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Cuộc thi vui về Dự đoán SCT của cọc do GS Bengt Fellenius đề xuất - Diễn đàn của các kỹ sư kết cấu Việt Nam

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Old 22-05-2011   #1
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Default Cuộc thi vui về Dự đoán SCT của cọc do GS Bengt Fellenius đề xuất

Vừa qua, GS. Bengt Fellenius có tổ chức một cuộc thi vui về dự đoán SCT của một cọc nhồi tiết diện nhỏ (DK400). Số liệu đầu vào có SPT từ 5 hố khoan và một số chỉ tiêu khác như dưới đây và trong file đính kèm.

Thư số 1:

April 30, 2011

Hello Good Friend!

On Monday May 9, I will attend a full-scale static loading test on a strain-gage (VWG) instrumented continuous flight auger (CFA) pile, as per the below details. Would you care to try your hand at predicting the outcome of the test in the form of a total capacity value and a load-movement curve where the pile capacity is indicated? Maybe also indicate the load-movement curves of the pile toe and pile shaft?

I was challenged to provide a prediction -- not that I need much challenge; I enjoy sticking my neck out. Then, I thought, it would be extra fun to be in the company of my good friends.

If you e-mail me your prediction at the latest on May 8, I will send you mine. And, after the test, I will send you the test results.

The static loading test will be performed on a 400-mm diameter, 18.5 m long, strain-gage-instrumented CFA pile three weeks after its construction. In constructing the pile, a 30 % "overconsumption" of concrete was measured. The flow meter indicated a uniform flow and the pile is considered free of any bulges or neckings.

The test method will be the quick maintained load method with small increments applied every 10 minutes until the pile fails, pile head has been moved 3 inches, or the maximum capacity of the (good size) jack is reached.

The soil profile is indicated in the attached soil boring table. In summary, the soil profile consists of a glacio-lacustrine clay and silt to about 8 m depth with a water content of 40 % and plastic and liquid limits of 25 % and 60 %, respectively. The groundwater table lies about 1.5 m below grade and the pore pressure distribution is hydrostatic. The SPT N-index ranges from 8 to 12.

Below 8 m depth, the soil consists of glacial clay till containing interbedded sand and gravel. The water content ranges from 15 % through 25 % and plastic and liquid limits are 25 % and 60 %, respectively. The SPT N-index ranges from 11 to >50 with and average of 25. Drilling terminated in the till at 25 m depth in a sand layer, but other boreholes at the site show the till to go deeper than 25 m, so the sand is a local lens.

As is the geotechnical practice of the area of the test, no CPTU tests or laboratory testing beyond basic soil parameters are performed. What you see is what you get!

Too short a time to do it? Hey, this is the real world — a full week to give a reply, that's plenty compared to many projects. Give it a quick try!

Best regards,


Thư số 2:

May 2, 2011

Hello once more,

I have received many responses asking for additional information. I very much appreciate the interest. Here is a bit of additional information, although, please note that the prediction is a low-key effort and we should not take it (nor ourselves) too seriously. It is for entertainment, mainly.

(1) the pile will be tested from grade and there is neither excavation nor fill placed at the site.

(2) the test is a conventional head-down test and the reaction will be from four anchor piles constructed to the same depth as the test pile. The test load is from a 500 ton jack. (Nobody in charge of the reaction arrangement has had the opportunity to optimize the system. It is sturdy, but may show excessive movement well before 500 ton).

(3) the pile has an 18- m long reinforcement cage stopping 0.5 m to 1.0 m above the bottom of the augered hole. The casing consists of five 20-mm rebars. Spirals (stirrups) have a 9-inch spacing.

(4) the clay till an eroded, transported, and re deposited glacial till interspersed with occasional layers of silty sand and silt (note the correction of water content values in my previous message) To put you on equal footing with me, the project site is in the same general geology as the tests I wrote up in my paper to the Schmertmann ASCE GSP volume (full citation below). You can download a copy of the paper from my web site: "270 Schmertmann Volume ASCE.pdf".

Fellenius, B.H., 2008. Effective stress analysis and set-up for shaft capacity of piles in clay. Honoring John Schmertmann "From research to practice in geotechnical Engineering", The Geo-Institute of the American Society of Civil Engineers, Edited by J.E. Lair, D.K., Crapps, and M.H. Hussein. ASCE Geotechnical Special Publication, GSP 180, pp. 384 - 406.

Note, the N-index at 16 m depth is smudged which has masked the value. It is 19 bl/0.3m not just 9 bl/0.3m.

I don't think the above information will make anyone who already sent me a prediction (oh boy, there are some fast persons around!) want to adjust their prediction. But, if so, please feel free to do so. The cut-off date is next Sunday the 8th.

Best regards,


Thư số 3:

May 4, 2011

Hello all,

I have received a couple of requests for more soil information. Sorry, I do not have much. However, I do have the N-values determined in three other boreholes at the site, BH2 - BH3. They are all within about 30 m to 50 m from each other and the test pile. The soil descriptions and water contents are very much the 'same' as shown in the borehole log I sent you with my first message. Today, I received a borehole log containing only N-indices from a new borehole the Owner had drilled about 2 m to 3 m away from the test pile. The N-indices are added to the below table. The new borehole was by a different drilling company.

(m) (N) (N) (N) (N) (N) BOREHOLE

1 10 7 6 7 10
2.5 7 11 7 9 5
4 5 >50 8 11 6
5.5 5 - - 8 10 4
7 9 24 13 38 4
8.5 31 >50 24 16 9
10 >50 26 >50 40 8
11.5 >50 13 - - 11 >50
13 25 14 26 28 >50
14.5 25 17 20 33 >50
16 19 17 14 32 >50
17.5 26 19 12 29 >50
19 22 32 21 29 >50
20.5 19 5 25 >50
22.5 23 22 >50 24
24 >50 14 18
26.5 21 13

If anyone in the light of the above wants to adjust his/her prediction, be welcome to do so. I made my own prediction having only the results of BH1 and I see no reason for adjusting it. I have received about a dozen predictions of capacity and load-movement. Great, I welcome more, though.

To those who send me a prediction and/or express interest in hearing how it all worked out, I will e-mail my own prediction on Sunday night and then later in the week send you a message with the main results.

Best regards,


Xin mời mọi người tham gia dự đoán và KQ từ chính GS. sẽ được đăng tải sau một tuần nữa.

File Kèm Theo
File Type: pdf 11-04-30 Prediction Soil Boring Table.pdf (80.3 KB, 34 lần tải)
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