Author

Steve Peregrine BEng Hons, Senior Technical Manager

Page Contents

Introduction

This advice note intends to describe how and why CCTV drainage inspections are graded in terms of their hydraulic performance and their physical condition.

This information is commonly used to ascertain suitable rehabilitation procedures for pipes and to prioritise commercial workflows.

The user should always consider the grading information produced by software applications such as WinCan to be a guide which is often auto-calculated according to a defined standard and structure, but at the same time the accuracy of the results can only be as good as the input data from the surveyor.  Good data into the inspection report at the front end will lead to good grading results.

Documentation

CCTV drainage inspection criteria in the UK is described in the MSCC5 (Manual of Sewer Condition Classification, 5^thEdition, WRC) for detailed inspections which also has a simplified subset of details for domestic inspections.  The domestic inspection parameters are also reflected in the DRB4 (Drain Repair Book, 4^th Edition, WRC).

The MSCC5 (similarly the historical MSCC4 and MSCC3 editions) does not detail any values or methods for calculating grading scores in pipe inspections.  This information is contained in the documentation of the SRM (Sewer Risk Management, 5^th edition) service as provided online by the WRC (Water Research Centre), available online at srm.wrcplc.co.uk

The similarity between the edition numbering of the MSCC and the SRM is nothing more than a coincidence, but it is fair to say that both documents have grown thrown various iterations and continue to do so.

Within the WinCan VX software application in the UK and associated countries, MSCC3 and MSCC4 inspections are scored according to the SRM4, while MSCC5 inspections are scored according to the SRM5.

MSCC4 and MSCC5 domestic style inspections in WinCan are scored in accordance with the DRB4.

For the avoidance of doubt the SRM calculations cannot be used for condition grading with MSCC4 or MSCC5 domestic coding catalogues.  These code sets are simplified with good reason and many of the assessment parameters required for the more complex scoring have been removed, rendering the calculations almost impossible to make.  Only the manual DRB scoring can be applied to these standards.

The final and very-much specialised scoring system to be considered is that included in DDMS (Drainage Data Management System) inspection standards for highways and motorways.  The values and calculations included in these documents are similar to those described by the SRM, but definitely not exactly the same.  The details of these scoring systems are not described here.

Assessment Criteria

The SRM describes grading values of 1 – 5 for both observations and inspections where 1 is excellent condition and 5 is the most severe deterioration.  With these grades, there are two subsets of criteria based on the service hydraulic performance of the pipe (how well it works) and the structural integrity of the pipe (its physical condition).

Within the MSCC, there are 4 categories of observations:

• Construction – these codes describe the physical attributes of the pipe and are not scored.

• Structural – these codes describe physical defects in the wall of the pipe and are scored.

• Service – these codes highlight the performance of the pipe and are usually parametrised according to a ‘loss of cross-sectional area’, and these are scored.

• Miscellaneous – any other codes which are not scored.

On the whole (not exclusively), structural observation codes will have a structural grade and the grade value is determined by a defined score in the SRM.  Similarly, with the service codes except that the service scores are prioritised according to the loss of cross sectional area, so a 5% silt level observation will not grade as heavily as a 30% silt level.

So, it is perfectly reasonable for a pipe to be graded as SER 1 and STR 4 (Service grade 1 and Structural grade 4).  These results describe a pipe that is in poor condition but is flowing just fine.  In real terms, this would be a pipe that has significant structural defects but is still open and not causing blockages.

By contrast, we can have a pipe that is SER 4 and STR 1 and these figures describe a pipe that is in great condition but doesn’t work.  This would be a pipe that has little or no structural defects but is blocked with heavy silt deposits or significant root intrusion as examples.

