Hughes, C.G.C, Mokhtari, A. & West, B. (2013)

Preservation and capping of a mineshaft under SH25 in Thames, New Zealand Proc. 19th NZGS Geotechnical Symposium, Ed. CY Chin, Queenstown

Preservation and capping of a mineshaft under SH25 at Thames, New

Zealand

C G C Hughes Opus International Consultants Limited, Hamilton, New Zealand

chris.hughes@opus.co.nz

A Mokhtari Opus International Consultants Limited, Hamilton, New Zealand

Ali.Mokhtari@opus.co.nz

B West Opus International Consultants Limited, Paeroa, New Zealand bradley.west@opus.co.nz

Keywords: Thames Goldmine, Pump Shaft, SH25, bracing of shaft walls, capping.

ABSTRACT

A number of engineering challenges were faced in the design of the shaft wall support and

capping of a mine shaft under State Highway 25 (SH25) at Thames on the North Island in New

Zealand.

A ‘hole in the road’ was reported towards the end of April 2012 and over the next few days an

abandoned mine pump shaft 3m wide by 4m long was carefully exposed by the highway

maintenance contractor. The design of the remedial options and the subsequent construction of

the mine shaft cover and the reinstatement of the highway were carried out by engineers from a

number of disciplines encompassing the highway network maintenance, structural, geotechnical

and archaeological teams.

Conventional treatment of mineshafts is to infill and cap to ground level. However, in

agreement with all parties it was decided to try and preserve this unique bit of New Zealand

heritage by shoring and capping the exposed shaft. The final design comprised a metal plate

held in place by props to brace and support the shaft walls. Precast prestressed reinforced

concrete slabs were then placed on rubber strip bearings to span the shaft before backfilling the

remainder of the excavation and restoring the road pavement.

The construction phase was carried out quickly, safely and with no issues on site. Propping and

capping of this shaft was a unique first in New Zealand and the success of this project came

about from good liaison and understanding between each of our teams.

1 INTRODUCTION

On the 18th April 2012 we were called out to inspect a hole that had appeared in State Highway

25 in Thames, North Island (Figure 1). It became quickly clear that there was a bigger problem

at the site than initially anticipated and further investigation was required to expose the extent

and origin of the hole. New Zealand Transport Agency (NZTA) engaged Opus to carry out the

investigation and to provide detail design drawings for the remediation in order to get the road

open as soon as possible.

The purpose of this paper is to present the investigation findings, the unique design challenges

and to describe the sequence of uncovering of the shaft and the construction process from start

to finish for the remediation works.

Hughes, C.G.C, Mokhtari, A. & West, B. (2013).

Preservation and capping of a mineshaft under SH25 in Thames, New Zealand

Figure 1: Site Location Plan Figure 2: Hole in SH25 at start

of investigation

2 INVESTIGATION OF THE SHAFT

Over the next few days a mine shaft 3m wide by 4m long was carefully exposed (Figure 2 to 4)

by the Contractor (Downer). The shaft was found to be lined by stone blocks with an average

block size 0.5m by 0.5m by 1m. Timber beams spanning between the shaft wall were also

uncovered.

Figure 3: Mine Shaft as uncovered Figure 4: Excavation extended around shaft

The groundwater level in the shaft was 4m below the top of the shaft wall and the apparent void

of the shaft was 13m deep.

We then extended the excavation beyond the shaft sides to uncover the full extent of the stone

block lining at the top and to try and define how thick the walls may be.

Hughes, C.G.C, Mokhtari, A. & West, B. (2013). Preservation and capping of a mineshaft under SH25 in Thames, New Zealand

A survey team was then commissioned to carry out a topographical survey in order to map out

the shaft dimensions and layout, and the extent of the stone blocks.

Mining related fill (brown gravelly sand) was exposed to the side of the stone blocks. Scala

tests were carried out in this fill and proved the material to be loose to medium dense in nature,

becoming dense from approximately 2.2m depth.

3 DISCUSSION ON THE CAUSE

From the initial inspections carried out, we determined that there may have been additional

timber beams spanning the shaft and that these had rotted away over time. In combination with

possible settlement of fill within the shaft, this had allowed a void to form, and the material

spanning the shaft then settled, forming the hole in SH25.

