Consolidation in clay soil 2

[eit2dot]

We are building a 40' highway on A-7-6 (clay) over a railroad. The max embankment to the approaches of the bridge is 35' and will be using A-7-6 as well as embankment material. MSE walls as high as 20' will be built to hold the embankment on each side where the railroad goes underneath the bridge. Yes we have essentially floating walls and this bothers me. According to our geotech consultant, the assumption is that the embankment is not saturated and that primary consolidation is essentially compression and all settlement will occur during compression and that no secondary consolidation will occur. I am having a hard time believing this. can anyone explain this to me? Thanks a lot.

 

[BigH]

How deep is your clay bearing stratum? What is its consistency - soft? firm (or medium stiff for you Yanks)?, stiff, very stiff? What are the estimated times for the bearing clay to consolidate? Are you installing any wick drains to speed up settlement, etc.? (in India we built a similar structure on 6 m of very soft clay and used wick drains and stage loading so we wouldn't have a shear failure. 11 m high wall (35 ft) settled about 1.1 m (40 inches) during construction.) Secondary consolidation will occur - but may be "so far in the future" that it is judged minor for the structure. A lot depends on the answers to the question I put forth above. In any event, make sure you have bridge end slabs extending out over the fill to take the "bump" out that will occur at the fill/bridge interface. Also, if you have time, I would suggest that you add a surcharge of 2 m or so in the last 15m of the approach and let it sit as long as possible, then scrape off and put in your pavement structure - this will help in reducing the bump's magnitude.

 

[fattdad]

I have no problem considering the embankment as compressible. It won't be saturated and by virtue of compaction it will be overconsolidated. Assigning the modulus value deserves some careful thought. . .

I'm not sure I'd be as generous for the clay below the foundation. I'd want to have some 1-d consolidation data and I'd want to consider the change in stress. If the stresses are below the preconsolidation pressure(s), then I'd be o.k. with compression. Careful thought on the modulus value(s) would be my biggest concern. That said, if you have the consolidation data, why not just use Cr and time readings to determine the magnitude and rate?

f-d

¡papá gordo ain’t no madre flaca!

 

[oldestguy]

I'd ask the getoech what percent compaction is planned for the fill. For A-7-6 material I'd expect optimum to be a lower moisture content than where the fill came from. Will there be expansion in the future should moisture content return to the values it "liked to have" in the cut? Moisture changes in the future would be my main concern, resulting in volume changes. What has been the experience in the area for volume changes? Some areas are prone to plenty of trouble.

 

[eit2dot]

Depth of clay is around 8 to 12 ft. Underlying it is shale bedrock with varying thickness of 2 to 15 ft accross the site. Ground water is encounteted at 9.5 ft below ground surface in one of the test holes and none was encounteted in the remaining 13 bore holes along 1.4 miles stretch. I have no idea what the consistency is but will let you know once I find out. Estimated consolidation is 7.5 in over a period of 5 months. A slab extending about 150 ft will also be installed on sleeper slab. Another problem i have is that at phi 28 deg and cohesion at 100 psf the mohr circle is above fracture line. I dont know much about geotech engineering hence i am having a hard time interpreting the test results.

 

[eit2dot]

Fattdad, I will look at the data per your pointers, I do have it. I just have difficulties understanding it, its been a long time.

Oldestguy, that is the biggest concern since previous projects in the same geo area are bumpy in cut areas. It is recommended that we over excavate it and fill with ganular materials.

I guess the favor I am going to ask next is that if you have anything else to add on what to look for as I do have the CU and 1d tests I will research more about it so I can interpret the report correctly and understand the consultant. I cant argue my concerns as I cant support my logic.


Thanks a lot, appreciate it so much.