The SRM calculation methods are clearly defined based on several parameters and can be auto-calculated by software applications to output the final grades.  However, there is no such auto-calculation for the DRB grading system.  The system described in the DRB is completely subjective and at the opinion of the inspecting engineer.  This system uses a ‘traffic light’ (A, B or C) list to describe the structural condition of the pipe (A = good, C = poor) and a ‘Serviceable’ or ‘Not Serviceable’ approach to the hydraulic performance of the pipe.  There are clear definitions of how these should be measured and qualified in the DRB including photographs of examples.

For the purposes of this document, only the SRM grading system shall be described from here onwards.

Calculations

Behind the 1 - 5 grades, there are score values for the applicable observations which are compiled to offer the simple yet informative grade at the end of the inspection.  The values of the scores are not important here, but suffice it to say that individual structural scores range from 1 – 165 and individual service scores range from 1 – 20 and further information on these is available in the SRM.

The service scores are common across all types of pipe, but there are different structural scores for brick/masonry sewers and piped sewers.  The WRC drain coding standard and output file formats only allow for a single pipe material to be defined in the header details, so if a pipe is brick at the beginning of the inspection and is recorded in the header details as being brick, but then changes to concrete half way down (as described by a change of material observation code), the entire pipe will be scored according to the brick scoring rules.

For structural scoring calculations, the length of the pipe is divided up into equal 0.1m segments starting at the nearest starting chainage (should be zero metres in a good inspection) and then the scores for all the qualifying observations within each 0.1m segment are summed together.  The final structural grade of the pipe is based on the 0.1m segment that has the highest total structural score.

For service scoring calculations, a similar calculation is taken as for structural, except that that in addition, the mean score for all the 0.1m segments is also calculated as well as the peak score, and the final service grade of the pipe is based on the mean or peak value that gives the worst result.  Experience and history have shown us that it is extremely rare for the mean score to trump the peak score in service calculations.

For both structural and service scoring calculations, additional information is also calculated regarding the scored length of pipe and the number of qualifying defects within that length.

In addition to all of the above calculation information, we also consider continuous defects:

• Repeating continuous defects are defined as defects that repeat at at least 3 out of 4 of all the joints within the continuous length.  With this in mind, it is extremely important to define the pipe joint length for the sewer in the section header information and this field is often omitted by operators.  If the field is left blank, then WinCan assumes a length of 1m for calculation purposes.  Imagine a continuous defect that is 10m long and the joint length is 1m – there can be 10 or 11 joints within that length, so the defect will be scored 10 or 11 times.  Now imagine the same 10m continuous defect is in a pipe with 3m joint lengths – now the defect is only scored 3 or 4 times.  Naturally, there can be no repeating continuous defects in brick or masonry sewers since they have no joints.

• Truly continuous defects are defined as defects that extend for more than 1m in length.  In this case, the scores are calculated for each whole metre or part metre within the continuous defect length, and each 0.1m segment within that length is assigned the score value for the defect.

So, continuous defects have a significant effect on the total scores for inspections and the final grades that are output.

Within the WinCan software family in the UK, the user is not normally able to see the observation scores in the user interface, and only has access to the final 1 – 5 grades.

Inspections Vs Observations

Having divided the scores and grades into service and structural camps, we also divide the 1 – 5 grades into observation level and inspection level results.  According to the SRM4, each qualifying observation can have either a structural or service grade, but not both.  In the SRM5, there are a small number of observation codes that have both a structural grade and a service grade.  These are described as structural defects that have a service impact, so the primary score is set against the structural defect.  Codes affected by dual scores in SRM5 include the deformation, crack and fracture groups.

Open

Where to locate the observation and inspection scoring data in WinCan VX.

At the end of the inspection, the final ‘overall’ grades and scores are calculated for the pipe, so in the final analysis we have;

• individual service and structural grades, and

• pipe overall service and structural grades.

