Subject II -Intermittent to Continuous supplies-Introduction

In many countries Intermittent supply for a couple of hours a day is the norm. Historical information on this subject for Indian towns is probably not easily available, but if available with any reader pl do share.
I have seen a paper re Delhi's water supply according to which Delhi had continuous supplies prior to 1960 and the situation kept on deteriorating after that.
Let us consider the main issues which determine whether we can give continuous supply and then the hours of supply that can be maintained in a particular system.

  Demand

Demand is defined as the quantity of water that meets the requirement and even though more water is available it is not consumed. The  demand of various types like domestic, commercial,industrial etc all vary for different communities for different reasons. Domestic demand depend upon habits of people, climatic conditions, water use facilities available,housing conditions,etc. While I was in Tata Consulting Engineers number of studies were carried out for different cities eg  Ahmedabad, Surat,Mumbai, Delhi, Chennai,  etc etc. An assessment was made of the water demand based on existing and projected housing conditions and the assessed demand in different categories of housing for a particular city. In general these figures were much higher than the supply figures recommended in the Manual on water supply published by Central Public Health Engineering Organisation (CPHEOO) of the Government of India. I am making a general statement and not giving the actual figures, as I do not have these reports now. I am sure some of my collegues or those currently in Tata Consulting Engineers (TCE) can produce an interesting data on demand projected by TCE and compare it with the norms of CPHEOO and publish under this blog.

One of the important components of demand is leakage. If we assess the percentage  leakage  at a point of time in Indian cities we  find that demand  far exceeds the availability of water. Naturally it will be desirable to bring down the leakage demand than to try and make effort to satisfy it.

Genesis of Intermittent Supply

Once it is evident that we do not have sufficient water to meet the demand, system cannot remain charged continuously for 24 hours as water at the source( outlet of treatment or reservoir at the Head works) will be finished and thereafter only air can be supplied. Therefore it becomes necessary to cut short the hours of supply.There are also issues related to the pressure in the system but we will take them up later.
At the moment it will suffice to conclude that if we cannot meet the demand we have to resort to intermittent supplies.  The leakage levels in many of the cities have been found to be of the order of 200 liters per connection per hour against a desirable limit of 10 liters per connection per hour. In fact it will not be out of place to mention that demand for leakage under 24 hour supply system, in Indian Cities in general , will be much higher than the requirement for all other purposes put together. Therefore till the overall demand can be reduced there is no possibility of giving continuous supply.

Stiffness considering embedment and native soil

The soil stiffness is based on the width of the trench, the embedment material and trench wall material, their densities or percent compaction. As explained in an earlier blog

E' = Sc . E'b
 Sc values can be obtained from the above table or the above graph or with the following formula

This enables us to obtain the value of E' the soil stiffness ,duly taking into account the native soil stiffness and embedment or backfilling soil stiffness.This is the most important factor for determining deflection of flexible pipes,particularly those which do not have a high pipe stiffness.
The important finding in the last decade is to consider stiffness of the soil in which the trench is dug and that of the soil in which the pipe is embedded.It is also clear that it may not be worthwhile to increase compaction beyond a point as it is expensive and it may be better to borrow and replace the soil..
We will consider these issues in detail after we deal with live loads and soil loads.
However by requests from many sources we are first moving to deal with the subject of how intermittent supply can be converted into  continuous supply.

Native soil Parameters

The table below has been copied from USBR and indicates native trench wall material properties. The link is give in the previous blog.

Material Type 1: Based on SPT, Cone Penetrometer Test (CPT), or in-place field density tests, the trench wall material has an E′n ≥ 2500. Minimum clearance between the pipe and the trench is all that is required.

Material Type 2: Based on SPT, CPT, or in-place field density tests, the trench wall material has an E′n ≥ 500 < 2500. The trench width will vary depending on the deflection requirements and the ratio of the stiffness between the embedment material and the trench wall material.

Material Type 3: When the trench walls are extremely soft (E′n < 500) and provide minimal pipe support.

When Material Type 3 soil conditions are identified prior to construction, design methods can be used to avoid a five pipe diameter wide trench. These methods are:
1. Specify only rigid pipe on an improved foundation.
 2. Increase the pipe stiffness factor (EI/r3 ) by increasing the wall thickness or increasing the thickness of mortar coatings, or both. Note: The haunch area still requires compaction.
3. Encase the pipe in reinforced concrete.
4. Completely remove Material Type 3 and replace with a suitable compacted material to a calculated trench width that will prevent excessive deflection.

E'n values can also be taken from the following:

Embedment Material Parameters

 we have copied below the table 5 from USBRs publication ' Methiod of Prediction of Flexible Pipe  Deflection' with the following link

https://www.usbr.gov/tsc/techreferences/mands/mands-pdfs/m-25report(pipedeflection)_rev08-05-15.pdf

Each E′ value in tables 5 and 6 is a typical value (i.e., for each category of soil type and classification, about one-half of the measured deflections were higher, and about one-half were lower, than the deflection predicted using the E′ value shown. To reduce the possibility that the actual deflection will exceed the predicted value, a design factor, Fd, is used.

Time-Lag Factor, Tf The time-lag factor, Tf, is used to calculate the increase in deflection of a flexible pipe with time. A flexible pipe continues to deflect over time for two reasons: Method for Prediction of Flexible Pipe Deflection  1. Increase in the soil load on the pipe. 2. Consolidation of the soil at the sides of the pipe. Table 5 gives values of Tf for use in the Reclamation Equation. The values are the ratio of long-term deflection to the initial deflection. The initial deflection is the deflection on the day the backfill was completed. For calculating initial deflections, a Tf value of 1.0 should be used

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.