Attribute 1 & 2 - Engineering Knowledge

Question 1

Describe the loads acting on wall formwork during a concrete pour?

Answer 1

Loads on formwork can be further divided in horizontal and vertical loads. Lateral Pressure of Fresh Concrete is the main design load for formwork. The freshly placed concrete behaves temporarily like a fluid, producing a hydrostatic pressure that acts laterally on vertical forms.

Horizontal forces result from wind and concrete dumping and should be opposed by properly designed supports. There’s also lateral pressure, which is a result of accumulated depth of placed concrete.

Question 2

What are the two types of piles?

Answer 2

End-bearing piles: These piles transmit loads directly to a strong layer of soil or rock below the surface. The bottom of the pile rests on this layer and presses against it to keep from shifting.

Friction piles: These piles transfer loads by friction between the surrounding soil and the surface of the pile.

Question 3

What are push pulls props doing?

Answer 3

For vertical alignment and transferal of wind loads.

Question 4

What is the difference between threaded bar and Dywidag?

Answer 4

Dywidag doesn’t work well in shear however threaded bar does. Dywidag is made from high tensile steel and has a working load of 90kN for 15mm diam. Dywidag is better used for axial loads.

Question 5

What are the stages of the temporary works checks? What are you looking for?

Answer 5

I check if the formwork has been erected as per the drawing. Are the push pulls/ frames in the correct place, are the ties at specified height, and have the had the correct materials been used e.g. right diameter of bar. If there is any discrepancy between what is installed on site and the design, then this must be referred to the TWC or temporary works designer who will then check to see if this is acceptable prior to pouring.

Question 6

What were the challenges faced when working so close to the water table?

Answer 6

Dewatering systems had to be put in place to constantly remove the water. This was pumped to a filtering system where the water was cleaned of silt.

Question 7

How do piles transfer loads?

Answer 7

Transferal of load through friction between the pile surface and soil. Ground bearing piles are another type of pile that transfer loads from the structure to a hard form of ground beneath which prevents movement of the structure above.

Question 8

When designing an excavation support what are the considerations designers must make?

Answer 8

Ground conditions, depth and how the supported excavation will deal with surcharges and ground materials.

Water ingress should also be considered as this can change the deeper you go. With the addition of hydrostatic pressure being involved.

Supports are not just necessarily trench boxes and trench sheets but also batters, these must be designed to take the weight/ surcharge of plant operating close to the edge.

Question 9

What is the purpose of a redline drawing?

Answer 9

They record changes that have occurred over the construction phase of a project- these are manually recorded onto the design drawings and writer.

Question 10

What items are included in the handover pack?

Answer 10

Specifications to which the building has been designed/ built, scope of works, concrete mix designs used, a copy of approvals, MAR’s, redline drawings.

Question 11

Difference between permit to strike and permit to unload?

Answer 11

A Permit to strike refers to a formwork system that would be able to be removed once the concrete pressure no loading on the formwork/ falsework. A permit to unload involves the manual unloading of a system once it is deemed the structure can support itself e.g. RMD propping system.

Question 12

How do movement joints work?

Answer 12

Movement or expansion joints work by allowing the structure either side to expand and contract from induced movement caused by temperature changes. They are often found between sections of buildings, roads, and bridges. This is a way to prevent cracking in a long spanning structure.

Question 13

What type of piling did they use? What did they put into the void before the reinforcement cage is inserted?

Answer 13

CFA piles for ground bearing piles. Concrete filled the void prior to installing the cage which they then used for a crane to drop the cage into position.

Question 14

What stops the deep excavation collapsing in on itself?

Answer 14

In this instance the auger keeps the excavation open along with the concrete once the pile has been drilled. In instances with the D-Wall, Bentonite keeps the excavation open.

Question 15

How do they place the concrete in the deep excavation? What is the reason for doing it in this way?

Answer 15

Through the Auger. In other instances, they use a Tremie Pipe in order to eradicate the possibility of concrete segregation and a high drop height.

Question 16

In being a young engineer, regarding concrete technology, what grade of concrete are you using?

