6-step phased approach to re-open the non-essential sites in Scotland

The 21th of may the Scottish Government announced the 6-step phased approach to re-open the non-essential sites. The phases proposed are the following ones:

• Phase 0: Planning
• Phase 1: Covid-19 Pre-start Site prep
• Phase 2: “Soft start” to site works (only where physical distancing can be maintained)
• Phase 3: Steady state operation (only where physical distancing can be maintained)
• Phase 4: Steady state operation (where physical distancing can be maintained and/or with PPE use)
• Phase 5: Increasing density/productivity with experience

One week after, on May 29^th the Government announced that the no-essential works could start to move to Phase 1. Under this phase, contractors can begin to prepare the sites to “facilitate the physical distancing and enhanced hygiene, including Installing new or expanding existing site welfare and toilet facilities, “one way” systems and marking 2m distancing throughout the site”.

During the first week of June, several companies [1] [2] [3] have been adapting their sites to the next phase. However, even the health and safety requirements are satisfied, progressing to Phase 2 is far from being automatic. The government must be consulted before proceeding to the ‘soft start’ phase (Phase 2), and it must be in line with public health advice. Also, Phase 2 will not take place until a minimum of 2 weeks after the updated advice is published. After more than one week of site preparations (from the 29^thof May), such announces have not been published yet, so it is unlikely to move to the next phase in two weeks’ time.

Furthermore, most of the works will not be able to be carried out in Phase 2. The guide remarks that in Phase 2 “no element of non-essential works requiring the use of PPE above the standard industry use, will commence at this stage”. As I understand, the soft start permits only a limited number of people to work when physical distancing can be maintained, and no special coronavirus PPE is needed.

The details of phase 3 will be issued within the Phase 2 update. As well as the previous stage, in phase 3 only works than can ensure physical distancing without the need of PPE could be carried out. It is suggested that under this phase, only 30% or 40% of the original workforce will be able to return depending on the site parameters. The objective of this phase is monitoring a supervising the activity to reach a steady-state operation. Therefore, the interchange of information between the sites and the Government would be essential in this phase.

In Phase 4, PPE could be used to ensure physical distance, and consequently, more workforce will be able to return to the sites. This phase only will be possible if the supply of the PPE equipment is guaranteed. Around 10% of the original workforce is expected to return thanks to the use of PPE, which means 40-50% of workers on site.

Lastly, in Phase 5, the workforce will be increased progressively based on the experience gathered in the previous phases.

The guide sets a long journey for sites to return safely to work as we remember. According to the MSP Kevin Stewart, progress beyond these stages will clearly depend on a range of factors, including the industry’s ability to demonstrate fully compliant working practices in place, the confidence of the workforce and its trade union representatives in those arrangements, and wider supporting health data.

The following image summarizes the re-start model.

Construction Scotland re-start model

Non-essentials sites re-opening in Scotland

While construction firms in England, Wales and Northern Ireland have been “actively encouraged” to re-open the construction sites (with safety measures in place), the categorized as non-essential works in Scotland remain closed up to date. The Scottish Government considered necessary a 6-step phased approach to re-open the non-essential sites safely, a progressive approach that takes more time to re-open the sites.

The divergence among approaches lies in the top priority set by the Scottish Government: guarantee the safety and wellbeing of the construction workers and the country. The Government considers the construction sector and its supply chain a non-essential business sector, except when supporting an essential or strategic activity for the country as could be energy projects, maintenance operations or repairs. Consequently, the non-essential projects must adhere to a “slow and steady” guided re-opening process in order to contain the COVID-19 spread. However, how is the Government putting into practice this approach against the economic pressures for an immediate re-opening?

It must be said that the Scottish Government did not force by law to close the construction sites. The Government is relying on the goodwill of the industry to follow their guidance instead of imposing an instruction upon them. However, new health and safety requirements to deal with the COVID-19 in the workplace are obligatory and forced by law, and then, the police have the power to enforce these obligations.

Even the Government is not forcing to shut down non-essential sites; it is hard to image re-opening a site against the Government advice. The scenario is far to be appealing: supply and services shortage as the majority of the industry is shut down, bad press for contradicting the public advice and high health and safety risk assumed are only a few examples.

