Dear Thapa,

Ground water problem can be solved by dewatering the ground water table to the safe depth below the depth of cut. Silty Sand can easily drain. If possible think of freezing.

The shear strength of the silty sand should be known.

For low shear strength material the process should be carried out very care fully.

Divide the entire depth of shotcreting into different equal number of layers and carry out the process in step by step. Excavate and apply shorcrete on soil by layer by layer until reaching the desired depth.

Also you can refer the following.

https://www.rms.nsw.gov.au/documents/projects/...design/shotcrete-design-guidelines.pdf

https://web.iitd.ac.in/~tanusree/courses.../02Standard%20Practice%20for%20Shotcrete.pdf

http://www.mining-technology.com/contractors/tunnelling/shotcrete-technologies

http://www.bamts.ru/eng/job/nabrizg.php

http://www.constructionworld.in/News.aspx?nId=V2yrqQr+3eQmgYlogOhWcg==

Firstly, shotcreting on its own is not a stabilization method. So if you are talking about a steep slope >70deg you will need to provide some form of stabilization i.e. soil nails.

Secondly, yes you will need to provide methods to control surface and ground water flows.

The Hong Kong Geotechnical Engineering Office have plenty of guidance on slopes and stabilization including drainage methods on their website.

http://www.cedd.gov.hk/eng/publications/geo/

Here the main problem is loose permeable soil formation. You may use reinforcement with nailing (anchoring) to protect the soil layer from collapsing. To manage (not control) the groundwater or surface water flows, sufficient weep holes or surface conduits should be provide.

You may also refer to the attached paper.

Conference Paper Mitigative Measures for Landslides Prone Zones in the Western Ghats

Dear Velusamy Gobinath, Chris Bridges and N B Narasimha Prasad

Thank you very much for valuable suggestions.

Here, I have added more about the slope.

The sloe is more than 80 degree comprising of highly weathered and fractured micaceous schist at bottom, red colored sandy soil with gravel fraction at the middle and fine soil at the top. The height of the slope is 15 m. Till now, we are planning  to construct retaining wall up to 5 m and back fill by soil and shotcreting at middle and  top soil part. But, Soil is continuously falling from the slope disturbing on retaining wall construction.

Could we ad more on how to stabilize this slope?

Here is the attachment of photograph of slope.

Thank you very much to all for your valuable time and suggestion.

This looks incredibly unsafe.

Get people away from the slope and get yourself a geotechnical engineer who can advise.

Thapa,

It seems that your team is doing work that they are not familiar with. This is a warning and not a criticism. The issue is not the shotcreting method. It is that 17.0 m of excavation, a significant thickness of which is into soil, not rock, is a big risk. If you have started excavation and did not face collapse, you must have been lucky. The shotcrete is only an erosion control method. It does not prevent the collapse of the soil should it intend to do. It can only be a local structural element if used together with reinforcement mesh or net, in a context of soil nailing.

All these answers of my colleagues and myself are conceptual and based on the rough data you communicated to us. You need to consult with a specialist and provide him with the full data, with best practice being to invite him to visit the site as well. Sorry of my advice is disappointing, but it is rather protective to your team at first place.

Kind regards,

Hesham Elshazly

Dear Thapa,

It is serious problem. why you excavated this much steep slope. I immediately ask you to consult geotechnical expert.  Better you reduce the steepness of the slope by dumping some materials in order to improve the safety.

The thickness, slope and more importantly, property of the soil (micaceous clay) all these indicate your planned activity as a very high risky proposal. Coupled with this if you have saturated condition underneath, then it is much more risky and definitely prone for landslip. Hence as already advised by some experts, take the help of a geotechnical engineer.

I am very grateful to all  for valuable and constructive  suggestions and comments despite of you busy time and schedules.Your suggestions and ideas definitely help us to find  the best solution for stabilization of the existing slope.

Millions of Thanks, Again !!!

Regards

Ishwor Thapa

Dear ishwor

Before applying for the support by shotcreating on such type of soil slope it should understand the drainage system of the area and groundwater depth, these two things are very crucial factors which influencing the shear strength of soil. therefore proper drainage system should be designed and implemented. 

for that, you can apply the weep holes and subsoil drainage system.

This slope seems very fragile, thus it is better to use the wire-mesh with shotcrete.  

hope this may useful. 

Thank you.

Dear Ishwor, Chris is absolutely right.  If you have access to the land above the slope, then the best thing you can do is to unload the existing cut slope.  If you don't have access then get it at any cost.  Start removing earth materials so as to create at least two berms (stepped slope).  The position of the berms and the inclination of the cut is to be estimated by experienced geotechnical engineer of engineering geologist with relevant qualifications.  Drainage of the entire slope will be done by means of sub-horizontal drains into the sand layers and the weathered schist.  Make sure that the water collected at the berms is also drained downslope safely.  Ground freezing is not applicable for slopes but for tunnelling under river bed coarse grained materials plus it is massively expensive and tricky to do.  So forget it.  Shotcrete can be applied with wire mess only as a surface protection.  A 18m high slope will easily crush any shotcrete lining if it fails (in any mode).  So.  get the slope unloaded with a proper design advice, drain permanently and create stability conditions and drainage before any shotcrete is applied.  Make sure that you do not plug the drains while applying shotcrete.  Depending on the design recommendations and the probable slip surface, you will need to install soil nails of appropriate length into the slope. In all other cases you will not eliminate risk and you will waste money.     

Shotcrete is the same as concrete when evaluated as a material and its exposure to potable water. Potable water stored in concrete tanks with direct exposure to the concrete (no coatings) has not exhibited any significant rise in alkalinity. Exposure of a tunnel in a groundwater aquifer would seem to have much less contact area per volume of water contained in the aquifer, such that any rise in alkalinity would be miniscule. Because concrete in the U.S. is universally accepted for the storage and transport of potable water, I’d assume that the use of shotcrete in your tunnel would be perfectly acceptable.

Also I recommend you to read the Q&A in this useful website:

https://www.shotcrete.org/pages/products-services/technical-questions-archive.htm

Good luck

Milad

I can only endorse what others have stated earlier - this slope appears to be in a very unstable state and shotcrete  will not solve this issue. Indeed, your proposals seem to me to potentially expose the workers who will execute your plans to a very high and totally unacceptable level of risk.

I urge you to seek expert guidance as a matter of urgency.

Like many have cautioned I again stress that the proposal put forth by you is highly risky considering the fragile geological and hydrological environment. In a some what similar condition in Kerala State, India, there was a major landslide occurred, eroding fertile soil cover and damaging plantation crops. So handle the problem with utmost care and taking the help of experts in the field of geo-technical engineering.

Article Recurring Landslides in the Western Ghats – An Environmental Hazard