basic question about coefficient Ka
basic question about coefficient Ka
(OP)
Hello
i need help fast , if you substitute a certain amount of back fill behind a retaining wall with gravel for drainage . What will be the coefficient of earth pressure Ka to take ? will it be the Ka of the gravel or that of the true backfill behind (for ex clay) ?
i need help fast , if you substitute a certain amount of back fill behind a retaining wall with gravel for drainage . What will be the coefficient of earth pressure Ka to take ? will it be the Ka of the gravel or that of the true backfill behind (for ex clay) ?





RE: basic question about coefficient Ka
What soil paramters are you using for clay and gravel?
Assuming friction angle of 32 for gravel and 24 for clay then using Ka for Clay would be more conservative.
A sketch with soil parameters would help a lot.
RE: basic question about coefficient Ka
RE: basic question about coefficient Ka
RE: basic question about coefficient Ka
RE: basic question about coefficient Ka
If the reverse of that (12m wide by 3m deep), then no question it is gravel Ka.
The active pressure from the clay will be attenuated in the gravel, but the gravel still has a reasonable component.
RE: basic question about coefficient Ka
A general rule is if you go a short distance behind the bottom of the wall or back of footing (0.5m?) then slope up at a 1:1 wedge, you can use the strength of the wedge material for the Ka calculation. If you are going to have a vertical interface between materials, you may have to consider the Ka from the material behind the gravel.
Keep in mind if the strength of the "clay" is low then global stability issues can come into play as a slide plane may go around the gravel zone and under the wall. Sometimes it is not all about lateral earth pressure.
RE: basic question about coefficient Ka
and yes its a cantilever wall
RE: basic question about coefficient Ka
The bigger challenge is in calculating an effective Ka for a cohesive material at a height of 11m since earth pressure equations are generally based on cohesionless materials. Cohesion of 50kPa (1000 psf) is significant but can it be counted on for the life of the structure? The conservative method is to not use the cohesive component and just deal with the high calculated pressure of phi = 20 deg.
You would have to draw a 1:1 line from the bottom of the footing to the upper right and fill that zone in with gravel to use lower calculated pressure for the complete analysis.
RE: basic question about coefficient Ka
RE: basic question about coefficient Ka
One of the first ones on the list is:
http://www.dot.ca.gov/hq/esc/techpubs/manual/bridg...
California Dept of Transportation is similar to other highway docs and the AASHTO code. Concrete walls have been designed for years so lots of info available.
RE: basic question about coefficient Ka
Question for killswitchengage: Is this wall being constructed in a cut situation or is the retained soil going to be a fill? If a cut situation, the proposed wall is very high and it may be more economical to build a permanent anchored wall. If a fill situation, it would be more economical to build an MSE wall. An 11 m high cantilevered concrete gravity wall is a monster.
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RE: basic question about coefficient Ka
This solution was advised since under the wall there is a public workout . Above the wall to some distance there is an important road . Beside, it was very difficult to get soil sampling because of the tight configuration of the site . We thaught about using anchored wall but we have no idea what's under the road which is about 20 m away from the wall
RE: basic question about coefficient Ka
killswitchengage, your response really did not answer my question about the wall needing to be built in a cut or fill situation. Also, the road being 20 meters away from the wall is a fairly great distance. Tieback anchors usually are shorter than 20 meters (except maybe for landslide situations) and angle downward which shortens the horizontal projection beyond the face of the wall. Therefore, tieback anchors, if used, may not extend under the road. Designing any wall, especially one that is 12 meters high, without soil borings or a geotechnical report is not wise. I would get soils information first. Then I would chose a wall type (cut wall, fill wall, gravity wall, non-gravity anchored wall, etc.). Last, I would worry about the earth pressure coefficient.
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RE: basic question about coefficient Ka
RE: basic question about coefficient Ka
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RE: basic question about coefficient Ka
If i am to be honest , the biggest problem we are facing is that the so called clay; is actually fill made of loose clays with some cobbles . Sampling failed to get any sample not only due to soil low quality but mostly due to the inexperience practice of our drill team . We had to guess soil shear strength from its visible behavior which its loose .
We thought about using cantilever wall with micro pile foundations but the client denied the solution ( He will change his mind afterwards) .Using tiebacks anchors needs firm ground and the only solution in my mind is to anchor it downward into the fresh limestone under the so called clay , but it will not solve the height issue .
I thought also about using a 7 m high wall and the correct the material above the back fill to an angle equal or less than Phi , but i think its unfeasible since the road above and a path to nearby houses could be in jeopardy
RE: basic question about coefficient Ka
loosesoft clay, and a very high wall? All you need next is a ground water problem. Sounds risky and very expensive for building shelters for fishermen. I also suspect that you could have a global stability problem with the completed wall. Checking global stability will be hard with no soil properties or ground water information. Seems to me that you will need a multi-tier, tiedback, retaining wall with long tiebacks and possibly long soldier beams or sheet piling. You have your work cut out for you. Good luck.www.PeirceEngineering.com
RE: basic question about coefficient Ka
Anyway , i should remind you all that under the fill material to be retained there is very good quality limestone then a succession of limestone and micaschist. I am thinking either build a MSE with geogrid ( to hell with the shelters ) or use tieback anchored to the limestone floor.
RE: basic question about coefficient Ka
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