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RC Retaining Wall - concrete additives required?

RC Retaining Wall - concrete additives required?

RC Retaining Wall - concrete additives required?

(OP)
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

RE: RC Retaining Wall - concrete additives required?

I would also show a foundation drain and backfill with a freely draining material. I never specify waterproofing systems on retaining or masonry walls. There is too much liability and it is just another thing i have to stay on top of for new developments. I put a note "waterproofing by contractor as required"

RE: RC Retaining Wall - concrete additives required?

Why do you want to waterproof a retaining wall? Typically we only waterproof occupied or liquid retaining structures. Drainage mitigation behind the wall is a good idea but that is different than waterproofing. You will also need a drain running along the bottom of the wall and/or weep holes through the wall. You need to provide a path for the water to go once it reaches the bottom of the wall.

RE: RC Retaining Wall - concrete additives required?

For a 200 wall, do you need two layers of rebar? and you should batter the wall into the soil by 70 to 100mm... so, when it rotates, it becomes more vertical. Also free draining back fill and drains at 1200 with rodent screens.

Dik

RE: RC Retaining Wall - concrete additives required?

I don't think I've ever specified waterproofing for a soil retaining wall.

By the way, why no toe on the wall footing? Are you near a property line?

RE: RC Retaining Wall - concrete additives required?

WARose... same here, just clean free draining backfill, filter cloth and drains...

Dik

RE: RC Retaining Wall - concrete additives required?

Quote:

WARose... same here, just clean free draining backfill, filter cloth and drains...

And you can count on those drains getting clogged eventually.......ergo I always check it with the (additional) hydrostatic pressure. (If applicable.)

RE: RC Retaining Wall - concrete additives required?

Never had that problem... the geotextile between the clean stone and existing soil keeps the clean stone, 'clean'.

Dik

RE: RC Retaining Wall - concrete additives required?

Quote:

Never had that problem... the geotextile between the clean stone and existing soil keeps the clean stone, 'clean'.

Most of the time my backfills are not that neat/organized. smile

Anytime they have been....the contractor gripes to the owner, and then I hear about it.

RE: RC Retaining Wall - concrete additives required?

I insist... and have standard details that are included in the contract documents. I ask the contractor what sort of credit he is prepared to offer... I then explain to the owner about the long term viability of his wall and the redesign and the added loading for hydrostatic surcharge. Never been an issue.

Dik

RE: RC Retaining Wall - concrete additives required?

(OP)
Hello everyone,

The only reason I wanted to provide waterproofing to the retaining wall is because I have seen crystalline waterproofing being used on slab foundations for residential houses here in my country,
therefore i was just wondering if they also use it on retaining walls since they are mostly in contact with soil.

Thank you everyone for your advise, I will make sure to add the detailing for the backfill with a freely draining material and also add a drain pipe at the bottom of the wall, however where is that drain pipe supposed to take the water to? Also how am i supposed to provide an inclination to the drain pipe?

Quote (WARose)

By the way, why no toe on the wall footing? Are you near a property line?

You are right i am exactly on the boundary of a plot which needs to be filled with soil to increase the level height in order to reach the level of the road

Quote (dik)

For a 200 wall, do you need two layers of rebar? and you should batter the wall into the soil by 70 to 100mm... so, when it rotates, it becomes more vertical. Also free draining back fill and drains at 1200 with rodent screens.

According to my calculations no actually i do not need two layers of bars, but i am being carried away by other engineers designs here in my country, which makes me feel a bit worried to go along and use only one layer while an experienced engineer uses two layers

What exactly do you mean by "batter the wall into the soil by 70 to 100mm"

The drains at 1200 with rodent screens you mention are called weep holes?



Anyone have a picture of the drain pipe with geotextile used on a retaining wall?

RE: RC Retaining Wall - concrete additives required?

(OP)
Any idea where I should drain the water to from the bottom of the retaining wall? Please have a look at the pictures below ti get an idea of the site layout


Blue colour in the plan view below is the retaining wall





[img https://res.cloudinary.com/engineering-com/image/up...