The Scottish Government position of not enforcing the site’s lockdown could open a debate about who is responsible for the delay and cost of shutdown. Even several public authorities are implementing measures to help the industry under these extraordinary circumstances (like the job retention and business interruption loan schemes), the government is not openly assuming the responsibility and then eluding a strict compromise.

According to the letter from the Kevin Stewart MSP on the 21st of May, the coronavirus construction guidance is a consensual decision between the stakeholders. The arrangement between the parts seems sensible; The Government agrees to help the industry to follow the guidelines and the industry accept to apply it. Then, in certain terms, the Government is accountable.

Another issue is the lack of resources of the public authorities to enforce the requirements on sites. I found quite intelligent to involve the industry in such a strategy instead of forcing them. If the industry as a whole takes an active part in the re-opening process, it is more likely that the companies embrace the guidance and its philosophy. Still, the Government is relying on the cooperation of the industry.

There is still a long way until we see the effectiveness of the approaches on both sides of the border. Under this approach, the Scottish Government has the challenge of keeping the commitment of the industry to re-open the non-essential sites as planned and, this commitment only will be possible if the financial support to the companies meets their needs and arrives on time. By now, the time is running against the Scottish side, but maybe the price paid is too high for those who went for a quick recovery.

Testing of gravity sewers

According to Scottish Water, in Scotland gravity sewers must be inspected and tested in two stages: as the work precedes and after backfilling sewer line. The checks requested in Sewers for Scotland (Scottish Water, 2018) follow the advice on testing requirements of BS EN 1610:2015 “Construction and Testing of Drains and Sewers”. Actually, the testing requirements for sewers for the UK are also based on the same British Standard, and consequently, the checking procedures are identical (Water UK, 2018).

The tests done after laying the pipes and before backfilling or concreting are done to ensure the correct assembly of joins and detecting possible workmanship and site handling damage.

Test performed after backfilling may guarantee that the sewers have not been compromised during the completion of the works and meet all the standards. Also, the testing process entails the contractor to clean the sewers before the handling.

Scottish Water in Sewers for Scotland V.4 (2018) requests the successful completion of the following tests/inspections for gravity sewers (foul and storm lines):

• Test as the work precedes:
• Air or water test (clauses 4.7.4. and 4.7.5. respectively). 
• Tests after backfilling: 
• Air test of the sewer line (between manholes) (Cause 4.7.4.) 
• Visual inspection: CCTV survey (Clause 4.7.6) 
• Infiltration (Clause 4.7.7) 
• Water-tightness (Clause 4.7.8) 

As follows, all the tests on the previous list are briefly described highlighting some aspects that help me to understand better these practices. I have not entered into detail, for further information is recommended to consult the official codes and guidance (references).

Tests as the work precedes

Air test for gravity sewers

The industry prefers the air test than the water test because it is less time demanding, more straight forward and avoids additional safety precautions needed when using the higher test pressures. Both of them serve the same purpose, and the acceptance of the sewers entails pass only one of them.

The test consists of blocking all the inlets of the sewer line tested and check if the pipes assembled could maintain a pressure of 100mm head of water pumped into the system for 5 minutes. The test is successful if the air pressure remains above 75 mm head of water after these 5 minutes.
 

There are two reasons why the test is done before backfilling: first, to be able to solve any deficiencies on time and second, because the test could be not valid due to possible water infiltration after backfilling.

A sewer is only accepted if the sewer passes the test. If not, the contractor should find and correct the issue and repeat the test. However, failure to pass this test is not conclusive; the test could give a fake fail. It is essential to ensure that the testing process is carried out correctly. Some reasons for a false test failure could be the following ones:

• Damaged equipment.
• Installation of the test equipment. For example leaks on the connections, valves not closed correctly or Inflatable Stoppers against the inside of the pipe not wholly sealing the pipe. 
• Insufficient time for stabilisation: Temperature and weather has a substantial effect on the test and stabilisation. In conditions of hot weather, due to the expansion of the air when heated, more stabilisation time could be required. On the other hand, testing below 1 Celsius degree could result in a significant loss of pressure during the test and the consequent test failure. In this case, more stabilisation time could be needed as well. 

