The surficial physical deteriotation behaviour of Neogene volcanosedimantery rocks of Eskisehir-Yazilikaya (NW-Turkey) A.Binal, K.E.Kasapoglu & C.Gökçeoglu Geological Engineering Dept., Hacettepe University, Ankara, Turkey. ABSTRACT In this study it is aimed to investigate surfacial deterioration of Neogene volcanoclastics in Eskisehir-Yazilikaya area where the historical Midas monument is located. Influences of freezing and thawing cycles on physical and mechanical parameters of these rocks have been investigated by performing freezing and thawing test on the samples which have not been affected from deteriotation also by taking into account the climatic characteristics of the laboratory environment. Physical and mechanical tests have performed on both the fresh samples and the samples subjected to various numbers of freeze-thaw cycles. As a result, when the climatic conditions of the study area and the physical properties of the rocks are taken into account, it is found that freezing and thawing is a process of physical deterioration. Thus, the fragmentation and degradation, observed on the surfaces of the rocks in the field, are also observed after freeze-thaw tests. 1.INTRODUCTION The study area situated to the South of Eskiþehir in Central Anatolian (Figure 1) has been inhabited for thousand of years. Since, the Neogene volcanoclastics that crop out extensively in the area are soft and easy to carve, numerous ancient carvation and excavation within these rocks stand as very attractive historical monuments. The most important of these is the Midas Monument.

However, climatic conditions prevailing in

the area favour for freezing and thawing cycles which deteriote the surface of these monuments significantly.

In order to generate technical measures

against deteriotain a series of measurements and tests were made on the volcanoclastics that comprise ignimbrite and tuffs in general.

Porosity of the ignimbrites and tuffs varies

between 25% and 40%. These rocks, because of their high porosity, become saturated easily during rainy periods; and some microfractures occur when the air temperature drops below

0°C. These microfractures is the main cause of physical deteriotation of rock surfaces. Deteriotation behaviour of rocks with higher porosity is developed rapidly (Lienhart, 1988).

Therefore, influences of freezing and

thawing cycles on physical and mechanical parameters of these rocks were investigated by performing freezing and thawing test on the samples which have not been affected from deteriotation also by taking into account the climatic conditions of the laboratory environment.

The Karaören formation which is the subject matter of this study, overlies the continental tuffites and base rocks with an unconformity; and its upper boundary is also overlain again by sedimantery tuffit of this facies two kinds of ignimbrite with different degrees of welding and a tuff levels underneath have been determined. The age of studied rocks are Neogene (Yalçin, 1989).

Fig 1. Location and simplified geological map of study area 2.GEOMECHANICAL PROPERTIES OF THE STUDIED ROCKS Due to their differentiation in welding degree, the studied Neogene volcanoclastics expose at three different level. Therefore, the geomechanical charcateristics were investigated in three different groups namely the pink ignimbrite (PIgn), white ignimbrite (WIgn) and tuff (T).

To determine the physical deteriotation

degree, the Schmidt hardness tests were applied following ISRM(1981) procedure and direct observations were made in-situ. Total 6 representative block samples were taken from all units. In the laboratory studies, uniaxial compressive strength, apparent porosity, water absorption, unit weight and slake durability index tests were carried out on the core samples taken from these blocks. All tests were performed according to ASTM (1990).

Joint compressive strength (JCS) values

were calculated using Deere and Miller (1966) equation.All geomechanical parameters of studied rocks are given on Table 1.

As seen in Table 1, the studied rocks have

very high porosity. Therefore, the surface water infiltrates easily to rocks and the microfractures are occured as a result of freezing of pore

water. For this reason, the influences of freezing-thawing cycles on physical deteriotation of rocks were studied. 3.FREEZING-THAWING TESTS One of the important factors that enhances the surficial deteriotation, is freezing and thawing cycles. The winters of the area are rainy and cold. The average precipitation in December, January and February is 364.9 mm and the average temperature for same period is -2.2°C. An isoline map of the freeze-thaw severity index of Turkey (Figure 2) was produced using temperature and precipitation data of 100 Meteorology Station. Each data group includes 20 years (for the period 1971-1990) moist freeze-thaw index (ASTM 1990) method was used for preparation of isoline map of the freeze-thaw severity index of Turkey. The annual freezing and thawing cycles of study area is determined as 20 using this map. Because of this, maximum freezing and thawing cycle was taken as 20 during laboratory studies. After each 5 cycles, the core samples were photographed for direct evaluation (Figure 3). As seen in Figure 3, the physical deteriotation behaviour can clearly be observed.

