Geotechnical sub-soil exploration by PRCS for the Wangchu Bridge site at Charkilo. Providing foundation analysis for Bhutan Education City development.
Name of the Client:
Department of Roads, Ministry of Works and Human Settlement
Project Description:
DHI Infra Ltd is planning the Bhutan Education City Project. It is Government’s commitment to provide the ancillary facilities such as roads, bridges, water, electricity & communication.
The main access road to the Education City is from Charkilo with a bridge over Wangchu. The access road whould be a double lane with of about 5 km with a bridge over Wangchu measuring approx. 50 mtr for a permanent bridge of 50 mtr span, detail subsoil investigation is required to be carried out.
Scope of work:
- Collection of Hydraulic data.
- Calculation of design discharge and linear Waterway.
- Sub-soil exploration and determination of engineering properties of soil.
- Preparation of sub-soil profile & Determination of bearing capacity of soil at various depths.
- Submission of Report.
1. Collection of Hydraulic Data
The Collection of Hydraulic data shall include Collection of history of flood discharge, maximum HFL, maximum velocity of flood flow, river flow characteristics, rainfall intensity and catchments area and characteristics of catchments area.
The general guidelines provided in IRC: 5-1998 Clause 102.3 shall be followed in the Collection of the hydrological data, the Hydromet Services Division of the Department of Energy may be consulted.
2. Calculation of Design Discharge and Linear Water way
The design discharge shall be calculated using mínimum three methods as per the guidelines given in IRC: 51992 Clause 103. Similarly, linear water way shall be calculated based on the methods and the formula given in IRC:5:1998 clauses 104.
3. Determination of boreholes locations
The tentative borehole locations are shown in the Drawings attached. However, the consultant shall decide the exact boreholes location and depth for collection of sub-soil simple in consultation with Bridge Division. Total of two bore holes shall be done – one each on the two abutments.
The depth of the boreholes shall not be less tan 20 meter from the surface unless confirmed refusal is encountered prior to achieving the prescribed depth. The engineering properties of the sub-soil strata at the location shall be worked out as detailed.
4. Sub-Soil Exploration and Determination of Engineering Properties of Soil
The boring for collection of sample shall be done at the proposed location of abutments and the arch foundations as per approximate span arrangement fixed.
The sub-soil investigation shall broadly consist of two stages, preliminary Investigation and Detailed investigation. In general the investigation procedure and collection of data shall follow the guidelines provided in IRC 78:2000. The scope of sub-soil investigation in brief are as below.
A. Preliminary Investigation
i. Preliminary investigation shall include the study of existing geological information, previous site reports, geological maps etc., and surface geological examination. These will help to narrow down the number of sites under consideration and also to locate the most desirable location for detailed sub-surface investigation.
B. Detailed Investigation
i. Based on data obtained after preliminary investigations, the bridge site, the type of structure with span arrangement and the location and type of foundations, programme of detailed investigations, etc. Shall be tentatively decided. Thereafter the scope of detailed investigation including the extent of exploration, number of bore holes, type of tests, number of tests etc., shall be decided in close liaison with the design engineer and the exploration team, so that adequate data considered necessary for detailed design and execution are obtained.
ii. The exploration shall cover the entire length of the bridge and also at either end a distance of zone of influence i.e. about twice the depth below bed of the last main foundation to assess the effect of the approach embankment on the end foundations.
Generally the sub-surface investigations should extend to a depth below the anticipated foundation level equal to about one and a half times the width of the foundation. However, where such investigations end in any unsuitable or questionable foundation material, the exploration shall be extended to a sufficient depth into firm and stable soil sor to rock.
iii. The scope of the detailed sub-surface exploration shall be fixed as mentioned in B(i) and B(ii). However, as a general guide it shall be comprehensive enough to enable the designer to estimate or determine the following:
- Engineering properties of the soil/rock.
- Location and extent of weak layers and cavities, if any, below hard founding strata.
- The sub-surface geological condition, such as, type of rock, structure of rock i.e. folds faults, fissures, shears, Fractures, Joints, Dykes and subsidence due to mining or presence of cavities.
- Ground water level.
- Artesian conditions, if any.
- Quality of water in contact with the foundation.
- Depth and extent of scour.
- Suitable foundation level.
- Safe bearing capacities of foundation strata.
- Probable settlement and probable differential settlement of the foundations.
- Likely sinking or driving effort and
- Likely construction difficulties.
C. Method of taking soil sample
The size of the bores shall be predetermined so that undisturbed samples as required for the various types of tests are obtained. The Method of taking samples shall be as given in IS:1892 and IS:2132. The tests on soil samples shall be conducted as per relevant part of IS:2720
D. Foundations Requiring Shallow depth exploration (Open Foundation)
These shall cover cases where the depth of exploration is not deep and it is possible to take samples from shallow pits or conduct direct tests like plate load tests etc. This will also cover Generally the foundation soil for approach embankments, proactive works etc.
i. Tests shall be conducted on undisturbed representative samples, which may be obtained from open pits. The use of plate load test (IS:1888-Method of Load test on soils) is considered desirable for ascertaining the Safe bearing pressure and settlement characteristics.
A few exploratory bore holes or soundings shall be made to safeguard against presence of weak strata underlying the foundation. This shall extend to a depth of about 1 ½ times the proposed width of foundation.
ii. The tests to be conducted at various locations for properties of soil etc., are different for cohesive and cohesionless soils. These are indicated below and shall be carried out wherever required according to soil type.
Cohesionless Soils
a. Laboratory Tests
i. Classification test, index tests, density determiantion etc.
ii. Shear strengths by triaxial / direct shear etc.
b. Field tests
i. Plate Load Test.
ii. Standard penetration Tests (as per IS:213)
Cohesive Soils
a. Laboratory Tests
i. Classification test, index tests, density determination etc
ii. Shear strengths by triaxial/direct shear etc.
iii. Unconfined compression test (IS:2720 Part X)
iv. Consolidation test (IS: 2720 Part V)
b. Field Tests
i. Plate Load Test
ii. Vane Shear Test (IS:4434)
iii. Static Cone Penetration Test (IS: 4968 Part III)
E. Foundation Requiring Large Depth of Exploration
In this group are covered cases of deep open foundations, deep Wells, pile foundations etc where the use of boring equipment, special techniques of sampling, in-situ testing etc., become essential. In addition to the problems of soil and foundation interaction an important consideration can be the soil data required from constructions. Often in the case of cohesionless soils, undisturbed samples cannot be taken and recourse has to be made to in-situ field tests.
i) The sub-surface exploration can be divided into three zones:
- Between bed level and up to anticipated maximum scour depth (below H.F.L.)
- From the maximum scour depth to the foundation level and
- From foundation level to about 1½ times the width of foundation below it.
ii) Sampling and testing (in-situ and Laboratory) requirement will vary in each case and hence are required to be assessed and decided from case to case. The sub-soil water shall be tested for chemical properties to evaluate the hazard of deterioration of foundations where dewatering is expected to be required, permeability characteristics should be determined.
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