Answer 16

C40/50 - 20mm aggregate with waterproofing additives in some cases.

Question 17

What is the correlation between 7-day and 28-day concrete cube strength?

Answer 17

7 day strength is usually around 60-65% the 28 day strength (for a mix with GGBS).

Question 18

What would you do if the gang ask the concrete delivery to add water to the mix?

Answer 18

I would check the ticket to see if any water was able to be added to the mix. Adding water reduces the cement/ water ratio and therefore increases the amount of water lost during the hydration process.

Question 19

What are the long-term implications of adding water to the mix?

Answer 19

Water is required to hydrate the cement where it chemically combines to make the cement hard. Additional water is also designed into the mix in order to make it workable- this has an affect on the concrete by increasing the porosity of the mix thus reducing strength and durability of the mix. Due to the large proportion of concrete containing aggregates, it is essential to consider the moisture content of aggregates as this can have an adverse effect on the quantity of water to be added.

Question 20

What is the difference between curing and hydration of concrete?

Answer 20

Curing is the process of maintaining adequate moisture content within a proper temperature range to aid cement hydration at early stages.

Question 21

What are the benefits of using GGBS as opposed to traditional CEMI mix?

Answer 21

Lower carbon emissions as it is a bi-product from iron smelting

Slower hydration process which therefore reduces the temperature differential. This is useful in large volume pours as decreases the risk of early age thermal cracking.

The chemical reaction between cement and water where the cement paste hardens. Heat is a bi-product of this reaction. Cement initially starts a a paste before stiffening gradually- the hardened state of this on mass is what gives concrete its strength. Strength gain will continue provided moisture is present.

Question 22

As the contractor, what actions would you take regarding the communicating to the designer?

Answer 22

Raising an RFI for general construction query’s, technical queries and clarifications on designs.

Question 23

Can you explain the difference in principles between designing temporary works and permanent works?

Answer 23

Temporary works are the part of the construction project that allow the permanent works to be built. These are often removed after use.

Shorter design life

Stressed up to a higher capacity

Components are planned to be re-used

Have to be sensitive to changes and account for assembly tolerances

Question 24

What is the purpose of a retaining wall?

Answer 24

Restrain soil or engineering fill at an angle steeper than the materials angle of repose.

Question 25

What loads had to be considered on the piled walls/ liner walls/ capping beams?

Answer 25

Soil and pore pressure from the sides, heave and pore pressure from the bottom.

Question 26

What are the roles of cement, aggregate, water in a concrete mix?

Answer 26

Cement: binds together the ingredients of concrete.

Aggregate: reduces production costs and increases resistance of mixes. crushed aggregates significantly affect the properties of freshly-mixed and hardened concrete – making it more compact, decreasing its permeability (which makes it more water-resistant), and modifying its heat retention values.

Water: reacts with cement to cause hydration of the mix- this generates heat, allowing curing and meaning the concrete becomes hard and solid.

Question 27

What does workability mean of concrete and why is this important?

Answer 27

How easy concrete can be placed, consolidated, and finished with a minimal loss of consistency.

Question 28

What concrete test was carried out at Old Oak Common? How was this process controlled?

Answer 28

Workability of this concrete was tested by a flow test as it was and F5 mix. We were looking for between 560-660mm. If concrete came in dry, water may be added at the discretion of the site team and as per the concrete ticket. If it was too wet, the concrete was sent away as excess water would result in greater segregation between aggregates and increase the water to cement ratio- Both have a direct effect on the compressive strength of the concrete.

Question 29

What is LOLER?

Answer 29

Lifting Operations and Lifting Equipment Regulations 1998.
This places duties on people and companies who own, operate, or have control over lifting equipment.

Question 30

Which parties need to know about the overloading of the piled wall should it occur?

Answer 30

Sub-contractor

Temporary Works Designer

Principal Contractor

Question 31

What were the challenges with working with the CESWI specification?

Answer 31

CESWI was a detailed specification but there were some references to other specs and guidelines. When writing ITP’s there were often cross references to guidance’s like sewers for adoption that were pretty vague.

