Published: 30 April 2020 | 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 sonic drilling as an investigation technique in the North Island and, as a result, this methodology must not be used unless its suitability for the works as designed is agreed with the road controlling authority. The new requirement applies from the date of this notice.
This note is to advise that there is an issue with the use of sonic drilling as an investigation technique in the North Island and, as a result, this methodology must not be used unless its suitability for the works as designed is agreed with the road controlling authority. The new requirement applies from the date of this notice.
There has been an increase in the use of sonic drilling in ground investigations being undertaken for Waka Kotahi NZ Transport Agency. This form of investigation in New Zealand gained prominence in the South Island following the Christchurch earthquakes. With the rebuild, the resulting ground investigations needed to be done with greater efficiency while still effectively penetrating dense gravels such as the Riccarton Gravels.
Whilst sonic drilling allows penetration of dense gravels it can also assist in progressing drilling in many other materials reducing time and therefore apparently reducing cost. However, sonic drilling is known to affect the Standard Penetration Test (SPT) results, generally reducing their value. This results in potential overdesign of foundations and geotechnical structures, with increased cost to road controlling authorities. The processes and procedures outlined in this update must be followed by Waka Kotahi, and may be used by any other road controlling authority, for all sonic drilling investigation works undertaken from now.
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 sonic drilling as an investigation method without proper consideration of its suitability to perform the role has had some unacceptable consequences. 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 concerns about the need to properly introduce new techniques whose impacts are understood.
The purpose of this note is to ensure that sonic drilling is only undertaken in materials where these affects are either well understood and/or can be adjusted for. Drilling in the North Island using sonic techniques can only be undertaken on Waka Kotahi projects by way of an approved departure through the proper process. The departure would need to demonstrate significant benefits for Waka Kotahi over conventional drilling methods at reasonable cost and acceptable risk. Specific requests for approval, through the departure process, will be required to be made through the geotechnical team. The departure request will require sufficient information about the soil materials likely to be encountered in order to properly evaluate the proposal.
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. Advice of these changes will be promulgated by:
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.
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, particularly walls, 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.
Author: Waka Kotahi New Zealand Transport Agency
Published: April 2020
Version: 1
Found at: https://www.nzta.govt.nz/roads-and-rail/highways-information-portal/technical-advice-note/
If you have any questions or feedback, please contact the Lead Advisor - Geotechnics at geotechnics@nzta.govt.nz
[signed]
David Darwin - Chair, Ratification Group
Vanessa Browne - Delegated Approver
