (m) The deep investigation showed the ground to be capable of having SLS Settlement <100mm in the upper 10m of soil profile and so is in the Minor to Moderate category as shown in Table The global lateral movement category for this site is Major. The lateral stretch of the ground across a building footprint for TC3 (at ULS) for this site is Major. The Geotechnical Report has provided relevel and rebuild solutions.

4 Consulting Professional Engineers Page 4 4. STRUCTURAL OBSERVATION 4.1 House Foundations There are cracks to the perimeter foundation wall to the main area of the house. The crack sizes are up to 7mm. The wall appears to be plumb. There are no apparent voids under footings. Some piles have settled vertically. Some piles are out of plumb. The connections between the piles and the bearers are structurally intact. There is a concrete slab in the corner of the Living Room next to the fireplace which has been covered by carpet. The homeowners have previously had an independent engineer inspect this and they apparently commented that the weight of the concrete slab combined with the weight of the fireplace has caused some settlement and the steel beam in the roof to move. This has caused damage to the ceiling and joints have opened up. However, this independent report was not available to Redco. There are cracks to the perimeter footing of the Kitchen slab. There are no apparent voids under footings. The concrete floor slab in the kitchen has minor cracks up to 1mm Ground Floor Level The floor is undulating but has not stretched. A floor level survey was carried out during the inspection using a Zip level. The maximum overall floor level variation is 48mm across the house. The timber suspended floor and the concrete slab in the kitchen has floor slopes >0.5% in the areas shaded on the Floor Levels Plan Exterior Walls There are major cracks up to 25mm to the mortar of the block walls. The walls are 20mm over 1.8m in height out of plumb in places. There is no separation between the walls and the perimeter foundation wall. There are horizontal cracks to the foundation at the junction between the concrete block wall and concrete foundation wall. There is cracking at door and window openings. The concrete block has separated from the soffit up to 4mm in some areas. The timber joinery has separated from the concrete block walls in some areas Interior Walls & Ceiling The interior linings have cracks at opening corners. There are cracks to the interior lining panels. There is no separation between the ceiling and wall intersection. The interior walls are 20mm over 1.8m in height out of plumb. There are no windows and doors jamming. There are no nails or screws popping through walls Roof The roofline is not sagging. There is no cracking to the roof structure. The roof bracing is intact and structurally sound. Refer separate Roof Inspection Report for damage to the roof cladding Chimneys The chimney at the Living Room is still intact and the part of the chimney above the roof is temporarily braced. There are cracks throughout the chimney. 4.2 Garage Foundation & Floor There are no visible cracks to the concrete foundation perimeter footing. There are no apparent voids under footings. The concrete floor slab has minor cracks up to 1mm. The floor level survey carried out during the inspection using a Zip level shows the concrete floor slab has a maximum overall floor level variation of 28mm across the garage. The concrete floor slab has floor slopes <0.5%, but is falling from front to back Interior & Exterior Walls/Roof There is no damage to the interior and exterior walls. Refer separate Roof Inspection Report for damage to the roof cladding. 4.3 Outbuilding Foundation There are no visible cracks to the perimeter footing. There are no apparent voids under footings.

5 Consulting Professional Engineers Page Floor One area of the marble finished floor had a minor crack. A floor level survey was carried out during the inspection using a Zip level. The maximum overall floor level variation is 20mm across the gym part of the outbuilding; however the floor covering is built to look as an uneven surface so the floor levels were not able to be measured accurately. Levels were not obtained in the shed are of the outbuilding for this reason Interior & Exterior Walls/Roof There was no damage to the interior and exterior walls. Refer separate Roof Inspection Report for damage to the roof cladding. 4.4 Retaining Walls The damage and observation of the retaining walls is summarised in the table below: No. Material Damages Cracks Rotation Observation 1 Masonry blocks Cosmetic No Cosmetic cracking only. No structural remediation required. 4.5 Site Ground Damage There are no cracks to the ground at this site and no evidence of liquefaction Driveway There is no damage to the loose metal driveway and no cracking to the asphalt driveway Landscaping Items There is differential settlement to the north-east concrete paved courtyard surrounding the water feature up to 35mm vertical and 15mm horizontal. The concrete steps leading to Bedroom 2 on the north-east elevation have been damaged. The concrete pavers have broken and there is some separation from the house in this area. There are cracks to the cut marble entrance to the Kitchen.

6 Consulting Professional Engineers Page 6 Outbuilding Garage House Floor Levels Plan House, Garage and Outbuilding

7 Consulting Professional Engineers Page 7 5. STRUCTURAL ASSESSMENT Reference is made in this section to the MBIE Guidance on repairing and rebuilding houses affected by the Canterbury earthquakes dated December House Foundation & Floors This foundation and floor is a Type B2, in terms of Table 2.1 of the MBIE Guidance. The maximum variance in floor levels is <50mm (48mm) but some local floor slopes are >0.5%. Using the indicator criteria contained in Table 2.3, Column 2 applies and No foundation relevel considered necessary but some local floor levelling will be required. KEY House Garage Sleepout Generally, floor level variations can be attributed to as-built conditions, historical static settlement and earthquake induced settlement. It is difficult to differentiate and gauge the type of settlement to the building as pre-earthquake floor levels are usually not available.

