Holdfast Training Services Trial Blog

Design process

Background

The method of getting sewerage from the current outfall point in the Thames into the new Thames Tideway sewer is via a series of chambers and a 30m shaft. At the Putney site these chambers are located on a foreshore site. To facilitate the construction a cofferdam is required. 

This blog looks at the approach taken to decide how to size up some piles for design development.

Putney Embankment Foreshore – End state river wall is the rectangular box in the foreground.

Durability

Tideway (the client) require a 120 year design life for all structures. This is onerous but after Guz’s recent blog on his river travels I can see why there is pressure to prove everything meets the design life. 

Tender stage

Tender documents had the permanent cofferdam as a secant wall. To enable construction an outer vast temporary sheet piled cofferdam was proposed.

The blue dotted line is the extent of the tender document sheet pile wall. The orange box is the proposed site location.

The contractor’s tender submission was an optimised design by combining cofferdams to have a single thicker sheet piled solution which would satisfy temporary and permanent conditions.

Contractor’s tender submission – Sheet pile wall to act as a temporary and permanent wall.

The issue

Proving a sheet pile will last for 120 years is a bit tricky. The River Thames is fresh (ish) water but the fact sewerage is being discharged nearby creates a risk of microbial induced corrosion which erodes steel and makes arguing the case that all will be fine (as in only 1 or 2mm rather than 5 or 6mm difficult). Unfortunately the UK national annex is more onerous than the main eurocode, I.e. assumes more corrosion occurs. 

Plan B

The contractor is keen to use a cofferdam which works in both temporary and permanent cases so have asked the Arup-Atkins joint venture to look at tubular piled options. Steel with concrete encased inside. This can be split into 2 cases. 1. Steel is non-sacrificial and last 120 years and 2. It does not.

Tubular pile option – outline sketch only

How does the cofferdam get built?

1. Modelling the construction sequence for either case is the same:
2. Install cofferdam
3. Install top prop
4. Dewater
5. Excavate to formation level
6. Install base slab (acts as a bottom prop)
7. Wait 2 years while shaft is excavated
8. Move to drained conditions.
9. Re-fill cofferdam/construct internal chambers to underside of prop level, back to undrained conditions.
10. Remove temporary top prop.
11. Finish cofferdam/structures, apply surcharge
12. Excavate and place scour protection.
13. Allow for some worst case tidal and tidal lag.
14. Long term case – move to drained conditions.
15. Case 1 – steel remains, Case 2 steel does not.

600mm tube option shown below.

Stage 5-8 in stages above – excavation to full depth with base slab at drained conditions

Final stage with deflection of wall shown in red (105mm) and bending moment in green (1MN)

Stiffness

Understanding shear, bending and deflection throughout the sequence was key. Various pile diameters were tried but this was simply an adjustment of stiffness in the Frew model. EI based on section size, steel and concrete Young’s Modulus. 0.7EI and 0.5EI were used for temporary and permanent concrete strengths. The difference for the 120 year case comes at the final stage where only the section properties of a concrete section above bed level can be taken into account, i.e. a reduction is stiffness. More onerous is the reduction in moment capacity the section has without a steel pile included.

The results

After trying various tubular pile sizes I got to a 600mm pile for both cases above as worst moments were in temporary case (max excavation rather than maximum fill height), so loss of steel section was not an issue as final stage was still below moment capacity.

Unfortunately I made a stupid error in the fill material (somehow I assigned a granular fill a high cohesion value) within the cofferdam which meant I had lower bending moments in the long term than I should have. That was identified before presenting to the contractor so I only lost face internally.

The amended results showed that a 600mm diameter pile works in the non-sacrificial case and a 900mm diameter pile works in the sacrificial case (due to reduced moment capacity). 

End state deflections were somewhat high (100mm) but all of the modelling was done at DA1 combination 2 which is not the case at SLS. Also some of the water profiling is a little unrealistic (high level inside the cofferdam, very low tide outside). There is, however, some assumed stress lockins. For example early stages see the wall bend inwards, later stages see it pushed outwards. Difficult to know if this will actually occur so this is a probable case and greater deflections might be expected.

