INTRODUCTION
Arapuni Dam, a 64 m high curved concrete gravity structure founded on Quaternary-age ignimbrites, has experienced several seepage flow incidents since its construction between 1925 and 1927. After grouting of the latest flow in 2001, the longer-term future performance of the dam was investigated by triple-tube coring of the foundations, laboratory strength testing, weir and piezometer monitoring of responses to induced changes in pressures in the foundations, and characterization of water chemistry and isotope signatures as well as temperature. The paper describes the use of these techniques to identify four subvertical zones (fissures) controlling the pattern of foundation seepage along joints formed in the Ongatiti Ignimbrite, many of which were either open or infilled with erodible nontronite clay. Recognition of the four fissure zones has enabled the design and construction of four discrete 90 m high cut-off walls through the dam and into its foundations, rather than treatment across the whole dam foundation.
Continued safe performance of dams requires ongoing surveillance in conjunction with an active dam safety assurance program, in particular as they age. The 64 m high Arapuni Dam on the Waikato River in New Zealand (Figs. 1 & 2), which was constructed between 1925 and 1927 [1], provides such an example. During its operational life seepage flows have suddenly increased several times without apparent cause (e.g. [2]). The latest incident was repaired in late 2001 by targeted grouting under full reservoir conditions [3].
The dam is an integral element of the chain of eight hydro electric power stations along the Waikato River owned and operated by Mighty River Power (MRP). The oldest of the dams, Arapuni impounds the reservoir for a 186 MW powerhouse at the end of a 1 km long headrace channel downstream from the dam on the left bank. Following the 2001 grouting, evaluation of the safe performance of the dam in light of possible future leakage incidents culminated in construction of cut-off walls through the dam and into its foundations with the reservoir still in service [4]. This paper describes foundation investigations aimed at assessing the stability of the dam and the design of the foundation cut-off walls.
(Figure in full paper)
GeologyArapuni is located in an area of flat-lying ignimbrite flows originating from volcanic eruptions over the last 1.2 million years. Three flows (Ongatiti Ignimbrite, Ahuroa Ignimbrite, Manunui Ignimbrite) are present in the narrow Waikato River gorge in which the dam is located (Fig. 2). They are dominantly sinter (point) welded so that their unconfined compressive strength (qu) values are generally <10 MPa. Defects (joints) in the ignimbrite rock masses, which formed during cooling of the ignimbrites after their emplacement, typically have subvertical attitudes.
The dam footprint is founded on the lower 40 m of the Ongatiti Ignimbrite where there is some variation in welding, in particular in the more welded ‘hard zone’ immediately below the base of the cut-off wall (Figs. 3 & 5).
