Retaining Wall Strut - Youngs Modulus or Pre stress (kN) per load

[EireChch]

Hi all,

I am using Wallap to design a 3.5m high basement wall. The soil is made ground sand/silt layer to 1.5m over clay to 20m plus (shear strength of 50-125kPa). There was some discussion on what type of retaining wall to use but we have arrived at a secant piled wall. For a worst case ULS event (i.e. factored soil parameters, groundwater at 1m below surface and 10kPa surcharge), to achieve a FoS of 1.2 i need roughly 12m of embedment. (Seems excessive to me,but thats what WALLAP is telling me, i suppose there is a lot of load with reduced soil parameters. I am newish to Wallap so my design will be check by another engineer and also by my line manager.

The top of the wall will eventually be restrained with a concrete slab so i would like to consider that as a prop/strut force.

When modelling props/struts in WALLAP gives the options of youngs modulus or Pre stress or both. What would be the most appropriate way to model this as to simulate the restraint provided by the slab. Not specifically i WALLAP but i general good practice.

 

[HENRYZAU]

Is this a real project? If yes you need to consult an experienced engineer otherwise you are wasting your client's money.

 

[EireChch]

Yes this is a real project. I have some very limited Wallap experience, this job was given to me as a "good one to start with". I will be discussing this with my senior but would like to go to him some of my own recommendations. Ive been going through text book etc. but wanted to come here too find some "real world" information.

And yes there may be some clients money wasted but isnt that what happens! Some times you have to learn on the job. Plus i am only charging for half of my time.

 

[HENRYZAU]

For a 3.5m deep exactions in stiff ground, sheet pile wall should do the job. Is this in a built-up area? Any settlement/ lateral displacement limit?

It's inappropriate to use factored soil properties in Wallap.

 

[EireChch]

The wall is to take an axial load also (thats a separate issue) so that why CFA secant wall was used i understand.

Why would it be inappropriate, can you expand on that?

WALLAP has a function that allows factored soil properties, these have to be used in accordance with Eurocodes.

Deflection is generally limited to 30mm for the SLS case and we generally state that WALLAP over estimates deflection by 50-60% adn therefore we would expect 10-15mm actual. I understand this 'rule of thumb' is based on case histories of embedded walls in London Clay.

 

[Retrograde]

If you have already factored the soil properties do you then need to apply a FOS of 1.2 on top of that?

 

[HENRYZAU]

EC, you may use sheet pile as a temporary wall and install a permanent RC wall and backfill in between the two. This should still be cheaper than secant pile wall.

Factored soil parameters have no physical meaning and could give you wrong depth of max bending/shear in finite element/difference analyses. Using factored parameters in hand calculations is alright.

My experience on Wallap is its prediction is reasonable (use pseudo 2-D option) if it's a symmetrical problem. Only when the excavation is too narrow you need a full 2-D numerical program like Plaxis/Flac.

Good luck

 

[EireChch]

Retro - apparently we do. I think Eurocodes give the designer the power to adopt an appropriate FoS. Factors of safety of 1.5 - 2.0 are common for retaining walls. I considered 1.2 to be on the low side, however with factored soil parameters i considered this ok. Interested to hear others opinions.

HZ - Secant piled walls are more common for permanent works than Sheet piles. Sheet piles are definitely quicker to install but the additional RC wall inside would slow it down. I would think a secant wall with a shotcrete finish would be more practical. I suppose there are a lot of factors to consider.

Re Wallap and Factored parameters - would it matter where the max bending and shear is located as long as the values were correct and that the wall/reinforcement had sufficient structural capacity to resist them.