Selection of Core Material for an Earth Dam in the Lower Çekerek Basin, Tokat, Turkey
Department of Geological Engineering, Cumhuriyet University, Sivas, Turkey
Department of Civil Engineering, Cumhuriyet University, Sivas, Turkey
This study aims at determining the suitability of the impervious to semi-pervious soils in Çekerek basin (Tokat, Turkey), as core and filter materials in an earth dam. Laboratory experiments were carried out on 20 samples from the study area for determination of geotechnical properties. Then the materials were evaluated according to their maximum dry unit weight (gdmax), optimum water content (wopt), specific gravity (Gs), liquid limit (LL or wL) and plasticity index (PI or Ip). All the parameters (except, of course specific gravity) indicated the undesirable characteristic as an impervious fill material. Especially the high value of optimum water content was judged as restricting the usefulness. Relative desirability rating for core and homogenous fill was determined to be 9 (very low level of desirability) for CH group clays in the colluvium. Although the clay materials from the colluvium would be impervious after compaction, it is thought that problems would be occurred when they used as core or homogeny fill material in an earth dam. According to the results of index, water suction, quantity of clay ball, strength loss after frost and Los Angeles abrasion tests on the samples from alluvium, these materials were evaluated as desirable as pervious/semi-pervious material, although the strength loss after frost have the value greater than the proposed ranges. So, due to the warm climate conditions of the study area, this deviation would not be a problem when they were used in the filter zone of an earth dam. This material can also be used in upstream and downstream fill covering the core of an earth dam, due to their gravel contents, because of SP group material desirability rating of 4.
KEYWORDS: Alluvium, colluvium, core, fill, filter, Lower Çekerek, Tokat.
This study aims at determining the suitability of the colluvium and alluvium in the Çekerek basin (Tokat, Turkey) as an impervious material for core and filter material in an earth dam. The study area is situated 45 km southwest of Zile (Tokat, Turkey) (Fig. 1).
Lower Çekerek basin is a proposed location for construction of a dam. Determination of the suitability of the geological units in this basin as a construction material will be important to supply the material for the construction in the planning stage and in the future, especially in a financial point of view. When the construction materials are required, engineers often have the opportunity to choose between several borrow sites. Their selection is based on their engineering properties, the cost of transporting them, and other factors. Often engineers require that the materials be at least as good as those onsite, but sometimes other criteria might apply. Because large quantities of materials are required, earth dams must be built from the material areas that are nearby (e.g., Anderson and McNicol, 1989).
Figure 1. Location map of the study area.
In the study area there are seven geological units, and they are from oldest to youngest; ophiolitic series, crystallized limestone, volcanic series, red marl series, lagunar limestone, colluvium and alluvium (Fig. 2) (D.S.İ., 1983).
Figure 2. Geological map of the study area (1. Alluvium, 2. Colluvium, 3. Lacustrinal limestone, 4. Red marl series, 5. Volcanic series, 6. Crystalline limestone, 7. Ophiolithic series).
Mesozoic ophiolitic series is the oldest of seven units in the study are, and observed in the northwest of the study area. Crushed, jointed, weathered ophiolitic series contain diabas, serpentine, schist with clay and tuff.
Crystallized limestone observed in southeast and southwest of the study area, of Mesozoic age, has jointed and crushed structure, and these joints had been developed in the two-three direction, systemically. In these grey-white massive limestones, fault brechia and clay had been observed along the joints.
Volcanic series, of Tertiary age, is distinguished stratigraphicaly by the presence of andesite, basalt, tuf and agglomerate. Volcanic series is observed in the north and west of the Lower Çekerek basin, in the large areas. Layers having the bedding plane of N20E/22° SE were observed in the parts formed by tuff-agglomerate-brechia.
Tertiary red marls outcrops in the southeast of the study area consisting the lower quantity of gypsum.
Grey-white Miocene lagunar limestones were observed in the northwest of the study area as a cover on the other Tertiary units. Layers have the bedding plane of N35E/10° SE.
Colluvium of Quaternary age consists of blocks and clayey gravel. It is observed in accumulation cones of creeks joined to the Çekerek River and in the slopes along the valley. Colluvium has the thickness of 9-10 m (D.S.İ., 1983).
Quaternary alluvium lies along the Çekerek River, to be widen and narrow, place to place. It is observed near to the Büyükisa village in the east of the study area. Alluvium having a thickness of 27 m (D.S.İ., 1983) consists of gravel, sand, silt and clays.
According to the first data obtained from the field studies on the study area, the units being used as pervious and pervious/semi-pervious material in structures were determined as clayey colluvium and gravel-sand-silt-clay alluvial soils. To determine the usefulness of the materials 35 disturbed samples (20 from colluvium and 15 from alluvial soils) were taken by using canal method, and the results obtained from these samples were evaluated.
