Issue 2 - September 2013

Update 2 provides clarification and further information on technical issues relating to the residential guidance (Repairing and rebuilding houses affected by the Canterbury earthquakes). These issues result from new information or feedback received on the guidance since its publication in December 2012.

20. How is the guidance document applicable to additions in TC3?

(Guidance document reference – Part C, sections 13.1 and 15.1) [link to section in document]

The Guidance document was written for rebuilds or repairs to existing houses, and not specifically for additions. The same principles can be applied to additions. However, where this will result in differing foundation types for the addition, the future performance of the house must be carefully considered. In some cases a carefully detailed seismic separation joint could be used to minimise the damage that will occur in a future event (and protect amenity by being readily repairable). In other cases this may not be possible, and the entire building will need to be upgraded to the same foundation system. Alternatively an application to the Building Consent Authority for a modification or waiver of the Building Act can be made. For the latter a sound justification is needed, and a consideration of the current risk for the existing house.

Any addition up to 3 square metres, with no more than 1.5m additional width added to the existing building, can be made without soils testing if a simple foundation and floor system (to match the existing) is used, and the new foundation section is well tied into the existing foundation. Larger additions or multiple additions require soils testing and specific engineering input, including an understanding of the existing foundation, how it has performed to date, and how it might perform in a future large earthquake.

21. Are there durability issues with grout injection (cement or urethane)?

(Guidance document reference - Part A, Appendix A1) [link to section in document]

MBIE understands that at least two companies are able to offer 50 year minimum durability warranties on grout injection products, both cementitious and urethane based.

22. Under the following circumstances: where the property is located in TC3, and there are static bearing issues (either soft ground, fill or peat problems), and a pile founding layer cannot be identified can a relevellable concrete surface structure (as per section 15.4.8) that is supported on deep piles designed for static conditions only be constructed?

(Guidance document reference – Part C, section 15.4.8)

Yes, as long as the site fits the criteria for a relevellable concrete surface structure – in other words SLS settlements over the upper 10m are less than 100mm. The purpose of this ‘composite’ design should be well signalled on design documents to avoid false future performance expectations.

23. If the geotechnical report for a TC3 site demonstrates that the site is suitable for TC2 foundations, will the official designation for the land be changed?

(Guidance document reference – Part A, section 3.1

No. The official Technical Category designations, being an area-wide guide for appropriate levels of site investigation and assessment, will not change and will remain in place.

24. Can an Engineering Technology Practitioner (ETPract) substitute for the CPEng requirements in the guidance?

(Guidance document reference – Part A, section 3.4, and Part C, section 13.1, and various other references)

No.

25. When constructing a Type 2 surface structure on an extended Type 1 or 2 ground improvement option as per section 15.3.4 of the guidance, can the compacted gravel hardfill be omitted from the surface structure construction?

(Guidance document reference – Part C, section 15.3.4)

Yes, as long as geogrid reinforcing is still incorporated in the upper 600mm of the improved crust.

26.Could you clarify the design philosophy to be followed to determine the depth of ground improvement required for TC3 land?

(Guidance document reference – Part C, section 15.3.4)

In the first instance, refer to section 15.3.4. If using one of the standard MBIE deep ground improvement options included in the Guidance where depths of liquefiable soils exceed 10m, the treatments need to extend to where residual total settlements (not index values over the top 10m) from SLS do not exceed 50mm and ULS settlements do not exceed 100mm. It is recognised that in many cases this might be uneconomic, and in those cases the designers will need to revert to specifically engineered solutions.

27.Do the specific engineering design provisions to address lower than standard bearing capacities for TC1 and TC2 in section 3.4.1 also apply to TC3 sites?

(Guidance document reference – Part A, section 3.4.1)

Yes, where static bearing capacities do not meet specified index values, specific calculations can be carried out by an appropriately qualified geotechnical engineer to examine actual applied stresses compared to assessed bearing capacities.

28.Are TC3 Type 1, 2A and 2B surface structure foundation systems suitable for two-storey construction where lightweight roof and medium weight cladding is specified?

(Guidance document reference – Part C, section 15.1, Table 15.1, and section 15.4)

Yes (see also Question 35).

29.The structural plan shape design principles set out in section 11.2, page 11.3, appear to be a requirement for all TC3 foundations. Is this the intention?

(Guidance document reference – Part C, section 11.2)

Structural plan regularity is required for all surface structures from section 15.4. For ground improvement options, either the overlying foundation or the ground improvement layout should be regular in shape. For other forms of construction, regularity is recommended but not ‘mandatory’ as such – but given the potential ground movements on liquefiable sites a prudent engineer would be unlikely to recommend a highly irregular floor plan.