Question 32

What other methods can be used to test integrity of piles?

Answer 32

Low Strain Impact Integrity Test

Crosshole Sonic Logging

High Strain Integrity Test (Dynamic Analyzer)

Thermal Integrity Profiling

As per guidance of CIRIA R144

Question 33

How does the Sonic Echo method work?

Answer 33

B striking the pile head with a light hand-held hammer and recording the response of the pile using a motion transducer (i.e. accelerometer) coupled to the pile head. The hammer strike (blows) generates compressive stress wave that will travel through the pile. The speed on which this wave returns to the accelerometer is measured. If the wave returns quicker than expected, then substantial cracks can be assumed to be within the pile.

Question 34

What would happen if a pile did not pass the integrity test?

Answer 34

If a pile passed, SOCTOEC would issue a NDT certificate. If not, then remedial works would have to be done. Firstly, if the pile did not pass due to cracks at the top of the pile, then this could be grouted. Furthermore, to ensure pile stability there is a way of reinforcing the strength of a pile by digging down to a depth and covering the pile with FRP laminates or GRP jackets.

Question 35

How did you ensure that the reinforcement was not damaged during the pile cropping stage?

Answer 35

The top of the reinforced cage did not have any links within it and therefore was only vertical starter bars that were left protruding. DE bonded foam was wrapped around these starter bars to ensure that the concrete could be cropped and come away easily from the starter bars.

Question 36

What was the type of geogrid used and how does it work? How would geogrid fail?

Answer 36

Geogrid works by interlocking the granular soil material placed over it. The open sections allow for the confinement of material within therefore increasing the overlying granular fill. This then increase the shear strength of the soil.

Question 37

What is the purpose of the blinding at the bottom of the excavation?

Answer 37

To act as a reinforcing prop at the bottom of the sheet piles as well as provide a clean work area for those installing the pipe.

Question 38

What are guidance/ documents are you to be aware of for aiding you in writing Engineering Design Briefs?

Answer 38

Eurocode 1 – General Actions on Buildings – Wind Actions (BS EN 1991-1-4:2005 and National Annex)

Manual for the design of building structures to Eurocode 1 and Basis of Structural Design (IStructE, 2010)

Formwork – A guide to good practice, Concrete Society (3rd Edition, 2012)

BS 5975:2008+A1:2011 – Code of practice for temporary works procedures and the permissible stress design of falsework

Concrete pressure on formwork (CIRIA Report 108, 1985)

Question 39

What was used to ensure the concrete was up to strength prior to striking formwork.

Answer 39

Concrete cubes were taken for every pour and crushed at 7-, 28- and 56-day intervals. In some instances, thermocouples were used.

Question 40

How is temperature converted into concrete strength?

Answer 40

By knowing the concrete temp at the core, you can gather at what stage the concrete is in during its curing/maturity process. A calibration will be made during trials so then an estimate of concrete strength can be made.

Question 41

Were there any instances that involved concreting in cold weather? How was this process managed?

Answer 41

The temperature of the concrete was checked upon arrival to ensure that it was above 5 degrees. This prevents the risk of water freezing during concrete placement. Concrete pours were planned for later in the day as temp rises. If frost was expected pours would be covered with polythene. Reinforcement and formwork were checked prior to placing to ensure that all the elements were free from frost and ice. Frost blankets were used after the pour to ensure concrete stayed at 5 degrees for first 48 hours (these were lapped by 300mm). Formwork must not be struck until concrete has reached 5N/mm2. Concrete surface was then covered wit blanket to prevent shock.

Question 42

How was the concrete placing process managed?

Answer 42

Concrete placing was carried out via a mobile pump, skip and static line. This was setup on a previously tested pile mat and temporary works supervisor checked the outrigger position to ensure they were in safe/steady positions. Test runs of the boom were setup prior to ensure the position was adequate.

Question 43

What is top down construction and what are the benefits?

Answer 43

Use of the permanent internal structure as the temporary propping to the retaining wall, cast in a top-down sequence.

The higher-level slabs are cast before the lower-level slabs to act as horizontal frames for wall support as the excavation progresses.

