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Infrastructure Development Code

CS-15.1 Subgrade Layer

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The subgrade layer at the time of completion of the earthworks is deemed to mean the road alignment from 100mm above design subgrade level to 500mm below design subgrade level and extending from the centreline of the road to 1.0m beyond the back of the kerb and channel (for road with kerb and channel) or to the edge of the road shoulder (for roads without kerb and channel). Upon completion of the kerb and channel the subgrade is deemed to extend from face of channel to face of channel.

  1. The Subgrade layer shall be constructed to meet the requirements of the pavement design and TNZ F/1. Wherever possible the natural in situ materials shall be used in construction of the subgrade by compaction or other methods of modification to meet the required subgrade strength.
  2. Where the in situ material is unsuitable to be used as subgrade or is otherwise excluded from use, it shall be replaced by imported subgrade material. The imported subgrade material for the pavement shall be fit for purpose and shall be subject to approval by Council before use. Options may include "run of the pit" sand, good quality local ashes or rhyolite/ignimbrite, selected quarry run rock, Gap100 aggregate, Gap65 aggregate or other material approved by the Engineer. The suitability of alternatives will need to be demonstrated.
  3. The subgrade material, whether in situ or imported, shall be compacted to a depth of not less than 500mm. It shall be placed in layers not exceeding 150mm (compacted thickness) and as close as practicable to optimum moisture content.
  4. The material shall be compacted to the specified design CBR. Measurement of CBR value achieved shall be undertaken in accordance with IT-4.2.1 Subgrade Testing Carriageways.
  5. The entire surface of the compacted subgrade shall be made smooth, firm and uniform, by blading, grading and rolling, approximating the crossfall required on the final surface.
  6. The reduced level of any point shall be within the limits 0mm above to 20mm below the designed or nominated level as establish by stringing.
  7. The surface shall be finished so that all points are within 15mm below a 3.0m straight edge laid at any point on the surface.
  8. Compaction shall cease if the material shows signs of excessive weaving or heaving and shall not recommence until the problem has been resolved.  This is usually caused in the local ashes by over-working the material or using the wrong compaction gear e.g. excessive vibration.
  9. After trimming out, the completed subgrade shall be tested in accordance with IT-4.2.1 Subgrade Testing Carriageways and IT-1 Apx A.1 Inspecting and Testing Summary to establish the constructed subgrade strength.
  10. If the compaction of the subgrade layer does not meet the required criteria then the following options are available for consideration:
    1. The Contractor may opt to carry out further compactive effort to achieve the required level of compaction.
    2. The Engineer may choose to undercut the subgrade layer and replace with sub-base metal. Once compacted, the surface shall be trimmed to grade and retested to prove the required strength has been achieved.
    3. The Engineer may, subject to the approval of Council, opt to correct the non-compliance of the subgrade by means of one or more of the following remediation methods: the use of geogrid and/or geofabric, stabilisation of the subgrade, stabilisation of the sub-base aggregate, stabilisation of the basecourse aggregate, a combination of the above.

It should be noted that approval by Council of the application of one or more of the above options does not relieve the Engineer of responsibility for attaining the required final pavement strength.

CS-15.1.1 Recovered Material

  1. Recovered material may be specified for use as the sub-base layer for the construction of a new pavement, subject to prior approval by Council.
  2. Where recovered material is to be used and there is a shortfall, the recovered material shall be placed first and the imported subgrade aggregate to make up the shortfall, placed on top, subject to suitable depths of each being achievable for effective compaction.
  3. Recovered road

 pavement for reuse shall have a grading curve within or close to (+/- 3% at any sieve size) the grading of the specified subgrade aggregate.

  1. Recovered material with obvious clay intrusions will not be used in the sub-base.
  2. No seal or asphalt from the old pavement should be included in the recovered material to be used.
  3. Other than the recovered materials consequential characteristics, the pavement

 layer shall be prepared as specified.


Definitions in this section

Contractor

Council

Design

Engineer

Pavement

Road

 

 

 

 

 

Tauranga City Council, Private Bag 12022, Tauranga, 3143, New Zealand  |  Terms of use

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