Deflection of Pipes

In engineering of  flexible pipes, deflection of pipes is a key parameter. It depends upon the load to which it is subjected, Pipe stiffness factor and the Soil stiffness factor. We will discuss ' Loads 'in later blogs. Pipe stiffness is dependent on the properties of the pipe and is unique for a particular pipe and is universally accepted. Soil stiffness factor is emperical in nature. After lot of observation in the field and lots of measurement, emperical formulae have been derived  to assess the soil stiffness factor. In India, to the best of my knowledge, neither sufficient work has been done to assess the soil stiffness factor nor guidelines have been provided for design or field work. In the next few blogs we would suggest what we could do.
There are many organisation from different countries who have addressed this issue, as it is of vital importance. However,  United states Bureau of Reclamation (USBR)  has worked on this issue for more than thirty years and has been providing guidelines based on field observations . Earlier it was believed that the soil stiffness factor depends on the depth of  embedment , the factor increasing with the depth. However it is now concluded that it is not so and it does not vary with depth. It was always  believed that  the properties of the native material in which the trench is being formed also contributes to the Soil stiffness factor. Now a relation has been established,to arrive at the combined Soil stiffness factor E', considering both native soil stiffness and bedding soil stiffness. , USBR has suggested the following equation:

                            E' = Sc . E'b
where
                          Sc  is a correction factor depending on the ratio of E'b and E'n and the ratio of  B and D
                          E'b  is Soil stiffness for the bedding soil
                          E'n is Soil stiffness for native soil.
                          B is the trench width at the springline
                          D is the pipe diameter.
We will discuss about E'n. E'b, E' and Sc in the next blogs.
The work carried out by USBR is based on Unified Soil classification System (USCS) and standard method of determining percentage compaction. Therefore it can be applied anywhere in the world.
The Soil Stiffness factor is generally classified as follows:
1. High Compaction with more than 95% proctor density
2. Moderate Compaction with proctor density value between 85 to 95%
3. Slight compaction with proctor density value being less than 85%.
4.Dumped.
I am personally of the view , that with the current state of  art of compaction and monitoring, it is better not to consider designs with high compaction over 95% because  it is difficult to achieve and we have not done it on regular basis. Also it will be possible to provide optimum designs without going to that level of compaction. I am also not in favour of 'Dumped' category. If the backfill is just dumped, the chances of settlement are high and the surface could cave in. I would suggest that slight compaction may be the minimum category. Generally the specifications for  back filling are strict even though it is not required. We should re look at specifications for the two categpories Slight compaction and moderate compaction. In moderate category  field observatios to categorise the soils, monitoring of proctor density etc should be strictly followed and all records maintained.