Cladding

The front face of the river wall has stone faced cladding panels which need to be fixed to the piled wall. The connection detail raises questions on durability – casting in stainless steel brackets works but if you touch a stainless steel bracket against a mild steel pile you get greater corrosion so some thought is needed to avoid that. Additionally, final deflections up to 100mm although equate to a small angle mean the top of the cladding is off-set further in order to backfill between the cladding panels and wall. This means the cladding load acts at a greater distance causing more deflection. It also makes it harder to pour something in between the piles and the cladding panels. 

Design meeting

After a week of looking at all the options the proposed solution from Arup-Atkins (us) was the 600mm pile thick enough to corrode and be acceptable in the long term. I.e. a section size that was acceptable in the long term and then a corrosion allowance added. The contractor is now going to price this option. Although more expensive than a sheet pile option, it is cheaper than doing the vast sheet pile cofferdam and there are significant savings in temporary and permanent works (fewer internal props and ties needed because of the stiffer piles).

There was sensible input from the contractor about ease of installing 600mm diameter piles versus 900mm as well as lots of points about presenting a solution which meets the client’s durability concerns.

Outcome

We wait and see what Tideway say – they may reject all options and say build it as per the tender documents. If this is the case then I wonder what the point of early contractor involvement is.

Lessons

The whole issue and reason for having to look at alternative options is because of durability concerns raised by the client. The optimised contractor solution was/is a sensible plan and on a pain/gain contract it is worth trying to make savings.

The best thing I see having gone once around the process is developing multiple options to give the contractor the choice of what to present to the client. This enables a solution to be put forward on technical merit.

The plan to get the tubular pile costed before presenting to the client presents risk (for the contractor) has failed because of how busy the commercial team is right now which might turn out to be an issue. 

I have learnt a lot about the importance of how something is to be done without the need to go into detailed calculations – certainly the case at concept design stage.

Temporary works jetty solution to build the new site. All by river approach demonstrated with barge removing spoil.

Following a site mtg we have a much clearer understanding of the issue at hand. The mech contractor, Envar, have sent us a mark-up (as requested) of the location and sizes of all holes made by the builder to initially increase air flow; which it has.

So why are we not getting the correct RA flow to the AHU? It also came to light that there is heavy congestion (as shown in fig 1) around a services (namely ducting) choke point (about 1 m²) which is restricting the RA flow. The congestion issue is unique to this level due to the balcony which a severely reduced the overall free area by approx. 1 m². So, in theory we could turn the closest partition wall into a sieve and we would still have the problem. We can’t practically do this though as there isn’t enough space in this particular partition.

Therefore, we still need to get the balance of air from somewhere which the AHU fan is drawing in. So with the choke point in mind this will have to come from the SA side (the corridor) outside the plantroom. This corridor is open to the large open-plan office area but air flow either end is stopped by pneumatic actuated doors (for security as these areas are sub-tenanted). So what? The makeup air would be mostly SA and not RA as intended. So how does this affect the CO² levels?

Below I’ve recalculated the area of hole required which would be incorporate as either a door grille in the plantroom door, or, depending on size, it could turn out to be a complete louvered door.

Figure 1. Return Air Choke Point in Plenum.

Calculations

A quick calculation to check the hole size required:

Q = 8.839 m³/s. This is the entire SA flow rate that is trying to get back to the AHU as RA.

A = 1 m². The approx. area of free flow through the choke point.

Therefore, V = 8.8 m/s. That’s pretty quick and why it has also been reported that a few ceiling titles are flapping around near the exit hole from plenum into plantroom. These are being addressed by replacing with egg crate extract grilles.

If we want to achieve a steady flow rate of 5 m/s then this gives an A = 1.77 m². Subtract from that the area we do have (1 m²) = 0.77 m² hole required.

