Please check the paper attached herewith. 

thanks a lot fr ur reply sir...but this literature tells about grouting in limestone bedrock overlain by soil. What about cavity treatment in Silty Sand/ Sandy Silt Soil which is not covered by any rock on any side. Also the water table level is just 2 mtr below ground level.

Sir,

if you have time please go through the book: 

Sinkholes and Subsidence

Karst and Cavernous Rocks in Engineering and Construction by Dr Tony Waltham, Professor Fred G. Bell, Martin G. Culshaw

I also attached a doc. with ground improvement techniques .

Regards

I think it is not common to have cavities down deep in to the ground, unless it is man-made (e.g. to transfer water as a canal). A preferred method is  to fill it with injection materials.

i agree with Behzad

Is this silt/sand cemented material or not? If not, is it possible to have an unsupported cavity in this kind of material? Pure sense tells me that it would collapse due to low strength of material in contrast to relatively high overburden. 

if the cavity is localized and upper layers have clayey content in them, it is then difficult for the overlaying soil to collapse as the cohesion will hold the soil together. Moreover, if the cavity we are talking about is not the exact cavity but only a patch of localized very loose soil then also the collapse won't happen and field tests (like SPT) will show 0 N values in those area.

What is the size of your "cavity"? Detailed geotechnical profile and high quality investigations would tell you more about this phenomenon. If the only data you have is based on SPT blow counts than.. Was the borehole bottom cleaned before an SPT test? Maybe loose material left form the drilling caused this confusion, this happened to me several times. How old are your sediments?

P.S. Apparent cohesion in clayey soils is not a constant, i.e. it is stress dependent. Several authors do not believe in cohesion intercept and find it equal to zero for all soils. The so called "cohesion" is a consequence of existence of micro-structure in soils (e.g. loess etc.)

size of the cavity is still not known, We have performed SPT at 5 different locations inside 50mtrX50mtr area. Out of 5 boreholes our casing collapsed 2-2.5mtr in 4 boreholes at a depth of 28-30mtrs. We are envisaging a cavity at that depth probably due to earlier water pumps (hand pumps) in those area which are generally placed below 30mtr from ground level. Although actual reason is still not available & also we have not performed any test to know the exact size of cavity.

Have you been thinking about some geophysical approach?

yes..GPR or MASW

Did you have sufficient depth penetration of GPR?

no idea.....we will see if it gets successful or not

Bhatia Ji,

The formation of underground caves in loose soil deposits constitutes an unusual case which presumes the function, in their formation, of two mechanisms:

(a) a mechanism for the removal of fine material (clay, fine sand, silt) from the initial bed

(b) the presence of soil beds with mechanical properties which will ensure a sufficient cohesion, so that they will function as a roof for the cave being formed.

As in your case the clay is acting as the roof and the sand is getting eroded by the circulation of the ground water. it is quiet common due to the absence of fine materials in sand.  

If you will share the type area then we can discuss the case broadly.

Thanks & Regards

Naveen