Sewers shall be tested regularly throughout the installation before any concreting or backfilling. Sewers for adoption (Water UK, 2018) recommends every three jointed segments (clause E7.3.2), but it could be done every 3 or 5 pipes. In general, it is recommended to test all the line before constructing the manholes in the extremes.

Lastly, I would like to highlight that the test is more severe for short and small pipelines than pipelines long with large diameters. The air test requests the same pressure requirements (pressure above 75mm head of water after 5 min) regardless of the diameter and length of the pipeline. As a consequence, in a short run of small diameter pipes that contains small air volume, minor loses of air will drop the pressure quickly, and then, it is easy to fail the test. It is recommended to do the test every 4 or 5 pipes for small pipes diameters (>150mm) to increase the volume of air tested and avoid the over-severity of this test. 

Water test for gravity sewers

The water test has the same purpose and philosophy than the air test: testing the water-tightness capacity of the sewer by measuring the loses of fluid in a pressured set of pipes.

In this case, water is the fluid used instead of air to give pressure. The pipes should be filled with water until the pipeline contains no less than 1 m head of water over the soffit of the highest point. The pressure in the pipeline’s lowest point always must be lower than 5m head of water. In such cases the pressure is more than 5m head in the bottom of the line, the pipeline should be tested on several stages.

The water test takes long to stabilise before proceeding. The pipeline should be filled with water over an hour to allow the absorption of water by the pipe material. Once the system is wet, the water level should be restored. 

The test consists of measure the water needed to maintain the water level (and the pressure) above 1 m head of water in the top of the pipeline. During the test, the water level is restored every 5 minutes, writing down the litres of water added. If the loses in 30 min do not excess 0.5 litres/m3, the pipeline passes the test. 

This test is more tedious and time-consuming than the air test but takes into account the difference of diameter and length of the pipeline setting different parameter. Also, the leaks are easy to detect.

Tests after backfilling

Air test of the sewer line

After backfilling, the pipeline should be Air tested again from manhole to manhole. This last air test is to ensure that the pipeline has not been compromised during the backfilling works, especially when compacting.

Visual inspection

The visual inspection usually includes the use of a CCTV survey and a light-line inspection that use light rings to measure deformations. All the net should be completely clean to carry out the visual inspection. Usually, the net is cleaned with water pressure and dewatered before the inspection.

The CCTV survey film any infiltration on the system, so it supports the infiltration and Water-tightness verification.

Infiltration

The pipelines and manholes should be accepted as satisfactory if the infiltration, including the infiltration into manholes, does not exceed 0,2 litres/m2 in 30 min, where m2 refers to the total internal surface area of the pipeline and manholes.

To perform this test, all inlets of the system shall be effectively blocked and any residual flow should be deemed to be infiltration.

Water-tightness

No discernible water flow penetration into the manholes and chambers should be detected. This could be checked when conducting the visual inspection and infiltration tests.

References:

• Scottish Water, Sewers for Scotland – A technical specification for the design and construction of sewerage infrastructure, Version 4.0 – October 2018
• Water UK, Sewers for Adoption – A Design and Construction Guide for Developers, Eighth edition – August 2018. 
• BS EN 1610:2015, Construction and testing of drains and sewers, September 2015

Commenting on the hydraulic parameters of the sewage drainage pipes

In this post, I will analyse the reasons why the hydraulic parameters are demanded on the design codes for the underground drainage, either for the foul and storm sewage. In other words, I will look for the fundamental purpose of those hydraulic parameters. I will focus on Scottish Water design parameters, comparing its requirements with different recommendations, codes of practice and guides. However, I am not entering in the mathematical analysis; there is enough bibliography available.

The hydraulic design of sewers entails finding out the pipe section and gradient. I would remark two variables that determine the hydraulic design of the sewage: the design flow and the relation section-slope-velocity. The design flow is the primary input of the net. On the other hand, the slope, the section and the flow velocity are parameters that the engineer can play with to transport all the sewage to the discharge point.