On a basic 2100 x 920mm door (leaving an edge of 100mm both sides) gives a door grille neck dimension requirement of 1100 x 720mm = 0.79 m². That’s the ‘free air’ area needed where a louver or grille would require being 50 – 60% bigger to take into account the area lost due to the vanes. So, it would need to be 1.26m², which is about 1800 x 720 = 1.29 m² which equates to pretty much the entire door when you take into account the structural edges.

So what? It would require attenuation on the plant side of the door to mitigate plant noises transferring back into the corridor, which shouldn’t be heard too much anyway as it is just a corridor and the open-plan office area is actually through a central kitchen/restroom area.

To check this I spoke to our acoustic engineer and used the dB data. The corridor was reading 52 dB, I then added 25 dB for removal of the current door and then subtracted 5 dB for adding a louvered door with some attenuation on the reverse side. This results in 72 dB being heard in the corridor, which is about 22 dB over what it should be (50 dB being the norm). Therefore, this is a problem and exacerbated by the frequency range of the particular plant, which will be a low rumbling (low freq) which louvers still let through.

It would therefore be better to install an ATD above the plantroom door but this idea was discarded due to a large duct run right next to it running down the length of the corridor. This also means that we can’t put any more egg crate extract grilles in as the one currently installed sits no more than 10mm below the ductwork so this too will be restricting the RA.

The only other option I can see is putting in a large or two smaller ATDs through the wall area into the plantroom. But even this has its issues: is there free space on the other side of the wall? How far away is the AHU and can we fit a ATD in? Being the base build, will the building owner approve it?

I have also been informed the AHU has an economy cycle (which I should have expected) where the ambient OA temp dictates how much OA is used rather than wasting energy in cooling RA when there’s ‘free’ cool air available from outside.

ATDs

I made a point of mentioning the need for ATDs in the cut-outs. With the extra holes present no occupants have complained of cross-talk, therefore Envar suggested they would get the builder to tidy-up the holes and leave them at that. Ironically, it seems like any holes made in the partition wall don’t need attenuating due to the ductwork congestion.

Introduction

Prior to Christmas I was given an interim task to investigate complaints of doors being hard to close with loud whistling noises coming from the plantroom door on level 4. The project is a typical tenancy office fitout with construction complete and the office space occupied by Synergy staff.

I was given an initial steer by the project leader; his view being restricted Return Air (RA) flow to the AHU.

Background

The design of the floor (slab to slab) uses the ceiling void as a large RA plenum. The plantroom, containing the AHU, acts as a large mixing chamber mixing the RA (from the plenum) with the Outside Air (OA), which enters through louvers in the skin of the building. It incorporates a CO² monitoring system that controls the motorised louvers to alter the amount of OA required to keep CO² levels within permissible limits.

The main floor space is split up in to a number of offices (various sizes) and larger open plan areas. To reduce the chance of cross-talk between these areas full-height partition walls were constructed. Whilst this met the acoustic requirements it created a subsequent problem; the RA was being severely restricted in its attempt to get back to the AHU.

To resolve this, the design incorporated the use of Air Transfer Ducts (ATDs) installed through the partition wall. ATDs are essentially fitted into a rectangular cut-out in the wall and consists of standard metal ductwork lined with acoustic material. This allows both air flow through it and simultaneously reduces carried sound, as shown in figures 1 and 2.

Figure 1. Basic ATD with Acoustic Lining.

Figure 2. ATD in Use.

Identified Issue

A typical problem found in RA systems, that utilise a plenum rather than a ducted system, is not enough RA finding its way back to the AHU. In this case the number of installed ATDs is insufficient to allow the correct amount of return flow and is creating positive pressure build-up. Effectively, the mixing chamber (entire plantroom) is being starved of its required RA (under negative pressure) and the CO² monitoring system, not seeing excessive CO² levels, won’t allow any more OA in. Therefore, the only available air remaining is from outside the plantroom in the corridor (which is under positive pressure) and is now being sucked (willingly) into the plantroom and creating excessive whistling noises. Figure 3 shows a basic sketch I drew to aid in visualising the situation.

Figure 3. Situation Hand Sketch.

