Engineering properties of red bed soil from Mae Moh Mine for covering coal outcrop in final mine pit wall

Abstract

Red bed soil is declared as the waste from mining of Mae Moh mine. It will be excavated and taken to the waste dumping area outside mine pit. But in the final plan, the thick overburden red bed soil of Huai Luang formation in the last pit C of Mae Moh coal mine is prepared to be the covering materials on the coal outcrop in the final pit slope. The red bed soil is divided into 10 parts. Part 9 and Part 10 will not be mined and planned to use as covering materials in the final pit wall. The soil samples were tested for classification using unified soil classification system first. After obtaining the soil groups, the samples from each group were conducted for basic engineering properties, compactibility and strength parameters which are uniaxial tests and unconsolidated undrained triaxial tests. The results indicated that the red bed soil was classified into three group names: sandy lean clay, gravelly lean clay, and sandy fat clay. Each group of red bed soil samples were tested for basic engineering properties which are moisture content, density, specific gravity and permeability. The moisture content of sandy fat clay is the highest among the three groups while gravelly lean clay is the lowest. The range of moisture content is 9.34% - 16.67%. Sandy fat clay has the lowest density and specific gravity whereas gravelly lean clay has the highest density and specific gravity. The density values range from 2.37 t/m2 to 2.43 t/m2 and the specific gravity values range from 2.21 to 2.27. The permeability of sandy lean clay is 9.02 X10-6 cm/s, while gravelly lean clay and sandy fat clay are 9.09 X 10-6 cm/s and 8.42X 10-6 cm/s, respectively. In compaction tests, the results show the lowest optimum water content value and lowest maximum dry density value from standard proctor test is gravelly lean clay which is 14.55 % and 1751.04 kg/m3 respectively. For sandy lean clay, the optimum water content is 16.59 % and the maximum dry density is 1760.26 kg/m3. The maximum dry density of sandy fat clay is 1763.03 kg/m3 and optimum water content is 16.76 %. In modified proctor tests, the optimum water content of gravelly lean clay is 13.77 %, the maximum dry density is 1867.4 kg/m3 Sandy lean clay is 14.29% of optimum water content and 1926.22 kg/m3 of maximum dry density. The optimum water content of sandy fat clay is 14.92% and the maximum dry density is 1934.36 kg/m3 According to the unconfined compression tests, sandy fat clay is very soft, but sandy lean clay and gravelly lean clay are soft in consistency. The sandy fat clay has the lowest strength, 4.87 t/m2 with 2.43 t/m2 of cohesion. The sandy lean clay has the highest unconfined compressive strength at 6.34 t/m2 and 3.17 t/m2 in cohesion. Gravelly lean clay has 6.31 t/m2 and 3.15 t/m2 in unconfined compressive strength and cohesion respectively. For the unconsolidated - undrained triaxial compression test (UU test), the undisturbed and remolded samples are conducted. The mixed red bed soil with the ratio of 6: 3 :1 from sandy lean clay, gravelly lean clay, and sandy fat clay are also tested. The results from the tests of undisturbed samples indicate that sandy fat clay has the lowest cohesion value, 3.35 t/m3 but the gravelly lean clay has the highest value, 5.84 t/m2, and sandy lean clay has a value of 5.60 t/m2. The friction angle of red bed soil is O degrees from UU tests. From the tests of remolded samples, the gravelly lean clay has the highest cohesion value of 3.69 t/m2, the sandy lean clay has 3.60 t/m2. The sandy fat clay has the lowest cohesion value of 2.95 t/m2. The cohesion value of the mixed red bed soil is 3.30 t/m2 The overall (conceptual) design and analysis of the slope covered with mixed red bed soil data are performed. The design fixed the thickness of 3 meters of red bed soil for covering according to the recommendation in EIA mine's report. Fourteen final pit slope sections are using Morgenstern - Price method from SLIDE program. The analysis results show that the low safety factors of 1.44 occur in the EW cross - sections of N6. But the highest safety factor of 4.23 is found in section N12. However, all safety factors from the design and slope analysis are more than 1.3.