Huntly East land subsidence due to coal mining Review ...€¦ · adjacent pillars, resulting in...

Level 2 TeRenCo House 86-96 Victoria Street P.O. Box 24 147 Wellington

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IAN R BROWN ASSOCIATES LTD

geo log i ca l e ng i ne er ing con su l t an t s

Huntly East land subsidence due to coal mining

Review following levelling survey

April 2010

Report for Waikato District Council

Project Number 1003

Huntly subsidence – review of survey data Ian R Brown Associates Ltd 1

1. Introduction

Ian R Brown Associates Ltd (IRBA) has been engaged by Waikato District Council (WDC) to

provide advice on issues of land subsidence over underground coal mine workings in the

Huntly East area (Figure 1). A scoping report, outlining what was known about the

subsidence problems, and the range of geographical and geotechnical data which were

available in the area, was presented to WDC in October 2008 (IRBA 2008).

In this report, we present follow-up studies which address a number of the recommendations

made in the 2008 report. In particular, we present an update on the status of the survey bench

mark network as well as results from a recent levelling survey, a review a study undertaken

for Solid Energy, and an improved understanding of the extent of past underground coal

mining in the Huntly area.

2. Survey benchmark network

Lands & Survey (now LINZ) benchmarks were established in 1978 prior to mining in the area

(Figure 1). As previously discussed, survey data for the network were presented in various

reports and WDC files up until about 1993.

Mr John Taris, a former LINZ employee, has reconstructed a digital file containing

benchmark elevation data for numerous surveys of the former Lands & Survey network

undertaken between October 1981 and June 1998. Unfortunately data are absent for many of

the benchmarks within the areas of maximum subsidence as identified in our previous report,

and the full range of measurement dates appears to be absent, therefore a full description of

the subsidence history of the area based on levelling data cannot be made.

A relevelling survey of available benchmarks in was conducted in October 2009. As many

sites as could be confidently identified were resurveyed, though a number of sites were not

able to be located. The locations of some survey sites which we previously reported have

been clarified using the updated data.

For most survey stations with data extending from 1981–2009, total subsidence is < 120 mm.

Between March 1998 and October 2009, subsidence was generally between 0 and 10 mm. A

small number of sites exhibited positive ground displacements. Slight increases in elevation,

recorded during the period 1994-1996, may be related to increased rainfall resulting in higher

aquifer water pressures. Two recorded sites had higher subsidence rates: E9 (17.5 mm/yr) and

E15 (7.4 mm/yr) over the last surveyed period (1998–2009).

The location of subsidence areas discussed below is shown on Figure 1.

Subsidence Area A:

Survey data for this area were absent from the data compilation. Limited points on the

periphery suggest the area affected by subsidence has not expanded since 1986.


Subsidence Area B:

Survey data for this area are generally absent from the data compilation. The only data

available for Subsidence Area B is Site E50 (up to November 1995), which shows a typical

pattern of rapid initial subsidence followed by much lower subsidence rate. This is consistent

with data depicted in Kelsey (1985/6) for sites E53 and E61. By May 1984, these sites were

going down 0.6–0.7 mm per day, declining over the last survey period (May 84-Nov 95) to

about 7 mm per year. There are no data to confirm what is happening subsequently, though

stations around the area are stable or show typical long-term subsidence rates, indicating

Subsidence Area B has not increased in size since Nov 1995.

Subsidence Area C:

Subsidence curves (Figure 2) show that subsidence in this area was ongoing prior to

recognition of problems at the ground surface in 1990. Subsidence rates were moderate until

Sept 1990, then 2 episodes of faster subsidence were recorded, namely Sept 1990-Nov 1993

(sites E42, E47, and E49), followed by more rapid subsidence (site E42, E43, and E49)

through to Nov 1995. Subsequent subsidence rates are low (indicated by E42) at 2 mm/yr.

Sites E42, E43, E47 and E49 show peripheral subsidence from Subsidence Area 2 (Sept

1983), suggesting maybe 90–230 mm of subsidence attributable to “far-field” effects, possibly

due to aquifer depressurization. New rapid subsidence from Nov 1993 was likely to have

been caused by localized pillar failure in Panel 4 workings, with the centre of subsidence near

the smallest pillar as discussed below.

Subsidence Area D:

Area D is not underlain by East Mine South Headings workings. Subsidence was noted in the

area in 1993 (Figure 3), though survey data indicate subsidence had commenced between

1981 and 1984 (23 mm at site E51). Rapid subsidence was observed between the Sept 1990

survey and the Nov 1993 survey (63 mm at site E102 and an additional 35 mm at site E51).

