Department of Building and Housing
Engineering Advisory Group
Overview of TC3 Foundation
Guidance Document
Update Briefing for BCAs
20 March 2012
Scope of Briefing
Deep geotechnical investigations
Vertical settlement and lateral
spreading categories within TC3
New/ rebuilt foundations
Repairing existing foundations
Timeline and communications
Deep Geotechnical Investigations
Required to provide understanding of ground
profile (highly variable) to inform foundation
repair and rebuild selections and specific
engineering designs
An area-wide co-ordinated investigation
programme will provide common base of
information
– 40m - 50m investigation grid (6 to 8 Ha) to build upon
existing 200m grid
Deep Geotechnical Investigations (2)
If we have this information, then many of the
repair and rebuild options won’t require site-
by-site deep investigations
– Particularly important in being able to separate
foundation repairs from rebuilds
– And indicating where full rebuilds can be in light
construction with simple foundations (surface
structures)
Needs agreement of all parties and an
appropriate co-ordinating leadership group
Sub-Categorising TC3
(1) Lateral Spreading (ULS)
TC2
TC3
Minor
Minor to Moderate
Major
<100mm
<200mm
200 to 500mm
Can generally assume building is in ‘minor to moderate’
category if more than 100m away from a watercourse or
other form of free edge that has or could spread
Sub-Categorising TC3
(2) Vertical Settlement (SLS)
TC2
TC3
Potentially
Minor
Minor to Moderate
Significant
<50mm
<100mm
>100mm
Requires information from deep geotechnical
investigations
Vertical
Settlement
(SLS)
Potentially
Significant
100mm
Minor to
Moderate
Minor to
Major
Lateral
200mm
500mm
Moderate
Spreading
(ULS)
TC3 Foundation Rebuilds (est 20-25%)
Overview of Options
Type
Objectives
Constraints
Negligible settlement in
No height and/ or material
Deep piles
both small and larger
constraints likely
earthquakes
Limits on some two storey/
Site Ground
Improving the ground to
heavy wall types and plan
Improvement
receive a TC2 foundation
configurations
Surface structures/
Only suitable for light
Readily repairable damage
shallow
construction, regular in
in future smaller events
foundations
plan
Site Ground Improvement Options
Densified crust
– Refilling and compacted
– Dynamic compaction
Cement stabilised crust
– Excavate and replace, or in place
Deep soil mix columns
– also LMG columns, stone columns etc
Two Types of Surface Structures
(1)
Enhanced NZS3604 foundations
– Suitable as Acceptable Solution for areas of TC3
where potential for moderate vertical settlement
and lateral spreading
(2)
New surface structure concepts with
lateral stretch capacity
– Now considered only likely to be applicable for
major lateral spread areas
Concrete Underslab
TC3 Foundation Repairs
(est 50-55% of total)
Objective: enabling repairs where possible
– Especially timber floor dwellings (Types A &
B, which represent ~65% of TC3 dwellings)
Rational approach, also based on accepting
minor damage in some cases from future
moderate events such as 13 June and 23 Dec
Repair Considerations
Proposed repair limits for timber floor
houses (est. up to 60% of houses in TC3)
– Replacement of up to half of piles, and quarter of
perimeter foundation beam permitted
Trade-off: replacement of some heavy tile
rooves and brick veneers
– To improve confidence of adequate performance in
future events (code compliance)
– Depending on level of damage and assessment of
future SLS settlement performance
‘Readily Repairable’
Accepting future minor damage in moderate
earrthquakes is the key to the viability of
both new surface structures and maximising
repairs
Consentable, but goes beyond normal code
interpretations
Key Extracts from Table 8.1 of
DBH November 2011 Guidelines
Key Terms
Interpretation
Continue to function
Occupiable as a dwelling
Minor damage to structure
Timber: able to be re-levelled using
standard procedures
Walls – interior: minor cracking at
lining joints
Some damage to building fabric and
Some cracking of lining junctions
lining
above doorways and windows
Readily repairable
Repairable without relocation of
occupants for more than four weeks
Consenting
Focusing on achieving Building Consents
through standard mechanisms
Guidance will provide reasonable grounds
for consent approvals
Councils likely to require peer review of
designs for at least the initial stages
Flood Risk Aspects
CERA & CCC working group
developing guidelines around floor
levels and flood risk
– Identifying where other policy inputs required
– Leading to update of CCC GD13
– Seeking solutions that avoid the imposition of
s73 notices on properties
Briefing Status and Timeline
Briefings of PMOs and their engineers and Councils
during February
Insurers and EQC briefed via fortnightly Regulatory &
Consenting Group meetings
Canterbury Geotech engineers have received proposed
geotech investigation criteria and repair approaches
QE2 Park trial full report to be issued - 23 March
Draft of TC3 Guidance Document to Minister – 30 March
TC3 Guidance Document – target publication date end
of April
Summary and Key Actions
Technical viability of new foundation types
established, and repair options close to
resolution
But there are issues associated with the
acceptability of future minor damage in
moderate events that affect overall viability
Complex communications and public messaging
involved if to be successful
Seeking to examine a range of scenarios with
insurers (including interaction with flood level
requirements)
Understanding future
Understanding how
land performance
standard house types
will perform
Land
Acceptability
House Types
to Insurers
Insurance
The Building Code
Interpreting building
regulations in a post-
disaster environment
Ensuring the best use of scarce technical resources during the recovery
