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#20-15 Use of timber poles for ground improvement

Published: | Category: Technical advice note | Audiences: Road traffic engineers & consultants, Roading contractors

This note is to advise that there is an issue with the use of timber poles for ground improvement where used in conjunction with a structure and outlines interim requirements pending the next Bridge manual update. This note shall apply from the date of publication.

Publication details

  • Author:
  • Published: 30 May 2020
  • Reference: 20-15
  • Contact:


This note is to advise that there is an issue with the use of timber poles for ground improvement where used in conjunction with a structure and outlines interim requirements pending the next Bridge manual update.  This note shall apply from the date of publication.

General information

Timber poles are an acceptable method of ground improvement for unreinforced earth fill embankments.  This is subject to demonstration of durability, required capacity for the design loadings, acceptable deflections and acceptability to regional consenting authorities.

Recently timber poles have been used for ground improvement purposes in conjunction with structures, including soil structures. This results in a conflict with durability and potential design working life, particularly where the ground improvement is supporting a structure.

The processes and procedures outlined in this update must be followed by Waka Kotahi NZ Transport Agency for all ground improvements using timber poles undertaken from now on.  The only exception is for projects tendered prior to this notice, unless agreed otherwise with the Transport Agency’s Lead Advisor – Geotechnical.  The requirements of this technical advice note shall then apply instead.

Purpose of the notice

Waka Kotahi must ensure that its practices deliver proper value and do not cause medium-term issues that are likely to increase costs.  In this case, the use of timber poles for ground improvement that supports a structure without proper consideration of their long-term suitability to perform their role, may have some unacceptable consequences.  This may lead to medium to long-term cost increases.

The purpose of the notice is to advise that there has been an issue and to ensure action to obviate further instances.  It also allows our suppliers to be more aware of our expectations for the importance of suitable design of structural features.


Timber poles may not be used for ground improvement in association with any structure or any supporting part of that structure. Waka Kotahi may consider a request for a departure to this requirement.  This will be subject to durability and structural capacity being demonstrated as consistent with the entire structure (ground improvement and effects on the structure supported on that area) and providing the required design working life. Such departures shall be in the form, and address the factors detailed in, 6.1.2(d) of the Bridge Manual, Third Edition, Amendment 3.

While all departures are subject to specific approval by Waka Kotahi, generally such departures are unlikely to be accepted for bridge and major reinforced concrete structures.  Departures may be accepted for soil structures, depending on a number of design factors.  These include, but are not limited to: ground conditions; design working life of the structure; maintenance regime and durability of the elements.

Communication plan

Our target audience for this advisory note is generally internal staff giving approvals and those external agents designing road structures on behalf of Waka Kotahi.  As a result, our communications will mostly be focussed internally, but external opportunities will be considered in order to communicate with the wider sector.  Advice of these changes will be promulgated by:

  • This note advising the amended practice
  • Direct information via the website and intranet
  • Delivery of presentations at the Project Managers and Maintenance Managers Fora.


The implementation should be fairly simple as the audience is specific and small.  The aim is to target that audience using the communication exercise.

Given this update is a single, specific issue, it was not considered necessary to conduct company-wide, specific training on the changes.

Effectiveness review

It is important to check that this this has been delivered effectively and the desired outcome achieved.  A review of the effectiveness of the changes will be undertaken by sampling the designs used for a number of structures used in road construction projects.  Should the advice not be being heeded, a larger sample will be taken to establish the problem size and a further communication exercise will be undertaken.

Publication details

Author: Waka Kotahi NZ Transport Agency
Published: June 2020
Version: 1

Further information

If you have any questions or feedback, please contact the Lead Advisor Geotechnical at



David Darwin - Chair, Ratification Group

Vanessa Browne - Delegated Approver