8 Consulting Professional Engineers Page 8 With reference to Figure 14.1 of the MBIE Guidance, 'Case 1: Local repair and/or minor relevel only required' is relevant for the House. There was no appreciable overall building settlement relative to the ground, and the site did not perform poorly. Therefore, repair foundation and relevel as necessary is appropriate. Lateral Stretch Potential The site is located 50m to the north of the Wairarapa stream. The lateral stretch potential of the ground across the building footprint for TC3 (at ULS) for this site has been assessed as Major, and the global movement category is also defined as Major. This assessment is obligatory due to the proximity of a significant waterway, but collaborating evidence of ground movement is not present. As per clause of the MBIE Guidance, careful consideration has been undertaken to determine the most appropriate remediation strategy. As the ground is not showing any effects from the close proximity of the stream, and given the location of the cracks in the foundation walls in relation to the direction of potential stretch, it can be concluded that the house has not been affected by the presence of the stream. Consequently a foundation repair is considered to be acceptable and appropriate. Although there is only 48mm overall floor variance, and 30mm variance over the foundation wall, the floor cannot be locally levelled by jacking and packing without levelling the block walls and areas of concrete floor. Consequently, a perimeter foundation and concurrent slab relevel by jacking is required. The MBIE Guidelines outline four options for jacking structures: 1. Using portable jacks; 2. Using piles (screw or similar); 3. Using engineered resin; or 4. Using thixotropic or low mobility grout. Option 1 is considered to be the most acceptable for remedial work as suitable bearing capacity can be obtained at 0.2mbgl. Some sections of the foundation wall require replacement due to cracking. These areas should be partially replaced as per Figure 4.2a of the MBIE Guidance. 5.2 Garage Foundation & Floors This foundation and floor is a Type C2, in terms of Table 2.1 of the MBIE Guidance. The maximum variance in floor levels is <50mm (28mm) and local floor slopes are <0.5%. Using the indicator criteria contained in Table 2.3, Column 2 applies and so No foundation relevel considered necessary. No local floor levelling is required. 5.3 Outbuilding Foundation & Floors The floor finishes to the outbuilding are built to be uneven, and so the floor levels could not be accurately assessed. The measured maximum variance in floor levels is <50mm (20mm) and local floor slopes are <0.5%. Using the indicator criteria contained in Table 2.3, Column 2 applies and so No foundation relevel considered necessary. No local floor levelling is considered to be required.

9 Consulting Professional Engineers Page 9 6. STRUCTURAL REMEDIATION The following items are required to repair the earthquake damage to this property: Contractor to carry out a detailed inspection of the subfloor to determine the extent of the damage, and notify the engineer of the findings for review. Relevel the foundation perimeter wall and areas of concrete slab in accordance with Part A of the MBIE Guidance using portable jacks, with grout fill under the areas of concrete slab. Replace damaged areas of foundation wall as shown on the remedial plan in accordance with Figure 4.2a of the MBIE Guidance. Relevel the House floor by jack & pack methodology in accordance with Part A of the MBIE Guidance. Replace any piles that are > 15mm over 1m out of plumb. Repair any cracks to the perimeter foundation wall and concrete slab areas in accordance with Table A4.1 of the MBIE Guidance. Relevel the Garage concrete floor slab using portable jacks or mechanical lifting with grout fill to correct reverse falls to the floor Repair any wall linings throughout the house. Any damaged wall bracing elements should be replaced with elements that match the bracing capacity of the replaced element. If required to be rebuilt, chimneys are to be designed and detailed in accordance with the Acceptable Solution of the building code (Acceptable Solutions B1/AS1, B1/AS3, C/AS1). The exterior walls needs to be repaired where required. All severely cracked areas and those areas of blockwork that appear to have loose blocks should be taken down and rebuilt, as well as the dislodged areas being replaced. Refer to the separate Roof Inspection Report for roof covering remediation. Remove and replace any disturbed or damaged landscaping elements. The concrete tiled courtyards and pathways will need to be replaced where required. House North elevation Garage West elevation 25mm step crack below window on north elevation Horizontal cracks to the foundation & block wall

10 Consulting Professional Engineers Page 10 No foundation or floor levelling required Relevel the House floor by jack & pack methodology in accordance with Part A of the MBIE Guidance. Replace any piles that are > 15mm over 1m out of plumb, according to specific engineering design Replace 2m of Foundation wall Relevel concrete floor slab using portable jacks or mechanical lifting with grout fill to correct reverse falls to the floor Repair any cracks to the concrete slabs of house and garage in accordance with Table A4.1 of the MBIE Guidance. Replace 5m of Foundation wall Remedial Plan House & Garage Relevel the foundation perimeter walls and concrete slab areas in accordance with Part A of the MBIE Guidance. Repair any cracks to the perimeter foundation wall in accordance with Table A4.1 of the MBIE Guidance.

11 Consulting Professional Engineers Page 11 APPENDIX A Geotechnical Investigation Report