This is an archival version of the original KnowledgePoint website.

Interactive features have been disabled and some pages and links have been removed.

Visit the new KnowledgePoint website at https://www.knowledgepoint.org.

 

Revision history [back]

click to hide/show revision 1
initial version
RedR TSS gravatar image
RedR TSS

This sounds like the 'black cotton soil' problem known to anyone building pavements in central Africa.

The issue is one of groundwater control. If this soil is silty then it will become very weak, maybe even fluid, when it is (a) wet and (b) subjected to cyclic loading, i.e. traffic. Loss of strength can be very dramatic with true silts. The options are either to stop it becoming wet by draining and capping it or to put a big thickness of free-draining gravelly fill to cushion the loading and assist drainage of the soil. 500mm or more of granular road base may be needed for the second option.

If the area is low-lying and subject to seasonal water logging then the option of keeping the subsoil dry is not practical and a concrete pavement is not likely to last very long as it will not have enough support. It may be possible to design concrete pavements to bridge short distances but that's hard to say from here.

Temporary road surfaces may do the job but they tend to be expensive - usually more so than gravel - so probably only useful for short lengths. You'd need a fairly heavy duty one, I expect. For example, a quick trawl of the web found https://www.groundprotection.co.uk/duradeck/duradeck.php which offers Duradeck polythene panels that claim to take loads up to 80T over soft ground. A 1200x2400 panel weighs 39kg and the basic cost is £299 ($540) per panel in UK (obviously, this is negotiable for quantity). There are other similar products, some available nearer to you - this gives an idea. The success of such a system will depend on how soft the ground actually gets.

An alternative could be to use a combination of a mesh to reinforce the granular base so that it does not need to be so thick. Whether that is cost effective depends on the availability/cost of mesh locally and the weakness of the ground with which you are dealing.

Another issue with these types of fine-grained soil could be frost susceptibility in which ground freezing expands and breaks up the soil structure, weakening it. I guess that will be the case during an Afghan winter. Again, not a problem if the ground stays dry.

I think that's probably as much I as can suggest without more information on the soil properties and groundwater conditions. I hope it helps give a bit of context. I might be able to get some practical info on temporary roads if it would help.

I suggest starting by investigating whether the road can be drained by digging ditches and installing culverts to avoid water logging, if there is time.

Regards,

Robert Hodgson

click to hide/show revision 2
No.2 Revision

This sounds like the 'black cotton soil' problem known to anyone building pavements in central Africa.

The issue is one of groundwater control. If this soil is silty then it will become very weak, maybe even fluid, when it is (a) wet and (b) subjected to cyclic loading, i.e. traffic. Loss of strength can be very dramatic with true silts. The options are either to stop it becoming wet by draining and capping it or to put a big thickness of free-draining gravelly fill to cushion the loading and assist drainage of the soil. 500mm or more of granular road base may be needed for the second option.

If the area is low-lying and subject to seasonal water logging then the option of keeping the subsoil dry is not practical and a concrete pavement is not likely to last very long as it will not have enough support. It may be possible to design concrete pavements to bridge short distances but that's hard to say from here.

Temporary road surfaces may do the job but they tend to be expensive - usually more so than gravel - so probably only useful for short lengths. You'd need a fairly heavy duty one, I expect. For example, a quick trawl of the web found https://www.groundprotection.co.uk/duradeck/duradeck.php which offers Duradeck polythene panels that claim to take loads up to 80T over soft ground. A 1200x2400 panel weighs 39kg and the basic cost is £299 ($540) per panel in UK (obviously, this is negotiable for quantity). There are other similar products, some available nearer to you - this gives an idea. The success of such a system will depend on how soft the ground actually gets.

An alternative could be to use a combination of a mesh to reinforce the granular base so that it does not need to be so thick. Whether that is cost effective depends on the availability/cost of mesh locally and the weakness of the ground with which you are dealing.

Another issue with these types of fine-grained soil could be frost susceptibility in which ground freezing expands and breaks up the soil structure, weakening it. I guess that will be the case during an Afghan winter. Again, not a problem if the ground stays dry.

I think that's probably as much I as can suggest without more information on the soil properties and groundwater conditions. I hope it helps give a bit of context. I might be able to get some practical info on temporary roads if it would help.

I suggest starting by investigating whether the road can be drained by digging ditches and installing culverts to avoid water logging, if there is time.

Regards,

Robert Hodgson

Robert