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Geotechnical Report

By Melvin Ramirez,2014-12-19 04:49
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Geotechnical Report...

    Addis Adama Toll Motorway

    Ethiopian Roads Authority Geotechnical Investigation and Design Report,

    0+000 16+500

    1. PURPOSE AND SCOPE

    This report is part of the deliverables that are to be submitted for the work of Section-1 of the Addis-Adama Toll Motorway. The report expounds the Geotechnical Investigation and Analysis that were carried out at the bridge, overbridge, and culvert locations and at deep cut sections found between km 3+500 and 17+000.

    In this section of the road, twenty boreholes were drilled as part of the geotechnical investigation. Out of these, thirteen boreholes were drilled at overbridge and bridge foundations positions, six at heavy cut sections, and one at box culvert foundation position. The geotechnical investigation was carried out to explore and assess the engineering properties of materials of subsurface strata. It was undertaken by running rotary core drilling, recovering samples (both disturbed and undisturbed) and conducting in-situ and ex-situ (laboratory) tests.

    The scope of the Geotechnical Analysis extends to the determination suitable footing depth, safe bearing pressures, anticipated settlement and recommendation of stable slopes. The details of the Geotechnical studies together with the desk studies and field reconnaissance were discussed in the subsequent sections of the report.

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    Addis Adama Toll Motorway

    Ethiopian Roads Authority Geotechnical Investigation and Design Report,

    0+000 16+500

    2. GEOLOGY AND GEOMORPHOLOGY

    The proposed alignment of the Addis Adama Toll Road traverses various types of

    geological formations. The geology of the project stretch is dominated by volcanic rocks with interruptions of Alluvial and Lacustrine deposits at some short stretch. The main geological formations along the road alignment are composed of:

    N = Nazret Series: Ignimbrite, unwelded tuffs, ash flows, rhyolitic flows, domes and n

    trachyte;

    NQ = Bishoftu Formation: Alkaline basalt and trachyte; tb

    Q = Alkaline basalt and trachyte; b

    Q = Alluvial basalt and Lacustrine deposits: sand, silt and clay;

    Nc = Trachyte, Trachy-basalt, peralkaline rhyolite with subordinate alkaline basalt.

    The topography of the project route is characterized by rolling and flat terrain and dendritic drainage pattern.

    The geologic formation of the project area as shown in the Geological Map of Ethiopia 1995 ed. is shown in the following plate.

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Addis Adama Toll Motorway

    Ethiopian Roads Authority Geotechnical Investigation and Design Report,

    0+000 16+500

    Addis

    Ababa

    Adama

    Plate #1 Geological Map of the Project Stretch.

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    Addis Adama Toll Motorway

    Ethiopian Roads Authority Geotechnical Investigation and Design Report,

    0+000 16+500

    3. FIELD EXPLORATION

    The subsurface materials at the bridge, overbridge, and culvert locations and on heavy cut sections were examined by drilling boreholes by rotary core drilling method. Two types of drill bits and samplers were used during the drilling operation. These are:

    1. Drill bits and samplers with diameter of 13cm for shallow drilling up to 1m; and

    2. Drill bits and samplers with diameter of 10cm for deep drilling beyond 1m. The drilling was carried out without casing. Instead of casing, as the drill advances deeper, bentonite slurry was used to prevent the overlying strata from collapse and keep the borehole open.

    The materials from each of distinct strata were sampled for laboratory investigation. Based on the material types and consistency, sampling of the recovered subsurface materials was undertaken in three ways. For the firm to hard cohesive soils and for non-cohesive soils, disturbed (representative) samples were collected; and for soft cohesive soils undisturbed samples were recovered. On the other hand, for rocks (of various degrees of weathering), rock cores were sampled.

    In-situ tests, namely SPT (Standard Penetration Test) and CPT (Dynamic Cone Penetration Test), were conducted at an average interval of 2m. The Standard Penetration Test was done on fine and medium grained soils. The test was carried out without liner. The Dynamic Cone Penetration Test, on the other hand, ran on granular soils on which it is difficult to conduct SPT.

    When a rock stratum was encountered, it was properly described after thorough scrutiny of its type, level of weathering and discontinuity. Besides, the parameters like Rock Quality Designation (RQD), discontinuity spacing and joint spacing, which are essential for rock mass rating were recorded by taking appropriate measurement of the recovered rock cores. All of the field procedures and observations were logged in a well-organized format. The format contains eight (8) columns for keeping a record of the downward run length, recovery ratio, material description, the SPT and CPT results, the sample locations, the laboratory test programs; and for taking any other additional remarks.

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    Addis Adama Toll Motorway

    Ethiopian Roads Authority Geotechnical Investigation and Design Report,

    0+000 16+500

    4. GROUND WATER MONITORING

    Ground water was not encountered in any of the boreholes during drilling and after short

    delays of the drilling operation. As per the information obtained from CCCC (the Client),

    ground water monitoring was carried out before commencement of the drilling operation

    and it was reported that the ground water level was found at deeper depths, around 90m

    below the natural ground level, after the rainy season.