Field observations emerged the thought of the colluvium extensively encountered in the study area to be used as an impervious material in structures. For this reason, index and compaction properties of 20 disturbed samples from colluvium were determined to evaluate the usefulness of the colluvium as an impervious material.
Related ASTM standards (ASTM D-421, DS-422, C-127, D-4318, D-2216) were used in tests performed to determine the index properties of the materials. Coarse sieve, fine sieve and hydrometer methods were used for grain size analyses. According to the results obtained from these analyses grain size distribution curves of the materials were plotted, and all of the samples were determined as fine grained (Figure 3). Specific gravity values of the samples varied from 2.68 to 2.75, averaging 2.72. Both grain size distributions and specific gravity values are placed within the values suggested for the material to be used in the clay core.
Figure 3. Grain-size distributions of the colluvium materials.
The highest void ratio was 0.90, the lowest was 0.62 with an average value of 0.75. Porosity varies from 0.39 to 0.47 with an average value of 0.42 (Table 1). Natural moisture content varies from 19.4% to 30.3%, averaging 24.9%.
Table 1. Test results for colluvium.
|Specific gravity, Gs||>2.60||2.68||2.75||2.72||0.021|
|Max. dry density, rd-max, g/cm3 (*)||>1.60||1.438||1.667||1.557||0.061|
|Optimum water content, wopt , %||15-20||19.5||28.3||24.3||2.383|
|Liquid limit, LL or wL, %||40-50||45||81||59.3||9.044|
|Plastic limit, PL or wP, %||-||19||27||22.1||1.971|
|Plasticity Index, PI or IP, %||14-20||23||58||37.2||7.695|
|Natural water content, wn , %||-||19.4||30.3||24.9||3.077|
|Porosity, n, %||-||39||47||42.1||2.438|
|Void ratio, e, %||-||62||90||75.1||7.531|
|Permeability, k, cm/s||3.2x10-9||4.6x10-7||3.5x10-8||-|
Materials were generally of high plasticity in that the specimens yielded liquid limits between 45% and 81% with an average value of 59.3%. The plasticity index extended from 23% to 58%, averaging 37.2% (Table 1). All of the liquid limit and plasticity index values obtained are over the limits suggested for the materials to be used in the impervious clay core.
According to plasticity chart (U.S.B.R., 1974) all of the materials are CH group (high plasticity, inorganic clay) (Figure 4). According to Williams and Donaldson (1980)s swelling potential chart (Figure 5) materials provide generally very high swelling potential, these values ranges from medium to very high. This swelling potential of the materials causes the problems in using as impervious material in the clay core, and restricts the usefulness of the materials.
Figure 4. Distribution of the colluvium samples on the plasticity chart.
Figure 5. Distribution of the colluvium samples on the swelling potential chart.
Compaction is the process of increasing the density of a soil by packing the particles closer together with a reduction in the volume of air. The most suitable case of this compaction is obtained at the water content, known as the optimum water content, at which a maximum dry unit weight is reached (Craig, 1978; Sutton, 1986).
Standard proctor compaction tests were performed on the materials taken from the impervious material areas to determine the compaction parameters as maximum dry density and optimum water content. In the compaction tests, the materials were compacted in three equal layers by using a rammer consisting of a 2.5 kg mass falling freely through 300 mm, each layer receiving 27 blows (BS-1377, 1975). By using the data obtained from the compaction tests, compaction curves, a typical of which is shown in Figure 6, were plotted, and by using these curves average values of maximum dry density and optimum water content were found as 1.557 g/cm3 and 24.3%, respectively.
Figure 6. A typical compaction curve obtained from Standard Proctor tests.
After obtaining an experimental compaction curve, the saturation curves of 90%, 95%, and 100% were plotted.
Permeability of the Compacted Soils
The permeability of the materials taken from colluvium was determined from the falling head permeability tests (ASTM D-2434) performed on 10 compacted samples. The permeability coefficients obtained from the results of these tests varied from 4.6x10-7 cm/s to 3.2x10-9 cm/s, with an average value of 3.5x10-8 cm/s (Table 1).
According to these permeability values obtained materials are placed in the very low permeable-impermeable class (Bell, 1993). These values are suitable values for impermeable clay core in soil embankment dams.
Index, water suction quantity, quantity of clay balls in the materials, strength loss after frost, and Los Angeles abrasion tests were performed on 15 disturbed samples taken from the alluvial soils in the study area to see whether this soil could be used as a pervious/semi-pervious material or not.