Until the 80’ in Scotland, foul sewage generated from domestic and industrial uses were mixed with the surface water (rainfall water) into a single or combined pipe. However, environmental and flood management considerations demanded to separate out in two different systems the foul sewage from the surface water. Nowadays, it is not usual finding a combined sewer system, so here we will be focus only on separate sewage systems.

Source: Scottish Water (https://www.scottishwater.co.uk/)

Design flow estimation

Foul line

The total foul flows would be the total discharges produced by domestic and industrial uses. Hypothetically, the wastage flow consumption is roughly the same as the water consumptions. However, for industries, it could not be accurate if water is added to the final product. The peak hours of consumption will be similar as well as the discharges to the sewage. Also, when estimating the foul discharges to the drainage system, the potential growth of the city or industries in the area must be considerate. So, the quantity of flow depends on the rate of water supply (consumptions) and population growth.

In the codes of practices (BS EN 752, BS EN 12056-2 and Sewers for Scotland v4.0) provide chads with average values of discharge according to the source of water consumption.

Storm line.

When rainfall takes place a part of it infiltrates into the ground surface, and the remaining part flows over the ground surface. The part of rainwater flowing over the ground surface (or runoff) needs to be drained through the sewers; otherwise, the entire area would be flooded.

The quantity of stormwater (or rainwater) that will reach sewers depends on intensity and duration of rainfall, characteristics of the catchment or drainage area such as its shape, imperviousness, topography, and the time required for the flow to reach the sewer.

The intensity and the duration of rainfall are determined according to the rainfall history of the area. In Scotland, the standard considered is 1 in 30 years’ period event of rainfall (return period or recurrence interval). That means the drainage has to be capable to drain the maximum event (storm) that could happen every 30 years’ time (average time or an estimated average time between events such as high-intensity storms).

The 1 in 30-year rainfall level of service could be increased if the flood risk assessment of the area demands it. According to the SEPA, in Scotland checks shall be made for the 1-in-200 year return period considering allowances for climate change and potentially new urban developments to ensure that properties on and off site are protected against flooding for all these possible scenarios. The design of the site layout or the drainage system shall be modified where the required flood protection is not achieved.

The characteristics of the catchment area due to its capacity to prevent flooding problems have an important repercussion in urban design. As said before, the rainfall than infiltrates into the ground surface do not produce runoff, so, the total runoff would be higher in impermeable surfaces such as roads, house roofs or concrete sidewalks than in a forest or green areas. As a consequence, the new urban designs trend is to avoid extended impermeable areas locating more green spots and infiltration systems.

For estimating the stormwater flow for the design of sewers, the following two methods are commonly adopted:

• Rational method: this method is useful for large catchments areas.
• Empirical formulae: The use of the rational formula for estimating the stormwater flow for the design of sewers is usually limited to small catchment areas or drainage areas, say up to about 400 hectares.

Hydraulic requirements: relation gradient-velocity

The sewers design (foul and storm) usually tends to optimize the excavation volume adapting the gradient of the pipelines to the ground level. However, the pipes transport the water by gravity, so the pipes should be laid at a continuous gradient in the downward direction to the discharge point under hydraulic requirements. Therefore, the pipes’ gradient is limited; subsequently, the pipes cannot always adapt to the ground level. Sometimes it is impossible to avoid deep excavations, pump stations to lift the water or backdrops when the pipes cannot face a steep slope.

The sewage pipe hydraulic requirements are usually related to accommodating a finite volume of sewage under certain velocity limits, which are commented as follows.

The sewers contain considerable particles in suspension. The heavier particles could settle down at the bottom of the pipe if the flow velocity is low. The deposition of sediments could result in the blockage of the pipe. In order to avoid silting of sewers, the laid sewers need to have a gradient that ensures a flow velocity capable of cleaning the sediment at different possible discharges. Therefore, the hydraulic design looks to provide a flow velocity to generate an automatic self-cleansing effect.

On the other hand, the maximum velocity is limited to avoid erosion or scouring problems due to the friction between the pipe and the suspended solids in the flow. Also, high flow velocities damage junctions and potentially generate leaking problems. The non-scouring velocity depends on the material of the pipe.

