Metehar Sugar.doc Improved Foundation Recommendation Final f

CONSTRUCTION DESIGN SHARE COMPANY1. INTRODUCTION 1.1 General Following an agreement signed on 21 st of April 2011 between Matahara Sugar Factory and Construction Design Share Company (CDSCo), Geotechnical works Business Unit of the Company has performed geotechnical investigation works at Sabure, Afar for the New Sugar Factory Project. The investigation comprises of core drilling, pitting, insitu testing, sampling and laboratory testing. 1.2 Location The project site is located in Sabure, Afar Regional State, locally known as Dankar, which is some 70km from Matahara town, which in turn is at 200km from Addis Ababa along the road to Djibouti. Location of the test points and layout plan of the project site is attached. 2. METHODOLOGY OF INVESTIGATION To get reliable and sufficient information about the surface and subsurface geology and hydrogeology of the project area, the following methodology was employed: • Planning of office and field work, • Rotary core drilling with in situ testing • Test pit excavation, • Sampling, • Logging, • Conducting different laboratory tests and technical report writing. 2.1 Drilling and Pitting The drilling operation was carried out using two mounted Koken /Trailer rigs and a BR50 drilling machine. New Sugar Factory at Kesem Geotechnical Investigation Report CONSTRUCTION DESIGN SHARE COMPANY The methods applied in the investigation were in compliance with the code, specification and standards of the American Society for Testing Materials /ASTM/. Plate. 1, 2 Trailer Mounted Koken Drilling Rigs New Sugar Factory at Kesem Geotechnical Investigation Report CONSTRUCTION DESIGN SHARE COMPANY A continuous rotary core drilling technique was used in order to obtain high quality cores. During drilling, complete geological materials reflecting the boreholes sections were collected as precisely as possible. In loose and compacted soils, a dry drilling technique was used starting with 116mm and reducing (when necessary) to 75mm diameter with tungsten carbide bits fitted to a single core barrel. As soon as soil or rock cores are removed from core barrels in each run, they were placed in standard partitioned wooden core boxes and described immediately on site. Finally, the boxes are properly labeled, photographed and transported to the store. For the purpose of visual inspection and completeness of data numerous test pits were excavated distributed within the factory’s structures, inspected, logged and backfilled. Moreover, additional test pits were excavated at the proposed road alignment of the factory and samples were recovered for modified proctor density and tests. New Sugar Factory at Kesem Geotechnical Investigation Report CONSTRUCTION DESIGN SHARE COMPANY Plate 3, 4 Test Pits 2.2 Standard Penetration Test (SPT) This test is performed to evaluate the degree of in site compactness and/or consistency and strength of soils. During drilling operation, at the bottom of the hole of required depth, the split spoon sampler is lowered and driven in to the formation by a drop of hammer of 63.5kg mass falling vertically through a height of 76cm. The method measures the resistance to penetration offered by the formation. It is measured on the basis of the number of blows of hammer required for a sampler tube penetration of 45cm. The number of blows counted for the last two intervals, 15cm each, is added to report as N-value of the test discarding the first 15cm penetration. 2.3 Sampling and Testing Desired undisturbed soil samples were collected, where favorable geological layers are encountered at depths shown on the log sheet of each bore hole and sealed with wax in order to preserve the natural moisture content and transported to the main soil laboratory of CDSCo. in order to perform the required tests. 3. GEOLOGY AND SEISMICITY New Sugar Factory at Kesem Geotechnical Investigation Report CONSTRUCTION DESIGN SHARE COMPANY 3.1 Regional Geology Since the area is located within the main Ethiopian Rift valley its regional geology is highly affected by volcanism, tectonics, erosion, and deposition. 3.2 Seismicity The methods of assessing likely earthquake intensity and frequency at a given site are complex, requiring reasonable judgment and collection of geological and seismic data. Due to this complexity for the structures with lesser magnitude, the tendency is to rely upon seismic risk maps. The maps are often published in national or state building codes, which recommend the engineering precaution to be taken in each rank of hazard in the map. Ethiopia is divided into zones of approximately equal seismic risks based on the known distribution of past earthquakes. By definition, the hazard with in each zone can be assumed to be constant. (Refer Table 1 below). From the seismic hazard map of Ethiopia, the project site falls under zone 4 which corresponds to maximum hazard. This map is based on the amplitudes to be expected during 100 years return period. The parameter α o , the ratio of the bedrock acceleration to the acceleration of gravity depends on the seismic zone (see the table below) Table1. Bedrock acceleration ratio α o New Sugar Factory at Kesem Geotechnical Investigation Report Zone 4 3 2 1 α o 0.10 0.07 0.05 0.03 CONSTRUCTION DESIGN SHARE COMPANY Figure 2: Seismic hazard map of Ethiopia Project location New Sugar Factory at Kesem Geotechnical Investigation Report P P CONSTRUCTION DESIGN SHARE COMPANY 3. 