ERSS - What on earth is that?

Many countries in Asia have romantic sounding nicknames, "the land of the rising sun", " the pearl of the orient" and I was attracted a while ago to work in the romantically sounding "Venice of the East" which after a few years dawned on me as perhaps being based on the simple recognition of the fact that cities are subject to widespread flooding and occasional subsidence.... Now for those living in Singapore, (and travelling . like me, down the PIE, KPE, MCE and AYE to get to work) eventually realise that we inhabit the "Island of the Acronym"  hopefully a tourist draw in its own right one day...

So to add one more to that evergrowing list, we have the "ERSS" or Earth Retaining and Stabilizing System. This is usually defined as "Any structure, structural system or other means used to maintain the shape of excavation during construction, earth filling or cutting" We are using such a structure to help us make a big wall in the car park adjacent to the tent plaza.

I suppose it could also be called PTPDS - That is to say the" Prevention of the Tent Plaza disappearing system" as it the civil engineering structure that is supporting the tent plaza whilst we dig out the basement car park and build a new retaining wall to support it. This is one of the more complex areas of construction in this project and this is a simple explanation of what we are doing..



This chap on youtube explains this "top down" construction method much better than me in this video but we have a few particular variations on our site that are not shown in  this video.

Step one for us is to drive in "Soldier Piles" these are simple steel I beams driven vertically into the ground in a line, (I suppose like soldiers standing to attention?) we then infill these piles with timber planks, called timber lagging...

Here you can see the soldier piles driven into the ground and being filled in with the timber lagging.

We then add another steel Ibeam horizontally between the soldier piles. This is called a waler and further reinforces this system and this prevents the solider piles from bending under the weight of the soil and other pressures behind them and coming out of line.

Soldier piles and waler complete - we can now move on to add more support  to the waler by bracing it back against the finished foundations. Whilst this is costly in terms of time as we have to wait for the foundations to catch up, it is by far the safest and most prudent way of construction in this school environment. 

Here you can clearly see the series of struts to add a supporting force to the waler by bracing against the foundations and ensure absolutely no chance of movement of the ERSS. Safety is paramount in this project.  Note the think layer of concrete on the slope of soil to be removed. This prevents erosion of the soil in the heavy rain of Singapore, as this soil erosion could cause pollution to the surrounding waterways.

A close up of the plate that we attach to the foundations which we then attach the structs to hold them firmly against the ERSS

Once these struts are in place, we can safely remove move soil and continue to add the timber planks down the soldier piles 

A nice top down view of a completed section of ERSS. It was completed in sections with progressive construction as each section was complete. In this photo you can see the area on the left being prepared for the foundations and the area on the right undergoing excavation.

The final strip of foundations being cast alongside the completed ERSS.  Once these foundations are complete, we can start to remove the struts. Overall a complex construction sequence completed safely and efficiently.  Phew...

Behind the finished retaining wall. We will now remove the steel (as much as we can!) and then backfill the gap with soil

Step 7 - Creating the Columns

Continuing the theme of trying to demystify the building process (and at the significant risk of making it sound too easy and losing my job to a team from grade 3!) the next step in the process now we have created the foundations are the columns. These very important often unappreciated part of the building will support the floors above.....This building is challenging as it has a number of spaces such as the exam hall, the small theatre and the drama studios that require column fee space (imagine watching a show with a column in the middle of the stage!) To reduce some of the complexity we elected to move these functions to the top most floor as they the structure above them is comparatively light..

Lets go back to the foundations... you might remember seeing the squares containing more steel, these marked the column positions, you can see the columns are laid out in very regular grid.

 This extra steel that you can see is added reinforcement to prevent 'punching shear'  A simple description of this would be the column punching through the structure...  Mr Hulk to the right is imparting some punching shear on this unfortunate wall. Even Mr Hulk would have problem punching through our column positions with our extra steel in place. An added bonus of the current design of the building I feel....

So now we have attached the steel work into the foundation and ensuring that they can take the pressure of the building above we can start making the actual columns...

As if to prove that its all common sense, we actually make a huge metal mould which is clipped around the steel reinforcing bars and then

Step 1 

Step 2 - Add more steel

Step 3 - Add the mould

Step 4 - Pour in the concrete

Step 5 - All Finished - Repeat 72 times per floor!

Further Progress on the other side of the foundations

We finished one side of the foundations and now we are using this platform to work to finish the other side of the building, with another HUGE continuous pour of concrete - Even bigger than the first one!

It took us 16 hours, 2 pepperoni pizzas, 3 portions of char siew rice, 12 coffees and 2 changes of clothes to finish this continuous concrete pour and the majority of the foundation...phew

Step 6 - the start of the building proper - Casting the foundations

Today is a milestone day as it represents the start of the some real visible construction - Yipeee!

Today we are going to casting zones 1 and 2 of 6 zones of the foundations that we eventually need to construct.




These 2 zones consists of 600 tons of rebar, which we will fill with 2295 m3 of concrete. This is one of the largest single casts in Singapore this year and certainly the largest one I have been involved in, we should be finished in around 16 hours. This is about as exciting as concrete gets!

