21 September 2019

Guest Article- Prof Dr T N Singh

Slope Stability analysis of Reliance Power Transmission Tower – 61,  along Western Expressway, Mumbai.

 

 

On the express highway  Reliance Energy Ltd (REL) has numbers of towers located on hillocks along the road side. Due to road widening work carried out by MMRDA, these hillocks were excavated and widened using explosive energy. These operations have affected the stability of hillocks. A study was carried out by IIT Bombay in year 2008 for three towers located at various locations i.e., Tower 53, 61 and 62. IIT Bombay team has suggested few strengthening treatment for tower number 53 and 62 at the site which has been executed by  REL. Prof. T N Singh of IITB along with Kunjjan Popat had also suggested special chemical grouting for tower number 61. REL has engaged TRIFIX to perform the job of chemical grouting at various vulnerable locations all along the slope. The work had  been carried out using special injection grouting equipments and Polyurethane based chemicals and completed in December 2008. After the completion of the grouting work REL consulted Department of Earth Sciences IITB, for assessment of improvement in slope at tower number 61. Prof. T.N.Singh and his team conducted the field investigation 2-3 times and taken required measurements at the site to collect the data for assessment and evaluation of stability of slope and it’s improvement after chemical treatment.

 

Mumbai region is mainly occupied by Deccan basalt flows and their acid and basic variants. The area under investigation consists of massive, vesicular, amygdaloidal and weathered basalt. The rock types of the study area is fractured and jointed in nature. Basalts are moderately weathered but the rate of weathering decreases from the bottom to top of the basement rock. Water seepages through fractured and jointed rock mass and vegetative covered found at some part of the area. The geotechnical field investigation has been carried out along the road cut section to understand the geotechnical properties of the rock mass. The rocks are jointed in some part of the road cut section.

 

The rock type in this location is dark brown to grayish in colour. The joints are uneven in nature. Sometimes bedrock shows horizontal opening between two consecutive layers. Distinct lithological contrast observed in this location where the rate of weathering decreases towards the top .

 

Factor of Safety without chemical grouting was obtained 1.86 indicates that the slope at location of Tower 61 is stable but few locations indicates high strain rate particularly at the top of a slope Maximum shear strain of 2.5e-05 has developed along the failure surface. Hence, chemical grouting was suggested to strengthening the all fractures and weak zones to avoid further widening as well as to prevent it from water ingress during rainy seasons.

 

To further confirm the soundness of rock, after treatment a vertical resistivity sounding survey was conducted at the depth of 45 ft and strength were measured for selected samples. The results of resistivity are given below.

 

After all these tests and confirmation, again a two dimensional numerical analysis has been done and was found that the overall factor of safety from 1.86 to 2.19. This indicates that the slope is more stable than the earlier. The maximum shear strain rate has drastically decreased after treatment particularly at the height of 16m at the top.

 

Location

Factor of Safety

Tower 61

1.86 (Before treatment)

Tower 61

2.19(After treatment)

 

 

Concluding Remark

Present study indicates that the slope is overall stable after the treatment strain rate has been reduced. However, proper drainage arrangement should be taken to avoid any engress of water through fractures. It is generally true for any slope.

 

 

 

Artilcle is co-authored by

Prof. Dr. T. N. Singh,Department of Earth Sciences, IIT Bombay and

 

 

 

 

Kunjjan Popat, I.R.P (Germany), Triseal Technologies (LLP)




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