The likely mechanism for this subsidence is aquifer depressurization. Subsequently, typical

long-term rates of 1-2 mm/yr have been recorded in this area.

Mine roadway area:

Kelsey (1986) indicated up to 100mm subsidence above the access roadway workings.

Extensive survey data through this area (Figure 4) demonstrate moderately rapid subsidence

between Oct 1981 and July 1987, followed by typical background rates of 1-2 mm/yr. One

site, E37B, shows a long-term trend of increasing elevation, possibly as a result of tree-root

growth causing heave in the footpath at the station site.

Northern area:

We are not aware of significant subsidence-related problems reported in the north of the study

area. Survey data indicate subsidence rates of up to 5-6 mm/year were experienced until

1998, with some sites now showing typical background rates of either 1–2 mm/yr (Figure 5).

Two sites in the northern area show ongoing subsidence.


Site E9 is located at the far north of the study area. Total subsidence is 248 mm over the

period 1983–2009, with a moderate rate of subsidence until 1998, and a higher rate (17.5

mm/year) recorded subsequently. Site E9 has the highest rate of subsidence presently

recorded at any site. The site is underlain by the southwest corner of the workings for the

Huntly East Mine North Headings which were developed later than the South Headings.

Site E15 is located north of the area where significant subsidence has occurred, with a total

displacement of 189 mm between Oct 81 and Oct 2009. The linear subsidence rate at site E15

is not typical of mine subsidence seen elsewhere in the area, and suggest another mechanism.

The survey data file indicates that localised settlement of a retaining wall is suspected at this

site. However, during the period 1993–1998, sites E9 and E10 had similar subsidence rates to

site E15, indicating that a mine-related subsidence mechanism is still required. Settlement

over mine roadways is likely.

3. Review of Solid Energy study

In 2003, Solid Energy North Ltd commissioned a report from Strata Control Technology

(SCT) Operations Pty Ltd to conduct a geotechnical assessment of the proposed North 6 panel

of the Huntly East Mine (SCT 2003). A significant part of this report includes assessment of

existing mine working at Huntly East, Huntly West and Taupiri Mines. SCT also undertook

an analysis of pillar stability issues in Panel 1 of the Huntly East Mine, underlying the NZED

hostel.

The SCT analysis shows that the pillar strength index (i.e. strength / load) for initial Panel 1

workings (3 m high roadway extraction) ranged from 1.7–2.3. Strength index values for 6 m

high roadway extraction decreased to 1.0–1.3 with smaller pillars having values of 0.9. These

low values are located near the axis of original surface subsidence, and the total subsidence in

the NZED hostel area is aligned parallel to these low strength pillars.

SCT conclude that the pillar system in Panel 1 has a marginal level of stability, and that,

should one pillar shed load (due to failure from overstressing) on to adjacent pillars, the

system could become unstable.

SCT also undertook a numerical model-based back analysis of the NZED hostel area. The

model demonstrated that failure of one of the smaller pillars would result in transfer of load to

adjacent pillars, resulting in yield and loss of stiffness in the system. This causes overburden

material to displace into the panel due to its inability to bridge the panel. The result is that

overburden is no longer supported, allowing subsidence to progress.

The SCT model and analysis appears to follow closely the observed subsidence events in the

NZED hostel area, both at the surface and in the mine workings. Previous interpretations of

the driving mechanism for the subsidence by Kelsey (1986, 1987, as discussed by IRBA

2008) downplayed the role of coal pillar collapse, because underground inspection showed

that the pillars were generally intact, with only minor spalling into roadways. The proposed

mechanism for the subsidence was mine roof collapse allowing dewatering and consolidation

of Tauranga Group sediments (especially pumiceous silts). The SCT study concluded that

partial collapse of smaller coal pillars was the initial driving mechanism for the mine roof

collapse and subsequent dewatering.


A driving factor behind the pillar collapse is likely to be an increase in overburden loading as

the ground elevation above the mine workings increases to the east. There appears to have

been no effective change to the pillar design to accommodate variable overburden thickness.

Subsidence still occurred in later parts of the mine working where pillar design was changed

in an attempt to improve stability at roadway intersections. The presence of smaller pillars

than was optimal to maintain stability in the revised design allowed pillar collapse to continue.

4. Extent of historical mine workings

We have compiled maps showing the extent of historical underground coal mine workings

throughout the Huntly township area (Figure 6). Data have been obtained from Crown

Minerals (as digital vector data) for the area north and east of the Huntly Power Plant. For

other areas, we have georeferenced (TM2000 grid) a set of copies of historic mine plans

which were supplied by Mr Jae Bateman, Solid Energy North.

