The effect of the alteration degree of ophiolitic melange on permeability and grouting

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Abstract

Knowledge of rock alteration can be used to give a reliable zonation of a dam foundation for grouting purposes. Rock alteration does affect both water-loss and the uptake of grout by the rock, and the existence of a close connection between water-loss and grout consumption has been accepted for years. However, the strength of this relationship is still debated and at the same time rock alteration is not considered to be possible indicator of grout consumption. In this study, the effect of rock alteration upon permeability and grout consumption has been investigated using the results of grouting at the Yenihayat Dam in Turkey. Using a geological field study, the alteration categories at the investigation area are identified, and zonation of the dam site according to these categories is made. The need for grouting was determined with the help of these alteration categories and these conjectures are tested using the results of injection work. Furthermore, the relationship between water-loss and grout take is investigated and a linear formula with a moderate explanatory power is established by statistical analysis. The formula also has a physical interpretation. This study shows that zonation according to the alteration degree can be a useful approach for grouting decisions. Similar studies at different sites and on different rock types are needed to verify the validity of this approach.

Introduction

To determine a need for grouting is a major concern in dam foundation treatment. If, with geological field investigations, a reliable zonation of dam foundation can be made, then better grouting estimate can be achieved. Our approach is to use the pattern of rock alteration, which affects permeability and hence grouting for a produced zonation map of the dam foundation.

In this study, the effect of alteration upon permeability and, hence, grout consumption is studied at the Yenihayat Dam in Turkey. In addition, the possibility of reliable zonation using fieldwork is demonstrated.

Ophiolitic melange is a preferred rock type in dam foundations. Just as any other rock type, however, ophiolitic melange is also subject to various degrees of alteration, and this alteration affects both permeability and grouting. For instance, cracking, etc. will increase these conditions whereas extreme weathering may decrease them.

The connection between permeability and grout consumption is expected, but the form and the strength of this relationship is still debated (Nonveiller, 1989). Lombardi (1985) denies such a possibility by pointing out the different flow characteristics of water and injected cements. By classifying the water-loss and grout consumption into low and high categories, Heitfeld (1965) showed that all possible combinations of water-loss and grout consumption were found in the applications. Similar results were obtained by Ewert (1981) and Karagüzel (1989).

On the other hand, the effect of rock alteration upon permeability has been discussed in general terms by Kutzner (1991); however, we know of no published study regarding this relationship.

The degree of rock alterations has been classified by various workers (DGEG, 1981, GSEG, Geological Society Engineering Group Working Party, 1972). Alteration categories found in this study area are correlated with the results of the Geological Society Engineering Group Working Party (GSEG, 1972).

Section snippets

Investigation site

The ophiolitic melange of Ankara is composed of dunite, harzburgite and proxenite at the lower part of the series; and gabbro, diabase, basalt and pelagic sedimentary rocks at the upper potion of the series, making a typical stratigraphic series for the oceanic crust. This allocthonous complex whose internal structure was partially preserved was estimated to be overthrust onto the area during the Upper Cretaceous period (Kılıç, 1995). The distribution of the allocthonous rocks in Central

Alteration on the ophiolite

A geological cross-section of the dam site was prepared using information from exploration holes (Fig. 3). The ophiolite at the dam site has been altered by different degrees under the influence of endogenic and exogenic factors. By considering the degree of alterations, three alteration categories were defined and delineated by field observations as follows:

  • 1.

    Moderate alteration: fractures and fissures, discolorations at the surfaces penetrating inwards; rock quality designation (RQD) is 25–50%.

Injection works

After taking into consideration the relevant factors (i.e. permeability, degree of alteration, and lithological properties of the foundation rock, the anticipated hydrostatic pressure) it was determined that an injection curtain 20–30 m deep was needed as a result of the investigations conducted along the dam axis (Fig. 3).

The contact injections were applied at two rows (Fig. 5) along both sides of the curtain injections, however, the result of these injections is not considered in this paper.

Alteration and permeability

The permeability status of the zones was further evaluated by using the water-loss results of 1350 sections (with 5 m length) from 227 injection holes. These data are presented in Table 2, where the water-loss values are given as ranked categories, in association with the information shown in Fig. 4. The same data are also presented as bar graphs in Fig. 6.

Here, the stages of alteration can be easily seen: the rock is heavily fractured by moderate alteration. Consequently, various size cracks

Alteration and grouting

To evaluate the grouting results, first, isarithmic maps of grout take (kg m−1) for each injection series were prepared (Fig. 7). The following observations can be made by looking at these maps:

  • 1.

    The grout take area is concentrated mainly in the upper part of Zone II (since its lower part is radiolarite) and especially in Zone III, which is mainly fractured serpentinite. On the other hand, grouting is a scarce event in Zone I, which became clayey with extreme alteration. Grout take at the

Relationship between water-loss and grout take values

Although a proportional relationship between permeability and grout take is expected, the correlation between the measured permeability and grout take in the same section of the injection hole is not a settled question and still debated (Nonveiller, 1989). Since water can penetrate more readily than grout into any void, threshold permeability must be reached before any grout injection becomes possible. At this threshold, 0.1 l min−1 m−1×1 bar is taken, which corresponds to the lowest rank of

Summary and conclusions

The necessity of grouting at dam sites is well known and the existence of a close connection between water-loss and grout consumption has been evident for years. However, the strength of this relation is still debated. On the other hand, rock alteration is not considered as a possible indicator of grout consumption. However, it is obvious that rock alteration does affect both water-loss and grout take of the rock.In this study, the effect of rock alteration upon the permeability and grout

Acknowledgements

The authors are grateful to the staff of the General Directorate of the State Hydraulic Works (DSI) in Ankara and the Sadri Sener Co., for their valuable contributions. Special thanks go to Göksel Türk, for many fruitful discussions and especially for his suggestions regarding regression analysis.

References (12)

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