What’s possibly exacerbating the issue is the CO² monitoring system. Because the RA is being restricted the CO² monitor is most likely reading low CO² levels, therefore signalling to reduce the OA intake as it’s not required to dilute the RA. This then creates increased negative pressure inside the plantroom and thus makes it easier to pull in air from the corridor due to the AHU fan demand. It then become a vicious circle as this extra air being pulled in from the corridor is most likely to come directly from the SA grilles, with little human traffic, meaning the CO² levels will low.

Depending on how much the RA is restricted will determine the severity of the symptoms found.

During construction there were a number of design changes, doors being relocated and the like, which caused the air flows to change slightly. This however, is not deemed a significant reason for the symptoms being experienced.

Resolution

The resolution quite simply requires increasing the number of air passages through the full-height walls. As stated this had already been designed for and implemented through the use of 5 No. ATDs, however, clearly more are required. Figure 4 shows the drawing mark-up with existing ATDs (blue) and my proposed design for additional ATDs (pink).

Previous Resolution Attempts

The mech contractor had already attempted to solve the issue by cutting holes in certain locations within the full-height walls. While this may have helped to alleviate the issue, it has not solved it.

Figure 4. CAD Drawing Mark-up.

Design Calculations

I conducted some basic calculations to determine the number of ATDs required, allowing the same rate of return air flow as that being supplied to the floor space, resulting in a more favourable neutral pressure.

The calculations included splitting the floor space into sections, boundaries based on the full-height partitions, where I then added together the SA from each diffuser in that section. I then worked out the area required (using Q = A x V based on a 5 m/s air velocity) to get that flow rate through the wall to the next section. I continued the same calculations for the other sections, remembering to carry through the flow rate from the previous section as the RA from the entire plenum space is trying to get back to the same point (left to right when viewing the dwg).

In total I calculated an additional 9 No. ATD2’s (600mm x 400mm) were required. In truth it’s only the cut-out dimensions that are actually required to solve the air flow issue, not the ATDs per se.

Design Considerations

There are important factors in positioning ATDs within a plenum: availability of space between the upper floor slab and false ceiling, space between other services, and understanding the use of the rooms either side of the full-height wall to name the most important. All these were considered in my design. The last consideration, the use of rooms, is particularly important as this could determine the possibility of negating the need for ATDs in a particular location altogether. That is not to say there wouldn’t be a cut-out, just no ATD installed, so still allowing the required passage of RA.

COAs

Two COAs were considered, with time, cost and quality in mind. However, before the suggested COA is carried out a number of RFIs must be answered by the mech contractor, these are: confirmation of the location and size of previously made cut-outs, and an estimate on the availability of space to install the ATDs. The location and size of previous cut-outs is important as these could be utilised and potentially increased, based on the calculated sizes above, as suggested in the COAs below.

COA 1

This would be conducted in two stages, both during out of hours so as not to disrupt staff working:

Stage 1 – Make the cut-outs in the full-height partition walls as indicated on the mark-up drawings.  This should solve the issue, however may lead to excessive cross-talk.  If the room pressures have been restored, the symptoms disappeared and there is no detection of cross-talk, or it is at an acceptable level, then the works will be complete.

Stage 2 – Assess the level of cross-talk, if it is not acceptable then conduct stage 3.

Stage 3 – Install the ATDs in the designated cut-outs, thus solving the cross-talk issue.

Pros and Cons

The main pro is: If stage 1 is sufficient there will be a substantial cost saving on negating the need for possibly all or at least some of the ATDs; each approx $1000 a piece depending on size.

The con is: If stage 2 is required then there would be extra labour costs due to the mech sub-contractors having to go back a second time to install ATDs.

COA 2

Do as COA 1 but all stages in one go.

Pros and Cons

The main pros are: Reduced labour costs for the mech sub-contractor and all the work being done in one go means minimal disruption.

The con is: The ATDs may be superfluous in solving the issue and therefore present an unnecessary cost.

Noise Level Consultation

I discussed the issue with our acoustic engineer and am confident that the overall cheapest option, that still achieves the required acoustic quality levels, is COA 1. This is based on not all the positions identified requiring ATDs. The exact number cannot be confirmed until a further assessment is conducted post stage 1 of COA 1.

