RC Retaining Wall - concrete additives required?

[kellez]

Hello everyone, I am designing a 3.5m high retaining wall of which 2.0m will be covered by infill soil. see picture attached

I have specified a layer of bitumen felt to be applied on the wall up to 2.2m high.
Next on top of the bitumen felt i have specified the installation of a geodrain membrane.

Do you guys think i will also need to specify additives to the concrete such as crystalline waterproofing?

Regards

 

[dik]

SRE... good point, and, one that I had not considered. Now, if you use 6" slump without SuperP... it might be self-levelling. I usually use a 4" max (not 4") slump for this type of work.

Dik

 

[kellez]

What about sliding resistance and frost depth considerations?

Sliding resistance has been checked and its fine.

The project is located in Cyprus and not in Greece, we dont worry about frost depth in my country, we have sun for 9 months a year and the only place we get minus degrees is high on the mountains. This project is not located on the mountains.

Kellez - For a "thin" (200 mm) wall, have the Contractor construct short lengths of the wall.

You are absolutely right the wall is too long to be poured in one go, Ive seen this be done the other day, while passing by a construction site and I was wondering why, now it all makes sense.

As the concrete works it's way down inside the forms, it coats the rebar with concrete residue. If the wall is placed quickly (a short length of wall) this is not a problem. If a long length of wall is constructed, it may take too much time to fill the forms. The residue on the upper rebar may have "set" before it is covered... affecting the bond strength.

WOW this is indeed great information and very important, thanks a lot for sharing with us, i would have never thought of this

 

[kellez]

Currently you have a cantilevered wall with no toe due to property line issues. I am concerned that your heel reinforcement bars will not be fully developed since you only have the stem to work with. Have you considered battering the rear side of the wall. A battered wall is thicker at the base, which will push your heel's flexure critical section rearwards. That will give you more room to develop the steel in the heel.

Below is an excerpt from ACI's Concrete Design Handbook showing the critical section for the heel. The CRSI Retaining wall design guide has a similar discussion about development length of heel bars.

Thanks for the advise, the stem reinforcement will actually continue down into the foundation and be part of the bottom layer reinforcment of the foundation, i will soon post a more detailed reinforcment drawing so that you may advice if this would help with the heel reinforcement bars to be fully developed

 

[kellez]

Ok guys I ve got a few questions,

1. If I use only one layer of bars inside the stem, should it be installed exactly in the middle of the wall?

2. The whole length of the wall is 121m, with 5 straight runs, each one with a length of 30m, 18m, 23m, 29m and 22m
Shall I use any expansion joints for the wall and the foundation?

Since i will be pouring the wall in several small lengths and not in one go, I could consider this as an expansion joint right?
However this is only for the wall, what about the heel/foundation of the wall? does it need expansion joints? shall the foundation be poured in parts as well?

Two more questions arise due to the question above...

a. Is the foundation and wall poured in one go?
b. What happens to the reinforcement since i will be pouring in several short lengths?

[URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1512149418/tips/Retaining_Wall_Plan_View_1a_mlnlxy.pdf[/url]

3. Shall I ask the contractor to use an attachment to the hose so that it may reach the bottom of the wall within the forms in order to avoid dropping the concrete from a considerable height?

 

[dik]

1. If I use only one layer of bars inside the stem, should it be installed exactly in the middle of the wall?

2. The whole length of the wall is 121m, with 5 straight runs, each one with a length of 30m, 18m, 23m, 29m and 22m
Shall I use any expansion joints for the wall and the foundation?

Since i will be pouring the wall in several small lengths and not in one go, I could consider this as an expansion joint right?
However this is only for the wall, what about the heel/foundation of the wall? does it need expansion joints? shall the foundation be poured in parts as well?

Two more questions arise due to the question above?