These two sets of data should always be considered on their own merits and can be extremely useful for recommending rehabilitation solutions.  For example:

•  A 100m sewer has little or no defects except for a hole (more than 3 clock positions) at 25m.  This single defect will give a STR 4 grade at the observation and will most likely make the entire pipe a STR 4.  Does this mean that the entire pipe needs to be rehabilitated?  Probably not, just a localised repair on the hole will suffice, and following the post repair inspection, the overall pipe will show a STR 1 or 2, so the rehab solution has successfully improved the condition of the entire length of the sewer.

• Another 100m sewer has minor continuous defects along its entire length such as maybe some continuous longitudinal cracks where an individual crack offers a STR 1 defect, but the continuous nature and the length of the cracks gives an overall grade for the pipe of a STR 4.  Does this mean that the entire pipe needs to be rehabilitated?  Probably, yes.

Interpretation of Results

There is no hard and fast rule regarding how the results should be interpreted since as has been stated already, the grading is offered as a guide only.  However, across the UK water industry and water engineering consultants, most are interested in grade 4 and 5 defects, with a smaller number considering grade 3 defects with a view to rehabilitation processes.

Many clients use their own consideration of the results based loosely on (as an example) ‘any sewer with a grade 3 or higher requires an engineering decision’.  What is involved in this ‘engineering decision’ is at the behest of the client, and maybe something that ranges from ‘we should inspect it again in 12 months’, to ‘we should dig it up and fix it tomorrow’.

It is a regular occurrence for the staff at WinCan’s office to be called by contractors and clients regarding the scoring outputs with questions relating to the severity of certain defect codes.  Usually, these questions come from domestic inspectors regarding displaced and open joints (usually graded as STR 1) in small pipes where they feel that the grade offered is not severe enough.

The interpretation of this comes down to a number of variables.  Taking the idea of a displaced joint as an example, there are a number of things at play here:

• The MSCC specifies that for a displaced joint to be classified as ‘medium’ severity, the user has to be able to see the entire thickness of the wall of the pipe, and anything more than 1.5 times the thickness of the wall of the pipe visible would grade the displacement as ‘large’.  Both medium and large displaced joints grade as STR 1.

• In a 100mm dia clay pipe, the wall thickness is approx. 15mm, so the dimension of the displacement has to be at least this amount before it even qualifies as a ‘medium displaced joint’, or 22.5mm before it qualifies as a ‘large displaced joint’.

• This displacement value can also be interpreted in a percentage of the diameter, so a 15mm displacement in a 100mm dia pipe is also a 15% displacement.

• The wall thickness of 100mm, 150mm and 225mm dia clay pipes does not vary very much. There is some increase as the pipes get larger, but not much.

• So, on a CCTV camera screen, a medium displaced joint in a 100mm dia pipe looks much more severe than a medium displaced joint in a 225mm pipe, and its effect on the hydraulic performance can be much greater in the smaller pipe than the larger pipe.

So, how can a JDM observation give a STR 1 in both a 100mm pipe and a 225mm pipe?  The answer is somewhat crude, but essentially the grading system must cater for all pipe sizes and there is no qualification of results based on the physical dimensions of the sewer.  So, the STR 1 for a JDM observation in 300mm dia pipe is perfectly agreeable, but unfortunately in a 100mm pipe it is often perceived to be incorrect.

At WinCan, we aim to apply the rules as they have been conceived and documented by the WRC, and we would encourage customers with questions regarding the values of grades and scores to accept that we are only applying the WRC rules, and we do not write the rules.

However, in all of this, there is a seed of hope for the small contractor and as always, good results come down to an excellent understanding of the various documents at play here:

Open

The WinCan Vx printed inspection report.

• Aside from the JDM and JDL codes for displaced joints, there is a third code JD where there is no M or L assigned to the observation.

• This code requires the user to enter measurement value of the amount of displacement in mm, but at the same time must only be used when the displacement is more than 10% of the diameter of sewer.

• So, going back to the ‘thickness of the wall visible’ displacement in a 100mm dia sewer where the wall thickness is approx. 15mm, this is 15% of the diameter, so the JD code is perfectly acceptable and legitimate in this case with a displacement value of 15mm.