5. LABORATORY TESTS

    The soil samples collected from each of the boreholes were subjected to the following laboratory

    tests:

    ; Atterberg limits (Liquid limit, plastic limit and plasticity index);

    ; Particle size analysis;

    ; Unit weight;

    ; Specific gravity and

    ; Based on the above tests the soil is classified by USCS System.

    On the other hand, the sound rock core samples were tested for their Uniaxial Compressive Strength

    and Unit weight and the weathered rock cores were tested for their unit weights.

    The results of the laboratory tests are appended at the end of the report.

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    Addis Adama Toll Motorway

    Ethiopian Roads Authority Geotechnical Investigation and Design Report,

    0+000 16+500 6. FOUNDATION AND SLOPE STABILITY ANALYSIS 6.1 Bearing Capacity Determination

    The allowable bearing capacities of the foundation materials were determined using

    Meyerhof and Terzaghi bearing capacity equations. For cohesive and non-cohesive soils, Meyerhof’s equations, based on SPT results, were used for the determination of allowable bearing capacity for 25mm settlement. The N-values

    of the SPT results were standardized to N values by adopting energy ratio and field 70

    procedure corrections in the following manner.

    NCN 70N1234

    Where η= adjustment factors from (and computed as shown) Table 6-1; i

     N = adjusted N for machine efficiency of 70%; 70

     C = adjustment for effective overburden pressure p’ (kPa) computed [Liao and No

    Whiteman (1986)] as

    1

    2?(95.76?) CN?)p'o??

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    Addis Adama Toll Motorway

    Ethiopian Roads Authority Geotechnical Investigation and Design Report,

    0+000 16+500

    Table 6-1 SPT Field Procedure Correction Factors

    Hammer for η 1

    Average Energy Ratio, Er

     Donut Safety

    Rope-Pulley Rope-Pulley Country Trip Trip/Auto United States 45 - 70-80 80-100 Japan 67 78 - - United Kingdom - - 50 60 China 50 60 - -

    Rod Length Correction, η 2

    Length >10m η = 1.00 2

    6-10m = 0.95

    4-6m = 0.85

    0-4m = 0.75

    Sampler Correction, η 3

    Without liner η=1.00 3

    With liner: Dense sand, clay = 0.80

    Loose sand = 0.90

    Borehole Diameter Correction, η 4

    Hole diameter: 60-120mm η=1.00 4

     150mm =1.05

     200mm =1.15

The N-values of the dynamic cone penetration test were converted to SPT N-values using

    the following relations.

    1?(N2N where N = SPT N-value corrected for ?)corrCPTCorr2??

    overburden pressure

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    Addis Adama Toll Motorway

    Ethiopian Roads Authority Geotechnical Investigation and Design Report,

    0+000 16+500

    The adjusted N-values of the SPT were then used as input parameters in the following Meyerehof’s bearing capacity formula.

     N N 5570NqK B ? F ad4FF 0.05 0.04 11

    F 0.08 0.06 22BFN?(3qK?) B > F adF 0.03 0.03 43FB??2

    F 1.20 1.20 4

    DK10.33~1.33 dB

    The allowable bearing capacity computed by such equations shall be adjusted by the following relation if the actual settlement is different from 25mm.

    Settlement(mm)(actual)qqadjusteda 25

    For clayey soils, the allowable bearing capacities were also determined by Skempton’s Formula developed for Saturated Clays. To use this formula, the unconfined compressive strength of the soil is determined based on SPT N-value as:

    q2C13N uu

    where C = undrained shear strength (kPa) u

    This value of Cu = C = cohesional resistance of the soil is used as input in the following Skempton formula.

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    Addis Adama Toll Motorway

    Ethiopian Roads Authority Geotechnical Investigation and Design Report,

    0+000 16+500

    qcND ultcf

    where q = ultimate bearing capacity ult

     Nc = the bearing capacity factor read from Skempton’s chart

     γ = unit weight of soil and

     D = depth of footing f

    The allowable bearing capacities were then determined using factor of safety of, FS= 3, on

    the net ultimate bearing capacity of the soil. The values determined by this equation are only

    used for comparison with the allowable bearing capacities determined by Meyerhof’s equations.

    The bearing capacities of the rocky foundations were determined using Terzaghi’s bearing

    capacity equation:

    qcNsqN0.5BNs ultccq

    (?(6Ntan45?)where ; q2??

    (?(4N5tan45?) ; c2??

    NN1 ; q

     c = cohesional resistance

     ф = angle of frictional resistance

    q = effective overburden pressure

     B = width of footing

     γ = Unit weight of the rock and

     s and s = shape factors cγ

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    Addis Adama Toll Motorway

    Ethiopian Roads Authority Geotechnical Investigation and Design Report,

    0+000 16+500

    The rock mass was rated using Beiniawski (1989) method of rock mass rating system. After

    the rock mass class was determined in such a fashion, the approximate values of cohesion

    and angle of friction were read from the same table and used in the Terzaghi equations for

    determination of the ultimate bearing resistance of the rock mass. The allowable bearing

    pressures were computed taking a factor of safety of, FS= 5.

    The bearing capacities computed for each of the foundation materials are appended at the

    end of the report.

    Beiniawski -1989

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