For more information on regularity, see the supplementary guidance ‘Regular structural plan shapes in TC3’.

30.The structural regularity principles set out in section 5.3, page 5.7, state:

“The representative floor plan which the development and modelling of these details has been based on is shown in Figure 5.4. The details in this section should only be applied to simple house plan shapes such as rectangular, L, T, or boomerang shapes.”
Is the intention to limit floor plan regularity to the concept shown in Figure 5.4 for all of the TC2 Options 1 to 5?

(Guidance document reference – Part A, section 5.3)

The aim is to maintain compactness in the foundation layout that will allow efficient spanning of a loss in support, should this occur. It is not necessary for TC1 and TC2 foundation plans to comply rigidly with the Figure 5.4 shape. House plans that fit the criteria for structural plan regularity in section 11.2, now modified by the supplementary guidance ‘Regular structural plan shapes in TC3’ are expected to perform satisfactorily in future earthquake events. For more information on regularity, see the supplementary guidance ‘Regular structural plan shapes in TC3’.

31. In order to achieve the MBIE Guidance target strength at seven days for cement stabilised ground improvement, the cement application rate in these soils needs to be increased (with the associated cost implications). This could be negated if the guidance could be relaxed to say achieving the criteria at 28 days. Would this be acceptable?

(Guidance document reference – Part C, section 15.3.4)

The strength needs to achieve the target strengths in the MBIE Guidance before it can be certified. In some cases it may be practical to relax the time to 28 days provided the contractor provides an undertaking to remobilise and undertake remediation of any defective works and these are completed prior to any construction of surface footings.

32. Is it anticipated that 100% of the soil tests required for ground improvement options given in the Guidance document must achieve the target criteria, or can an average value per layer be used to determine whether it is sufficient?

(Guidance document reference - Part C, section 15.3.4)

A statistical approach to soil testing is acceptable with 95% of tests exceeding the strength criteria provided that:

  • this is calculated from at least 20 measurements
  • that no two results which fail to exceed the criteria are adjacent (vertically or in plane) and
  • no single result is less than 80% of the target strength.

33. Stand-alone garages – how close can these be built to the main house before they are not considered ‘stand-alone’ for the purposes of the MBIE Guidance?

(Guidance document reference – Part A, section 5.6)

900mm minimum walk space is required between cladded walls, and 300mm minimum separation at roof level.

34. Are the LiDAR horizontal movement vectors ‘real’? Why do we need to design piles for 300mm movement away from lateral spread zones?

(Guidance document reference – Part B, section 8.2.5)

The LiDAR horizontal movement vectors available on the Canterbury Geotechnical Database need to be carefully interpreted. Before conclusions are drawn from this information it should be carefully considered within the context of the other available information and an understanding of the topography and driving forces for possible lateral ground movements in the area. Because of the complex analysis required to derive the horizontal vectors, the uncertainty in the horizontal displacement magnitude is about four times the uncertainty of the vertical ground level. So a typical error of about 100mm in the vertical ground level would correspond to an error of about 400mm in the horizontal ground displacement. Therefore, the LiDAR horizontal vectors are good for interpreting large lateral displacements over a wide area (eg area-wide lateral spreading along the main rivers), but are less suitable for interpreting smaller and more localised displacements.

With this limitation in mind, the EAG (Engineering Advisory Group) examined the LiDAR horizontal movement vectors, and the available crack mapping data, and identified general areas along significant waterways where there was evidence (or a reasonable possibility in a future large earthquake) of lateral ground displacements of more than 300mm occurring – this offset ranged from 50m to 200m, depending on the location (see Table 12.3). These offset distances are very generalised, and do not take into account specific geological features alongside the rivers which reduce the potential for lateral spreading (eg higher terraces of stronger soil). This assessment also identified two areas where “compressional slumping” had occurred in areas away from waterways, one in North New Brighton and one in Wainoni (see Table 12.2). These are large-scale topographic features, likely associated with historic dune formations, with about 2-3m of elevation change over about 1km distance. The topography has caused very subtle ground displacements to occur in these areas, producing only minor ground stretching, but because this small strain accumulates over a long distance it has produced significant absolute displacement in some cases.