Much of the northern part of the Huntly township is underlain by workings of the former

Taupiri Extended Mine, which operated from 1888 to 1924 (Kirk et al. 1988). Brown (1981)

indicated that in the Taupiri Extended Mine, pillar stability problems were experienced when

overburden depths exceeded about 110 m. Pillars were typically 150 m2 and extracted heights

were up to c. 5 m.

One roof fall which reached the surface was recorded between Waikato River and Lake

Hakanoa in July 1915 (Mines Department, 1983). Cover above the coal seam at this point

was about 20 m. The collapse appears to be located above a fault-controlled structural high.

An area of 0.8–2.8 ha was reported to have been affected by subsidence over the deeper

workings of the Taupiri Extended Mine (Mines Department, 1984). The location of this

subsidence is not known, and does not appear to have been reported in any documents

available to us.

The southern part of the Huntly township is underlain by workings of the former Ralphs

Mine, which operated from 1890 to 1916 (Kirk et al. 1988).

While a number of historical reports indicate minor effects at the surface from these old

workings, there is no record of significant subsidence in these areas.

5. Recommendations

Our scoping study identified the following issues which required further investigation, and

which would allow a full assessment of the risks associated with future subsidence. As

indicated, some items have been addressed and others require further consideration.

• Request a copy of the recent Solid Energy study of mine related subsidence in the

Huntly East area. We have received and reviewed this report.

• Compilation of all benchmark survey data and thorough review to highlight changing

patterns of subsidence since 1979. This was completed and is discussed in the present

report.


• Resurvey of monitoring network. The available points have been resurveyed.

• Compilation of WDC and Government records of property and infrastructure damage

to correlate response to subsidence with measured surface settlement patterns.

• Construction of a digital geological and geotechnical database and model.

• Compilation of existing piezometer records, collection of current piezometer data,

review of groundwater hydrology. In particular, the piezometer installed above the

Panel 4 area should be investigated to test whether dewatering occurred as predicted

during subsidence in that area.

• Analysis of geological and geotechnical data to determine whether sensitive strata

identified by Kelsey are present in areas where subsidence was recorded subsequent

to 1987.

• Preparation of a dewatering consolidation subsidence zonation map as recommended

by Kelsey (1987).

• Further assessment of the stability of the South Headings workings. We understand

this area is not flooded, and is presently used by Solid Energy for Mines Rescue

training. This task may have been undertaken by Solid Energy during their recent

study. The Solid Energy study addresses some of the issues, but only analysed Panel 1

workings, and no subsidence predictions are presented.

• Compilation of mine plan information and subsidence for historic working under other

parts of Huntly, and comparison with the Huntly East Mine workings. This is

presented in Figure 6.

• Suitable foundation designs which mitigate against subsidence damage should be

developed for the area. A draft document prepared by Ministry of Works and

Development in 1985 for Huntly Borough Council, but which does not seem to have

been finalised and adopted, provides a good starting point.


6. Conclusions

Survey data, where available, show that many areas within the Huntly Mine Subsidence

zone are subject to long-term subsidence rates of 1–2 mm per year.

Limited survey data indicates rapid subsidence experienced in the period 1983–1993 has

stabilised, but many relevant survey points have not been obtained to provide a definitive

answer about this.

Survey data indicate that previously mapped subsidence areas have not expanded since

1993.

Some subsidence has been recorded in benchmarks in the northern part of the area. We

have not seen any damage records for this area. Settlement over mine roadways is the

likely mechanism for subsidence in this area, with addition subsidence at site E9 being

probably related to the extensive Huntly East Mine North Headings.

The Solid Energy report provides further indications as to the origin of the subsidence, in

particular the recognition that sub-optimally sized pillars are likely to have failed, starting

a chain reaction of roof strata failure followed by dewatering followed by subsidence.

While most of the Huntly township is underlain by historical mine working, these are low

elevation areas. It appears that the higher overburden ratios in the Huntly East Mine

South Headings area due to increasing elevation, were sufficient to initiate instability in

the mine workings beneath.

Due to gaps in the survey data, we are unable to confirm whether Subsidence Areas A and

B, where significant subsidence occurred in 1983–1984, are presently stable.


7. Applicability

This report has been prepared for the benefit of Waikato District Council with respect to the

proposal submitted by Ian R Brown Associates Ltd. It may not be relied upon in other

contexts or for any other purpose without our prior review and agreement.

