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 - 560070

Study Area Luxurious residential apartments near Hosakerahalli, Bangalore

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

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

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

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

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 : 10-250Hz 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

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

Seismic Results

ERT Results

GPR Results

Comparison of SRT, ERT and GPR

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-3 150-160m 0ver 10m 0.5-6.5m

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.

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