3 Topography, Local Geology 3.3.1 Topography The area and its surroundings have flat topography. Currently it is further modified and leveled as preparation for construction. No hills are observed in close range to the site. A river called Kebena is located near the site. 3.1.2 Local Geology Surfacially the area is covered uniformly by grayish color silty soil. In order to establish subsurface geology and groundwater condition of the site 38 boreholes and 18 test pits (distributed within the different structures or blocks of the factory) were proposed by the client. Progressive observation and characterization was made in the course of drilling and it was agreed that since the site is constituted by uniform formations some boreholes were omitted. The formations were characterized according to samples recovered by drilling and visual inspection of the test pits. Detailed outcome of the drilling is presented on the log sheets while summary of the encountered units is presented below. A. Grayish Silty SAND This unit is encountered in all the boreholes. It covers the surface of the site as well as the top 5.50 to 7.0m. It is medium dense to dense, grayish, silty SAND/ sandy silt with occasional gravels. Standard Penetration Test conducted on this layer gave an N-value in the range of 11 to 24. This unit appears loose when observed from core recovered by drilling but when observed in the test pits the walls are intact and not easy to excavate using pick axe and shovel. This is further supported by the high N values. Occasionally this unit comprises high percentage of gravels and rock fragments, as can be observed in boreholes K33, K32, and K34. The following averaged design engineering parameters are considered to represent the layer:- Bulk Unit Weight = 14.4KN/m3 Specific Gravity = 2.49 Moisture Content = 10.3% Cohesion Value = 18.8KN/m2 Angle of internal friction = 25.70 New Sugar Factory Geotechnical Investigation Report CONSTRUCTION DESIGN SHARE COMPANY Free Swell = 10% Plasticity Index = 11.5% B. River GRAVELS and SANDS This unit is encountered underlying the top sandy silt/ silty sand soil. It is a granular material composed of rounded, smooth river gravels and pebbles mixed with sand. Occasionally pure river sand becomes dominant. Sometimes this unit is intercalated by thin layer of silty formation. Standard Penetration Test conducted on this layer gave varying N values in the range 23 to refusals. C. Sandy SILT This unit occupies the bottom part of all of the boreholes. It underlies either the top silty sand soil or the river sand and gravels. It is firm, brownish (wet) and grey (dry) sandy SILT with some clay. Occasionally this unit is intercalated by lamina of sand dominated formation. Groundwater is encountered on this formation; as a result part of this formation which lies below the water level is more cohesive than the part which lies above. Standard Penetration Test conducted on this layer gave an N value in the range of 11 to 43 with the most part being in the range of 14 to 19. The following averaged design engineering parameters are considered to represent the layer:- Bulk Unit Weight = 15.7KN/m3 Specific Gravity = 2.5 Moisture Content = 9.1% Cohesion Value = 8.4KN/m2 Angle of internal friction = 27.40 Free Swell = 12.3% Plasticity Index = 6.3% 3.3.3 Ground Water Condition Groundwater was encountered during drilling at depths in the range of 13.50 to 14.50m. The following averaged engineering parameters are considered to represent the water:- New Sugar Factory Geotechnical Investigation Report CONSTRUCTION DESIGN SHARE COMPANY Chloride Content = 116.5mg/c Sulphate Content = 77.7mg/c PH-Value = 6.9 4. FOUNDATION RECOMMENDATION 4.1General The geology of the site is uniform and it is composed of loose to medium dense silty sand with occasional gravels followed by a relatively thin layer of rounded gravels and pebbles and firm moist sandy silt soil is encountered to the end of the drilling. In some borehole log sheet the rounded gravel sand pepples will disappear and pure, loose to medium dense sand will be found below the top soil layer. In general the country has been divided in accordance with figure 2 (from the geological report) and table 1 in to seismic zones, depending on the local hazard .The site is located at the highly seismic area and seismic hazard shall be taken in to consideration in the design of the structures. 