4 RECORDS SEARCH

We reviewed the publically available geological

maps1 for the subject site and identified that the

site was underlain by recent alluvial soils

associated with the Firth of Thames.

Hydrothermally altered andesites are present below

the alluvial soils. The geological map identified

the shaft to be ‘Big Pump’ at the location of the

hole in the road.

The shaft was inspected by archaeologists and by

mine historians of the Thames School of Mines

and their review of records held identified that the

shaft was part of a water pumping system dating

around the late 1800’s which was used to drain the

mines on the Thames goldfield.

The former pump house floor and old machinery

were uncovered by the initial excavations (Figure

5). From review of the records held, the shaft itself

was thought to be close to 200m deep. When it

was decommissioned it may have been filled in.

Figure 5: Old Machinery in Shaft

5 REMEDIATION OF MINE SHAFTS, METHODS GENERALLY ADOPTED

Methods for dealing with unused shafts are varied across the world but normally involve at least

filling the shaft. Historically, the materials used to fill the shaft could be highly variable (refuse,

colliery waste, timbers, granular fill) and would typically be chosen from what was readily

available at that time.

As an example, general protocol for dealing with shallow untreated mine shafts in the UK

include filling the shaft (incorporating a program of drilling and grouting) and placement of a

reinforced concrete cap over the shaft. A detail for mine shaft treatment from the UK is

provided below (Figure 6):

Hughes, C.G.C, Mokhtari, A. & West, B. (2013). Preservation and capping of a mineshaft under SH25 in Thames, New Zealand

Figure 6: General arrangements and recommended design for a capped shaft (Sourced

from CIRIA Special Publication 32, 19842)

6 DESIGN CHALLENGES

The shaft under SH25 presented a number of remedial challenges, both from an archaeological

and an engineering perspective. These challenges were required to be overcome as quickly as

possible as our brief was to re-open SH25 at the earliest opportunity.

The unusual archaeological and historical value of the shaft meant that New Zealand Historic

Places Trust (NZHPT) required we come up with a suitable design that prevented damage to the

structure as much as reasonably practicable. This ruled out the option of simply backfilling the

shaft and placing a concrete cap over it.

Two remedial options were discounted at an early stage:

1) The shaft location is within a built up part of Thames and as such re-routing SH25 to

outside the influence of the shaft was not an option. A temporary diversion had been

put into place whilst SH25 was closed but this started to draw complaints from local

residents and increase the urgency to get SH25 open again.

2) The option of installing piles and then spanning beams to support SH25 was discounted

at an early stage due to time constraints and the high costs of large spans and piling.

Our geotechnical and structural teams came up with a design comprising installing precast

reinforced concrete bridge slabs to span the shaft and fill over the void. One of the benefits of

this option was that existing reinforced concrete bridge slabs were readily available from a

concrete precast supplier and as such this option could be implemented quickly by the lead

contractor.

There remains a minor risk with this option of catastrophic collapse of the shaft below the

propped zone. For this to happen, the fill in the shaft would need to substantially settle or be

Hughes, C.G.C, Mokhtari, A. & West, B. (2013). Preservation and capping of a mineshaft under SH25 in Thames, New Zealand

washed out in order to provide the space and room for the wall blocks at depth to move or

collapse into. In terms of likelihood of this threat, we considered this would be a rare

occurrence (happening less than once per 50 years).

This risk was put to and accepted by the highway authority (NZTA) after due consideration of

the large size and the excellent condition of the shaft wall stone blocks that line the shaft and the

presence of fill in the shaft from below 13m depth.

7 STRUCTURAL CONSIDERATIONS

Five reinforced concrete bridge slabs were available and a structural inspection of these was

carried out on the 10th May 2012. We were advised that the slabs (approximately 2290mm wide

x 5010mm long and 280mm thick) were part of a temporary bridge on a recent subdivision

construction site in Tauranga and were used for around 2 years.

There was some minor damage to the edges of a number of the slabs (Figure 7).

The most significant area of damage was to the corner of one of the units where the end of a

prestressing strand and a stirrup had been exposed. The exposed strand end and the exposed

stirrup were cleaned and coated with

two coats of bitumen.

Units with hairline crack and exposed

stirrup were used as bearing pads. The

remaining units with only minor

defects were used to span the shaft.