Each concrete truck that you see on the road can carry around 8.5m3  This cast will take around 270 concrete trucks who will have to come constantly throughout the day..

Heres one of the delivery notes for the concrete. The concrete we are using is grade 40 PBFC (portland, blast furnace, cement) instead of OPC (Ordinary portland cement) This is considered a green concrete as it uses the by product of industrial blast furnaces to replace the sand  More information on this type of concrete here.

We need the concrete to set slowly to prevent cracking, so as well as thinking carefully about its chemical composition and adding some extra chemicals, we also chill the concrete to around 27 - 30 degrees celsius before it is sent from the concrete mixing plant and we pour it all in a single 16 hour session without stopping...


The concrete will take around 4 weeks to fully gain its strength and we do a number of tests on the concrete both on delivery and progressively as the concrete drys to ensure that the concrete is performing as we would expect and can reach its required strength.

We are using three elephant pumps (yes they look like an elephant with a 35m trunk and minus the ears!) The concrete trucks back up to the pump and discharge their load, the pump them can move it to the far corners of the site!

Each pump can discharge around 60 m3 per hour and so we are estimating around 14 hours non- stop casting depending on site conditions.

Once the concrete is pumped into the metal cages, a team of men with vibrating metal probes help add some vibration to the thick concrete which helps it flow into all the nooks and crannies of the raft.

 Nearing midnight and almost finished.... The weather was kind to us.. and all the planning paid off.

 Our First Casting....2500M3 of Concrete, 16 hours!

Step 5 - Making the foundations - Designing and testing the quality of the concrete.

Testing the quality of the concrete.. 

Throughout the project, we will use huge amounts of concrete. Concrete is made up of a number of materials and these materials can be adjusted in quantities to give the concrete various properties. These properties could be in its eventual strength, in the way it flows, the speed it drys and even in its ability to resist water.

Getting this concrete mix right is essential - The ultimate strength of the building depends on it!

Like making a less than yummy cake?

The first stage of this journey is the design of the concrete mix, this is a theoretical design of the various components that might be used to make the concrete, sort of like making a huge, very unpleasant tasting cake!

This trial mix is then made and put through a number of test before being selected for the project.

Throughout the project , whenever concrete is delivered to site the same tests are done to ensure that the concrete delivered is the same as the concrete trial mix that was done at the start of the project...

A sample of the concrete delivered  to site and used in any construction is stored and labelled in these steel boxes 15cm x 15cm for later testing




After a few weeks it is then sent to a special outside lab for testing on its composition and also its compressive strength.......

Here is an example of the concrete test certificate, this one is from the grade 40 concrete for the raft, around 2 days from casting, you can see it is nearing 70% of its strength as it progressively dries.



The concrete also goes through a slump test before it is allowed on the site. This very simple test is designed to quickly check that the concrete does not have a too high water content as this would ultimately affect the ability of the concrete to chemically react and harden properly...





UPDATE: Here are some example of the tests of the concrete from the foundations

Step 4 - Making the foundations - Laying the steel for the foundations

Phew our soil passed our plate load tests with flying colours.... onto stage 4.



Once the soil has been compacted and the tests have been done, we cover the bare soil with a thin layer of concrete to stop it getting wet and soggy as this would weaken the soils ability to take the pressure of the building.







The foundation is made up of around  1440 tons of steel reinforcing bar (Rebar) which is twisted and shaped into boxes. (using our earlier elephant analogy that 288 big ones..)

The rebar has different names depending on its diameter. For this foundation we use 7 types of rebar

R6 - 6mm round Bar - mild Steel
T10, 13, 16, 20, 25, 32 - which is t for torsional and the diameter of the bar in MM.

You can see the steel rebar being laid out to form a grid, and if you look closely you can see were we put more rebar at the column positions, to prevent the columns "punching" through the foundation (the punching shear). The punching shear of these these columns positions is around 600 tons of pressure...most of our columns will be around 1m x 1m in diameter.

Laying the first layer of rebar, its a very unpleasant job under the sun all day!

The final project - A complete mesh (not mess) of steel, like a box, ready to have concrete poured into it...

Step 3 - Making the foundations - Testing the soil

well we finished Step 1 and step 2 and now we need to tackle

Step 3.......

Once we have excavated the soil down to the right level, it is time to do some more tests which will help us confirm the design of our raft foundation.

The test we do here is called a plate load test...

For this test we put a plate under load..... (you see its not that hard, is it?)

We choose around 10 areas around the site and then construct a series of pads, we place plates on the ground and then slowly in a series of cycles place the plates under increasing pressure using an hydraulic jack. We test these plates up to a pressure of 800 Kpa  (kilopascal) ... 80 tons to the ordinary man on the street.

This is 3 times the ultimate planned weight of the building on the ground





The plate itself is only 30cm x 30 cm  and this weight is the equivalent of around 16 full grown african elephants balancing on this single point...

I have heard a rumour that in the olden days, before the hydralic jack this service was an important source of revenue for circuses with elephants.

 Almost there,  80 tons of pressure.

For the test to pass we allow only a deflection of a maximum of 2.5 cm into the soil! If the soil fails the test, this area of the site will need to have this soil removed and a better quality of bearing material added in its place.








More detailed information on this process can be found here!