As follows, I will comment on the limitations imposed on the storm and foul sewers:

Foul sewers

The design requirements for foul gravity according to Scottish Water are as follows:

Section 2C - 2.23. Hydraulic design – Foul sewers. Sewers for Scotland – A technical specification for the design and construction of sewerage infrastructure, 2018.

Surprisingly, Scottish Water do not consider any maximum velocity for foul sewers. It only mentions that energy dissipation and safety measures may be required without further guidance than the requirement of bedding arrangement for gradients steeper than 1 in 10. As an example of other guidances, the maximum velocity has a value between 3 and 6 m/s depending on the conditions and local authority in Spain.

Also, it is remarkable that no matters the material of the pipe, the roughness parameter of the pipe is set in 1,5 mm for the calculations. This criterion is based on extensive research into the behaviour of drains and sewers in service (BS EN 752:2017).

Lastly, in some norms ask the designers that sewers (storm and foul) should be designed to run at 75% of pipe full conditions (Water UK, Canal Isabel II), while I couldn’t find any mention on it in the Scottish Guide.

Storm sewers

The hydraulic design parameters for storm sewers according to Scottish Water are as follows:

Section 2B - 2.7.2. Sewers. Sewers for Scotland – A technical specification for the design and construction of sewerage infrastructure, 2018.

The guidance set the minimum velocity of 1 m/s at pipe-full flow for self-clearing reasons, with the possibility to be reduced to 0.3 m/s if the discharges received by the pipeline are free of sediments. Other ways to ensure the pipe self-clearing in storm sewers could be checking the minimum velocity is archived for a reduced return period. In Spain, it is usually checked that the minimum velocity is higher than 0.6 m/s for a 2-year event, so that, the drainage system is expected to be cleaned at least one time every two years.

As well as for the foul sewage, Scottish water set the hydraulic roughness value for all pipe materials (0.6 mm). As said before, that value is established based on extensive research into the behaviour of drains and sewers in service (BS EN 752:2017).

Also, like the foul sewage, it is required by several norms that the drainage system should be designed to run at 75% of pipe at full flow conditions (Water UK, Canal Isabel II). In this case, the 1 in 30-year rainfall should not overcome the 75% of the pipe capacity.

References and useful related information:

• Scottish Water, Sewers for Scotland – A technical specification for the design and construction of sewerage infrastructure, Version 4.0 – October 2018.
• Water UK, Sewers for Adoption – A Design and Construction Guide for Developers, Eighth edition – August 2018.
• Sustainable Urban Drainage Scottish Working Party (SUDSWP), Water Assessment and Drainage Assessment Guide, 2016
• CIRIA, the SuDS Manual C753, London 2015
• Canal de Isabel II, Normas para redes de saneaminto – Version 2, 2016
• BS EN 752:2017, Drain and sewer systems outside buildings. Sewer system management, March 2018

Setting-out pipes

When laying pipes, it is critical to ensure the right position since the beginning because a small error in the gradient, level and direction could be unacceptable at the end of the line pipe.

Setting out is time-consuming, but it is common knowledge that site engineers are often rushed when setting out to give all the information to the squad to start laying pipes and run to the next job. Quality and safety should not be compromised for the program, but it is widely known that in construction, the productivity is in everyone's minds, time is money. Here I will try to compare several methods of setting out pipes analysing the pros and cons to meet the three goals mentioned previously: time, quality and safety. For this kind of task, the cost difference between methodologies is predominantly determined by the time needed to perform it. Therefore, the pure cost analysis has been overlooked.

Traditionally, the pipes have been set out placing pegs (position), and profiles (highs) along the pipeline with an offset from the centreline of the line (around 3m). The profiles and the pegs describe a reference line needed by the squad to start digging the track and laying pipes.

Layout, profile and section of the traditional setting out method (BM Sadgrove, 2007)

The availability of beam or line laser levels on sites gave more control, precision and productivity to these jobs. When laying pipes, this device creates a laser beam that gives a straight line at whatever slope needed that the squad uses as a reference. So once the laser is set, the construction process is monitored continuously increasing the accuracy and productivity of this task. The line laser levels, as expected, substituted the traditional straight line and also generated new methodologies. Then, the issue with this new technology is the method to put in place the laser at the start of the line with the right gradient, level and direction.