Figure 7: Bridge slab units at

contractor’s yard

Structural engineers assessed the slab

units to confirm that they are capable

of carrying HN-HO-72 loading as required by the NZTA Bridge Manual3.

8 GEOTECHNICAL CONSIDERATIONS

Two of the key geotechnical design considerations were for bearing capacity of the ‘abutments’

and the likely lateral loads that may be applied to the shaft wall.

We decided to utilise all of the bridge slabs and designed an arrangement to spread the applied

loads over as wide an area possible. Structural calculations were carried out to determine the

loads (factored and unfactored) applied to the stone blocks at the top of the shaft. We then

checked the bearing capacity (following Verification Method, B1/VM44) assuming the fill

properties estimated from the Scala test results.

We calculated the horizontal load acting on the mine shaft wall at each end of the contact area

and designed metal plates and props to counter this loading.

The final design incorporated use of concrete block units to counter a step in the stone block

floor that lined the shaft. The concrete components (i.e. precast slab and supporting concrete

blocks) of the bridge over the shaft would provide a 100 year design life as required by the

NZTA Bridge Manual.

Hughes, C.G.C, Mokhtari, A. & West, B. (2013). Preservation and capping of a mineshaft under SH25 in Thames, New Zealand

In addition we incorporated detailed requirements for preparation and coating of the metal

components (the plate and props) to ensure a design life of approximately 50 years. This means

that a maintenance check will be required towards the end of this design life in order to identify

metal components requiring re-application of corrosion protection coatings.

9 REMEDIAL DESIGN

A sketch detail of the remedial design is presented in Figure 8 below.

Figure 8: Sketch detail of the remedial option

The components used in the construction were treated where needed for corrosion protection.

This included both utilising galvanised metal components and treating the previously identified

damage on the bridge slabs.

10 CONSTRUCTION

Construction work progressed from the middle of May 2012 and initially involved preparing the

site for the concrete slab bridge units that were to be used span the shaft (Figure 9).

Hughes, C.G.C, Mokhtari, A. & West, B. (2013). Preservation and capping of a mineshaft under SH25 in Thames, New Zealand

Figures 9 & 10: Preparing the shaft for the placement of the bridge slabs

Once the foundation was set, concrete mass blocks (Figure 10) were put into place to counter

the difference in floor level. The latter part of the construction work (Figures 11 to 16) involved

mobilising two cranes to the site to install the steel plate and the props within the shaft, before

finally covering the shaft with the reinforced concrete bridge units.

Figures 11 & 12: Lifting the steel plates into position inside the shaft

Figures 13 &14: Installing plate, props then placing bridge units to cover the shaft

Hughes, C.G.C, Mokhtari, A. & West, B. (2013). Preservation and capping of a mineshaft under SH25 in Thames, New Zealand

Figures 15 and 16: Final concrete pour prior to backfill and pavement reconstruction

11 CONCLUSION: A SUCCESSFUL PROJECT OUTCOME

Design of the remedial options and

the subsequent construction of the

mine shaft cover and the

reinstatement of SH25 was carried

out by engineers and staff from a

number of disciplines

encompassing our network

maintenance, structural,

geotechnical and archaeological

teams.

SH25 was re-opened to the public

by the end of 15th June 2012

(Figure 17).

Figure 17: SH25 as reinstated

The success of this project came about from good liaison and understanding between each of

our teams and in particular from engaging all disciplines at the early phase of the design work to

agree the most appropriate and safe way forwards.

ACKNOWLEDGEMENTS This paper has been written with the approval of the New Zealand Transport Agency, who

commissioned Opus International Consultants Limited to undertake investigation and design of

the mine shaft remedial works.

REFERENCES

The Geology of the Thames Subdivision, Hauraki, Auckland, by Colin Fraser. New Zealand

Geological Survey, Bulletin No 10, dated 1910. Geological Map 1:10,000 scale.

Copy of Figure 35, ‘Construction over abandoned mine workings’, by PR Healey, JM Head.

CIRIA Special Publication 32, PSA Civil Engineering Technical Guide 34, dated

1984, reprinted 2002.

NZTA Bridge Manual, 2nd

edition 2003.

Verification Method, B1/VM4, Building Industry Authority, 1 December 2000.