The method has been used for deep excavation projects where tieback or anchor installation was not feasible and soil movements had to be minimized. There is a possibility of saving the overall construction time.

Question 44

What are some unique features of a water retaining structure?

Answer 44

Minimizing construction joints and the use of waterbar to avoid water penetration. Reinforcement and concrete are often beefed up since the load on the slabs/walls will be a constant hydrostatic pressure in comparison to non-retaining. Concrete could also be placed at a cooler temperature to minimize crack formation and concrete temperature from the wagons are taken on arrival at site. Construction joints also have exposed aggregates within them to ensure a good bond onto the next pour and a kicker to allow the formwork to bite onto it during installation.

Question 45

What does retarder do?

Answer 45

Retarder slows the rate of setting concrete. This means that the concrete can stay fresh for longer so that it gets to its final hardened state later. This allows for an easier construction joint to be formed if placed on top of a pour. It also allows for a more consistent pour and the reduction of cold joint occurrence.

Question 46

What does curing agent do?

Answer 46

A concrete curing agent forms a membrane over the top of the concrete slab while it cures. This stops the water near the surface of the slab evaporating too quickly, and hence helps to reduce cracking and dusting.

Question 47

The difference between cement, GGBS and PFA?

Answer 47

GGBS is a cement substitute manufactured as a by-product from the iron-industry. It is the most sustainable cement substitute such as fly-ash.

GGBS is supplied as a separate concrete component and added at the concrete mixer. It counts towards the cement found in concrete. In concrete it reduces the embodied CO2 by around 900kg compared to using one tonne of Portland Cement and increases concretes durability. Often GGBS is 70% or more when used with cement.

PFA is the ash that is a result of burning pulverised coal. It needs lime and water for the hydration process and is used 20-40% with a combination of Portland cement.

Question 48

What are some elements of a permit to dig?

Answer 48

A permit to did is defined as a specialised safe system of work through which certain activities can only be carried out by an authorised person. It should include:

Project details

Precautions to be taken before work is carried out-check sheet

Completion of work signage/close out

Question 49

What is CAT 1, CAT 2 & CAT 3 checks?

Answer 49

CAT 1= design checked by someone in your team

CAT 2= design checked by someone in the company but not in team

CAT 3= Design checked by external person

Question 50

Explain the load paths on formwork?

Answer 50

Concrete pressure

Formwork face

Secondary wailings

Primary soldiers

Supports & fixings

Question 51

What is the required strength of concrete before striking the shutters?

Answer 51

5N/mm2 as described in BS5975. Early strike cubes can be taken to ensure that the concrete has reached this strength.

Question 52

How do designers determine the prop loads?

Answer 52

Prop loads are determined through a combination of soil parameters, water pressures and expected thermal changes.

Question 53

What is the making good process?

Answer 53

Is a term that is often used in relation to defects which must be ‘made good’ through remedial works before a specified date after practical completion has been certified.

Question 54

Why can’t you use a poker to move the concrete?

Answer 54

A poker cannot be used to move the concrete- it should be used to remove trapped air within the mix. Using this to move the concrete will result in the segregation.

Question 55

What are the various RIBA Design stages?

Answer 55

0 – Strategic Definition.
1 – Preparation and Brief.
2 – Concept Design.
3 – Spatial Coordination.
4 – Technical Design.
5 – Manufacturing and Construction.
6 – Handover.
7 – Use.

Question 56

What were the two design options chosen for the UTX construction ?

Answer 56

Cantilever and single propped solution

Question 57

What are the ground conditions at Old Oak Common?

Answer 57

Piling mat - 6F2

Made ground - mixture of coarse and fine grained materials (concrete, brick, ash)

Alluvium- soft organic fine grained silty clay (typically found near SBS sewer)

London clay - stiff clay material

Question 58

What are the documents providing guidance for the design of retaining walls in the UK?

Answer 58

Eurocode 7 (EC7) - design approach 1&2, take worst case

CIRIA C760 - embedded retaining walls ‘non-contradictory complementary’