Recommended Solution

The recommend solution is COA 1 but I envisage having to discuss these actions with the mech contractor, as alluded to, to get answers to my RFIs. This may also involve going on site to discuss further.

Update

During the preparation of this blog I learnt, through a third party, that the mech contractor indicated it wouldn’t be possible to fit the ATDs as proposed due to lack of space in the partition wall around the plantroom. There were some other suggestions mentioned, such as, installing a large transfer grille in the corridor/plantroom wall but this would need to be quite large to solve the issue and would likely look pretty rubbish not to mention noisy. It would then require an attenuator on the plantroom side to attenuate the plant noise. I think the next step is to meet with the mech contractor to discuss options.

What did I learn?

A key observation, which extends to all projects, has been that no matter who you ask for advice, you will always get a slightly different answer. The key principles will be the same but individual engineers’ will be basing their ideas on their accrued experience. I suppose this highlights the meaning of engineering, derived from the Latin ingenium, meaning ‘cleverness’ and ingeniare, meaning ‘to contrive or devise’, and just goes to show there is more than one way to skin a cat. The challenging part is to learn how and why these little design nuances can aid in delivering a more technically, sustainable and economical solution and therefore aid in refining your design skills. The technical solution also needs to be balanced with any political considerations, especially when wishing to win any future work with the same client.

I also learnt about the disadvantages of plenum based return air systems and found this article helpful in understanding:

http://www.google.com.au/url?sa=t&rct=j&q=&esrc=s&source=web&cd=3&cad=rja&uact=8&ved=0ahUKEwjxgbbF9NzKAhVELg8KHbM0CyEQFggsMAI&url=http%3A%2F%2Fbookstore.ashrae.biz%2Fjournal%2Fdownload.php%3Ffile%3D2015Mar_044-047_EngNote_Taylor.pdf&usg=AFQjCNFMBmRONuxPkHoOOQXtChymqc4RJA

In Other News

I organised a surprise birthday present for the wife; a great view of Perth from 1500ft. It included a shot of JHG’s PCH project and the new sports stadium, managing contractor being Brookfield Multiplex (new alternate Ph2 attachment). I’ve also included a few other snaps…

Perth Children’s Hospital.

Perth’s New Sports Stadium – Still in the structural build stage.

Perth’s Central Business District.

WA Coast Line.

Happy Wife Happy Life!

Original article written for Sapper Mag:

On Sun 31 Jan 16 members of the Professional Engineer Training (PET) course embarked on the ultimate test of civilian toughness – Tough Guy 2016. Four elite members of the cohort were selected to compete; Captains Grant, Kiddie, Palmer and Parton had trained for literally hours and were in peak physical and mental condition as they journeyed over to the exotic lands of Wolverhampton for the event.

For those unfamiliar with the Tough Guy Concept the event calls for competitors to cover 15km of cross-country running interlaced with wet and muddy obstacles and hill sprints. The theme for this year was “The Somme” with the course culminating in an obstacle course named the ‘Killing Fields’ with electric fences, burning hay, concrete tunnels, a lot of cold water and endless mud.

Morale remains high for Kiddie and Grant. 

The ‘Toughened’ PET team celebrate.

Unfazed, the team donned their lycra, neoprene hats and X-Country trainers and fought their way to the front of the 3000+ people starting the race. At the starting cannon the team was immediately split into two groups and ran hard to get to the front of the pack; myself and Capt Parton found ourselves in a group of crazy Germans and Danes who remained with us for the rest of the event.  The next 10 miles blended into a haze of throbbing legs, ice-cream headaches and a lot of bruises.  Particular highlights of the course include seeing a fellow competitor dislocate his shoulder, a series of twenty 50m hill-sprints (some carrying a car tyre) and an alternating ditch-hurdle arrangement called the ‘Gurkha Grand National’.