4.2 Bearing Capacity Calculation Bearing capacity computation is made using different calculation methods giving stress to field tests. Site inspection results with engineering considerations are taken to give the foundation recommendations. Meyerhof approach is used for computing the allowable bearing capacity. This approach is calculated for 25-mm settlement using SPT–values (Standard Penetration Test values) as can be seen below. Q all = N x Kd B≤ F 4 F 1 Q all = N (B+F 3 ) 2 Kd B>F 4 F 2 B 2 Where: - Kd = 1 +.33(D/B) ≤ 1.33 N = Adjusted SPT blow count B = Width of foundation New Sugar Factory Geotechnical Investigation Report CONSTRUCTION DESIGN SHARE COMPANY D = Depth of foundation So = Allowable settlement For So=25mm N’70 F1 0.04 F2 0.06 F3 0.3 F4 1.2 Adjusted N- Value calculation Adjusted –N value = N-count x Cn x 1x 2 x 3x 4 η η η η Where: - Cn = Adjustment for effective overburden pressure 1 = Adjustment for hammer energy η 2 = Adjustment for rod length η 3 = Adjustment for liner η 4 = Adjustment for borehole diameter η In these equations N is the statistical average value for the footing influence zone of about 0.5B above footing base to at least 2B below. Note:-In these equations, footing width is a significant parameter. Obviously if the influence depth is on the order of 2B a larger footing width will affect the soil to a greater depth and strains integrated over a greater depth will produce a larger settlement. Considering the allowable settlement to be 25mm at the recommended foundation depth the following allowable bearing capacity is calculated for the following structures. No. Type of Structure Foundation Type Foundation Depth from N! "earin# Capacity New Sugar Factory Geotechnical Investigation Report CONSTRUCTION DESIGN SHARE COMPANY 1 Cane Handling Isolated Footing 3.0m 220Kpa 2 Boiler House 2.1.For the Building Isolated Footing 3.0m 200Kpa 2.2.For the Tanks Mat oundation 3.0m 2!0Kpa 3 Po"er #u$%#tation Mat oundation 2.&m 3&0Kpa ' Milling House Mat oundation 2.&m 3&0Kpa & Centriugal House Isolated Footing 3.0m 220Kpa ( #ugar #tore Isolated Footing 3.&m 1)0Kpa * +orkshop Isolated Footing 2.&m 200Kpa ! Pro,ess House Mat oundation 2.&m 300Kpa ) Pump House Mat oundation 3.0m 300Kpa 10 -iuser Isolated Footing 2.0m 1)0Kpa Additional Consideration • PH, Chloride and Sulphate content of soil was determined according to BS 1377, and all values are with in the permissible requirement (BS 3148). Therefore Ordinary Portland Cement can be used for the structures. 4.3 DESIGN CONSIDERATION FOR THE ROAD Engineering characteristics of sub grade soil, i.e. its CBR strength is used to design foundation of heavy duty pavements. It is clear that heavy trucks are expected to move around the service area within the premises of the factory for loading and unloading purposes. Considering this fact sub grade soil investigation has been conducted using test pits along with sampling and subsequent libratory testing. The entire site is composed of uniform geology as revealed by borings and pit excavation. For this particular case associated with road base, test pits TP19, 20, 21 and 22 have been dug to a maximum depth of 4m. The test pits were properly logged and representative samples were taken for laboratory testing form layers that are likely to be affected by traffic load. The laboratory tests conducted on the sub grade soil samples are • CBR test • Classification test (Atterberg limits and grading) • Natural moisture content. The summary of the soil test results are appended in the report. New Sugar Factory Geotechnical Investigation Report CONSTRUCTION DESIGN SHARE COMPANY 4.3.1. LABORATORY TEST RESULTS AND ANALYSIS The test pit logs depict that there exist two layers which are sandy silt soils. The most problematic in sub grades are expansive soils which does not exist at this site makes the situation very favorable for the desired purpose. 4.3.1.1. Laboratory CBR Laboratory CBR indirectly measures the shearing resistance of a soil under controlled moisture and density conditions. Therefore, Laboratory CBR tests were under taken on the sub grade soil samples to measure its strength. The CBR values are reasonably high enough which falls in the range of 9% -18% with one exceptional result of 3%. 4.3.1.2. Plastic Index The Plasticity of a soil gives very good indication of its expansion potential. Therefore, Atterberg limit tests were undertaken to asses its expansion potential and the results reveal that the soil is low plastic. Besides the swell test result also depicts that it is very far below the permissible limit which is 4%. 4.3.1.3. AASHTO Soil Classification After conducting Atterberg’s limit and gradation tests it is possible to classify sub grade soil as per AASHTO soil classification. In general, using AASHTO soil classification a material can be rated as excellent to poor as road sub grade material. Accordingly, the sub grade soil of the site in consideration falls dominantly in A-5 class and rated as fair to good as per AASHTO road sub grade rating. New Sugar Factory Geotechnical Investigation Report