Pipe laser positioning (BM Sadgrove, 2007)

The laser could be set using the profiles and pegs of the traditional method. However, the information given by the profiles has to be transferred to the bottom of the excavation (usually 2-4 m depth and an offset of 3m), losing precision. It entails the acceptance of wide margins of tolerance. The increasing demand for high-quality standards implied that a site engineer (with a total station) join the squad to set the laser on the bottom of the track. Notice that the sitting out process, in this case, is a critical task in terms of time because the squad cannot start laying pipes until the laser is set. Also, it demands the presence of a site engineer when required.

From my professional experience, I have been applying two methods which I describe and compare next.

1. Setting out the first concrete pipe

The first method consists of setting out the first pipe. Being the first pipe on position and buried to ensure it doesn't move, the laser can be placed inside of the pipe, giving the required baseline. So, setting out the pipe is key, which is tedious, especially with large and heavy concrete pipes. Also with this method, the direction of the pipe could be compromised because the two control points are at the beginning and the end of the pipe which usually is only 2.5-3 meters long for a full pipe or 0,6-1m long when setting out the stub and rocket.

The first step for this method is to level the gravel bed where the pipe will lay. It is a good practice to level the bed a bit high (around 10mm) because when placing the pipe, this margin probably is lost (especially with heavy concrete pipes). Next, the pipe has to be positioned in the right direction, checking that the two extremes of the pipe are inside the reference line. The following step is to check the levels in both ends to make sure that the pipe has the right gradient on the correct elevation. If the pipe is low, the gravel bed has to be risen, going back to the first step. If the pipe is high, the pipe could be "shacked” or pushed down until meeting the desired position. It is an interactive process; it could be the possibility after levelling the pipe that the direction has been compromised, which means repeating some of the previous steps.

After the first pipe is on place, the laser is placed inside the pipe with the desired slope programmed. The end of the pipe could be taken as a reference point to point the laser in the right direction. Still, it is highly recommendable that the site engineer goes as far as possible along the reference line to give another point to ensure the direction of the laser beam. Despite being accurate laying the first pipe, it is recommended to double-check the line after 3 or 4 pipes.

In terms of security, sometimes when checking the position and highs of the control points the site engineer hast to stand on the pipe with the measurement tools (prism and controller), which can be risky when laying big diameter pipes.

2. Setting out the laser in a concrete block

In this method, the laser is placed on a concrete block at the beginning of the line, usually in the centre of the manhole. As well as the previous method, the site engineer with a total station will join the squad to set the laser.

First, the concert block must be positioned. The top of the concrete block (or log) has to be on the invert level and it roughly needs to be in the right direction. The direction accuracy is archived when positioning the laser on the block. The second step is to set the laser on the block with the gradient desired. Next, the site engineer will give a reference point on the block and another reference along the line to set the direction. In order to minimise errors, it is recommended to provide the second reference point as far as possible. It is also recommendable doing periodic checks along the line. Once the first pipe is on place, the laser could be paced inside the pipe, and the concrete block retired.

Site engineer setting out the laser on a concrete block

Conclusion

After putting into practice these two methods, setting out a concrete block has been proved to be less tedious and less time-consuming than setting out the first pipe. Additionally, as a consequence of this time optimisation, the site engineer has more time to do other tasks.

In terms of quality, the concrete block method is more simple and straightforward than the rest of the methods described. It offers more accuracy, mainly because the process needs fewer adjustments. It is essential to mention that the site engineer with this method has more control over the process. He knows exactly the level of the block where the laser is placed, so the gradient could be adjusted to meet the endpoint at the right level.

Despite setting the laser on a concrete block seems to be the proper method to lay pipes, setting out the first pipe could be the only solution in narrow places without space to place the concrete block.

The main drawback of the two methods exposed is that require a site engineer with a total station when the squad needs to start laying a pipeline. In contrast, the traditional approach the setting out could be independent of the task of laying pipes.

References:
- BM Sadgrove (2007). Setting-out procedures for the modern built environment.
 (C709) CIRIA, London.