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.

Why this blog?

          The main purpose of this blog is to share the knowledge acquired  by me in the  last fifty years, working in the field of water. While the blog is intended for engineers and engineering managers, We will from time to time address issues of significance for public policy, administration , trainers and others. My focus has been  in distribution systems and would like to start with Pipes and Leakage.

           Pipe Engineering has been evolving over time and considerable changes in concepts have taken place and will continue to take place . Pipe soil interaction is now better understood and engineers are willing to consider support from soils in a big way.  Soil stiffness (modulus) is being recognised as the most important factor in pipe soil performance. In developed countries, the cost of compaction of soil to increase modulus is being compared to the cost of increasing thickness of pipes.The corrosivity of soils has  always been  seen as an important aspect for  metal and concrete pipes. It is necessary that the regulatory authorities recognise these facts and lay guidelines to achieve economy and optimization. I will attempt in the next few blogs to deal with this aspect.
          Different countries have different loading conditions for designs of Highways and bridges, which are used for  assessing  the super imposed loads on pipes. In India the governing Code is published by the Indian Road Congress  and Number of different classes of load trains are specified. Unfortunately there is no guidance in any code re what class of load should be used for what type of road. Also although the national highways does not permit any pipes or services within the carriageway there is a tendency to design the pipes for highest class of load. Is it necessary or desirable? Also We will make some suggestions for using specific class of loads for different type of roads, which could eventually be put into codes or manuals. I would also attempt to standardise the process for load computation. This should lead to optimum engineering design of pipes of different materials.
          Air in pipelines has played havoc, when not dealt with properly, both in terms of reduced quantity of flow and additional pressures exerted. I will like to discuss it with some cases.
          Thrust in pipelines due to change in direction is dealt with by thrust blocks or through pipes with restraint joints. Again there is the question of what support one can get from the soil? Tendency to not assume any support is unjustified. We will look into this aspect.
         
          One of the biggest challenge faced all over the world is to keep unaccounted water/ leakage in water systems to a reasonable level. Those ,who give continuous supply  cannot do so if leakages are not controlled. Leakages in Indian Cities and towns are being controlled by restricting the hours of supply and controlling the pressures. If we wish to have continuous supplies with reasonable pressure we have to do something about leakages. Unlike many countries where savings from reduction of leakage can pay for the effort to reduce it, in India savings are too meager  as cost of water is heavily subsidized. The methodology for leakage identification in intermittent supplies has to be different than for continuous supplies, as has been demonstrated. However Practitioners with continuous supply background  and their methodologies have been making India a guinea pig  for last half a century.  We will discuss the approach and methodologies we can adopt.We will make recommendation for public policy in this regard.


            This is an overview to start with. We will detail out more topics as we proceed, possibly with the interaction of the readers.I will request all who read this blog to share their experiences, success stories and failures which all teach us to do things the right way . I will also like to invite my friend, colleagues and professionals to feel free to contribute to this blog to create better understanding amongst practitioners in general , to improve our national and state policies and also improve codes, manuals and practices.