Densification of Saturated Sand Idea 1

[WWPierre]

Hello Ladies and Gentlemen,

I live on the delta of the Squamish River, halfway between Vancouver and Whistler, British Columbia, Canada. I am not a professional engineer, but I am a damn good engineering technician.

Our town has decided to adopt the principals of Smart Growth, and has settled on fairly high density in the Downtown area.

The town has recently acquired a brownfield peninsula of about 100 acres, the Oceanfront Lands.

http://squamishoceanfront.com/index.php

This is mostly man-made, having been dredged from the surrounding water. The elevation is about 3-3.5 M. Tides are about 2.6M either side of 0 geodetic. It has been determined that about 1500 residential units will be required to pay for the servicing and the amenities demanded by the people of the town. The area is subject to uncomfortably high winds from the water side in the summer. They generally rise at 10am and abate around sundown.

The townspeople have demanded that a great portion of the site be public lands. My proposal is to accomodate the residential requirement in 3 14 story mid-rise buildings with cast-in-place bents 33' apart connected by pre-cast panels and steel trusses. The floors would be concrete with hydronic coils on Q-decking.

There is a thick (enough) layer of saturated sand at about 0 geodetic throughout the downtown. Most new development is founded on this layer. The overlying 8' of "loon****" is excavated, and a mat of broken rock or pit run is hauled in, and compacted, either in layers with a vibrating roller, or with a large hammer mounted on a tracked excavator.

The 8' of loon**** removed from these sites is becoming a problem throughout the community. We are running out of places to get rid of it. I am proposing that it be trucked to the Oceanfront lands and used as a pre-load. After the buildings are built, this soil would be piled against and over the parking structures, making them invisible, and increasing the public space, while providing shelter from the wind.

Given the fact that we are hauling this non-structural fill away anyway, and we are going to be using it on site after construction, what if there was a way to vibrate the whole site after the pre-load was applied.

Here is my idea:

10' lengths of 4" pipe would be fitted with bearings on each end holding a shaft with an eccentric weight. These would be strung together and jetted into the ground to a certain depth on a certain grid spacing. An electric motor would be attached to the top, and the supply wire ran to a control station. The pre-load would then be hauled in, and after it was in place, the motors would be activated.

After settlement has taken place, the preload would be moved aside, the motors recovered,the vibrators abandoned and the structure built. The pre-load would then be replaced around the building to the depth of the roof of the parking structure, giving public access to the roof and the lower floors of the mid-rise.

The vibrators could be fairly cheaply made in any metal fab shop. The question is, what would be the effective lateral coverage of the vibratory action of each installation? If they were effective for a radius of 10', for instance, they could be installed on a 20' grid. If they needed to be 5' apart, it wouldn't be cost efficient.

If you turned them on for a week, would you get significantly more settlement than if you ran them for 10 hours? Would the water keep seeping out of the sand as long as you kept the vibrators working until the sand approached 100% density?. (the implication being that much larger and heavier structures could be contemplated.)

There is an opportunity to excavate about 600,000 cubic meters of granite nearby. Supposing the property were to be used temporarily for stockpiling this material, and the stockpiles were located where buildings were to be eventually built. Does it make sense for the community to allow the temporary use of this land for free, knowing that when all the product is sold, the stockpile sites would be ready to build on?

Thank you for reading this, I would appreciate any comment.

Regards,

Peter B. Legere
Consulting Visionary

 

[VAD]

Interesting thoughts and questions. There are may ways to look at your questions.

Before going further, I would like to ask a few questions:

1. Have you discussed your concept ith the geotechnical consultant that forms part of the board re the web link.

2. If other developments are founded on the sand layer,it seems that the process of compacting the rockfill through dynamic compaction has been employed, but you indicate that rolling with a vibratory roller is undertaken as well. Something tells me that the sand deposit is not a potentially liquefiable deposit.

I can ramble on but the bottom line from my perspective giving full credit to your thoughts is that much more information is required for any geotechnical engineer to provide good responses to your questions.

Rather bluntly, you will probably obtain a series of suggestions, and only you would decide what suits you. If you can go to a doctor and let know about your ailments, he will listen to you but will also want you to listen to him and will conduct his tests and analyse the problem with your ideas in mind plus his own.

Too often good geotechnical technicians have great ideas but sometimes fail to understand that there are issues that a professional engineer must come to grips with before making decisions. This goes for other professions as well. In my experience ideas such as yours have also led to better solutions to problems. So I like to encourage thoughts from technicians etc. I was there myself.

In summary, I would strongly suggest that you discuss your ideas with both the geotechnical and structural consultants assigned to the project. You may find that they mat determine that they are perfectly sound.

Having said the aforementioned not with any disrespect,I would give some thoughts on items listed below, if we were working together on this project:

1. Seismicity of area, site topography,

2. Susceptibility to liquefaction of sand deposit

3. Insitu characteristics of deposit - density etc

4. If 14 story structures, what is the soil stratigraphy with depth - depth of sand, material below sand etc

5. What effect would the vibration as you would like to provide cause if the deposit is potentially liquefiable. Would it cause lateral spread or would settlement be taken up by the "loon shit" sinking below the surface non uniformly perhaps. Is that desirable.

6. Would the structure have to be founded on piles taken below the sand layer. If so is there any benefit in undertaking your proposed type of compaction. Would the preload be sufficient to firm up the near surface soil. What height of preload would be satisfactory etc

7. What is the overall planning scheme for the grounds around the buildings. Roadways have to be built etc. Would settlement be an issue.

8. What has been the performance of other structures presumably built on this similar ground.

so on so forth.

Regards

 

[1Kat1]

1st I would like to say that VAD gave very good advice. 2nd, while I was reading your thoughts on the vibratory methods you were tossing around, I was envisioning the lab sieve, which works on a similar fashion. The material in put in the top of the sieves, which has the smallest sieve size. The entire machine shakes for about 5 minutes, until everything has made it through each sieve size. The remaining fines or lets call it sand is on the bottom tray. The sand is in a fluffed matter. Now that is dry sand of course. Now you take wet sand, and put it through the same sieve and you end up with wet balls that do not pass through the small sieve. Now you have to dry the material, in order to put it through the sieve. Sand in itself will have a high moisture value, in order to reach the required density set forth. I therefore might suggest having sample lab tested for optimum moisture. If the material is only slightly higher in moisture, than what is optimal, why not excavate deeper, and put in a dry material, and use static roll only. Sand has a low dry density value. By bringing in a lift of dry material, you will be allowing for the static roll, achieving required density, and lessoning the chance of water pumping up.