Is the foundation and wall poured in one go?
What happens to the reinforcement since i will be pouring in several short lengths?

https://res.cloudinary.com/engineer...8/tips/Retaining_Wall_Plan_View_1a_mlnlxy.pdf

3. Shall I ask the contractor to use an attachment to the

Rebar should be located on the tension side of the wall, ie., the side adjacent to the soil.

The wall should be divided into 5m or 6m lengths to accommodate shrinkage... I don't know if there are thermal issues. Maybe from heat if face of wall is facing south. By increasing the horizontal reinforcing percentage, you can increase the distance between vertical joints.

You can construct joints in the footing part of the wall, maybe at 10 or 12m lengths. Footing is not exposed to view and and also not subject to thermal cycling as much as the vertical face.

Usually the foundation is poured and then the wall poured on top of it a little later. The foundation should be wide enough to provide formwork support for the exposed face.

Vertical reinforcing to footing connection is normally accommodated by means of steel dowels from the footing into the wall to develop the tension in the wall vertical rebar.

Dik

 

[SlideRuleEra]

Kellez - By all means, pour the foundation separate from the wall for several reasons:

1. As Dik mentioned, the top of the foundation provides firm support for the wall forms.

2. If the foundation and wall are placed at the same time, hydrostatic pressure from fresh concrete in the wall will tend to "push" the fresh concrete in the foundation out the top of it's forms.

3. Placing the wall concrete (on top of a previously constructed foundation) greatly reduces the volume of concrete to be placed at one time. This gets back to the importance of placing the wall concrete quickly, as we discussed above. Short section of wall keep the contractor from having to construct, buy or rent a lot wall forms. The same forms can be reused many times on the job - saving money for everyone.

4. Having such a project with a long wall is a big help. The Contractor may have crews working on separated parts at the same time. Say, one crew doing the foundations. A second crew doing a section of wall on top a previously constructed foundation.

3. Shall I ask the contractor to use an attachment to the hose so that it may reach the bottom of the wall within the forms in order to avoid dropping the concrete from a considerable height?

No. The 200 mm wall is too thin. With a rebar mat there is no space for a reasonable sized hose without disturbing the rebar mat. Don't want to risk having the mat wind up in the wrong place. As a bridge contractor, we occasionally offered to make a wall thicker for "free". This would make the wall much easier (and cheaper) to construct. Of course, a competent contractor knows this at bid time and prices the work so that the "free" concrete is adequately covered in the bid. IMHO, "thin" walls to save money on concrete are false economy and fool only the Owner and Engineer.

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[kellez]

ok thanks a lot for the advise,

1. i will first pour the foundation and then the wall on top
2. wall to be poured in parts of 5-6m. this can be increased by increasing horizontal reinforcment

One more question

do you guys think I am overdoing with all the base preparation for the wall to sit on, what I mean is, is it too much to use

1. a crushed gravel base of 30cm?
2. then use a waterproof membrane on top
3. and then use a concrete layer of 10cm to allow for a clean base to work on?

please the details below

 

[dik]

Attached detail shows some features... quickly cobbled together from another project You can use this as a guide. I didn't quickly check the mods.

You can remove the reinforcing from the outside face of the retaining wall, modify the footing thickness, and footing reinforcing.
Dik

 

[SlideRuleEra]

...think I am overdoing with all the base preparation for the wall to sit on...

IMHO, both the waterproof membrane and the the concrete base (mud mat) add no value to the project and are wasteful of time an money. Why is a waterproof membrane even being considered?

My father taught me to get underground foundation work completed a quickly as practical. Things happen... rain, mud, high groundwater, etc.
I have found that advice correct 100% of the time.
Keep things basic and simple:

Excavate
Compact the subgrade
Place the gravel base
Form and place the concrete foundation
Backfill as soon as possible

Many Contractors can per perform excellent concrete work above ground. Few are competent below ground.

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[dik]

SRE... your daddy taught you well. I often see housing strip footing excavations filled with water and sitting for a few days after a rainfall... and if the contractor is on site, I often tell them they should pump out the water... their attitude is 'just another sidewalk supervisor'.