• The JD code is graded as a STR 4 which much better reflects the contractor’s opinion that a STR 1 is insufficient for the defect that they can see on the screen.

As has been stated at the very beginning of this document, good data in at the front end will give good results at the back end.

Reporting

There are a number of ways that grading data can be represented as outputs from WinCan VX and WinCan v8.  These include tabulated .pdf reports and spreadsheet .xlsx files which describe just the grades (and scores if required) associated with observations and inspections.

The most common of these is the standard WinCan CCTV inspection report for MSCC coded reports.  In the attached example, a number of useful areas of the report have been highlighted:

• The observation text strings are colour coded by observation category (Construction, Structural, Service or Miscellaneous)^[1] and the colour key is printed at the bottom of the page.

• The calculated numbers from the scoring maths are printed in 10 boxes along the bottom of the page with the first 5 boxes offering structural values and the second 5 boxes giving the service values.  These are the overall scoring values for the pipe.

• On the right-hand side of the page are the individual grades for the observations entered by the operator.  Because this example is a MSCC5 report, the FL code has both a structural and a service grade and these are represented as ‘3 / 2’ in the print out.  Where there are dual scores, the structural grade always comes first.

• Where continuous defects are entered, the grade is not calculated until the continuous defect is closed because the software needs to know how long the defect is before it can calculate the score and the grade.  Hence, there is no grade on the FL start code.

• The inspection Comments and Recommendations entries from the inspector should reflect the grades calculated in the report (inappropriately completed in this example).

As has been mentioned previously, good recommendations will always reflect the grades that have been generated by the software.  An example of a poor recommendation might be where a pipe has been graded as STR 1 and SER 3 and the contractor has recommended that this pipe be lined.  The data does not support this recommendation because the pipe is a STR 1 and is in ‘perfect’ structural condition, so why should it be lined?  The data actually suggests that this pipe should be cleaned, not lined.  Of course, after it has been cleaned and inspected again, there maybe additional structural observations that some to light which might support a lining recommendation, but that is something that only a future inspection and grading can highlight.

Grade Value Examples

The following are taken from Network Rail’s Specification for CCTV Inspection of Drainage as written by Mott MacDonald in 2013 and is considered by the author to be an easy-to-understand version of how the 1 – 5 grades should be considered by the report reader.  There are of course different variants of this type of list including some from the SRM, but they are all intend to state much the same emphasis:

Life Cycle of Grades

Any sewer can be inspected any number of times over any length of time, so the current graded status of a sewer is that when it was last inspected, regardless of how long ago that was.

However, sewers are dynamic and always changing their operational capacity and structural condition, so care should be taken when analysing CCTV graded inspection reports that are ‘old’.  How ‘old’ is another matter of opinion, but if there is any shadow of a doubt, the sensible option is to simply inspect the pipe(s) again and look at the most recent data.

Furthermore, as has been mentioned already, drainage observation coding is subjective by the inspector, and no two inspectors are exactly the same, but clients are encouraged to ascertain the competency level of their contractors and their operators in accordance with the relevant water industry training and standards.

WinCan v7 Scoring

A historical note on WinCan v7 grading - WinCan v7 included 2 versions of grading.  The visible observation grades were 'Hardwired' following the industry standard 'Examiner' software authored by Amtec Surveying.  This grading is obsolete & is not explained by this document.  SRM grading was also available as a separate report through the report generator in WinCan v7.

References

• Manual of Sewer Condition Classification, various editions, WRC.

• Drain Repair Book, 4^th Edition, WRC.

• Sewer Risk Management, various editions, WRC.

• Specification for CCTV Inspection of Drainage, Issue 2.0, Network Rail.

^[1] This is an option in the printing panel where the user can select category colours (used in this example), graded colours (each grade 1-5 has a different colour) or no colours (all text is black).

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