Opinions and recommendations contained in this report have been derived from the

information and data gathered during the course of our investigations.

No liability is accepted by Ian R Brown Associates Ltd nor by any Director, or any other

servant or agent of the company, in respect of the use of this report (or any information

contained therein) by any person for any purpose other than that specified in the brief.

8. References

IRBA (2008): Huntly East Land Subsidence Due to Coal Mining - Scoping Study.

Unpublished report for Waikato District Council, October 2008, IRBA Job 1003, 16pp.

Kelsey, P.I. (1986): An engineering geological investigation of ground subsidence above the

Huntly East Mine Area. Unpublished M.Sc. thesis, University of Canterbury.

Kelsey, P.I. (1987): An engineering geological investigation of ground subsidence above the

Huntly East Mine. NZERD Report P123.

Kirk, PA; Sherwood, AM; Edbrooke, SW (1988): Waikato Coal Region: a summary of

geology and resources. New Zealand Geological Survey Record 34, 28pp.

Mines Department, Ministry of Energy (1983): Huntly East Mine, Longwall feasibility study

Part I, Preliminary Assessment. Unpublished Coal Report CR102, Crown Minerals, Ministry

of Economic Development, 100pp.

Mines Department, Ministry of Energy (1984): Huntly East Mine, Longwall feasibility study

Part II. Unpublished Coal Report CR1488, Crown Minerals, Ministry of Economic

Development, 633pp.

Strata Control Technology Operations Pty Ltd (2003): Assessment of North 6 pillar design

issues at Huntly East Mine. SCT report CNZ2535, unpublished report for Solid Energy North

Ltd, 4 August 2003, 44pp.


FIGURES

Figure 1. Location map showing survey benchmark locations.

Figure 2. Time series plot of elevation data, Subsidence Area C.

Figure 3. Time series plot of elevation data, Subsidence Area D.

Figure 4. Time series plot of elevation data, Mine roadway area.

Figure 5. Time series plot of elevation data, Northern area.

Figure 6. Map showing extent of historical underground coal working, Huntly.

1790500E 1791000E 1791500E 5842500N

5843000N

5843500N

5844000N

E12B E13

E57

E34E34L

E38

E38L E37BE37A E37C

E36

E41

E40A

E40

E51A

E44

E43

E42

E47

E49

E50

E73

E39A

E72

E37

E56

E9

E67

E16A

E16B

E15

E71

E35

E41A E45

E69

E68

E104E105

E106

E102

E8

E10

E12

E16

E16C

E48A

E51

E51C

E52

E70

E81

E94

E95E96

E97

E98

E99

E100E101

E107

E108

E109

E110

E111

E112

EE

E108A

E108BE108CE114A

E114BE114C

E114DE114EE114F

E12A

E14

E33

E46A

E46

E42A

E39

E62

E63

E53

E61

E54E60

E55 E59 E65

E66

E55A

E58

E92

E90E87

E88

E89

E64

E11A

E38A

E39BE42B

E51B

E68A

E73B

E86

E100B

Figure 1. Survey data point locations. gp1003_base5.rlg - 1 April 2010

LEGENDWDC subsidence zone

Cadastral boundaries

SCM underground mine workings

Kelsey 1987 subsidence contours (mm)

L&S/Works surveyed subsidence zones

A September 1984 subsidence

B April 1983 subsidence

C 1990-92 localised sinkhole

D 1993 localised subsidence

Survey stations with data available

Survey stations with no data available

A

B

C

D

50

50

50

100

100

200

400

600

100

200400

600

800

-440.0 -440.0

-420.0 -420.0

-400.0 -400.0

-380.0 -380.0

-360.0 -360.0

-340.0 -340.0

-320.0 -320.0

-300.0 -300.0

-280.0 -280.0

-260.0 -260.0

-240.0 -240.0

-220.0 -220.0

-200.0 -200.0

-180.0 -180.0

-160.0 -160.0

-140.0 -140.0

-120.0 -120.0

-100.0 -100.0

-80.0 -80.0

-60.0 -60.0

-40.0 -40.0

-20.0 -20.0

0.0 0.0

20.0 20.0

40.0 40.0

E42

E42

E42

E42

E42

E42

E42

E42

E42E42

E42

E42

E43

E43E43 E43

E43 E43

E43

E43

E43E43

E43

E47

E47

E47

E47

E47E47

E47E47

E47

E48A

E48A

E48A

E48AE48A E48A E48A

E48A

E49

E49

E49

E49

E49

E49

E49

E49

E49

E50

E50

E50

E50

E50

E50 E50

E50

Figure 2: Time series - Subsidence area CVertical scale = millimetres vertical movement relative to original