Away from these areas of major (300-500mm) and severe (>500mm) global lateral movement, the LiDAR horizontal movement data does not have sufficient accuracy to quantify the magnitude of displacement which has occurred any more precisely. All that can be concluded with confidence from this data is that, in these areas, there has likely been less than 300mm of permanent global lateral movement over the course of the earthquake sequence. This conclusion is part of the rationale for the recommendation that all foundations in TC3 should be designed to tolerate at least 300mm global lateral displacement (or more in some areas) – there is no readily available observational dataset to subdivide TC3 further into areas with lesser design displacement.

Additionally, the potential for transient cyclic (non-permanent) displacement of liquefied ground during the course of earthquake shaking was considered when recommending the 300mm minimum design displacement value for TC3. Research by Tokimatsu & Asaka (1998) shows that peak cyclic shear strains of about 4% may occur during the course of strong shaking once loose soils liquefy. Accumulating this strain across a nominal 8m thickness of liquefied soil would result in a peak cyclic ground displacement of about 300mm.

The recommended 300mm minimum design displacement value also serves a purpose to ensure that all piles are designed to provide a reasonable minimum level of ductility or resilience, reducing the potential for undesirable brittle failure. This level of resilience is generally not difficult to achieve for well-designed piles, and there are a range of readily-available types of pile which are suitable (see Table 15.2).

Engineers with suitable competence in both of the specialised fields of earthquake geotechnical engineering and seismic pile design may choose to undertake detailed site-specific assessment of potential ground movements and specific engineering design of suitably-detailed foundations. However, engineers are cautioned that the prediction of seismic lateral ground displacements following liquefaction is subject to great uncertainties, so a careful approach to the issues identified above is recommended.

Ref: Tokimatsu, K. & Asaka, Y. (1998) "Effects of liquefaction induced ground displacements on pile performance in the 1995 Hyogoken-Nambu earthquake", Soils & Foundations, Special Issue Sept 1998, 163-177.

35. What details are required for Type 1 and 2 surface structures for flood areas where floor levels need to be higher than maximum heights currently provided for in the Guidance?

(Guidance document reference – Part C, section 15.4)

For Type 1 surface structures, the plywood skirt is relied on to brace the foundation around the perimeter of the plan. Any higher foundation than that shown in Figure 15.16 would require the use of two horizontal runs of plywood sheets and careful detailing of force transfer at sheet junctions, and is not covered in this guidance. The use of a Type 2A or 2B surface structure is recommended in this case.

More detail is provided below regarding the permissible heights of piles above the concrete slab in Types 2A and 2B foundations to accommodate both single storey and two storey construction with medium weight claddings. The following table provides height limits for four possible pile loading conditions:

Maximum distance between top of slab and underside of joists (m)Type 2A (150mm slab)Type 2B (300mm slab)
Single storey light weight wall/light roof 1.00 1.15
Single storey medium weight wall/light roof 1.00 1.00
Two storey light weight wall/light roof 0.75 0.75
Two storey medium weight wall/light roof 0.60 0.60

In a case where flood requirements determine that the floor level must be higher than those given in the above table, then diagonal bracing will be required in both orthogonal directions between the piles. Details of the brace size and connections should follow those provided in NZS 3604 for braced piles. One brace in each orthogonal direction is required for every 8m2 of floor plan for single storey construction (light and medium weight wall cladding) and one brace in each orthogonal direction for every 5m2 of floor plan for two storey construction (light and medium weight cladding). Plywood sheathing to the perimeter of the floor may be omitted if diagonal bracing is used but diagonal braces should be fitted on the perimeter of the floor plan as much as possible. Reminder: braced piles should not penetrate the ground beyond that shown in the Type 1, or Type 2A and 2B surface structure options in the Guidance, refer Figures 15.15 to 15.20.

36. What is meant by ‘Engineer sign-off’ for TC2 sites, as indicated by the lower left box of figures 4.1 and 5.2?

(Guidance document reference – Part A, section 4.2, Figure 4.1 and section 5.1, Figure 5.2)

Engineer sign-off for TC2 sites requires a CPEng signing the shallow geotechnical investigation report to confirm the technician’s work and that the TC2 foundation repair/rebuild option is appropriate. Refer to Appendix A of MBIE’s ‘Guidance on the use of Certificates of Work, Producer Statements and Design Features Reports by Chartered Professional Engineers under the new Restricted Building Work regime’

Repairing and rebuilding houses affected by the Canterbury earthquakes

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This information is published by the Your home’s Chief Executive. It is a general guide only and, if used, does not relieve any person of the obligation to consider any matter to which the information relates according to the circumstances of the particular case. Expert advice may be required in specific circumstances. Where this information relates to assisting people: