1. Geophysical survey for addressing geo-environmental problem due to unplanned rapid urbanization
Dr. P. C. Jha and Dr. H S Venkatesh
Presenting by
Dr. B. Butchi Babu
Engineering Geophysics Department
National Institute of Rock Mechanics
Bangalore

2. Study Area
Luxurious residential apartments near Hosakerahalli, Bangalore

3. Local Geology of Site
Precambrian granite and gneiss of the Indian Precambrian Shield and which are part of the peninsular granitic complex.
Granitic rock is in massive form and fine grained
Presence of Lataritic soils along hill slopes
Approach roads was made by backfilling of locally available soils
Gabion walls was made to stabilize the pavements
Drainage System
The direction of drainage channels orienting towards the Hosakerehalli Lake right below Hanumagiri hill
Naturally formed surface runoff water flowing channels were filled with construction debris

4. Continuous seepage through
Site conditions
Undercutting of road
Water seepage through soils
Cracks along road and sinkholes
Continuous seepage through
concrete walls

5. Problem Statement Assumption Repeated occurrence of slope failures
Formation of cracks, sinkholes and potholes
Breaking of underground drainage pipes
Bulging of gabion walls
Continuous seepage through concrete walls as well as through lataritic soils
Assumption
If the problem is due to leakage around the subsurface drainage pipe, this should be reflect in surface GPR study
If the problem is due to wash-out by any hidden water flow channel, this should reflect in surface seismic and resistivity study

6. Layout of Geophysical Surveys
Along approach road: Ground Penetrating Radar (GPR)
Between gabion and pavement: Seismic Refraction Survey (SRS)
: Electrical Resistivity Tomography (ERT)

7. Techniques used for Data Acquisition
SRT
Data acquisition system: ABEM Terraloc MK6
No of channels: 24
Source: 16lb sledge hammer
Receiver: 10Hz geophones
Geophone spacing:2.5m
Sampling Interval: 50µsec
Recording frequency : Hz
ERT
Data acquisition unit: SYSCAL Pro (10-channels)
No of channels: 48
Electrode spacing: 2.5m
Array configuration: Dipole-dipole
GPR/SFR
Transmitter& Receiver: Network Analyzer (HP8714ET)
Antenna type: Dipole (2m)
Operating frequency range: 1-250MHz
Survey Mode: Reflection
Tx and Rx cable length: single-core50m pair
Antenna step movement: 0.5m

8. 16lb sledgehammer for generation of seismic signal
Refraction data Acquisition at Site
16lb sledgehammer for generation of seismic signal
Acquiring data using ABEM seismograph

9. Seismic Results

10. ERT Results

11. GPR Results

12. Comparison of SRT, ERT and GPR

13. Demarcation of possible flow paths
Identity
Chainage
Width
Depth range
Stream-1
35-50m
Over 15m
5-10m
Stream-2
75-90m
2-7m
Stream-3 m
0ver 10m m

14. Conclusions
Integrated approach using SRT, ERT and GPR studies demarcated the possible flow path channels between approach road and gabion wall.
These flow paths might be the naturally exist before construction and are apparently blocked below the access road.
The probable reason for under cutting of roads, formation of cracks etc due to blockage of these flow paths and in the rainy season due to accumulation of excessive pore pressure may responsible for bulging of gabion wall etc.
Suggested for alternative drainage tubes along the flow path to reduce further damage and allowing of these flow channels in their natural paths eco-system was not disturbed.

15. Thank you