1981/10/01

1984/08/01

1987/07/01

1990/09/01

1992/11/01

1993/11/01

1994/11/01

1995/11/01

1996/11/01

1997/03/01

1998/03/01

2009/10/01

-150.0 -150.0

-140.0 -140.0

-130.0 -130.0

-120.0 -120.0

-110.0 -110.0

-100.0 -100.0

-90.0 -90.0

-80.0 -80.0

-70.0 -70.0

-60.0 -60.0

-50.0 -50.0

-40.0 -40.0

-30.0 -30.0

-20.0 -20.0

-10.0 -10.0

0.0 0.0

10.0 10.0

20.0 20.0

30.0 30.0

40.0 40.0

50.0 50.0

E100E100C E100CE100CE100C E100CE102 E102

E102 E102 E102 E102E102

E102

E102

E111 E111E111

E111

E111 E111 E111E51

E51E51

E51 E51

E51 E51E51 E51

E51

E99

E99 E99 E99E99E99

E99

E99

Figure 3: Time series - Subsidence area DVertical scale = millimetres vertical movement relative to original

1981/10/01

1984/08/01

1987/07/01

1990/09/01

1992/11/01

1993/11/01

1994/11/01

1995/11/01

1996/11/01

1997/03/01

1998/03/01

2009/10/01

-150.0 -150.0

-140.0 -140.0

-130.0 -130.0

-120.0 -120.0

-110.0 -110.0

-100.0 -100.0

-90.0 -90.0

-80.0 -80.0

-70.0 -70.0

-60.0 -60.0

-50.0 -50.0

-40.0 -40.0

-30.0 -30.0

-20.0 -20.0

-10.0 -10.0

0.0 0.0

10.0 10.0

20.0 20.0

30.0 30.0

40.0 40.0

50.0 50.0

E112 E112E112 E112

E112 E112

E112

E34

E34

E34

E34

E34 E34 E34E34E34 E34

E34

E34L

E34L

E34LE34L

E34L E34LE34L E34L

E34L E34LE34L

E36

E36

E36

E36

E36 E36 E36E36

E36 E36

E36

E37

E37

E37

E37

E37

E37 E37 E37 E37E37 E37

E37

E37A

E37A

E37A E37A

E37A E37A E37A E37AE37A E37A

E37A

E37B

E37B E37BE37B

E37BE37B

E37BE37BE37B E37B

E37B

E37C

E37C

E37CE37C

E37C

E37C E37C E37CE37C

E37C E37C

E38L

E38L E38L

E38L E38L E38LE38L

E38L E38L E38L

Figure 4: Time series - Mine roadway areaVertical scale = millimetres vertical movement relative to original

1981/10/01

1984/08/01

1987/07/01

1990/09/01

1992/11/01

1993/11/01

1994/11/01

1995/11/01

1996/11/01

1997/03/01

1998/03/01

2009/10/01

-250.0 -250.0

-240.0 -240.0

-230.0 -230.0

-220.0 -220.0

-210.0 -210.0

-200.0 -200.0

-190.0 -190.0

-180.0 -180.0

-170.0 -170.0

-160.0 -160.0

-150.0 -150.0

-140.0 -140.0

-130.0 -130.0

-120.0 -120.0

-110.0 -110.0

-100.0 -100.0

-90.0 -90.0

-80.0 -80.0

-70.0 -70.0

-60.0 -60.0

-50.0 -50.0

-40.0 -40.0

-30.0 -30.0

-20.0 -20.0

-10.0 -10.0

0.0 0.0

10.0 10.0

20.0 20.0

30.0 30.0

40.0 40.0

50.0 50.0

E10

E10

E10E10

E10

E10E10

E10

E12

E12

E12 E12 E12 E12E12 E12

E12

E15

E15

E15

E15E15

E15

E15E15E15

E15

E16

E16

E16 E16

E16 E16 E16E16E16

E16

E16

E8

E8 E8

E8 E8 E8E8E8

E8

E8

E9

E9

E9 E9 E9

E9E9E9

E9

Figure 5: Time series - Northern areaVertical scale = millimetres vertical movement relative to original

1981/10/01

1984/08/01

1987/07/01

1990/09/01

1992/11/01

1993/11/01

1994/11/01

1995/11/01

1996/11/01

1997/03/01

1998/03/01

2009/10/01

Huntly East land subsidence due to coal mining Review ...€¦ · adjacent pillars, resulting in...

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