Special mention is due to the commitment of Fred ‘the tank’ Kiddie who was running the race as part of his first weekend as a married man, fuelled only by a bottle of Lucozade and propelled by a pair of inadequate flat-soled CrossFit trainers. Also credit is due to the old man of the group, Capt ‘Eton’ Grant, who was competing on his 32^nd birthday and only managed to get round as he was following a young lady dressed as a bunny girl (whom he followed from Mile 1).

The team conquers another obstacle within the ‘Killing Fields’.

Three hours later the last of the PEW cohort had made it through the finish-line. Two of the group had finished in the top 150 runners and the team average time was in the top 25% of the field … not too bad for a group who spend most of the week in a classroom.

Thanks are due to Mid Kent College and 1 RSME Regt for supporting the team and allowing them to make such a good effort. Lastly, for anyone brave enough to take on the Tough Guy challenge next year a few tips; issued builders gloves are a must, neoprene hats are advised and more than two pints the night before should be avoided!

Capt Mark Palmer RE

Yesterday I attended a PIANC workshop at the ICE. PIANC is the International Navigation Association, the body that produces codes for ports and waterways and governs much of what I am doing during Ph3. Coincidently, the chairman of PIANC works for CH2M.
The day focused on the ‘future of design’, challenging and refining codes, and expanding design in to new areas such as seismic (previously not covered).
In parts, the technical aspect when straight over my head as I lacked decades of practical experience however, some of the key takeaways for the day were completely generic across design disciplines.

Some generic points:

Updating of codes – M/515 (Extending the scope of Euro Codes)
There was a public acknowledgement that the updating of ENs due was overly complex, poorly publicised and understood. The main concern were the pending changes (M/515) to load factors, partial and combination factors across all areas of design.

Code Bashing
There is a concern that code writers have become wrapped up in code-bashing. For instance, the ψ (trident for Brad) factor, in ENs is used for long term effects such as creep. So why is that factor appearing, in the same context, in codes dealing with seismic design…surely the impact of the ground accelerating back-and-forth is far worse than a little creep. PIANC are attempting to identify these cases and add clarity/remove confusion within their own codes. They are also offering their finding to the EN and BS committee’s but acknowledge that changing principle codes will not be easy.

The legal requirement of codes
ENs use the would Shall which legally implies you must do it. BSs use Should which implies a recommendation but is supported but an understanding that without just reason or better knowledge it should be considered as shall. The bottom line and advice from PIANC was to treat these as the same ‘a legal requirement’ and only challenge the code if you undertake some form of monitoring, statistical or physical modelling during the design phase.

BIM
Not a single engineer present (over 100 from a wide range of companies) had used BIM for any non-government project. Furthermore, when asked to what degree it had been utilised for Gov projects, only sniggering could be heard. There is no doubt that if used correctly BIM is a fantastic data managing tool that would support the H&S/project files etc. However, this is not happening. The key concern is that as-builts are improving but supporting cals, assumptions, etc are not available which makes future alterations difficult. PIANC is attempting to manage this by introducing the requirement to produce a ‘Facilities Operating Manual’. In essence the factors and design assumptions made at every step of design are to be logged and presented with the drawing pack. This is not optional if designing to their codes. It does not require companies to expose their calcs or software details but an understanding of inputs and outputs along with factors.

Summary
The day highlighted a genuine desire across the industry to refine the policies and the codes in use. The senior PIANC committee were open and seemed active to explore ideas being presented to them rather than trying to justify their latest publications. There was a acceptance that the ENs are so vast that using them as a baseline and working alongside was the most proactive way to progress rather than trying to alter, amend or refine them. The day finished with a short discussion on what, if any, new codes should be written.

Genuinely a good day that expended my understanding of how the codes are born, developed and published. It was warming to see that the industry steers the codes and is not simply constrained by them!

Does anyone have any recommendations for a sensible c’ for a soft sandy clayey SILT?

I’m thinking I’ll probably assume something around 5, then bash out a quick sensitivity analysis to see how much difference it makes to my foundation design then make a decision based on risk.  But I’d be very happy to hear if anyone has experience in this sort of thing?  – Damo I’m looking at you, and John obviously!