RE: RC Retaining Wall - concrete additives required?

Unless your concrete placement is really good... I'd not use 2 layers of bars in a 200 wall... 250 wall minimum for 2 layers. You're looking for segregation issues.

I'll see if I can readily locate a wall section.

Dik

RE: RC Retaining Wall - concrete additives required?

(OP)

Quote (dik)

Unless your concrete placement is really good... I'd not use 2 layers of bars in a 200 wall... 250 wall minimum for 2 layers. You're looking for segregation issues.

I'll see if I can readily locate a wall section.

Thanks for the advise, actually i have seen quite a few 200mm thick being build and poured just fine however the bars of each layer are alternating, what i mean is that one layer is shifted by a distance = half the spacing of the bars therefore the bars of each layer are not exactly on the same axis. see picture for clarification.

However another important reason to use only one layer is the cost savings


so from the top view of the wall it would look like this

RE: RC Retaining Wall - concrete additives required?

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.

RE: RC Retaining Wall - concrete additives required?

RE: RC Retaining Wall - concrete additives required?

kellez: You shouldn't stagger the vertical reinforcement. If same spacing, then the bars should be opposite each other. I should have added that it's the horizontal reinforcing that causes the segregation...

Dik

RE: RC Retaining Wall - concrete additives required?

What about sliding resistance and frost depth considerations?

Mike McCann, PE, SE (WA)


RE: RC Retaining Wall - concrete additives required?

Where in Greece is the project?

Mike McCann, PE, SE (WA)


RE: RC Retaining Wall - concrete additives required?

Kellez - For a "thin" (200 mm) wall, have the Contractor construct short lengths of the wall. Whether there is one rebar mat or two, there is no room to get a tremie pipe or concrete pump hose to get inside the forms. Agree with Dik about segregation and there is an additional concern:

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.

www.SlideRuleEra.net idea
www.VacuumTubeEra.net r2d2

RE: RC Retaining Wall - concrete additives required?

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

RE: RC Retaining Wall - concrete additives required?

(OP)

Quote (msquared48)

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.

Quote (SlideRuleEra)

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.

Quote (SlideRuleEra)

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

RE: RC Retaining Wall - concrete additives required?

(OP)

Quote (JoelTXCive )

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

RE: RC Retaining Wall - concrete additives required?

(OP)
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?

https://res.cloudinary.com/engineering-com/image/upload/v1512149418/tips/Retaining_Wall_Plan_View_1a_mlnlxy.pdf

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?

RE: RC Retaining Wall - concrete additives required?

Quote (OP)

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/engineering-com/image/up...

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





RE: RC Retaining Wall - concrete additives required?

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.

Quote:

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.

www.SlideRuleEra.net idea
www.VacuumTubeEra.net r2d2

RE: RC Retaining Wall - concrete additives required?

(OP)
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


RE: RC Retaining Wall - concrete additives required?

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

RE: RC Retaining Wall - concrete additives required?

Quote (kellez)

...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.

www.SlideRuleEra.net idea
www.VacuumTubeEra.net r2d2

RE: RC Retaining Wall - concrete additives required?

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

RE: RC Retaining Wall - concrete additives required?

(OP)

Quote (SlideRuleEra)

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.

RE: RC Retaining Wall - concrete additives required?

Quote (kellez)

...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.

www.SlideRuleEra.net idea
www.VacuumTubeEra.net r2d2

RE: RC Retaining Wall - concrete additives required?

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

Dik

RE: RC Retaining Wall - concrete additives required?

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

RE: RC Retaining Wall - concrete additives required?

(OP)
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?

RE: RC Retaining Wall - concrete additives required?

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).

www.SlideRuleEra.net idea
www.VacuumTubeEra.net r2d2

RE: RC Retaining Wall - concrete additives required?

(OP)
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


Quote (SlideRuleEra)

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.

RE: RC Retaining Wall - concrete additives required?

Quote (kellez)

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.

www.SlideRuleEra.net idea
www.VacuumTubeEra.net r2d2

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