A note on Geo-technical Investigation in Soil and Rock For a Highway Project in Hilly area By - Pushpendra Kumar, Deputy Manager, Geology, Rodic Consultants PVt. Ltd.
Sub-Surface
investigation/Geo-technical instigation/ Soil investigation is play a very
important role for a Highway project especially in a Hilly terrain like
Himalayas. It directly affected to cost of project as well as completion time
very badly if not executed as per standard. As we know, based on the
preliminary instigation we have finalized our project design and expected cost
with a time duration of the project. These details may be updated during
detailed investigation later. I f there
is major change is observed during
details Geo-technical investigation then all the things will be change sometimes
as design, project cost and the project time schedule. So it is suggested not to omit any sequences
of investigation as generally observered and planning of investigation at
different stage ( desk planning preliminary/reconnaissance, details survey,
type of field test required and sample selection etc) must be prepared by a
well experience expert. “Without a Geo-technical investigation, ground is a hazard especially in hilly terrain. You
pay for geotechnical investigation or will pay for delay or design up gradation
of project during execution.
During Geo-technical investigation correct Rock core logging and interpretation is very important
during core drilling activity. It must be ensured that maximum information
should be recorded during core drilling activity. During core drilling activity
final log must be included following information such as-
•
Project Name
•
Borehole number
•
Location and orientation of borehole
•
Drilling technique
•
Drillers daily record
•
Flush returns
•
Standing water levels
•
Litho-Unit
•
SPT no. In soft strata etc.
•
Sub-surface investigation
not just a core drilling but also includes pit excavation core drilling with
cone penetration test, field shear test, dilatometer test, permeability test
etc. as required by project and design team. It will also include collection
and preservation of soil/rock sample (disturb and undisturbed) from bore hole
borrow pit etc. Except SPT and CPT all other test will be conducted on samples
in laboratory by following IS and IRC standard.
• Depth and numbers of Bore hole taken.
Geo-technical Investigation depth for Structure
 |
| Depth of geo-technical investigation for Slide area |
 |
| Depth of geotechnical investigation for Tunnel area |
•
The depth of Bore hole for structure must
be 1.5 times of the width of foundation below foundation level or if RQD encountered >80%
continuously up to 3 meters. If suitable soil not encountered it can increase
up to 4 times. For embankment and guide bund depth must be 2.4 times of height
but not less than 20 m. For landslide the investigation depth should below 5-10 m from slip circle or failure plane. In tunnel area depth of the investigation must achieved up to 5-10 meters below invert level.
•
At least 1 no. of bore hole
taken for each foundation of Structure. One bore hole must be taken for
approaches at both side of bridge/viaduct at 50-100 meters from abutment
location. If any complication observed in lithology transverse bore hole may
taken to know the variation in underlying strata
• A soft clayey
strata must be investigated carefully and shall be mentioned specially as it
generally create problem during execution state in highway, structures as well
as in tunnels.
•
During investigation all
required data must be recorded as mentioned in IS:5313 and IS:4464. The samples
must be stored as per IS:4078.
•
The core recovery must be classified
according to Cl. 2405.6, very poor to excellent rock mass.
•
In-situ test for deformation
shale be conducted in field as per IS:7292,7317,7746, 1888 etc.
Rock cores and split spoon samples are known as undisturbed sample.
For details how sample taken may referred IRC:75.
The size required sample is given in table 2400-2. The length
of core sample must be 150-200 mm for 40 mm dia core and 300 mm for 100 dia core.
In Hilly terrain it is mandatory to taken bore hole for Hill slope
excavation and stabilization works. It can be decided based on cut slope height,
strata, and ground water condition.
If slide area exist in a project, one bore hole shale be taken for
each slide area. Other necessary investigation may be carried out as suggested
by expert.
For tunneling, seismic refraction and electric resistivity survey
shall be conducted with detailed core drilling to maximize in-situ
(underground) information at tunnel area. It is very helpful for design stage
as well as construction stage. Field shear test must be conducted to validate
lab test result for c and phi parameters. Dilatometer test also be carried in
bore hole for modulus of deformation of rock mass
Few Important IS code which is helpful other than MoRTH-2400 during
Geotechnical investigation:
IS:1494, IS:1888, IS:1892,IS:2131,IS:2132
IS:2720, IS:3720, IS:4464, IS4434, IS:4453
IS:4968, IS:5313, IS:4079, IS:7292,IS:7317
IS:1892,1979
IS:7746
IRC:78 and IRC:75
IRC-SP-91,2010
IRC:SP:48, 1998
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