Coarse sieve and fine sieve analyses of the alluvial soils were used for determination of the grain size distributions (Figure 7). According to Unified Soil Classification alluvial soils are generally classified as SP (poorly graded sand) by considering the uniformity and curvature coefficients. The average value of the quantity of the material passing No. 200 sieve for sands was 3.74%, with a range extending from 0.30% to 8.9%, while these quantity for gravels varied from 0.20% to 1.24% with an average value of 0.76%.
Figure 7. Grain-size distributions of the alluviums.
Natural unit weights (densities) for sands ranged from 1.75 g/cm3 to 1.85 g/cm3 with an average value of 1.78 g/cm3, as for gravels the average value of the natural unit weight (density) was 1.73 g/cm3, with a range extending from 1.66 g/cm3 to 1.82 g/cm3. Specific gravity values of sands and gravels were found as 2.68 and 2.65 respectively (Table 2). Grain size distributions, natural unit weight and specific gravity values so obtained were placed within the limits suggested for the material to be used as pervious/semi-pervious material in the filter zone in soil embankment dams.
Water suction, quantity of clay balls, strength loss after frost an L.A. abrasion
In determination of the water suction, the quantity of the clay balls in the materials, strength loss after frost (NaSO4 loss), and Los Angeles abrasion properties of the materials, sand and gravel compositions were separately evaluated (ASTM, 1970).
The average value of the water suction rate for sand compositions was 1.64%, while this value was 1.11% for gravel compositions. The values obtained are very close to the limits (max. 1%) suggested for the usefulness as an impervious material (Table 2).
Table 2. Test results for alluvium.
|Unit weight, g, g/cm3|| Sand |
|-|| 1.75 |
| 1.85 |
|Specific gravity, Gs|| Sand |
|>2.60|| 2.65 |
| 2.73 |
| 2.68 |
|<200#, %|| Sand
|Water suction, %|| Sand
|Quantity of clay balls, %|| Sand
|Strength loss after frost, %|| Sand
|L.A. abrasion, %|| Sand
The quantity of clay balls in the materials was calculated as 1.3% for sand compositions while this value for gravel compositions was calculated as 1.1%. The quantities obtained are in good agreement with the suggested limit values (max. 1% for sand and max. 2.5% for gravel) (Table 2).
In frost loss tests performed on materials, the lowest value of the strength loss after frost for sand compositions was 5.3%, the highest was 25.9% with an average value of 18.5%, while for gravel compositions, the lowest value of strength loss after frost was 8.4%, the highest was 31.9% with an average value of 24.4%. The values so obtained are over the suggested limit values (max. 8-10% for sand and max. 10-12% for gravel) (Table 2).
According to Los Angeles abrasion tests performed by selecting 100 per minute and 500 per minute for sand and gravel compositions, respectively, the lowest value of the abrasion for sand compositions was 4.6%, the highest was 5.5% with an average value of 4.8% while the value of the abrasion for gravel compositions varied from 20.5% to 25%, averaging 22.7%. The abrasion loss values obtained are placed below the suggested limit values (max. 8-10% for 100 per minute and max. 40-50 for 500 per minute) (Table 2).
RESULTS AND DISCUSSIONS
For the impervious materials to be used in the clay core of earthen dams, limits of maximum dry unit weight, optimum water content, specific gravity, liquid limit and plasticity index have been proposed: gdmax>1.60 g/cm3, Wopt= %15-20, Gs>2.60, LL= %40-50 and Ip= %14-20 (after Yılmaz and Karacan, 1996). When the materials were evaluated according to their maximum dry unit weight (gdmax), optimum water content (Wopt), specific gravity (Gs), liquid limit (LL) and plasticity index (Ip), all the parameters except specific gravity indicated the undesirable characteristic as an impervious fill material. Especially the high value of optimum water content restricts the usefulness. U.S.B.R (1987) pointed that the optimum water content is especially important factor.
Relative desirability rating for core and homogeny fill was determined to be 9 (very low level of desirability) according to the Engineering Use Chart (after Wagner, 1957; Attawel and Farmer, 1976) for CH group clays in the colluvium.
Although the clay materials from the colluvium would be impervious after compaction, it is thought that problems would be occurred when they used as core or homogeny fill material in an earth dam. So, it can be said that they are not desirable for using as a core or homogeny fill material.
According to the results of index, water suction, quantity of clay ball, strength loss after frost and Los Angeles abrasion tests on the samples from alluvium, these materials were evaluated as desirable as pervious/semi-pervious material, although the strength loss after frost have the value greater than the proposed ranges. So, due to the warm climate conditions of the study area (Akyazı, 1991), this deviation would not be a problem when they were used in the filter zone of an earth dam. This material can also be used in upstream and downstream fill covering the core of an earth dam, due to their gravel contents, because of SP group material desirability rating of 4 (after Wagner, 1957; Attawel and Farmer, 1976).
|© 2004 ejge|