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Fault Displacement Evaluation at Aviemore Dam

D. J. A. Barrell, K. R. Berryman, L. Mejia , D. F. Macfarlane, S. A. L. Read, R. J. Van Dissen and J. Walker

Setting

Setting for Aviemore Dam

Aviemore Power Station is a 220 MW hydro-generation facility owned and operated by Meridian Energy Ltd (Meridian). The dam straddles the Waitangi Fault and its design reflected different bedrock foundation conditions on either side of the fault with a 56 m high concrete dam founded on closely-jointed 'hard' greywacke and a 48 m high earthfill embankment founded on 'soft' claystones, sandstones and coal. The fault crosses the dam in the embankment foundations, and no specific allowance for fault movement was made in the dam design.

Dam Safety Review Question - is the Waitangi Fault active?

Answering this question involved a suite of paleoseismology investigations by geological mapping, fault trenching and age dating, including:

  • detailed logging of trench walls to determine the locations of most recent surface fault rupture events, the amount of fault displacement and the sense of slip,
  • collecting and dating samples from the trench walls, mainly by luminescence techniques, to determine the time since last fault rupture and the recurrence interval,
  • evaluation of fault displacement data for dam engineering assessment of stability.

Fault Characteristics

Late Quaternary (active) fault deformation

features of the Waitangi Fault include:

  • the bedrock strath at the base of the alluvial deposits (unit 4) is vertically offset by up to 3m,
  • the west side of the steeply dipping fault is upthrown (i.e. the fault has a reverse component of displacement). This is in contrast to the juxtaposition of bedrock across the fault (units 6 & 7), for which the east side of the fault is upthrown,
  • the fault plane of the most recent movements may be at the bedrock fault (e.g. T00/2) or up to 6 m west of it within Tertiary bedrock (unit 6 - e.g. T00/3 and T01/1),
  • tectonic warping (folding) close to the fault plane has occurred in association with fault rupture, particularly on the upthrown side. Displacements are sharper at the bedrock strath and diffuse upwards into a broader deformation zone in gravels, the sense of displacement is best characterized as oblique right-lateral reverse, with vertical separation for the most recent event (MRE) and penultimate event (PEN) of 1.5mand 0.6m respectively,
  • two, possibly three, recent rupture events were recognized over the last ca. 23,000 years,
  • the most recent rupture event was 13,000 to 14,000 years ago.
Avimore Log of SE wall

Log of south east wall of trench 01/1

Avimore Log of W wall

Log of west wall of trench 00/3

Most recent event

Dating most recent rupture event

Single - Event Fault Displacement Evaluation

Fault displacement parameters for engineering assessment of dam safety were derived directly from the site specific trenching, rather than using published correlations based on world-wide data. The assessment was performed by an expert panel, and included:

  • evaluation of the best field data - single-event vertical separation for individual or combined fault rupture events, and age control,
  • selection and weighting of three alternative estimates for median fault separation (low, medium, high), and coefficients of variation of each,
  • selection and weighting of three alternative estimates of recurrence interval for surface rupture of the fault. including consideration of evidence from dam foundation construction records

Simplified deterministic and probabilistic assessments based on the expert panel values gave a vertical displacement of 1.2 m at the 50thpercentile
level, which was adopted as it corresponds to the 95th fractile for an annual exceedance probability of 1/10,000.