Fig. 2. Isoline map of the freeze-thaw severity index of Turkey

After each 5 cycles (5th,10th,15th and 20th

cycles), geomechanical tests with made to fresh were applied (see Table 1). Based on the results obtained from laboratory tests and direct observations, it was concluded that specially tuff samples were disseminated by effect of freezing and thawing cycles.

It was found that water absorption rate

increases with increasing porosity. The water

0

2 cm4 cm

6 cm

4 cm2 cm 6 cm 8 cm 10 cm 12 cm 16 cm14 cm

WIGN-1 WIGN-2 PIGN-1 W-1 W-2

(a)

(b)

(c)

0

2 cm4 cm

6 cm

4 cm2 cm 6 cm 8 cm 10 cm 12 cm 16 cm14 cm

W-6W-5

WIGN-4PIGN-5PIGN-4

0

2 cm4 cm

6 cm

4 cm2 cm 6 cm 8 cm 10 cm 12 cm 16 cm14 cm

WIGN-6 PIGN-8W-9 W-10WIGN-7

Figure 1. Location and simplified geological map of study area

absorption and porosity values of tuffs increased approximately two times of those of the fresh sample after 20th cycle. Similar behaviour was observed in slake durability index tests. The minimum change was obtained from dry unit weight.

The Schmidt hardness test were not applied

on the tuff surfaces, because the tuffs are very soft. For putting forward the change of weathering class of ignimbrites, the following expression proposed by Singh & Gahrooee (1989) was used.

JCSwi

cσ=

where; wi : weathering index, σc : uniaxial compressive strength of fresh rock sample JCS: Joint compressive strength obtained from Schmidt hammer

The joint compressive strength (JCS) of fresh core samples of white and pink ignimbrites are 15 and 28 MPa, respectively. After the 20th cycle, these values was obtained as 10 and 14 MPa. On the other hand, the freezing and thawing cycles can affect alone from fresh to “moderately weathered” and “weathered” degree of weathering (Figure 4).

0,0

0,5

1,0

1,5

2,0

2,5

3,0

3,5

4,0

0 2 4 6 8 10 12 14 16 18 20 22 24Cycle No

Wi

W IgnP Ign

Fig. 4. The variation of weathering index with freeze- thaw cycles

4. RESULTS & CONCLUSIONS Since, the historical Midas Monument and Yazýlýkaya village are located at soft tuff level, the weathering and deteriotation behaviour of Neogene volcanoclastics in the study area is important.

When the climatic conditions and the

physical properties of the rocks in the study area are taken into account; it is found that freezing and thawing is a process which causes the physical deteriotation. Thus, the fragmentation and degradation observed in the field at the surfaces of these rocks, were also observed after freezing and thawing tests performed in the laboratory.

The main cause of the decreasing of slake

durability index of the tested rock samples were the development of microfractures at the near surface of the samples as the result of freezing of pore water. Therefore, under dry conditions (i.e.), if the voids of rocks is empty, freezing and thawing is not an important factor for the physical deteriotation of these rocks. ACKNOWLEDGEMENT This study was supported by Hacettepe University’s research fund (Project No: 95.01.010.021) REFERENCES ASTM, 1990. Standard test method for evaluation of durability of rock for erosion control under freezing and thawing conditions (ASTM D5312-90). The American Society for Testing and Materials, Philadelphia. pp. 1344-1346. DEERE , D.U., and MILLER, R.P., 1966. Engineering Classification and Index Properties for Intact Rock. Technical Report No: AFNL-TR-65-116 Air Force Weapons Laboratory, New Mexico, USA.

ISRM, 1981. Rock characterization, testing and monitoring - ISRM suggested methods. International Society for Rock Mechanics. Pergamon Press, Oxford, Brown, E.T. (ed.), 211 p. LIENHART, D.A., 1988. The geographic distribution of intensity and freeze-thaw cycles. Bulletin of IAEG, Vol. XXV, No.4, pp. 465-469.

SINGH, R.N., and GAHROOEE, D.R., 1989. Application of rock mass weakening coefficient for stability assessment of slopes in heavily jointed rock masses. Int. J. Surface Mining. Vol.3, pp. 207-219. YALÇIN, H., 1989. Stratigraphic and tectonic features of Kirka (Eskisehir) volcano sedimantery lacustrine basin of Neogene age (Turkish). Journal of Cumhuriyet University, Vol. 6-7, No.1, pp. 165-181.