Dik

 

[kellez]

IMHO, both the waterproof membrane and the the concrete base (mud mat) add no value to the project and are wasteful of time an money. Why is a waterproof membrane even being considered?

The reason for considering the waterproof membrane is for protecting the retaining wall from absorbing moisture from the ground.
Since the sides of the retaining wall will be covered with bitumen felt and some sort of drainage system will also be installed, why not also protect the bottom side of the foundation? Just wondering?

Please note that this wall will be plastered and painted, the aesthetics of the wall are very important, therefore i want to make sure that the wall will not need to be repainted every 2-3 years due to the paint coming off because of absorbed moisture. Its a very long wall 121m which would be really expensive to paint every 2 years because of pint flaking off.

I agree that the concrete base mud mat may is too much.

 

[SlideRuleEra]

...some sort of drainage system will also be installed.

When the drainage system has been designed, then the importance (or unimportance) of a waterproof membrane can be considered.

From a practical viewpoint, installing a large quantity of any membrane on crushed stone, covering it with concrete and expecting zero damage to the membrane is unrealistic. If (when) the membrane is damaged, water pressure guarantees that moisture gets to the other side of the membrane.

My advice... design an effective drainage system and forget the membrane.

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[dik]

SRE... more likely to work... and less hassle.

Dik

 

[dik]

Free draining backfill will go a long way to keeping the face of the concrete dry. Can also use Miradrain on the backface of the wll for positive drainage... If groundwater is a serious issue, you can use well points to draw the WT down or use French drains to get rid of the water... you need free drainage or a pumphouse to finally get rid of the water.

Dik

 

[kellez]

Hello guys,

quick question, how thick do you think the gravel base for the wall should be?

By the way the contractor for the retaining wall suggests that we do not need to add the gravel base, what you guys think?

 

[SlideRuleEra]

What are the soil properties of the subgrade?
Compared to the bottom of the footing, where is the water table?
The gravel must have been specified for a reason... what is that reason?
If the gravel is deleted, will the Owner receive a credit (money) for material and labor not used?

I believe you have a good Contractor. Sounds like he realizes the importance of keeping things simple (refer to my 3 December post).

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[kellez]

I am coming back to this post because the day we start excavations is near and i am bit worried, please see my comments in red below

My father taught me to get underground foundation work completed a quickly as practical. Things happen... rain, mud, high groundwater, etc.
I have found that advice correct 100% of the time.
Keep things basic and simple:

Excavate - top soil of 30cm will be removed underneath the retaining wall and then the necessary excavation will take place to achieve desired depth.
Compact the subgrade - How important is this? For me I think this is very important since we are not excavating very deep into the existing soil.
The contractor and architect keep saying that this is not needed however i disagree, please state how important this is.
Place the gravel base
Form and place the concrete foundation
Backfill as soon as possible

Many Contractors can per perform excellent concrete work above ground. Few are competent below ground.

You asked me about the soil conditions before but i must say that the only thing i know is that the soil there is full of big white boulders (weak stone), it can break with your hand, i also think this is mostly sandy soil.
I have also talked with the neighbours that recently build a house 50 meters away and excavated 4m into the soil, they said that the soil is very strong and its full of big boulders, weak stone.

 

[SlideRuleEra]

Compact the subgrade - How important is this?
For me I think this is very important since we are not excavating very deep into the existing soil.
The contractor and architect keep saying that this is not needed however i disagree, please state how important this is.

Well, it's important to have compacted soil... how much effort this requires depends on the soil properties. Where I work, proper compaction is a challenge, maybe not where this project is located.

As for the reason why compacted soil is important, from your other thread, take a look at the pressure distribution and forces on the soil as the retaining wall loading nears overturning (and sliding.. at the same time). The soil under and near the toe of the retaining wall has to be "at it's best" to perform successfully.

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