Stone Column Plate load Test

[gokcealtun]

I am writing from Eskisehir-Turkey. Stone Column method is already being applied in my city since 5-6years. And after application, plate load test is asked to be applied on the single column to control the Project service load of the stone column by the engineers of our Municipality. As an example, if a single Stone column has 15tonnes of a project load, in the plate load test, Municipality engineers ask to load that column up to 30tonnes to control the displacement under twice bigger load than the service load. They have this idea from pile plate load test according to ASTM pile plate load test standarts. Besides, the displacements are measured quite small as 2-3cm. under 30tonnes of plate load.

According to articles which concern Stone column applications, a single Stone column may carry 15-50tonnes of a service load according to the ground conditions. Eskisehir is a city based on alluvial ground. Thereby, in some foundations, single column may carry 15tonnes but in some foundations in another districts, it may carry up to 30-40tonnes.

However, our Municipality Engineers would like to control twice more load than the Project load, any machinery in Eskisehir is not heavier than 30tonnes. In this matter, although according to calculations, each Stone column can have a Project load of 20-25tonnes; They never let us make a single Stone column carries more than 15tonnes of service load. Besides, We don’t have CPT in our city, and SPT is not able to applied due to the foundation which can not be entered by ground investigation trucks. Furthermore, only sysmic test is not enough to control the Stone column application as the foundations are quite small.

In conclusion, I would like to learn the technical specifications of plate load testing for Stone columns. Is it an obligation to control the displacements under twice more loads than the Project load? I will be glad to receive technical specifications of Stone column plate load test, and ideas about controlling Stone columns if there are any new methods to proove that ground is improved.
Thank You.

 

[oldestguy]

It would appear that the local engineer merely wants to show that the stone column can carry twice the design load. This would imply that the size of the plate is not critical, but probably should not exceed whatever footing sits on that area. If you must use a plate, too small a size may not represent what a footing on that area would do.

In this instance I would apply the test procedure that ASTM has for piles but use the largest size plate that can reasonably represent how the final load will be applied. I do not have the book for pile load tests, but I am sure some others here can do that for you.

Then, the apparatus for the loading can be a variety of methods, which can include a single weight, or more likely two weights or anchors on either side, with a beam between them and of course a jack for applying the load.

For anchors, I believe ASTM has some limitation as to how close the anchors can be to the tested "pile". Typical load test pile anchors are similar driven piles, but I am sure for 15 tonnes per anchor that a suitable screwed-in anchor can be developed. If not one anchor for 15 tonnes per side, then two, or even 4.

ASTM then has criteria for acceptability of the test, based upon settlement rate, etc. Your local code may have some guide for this

The typical plate load test usage is for determining soil bearing capacity, such as a load per unit area designation and I do not believe that is your purpose.

 

[Rockmynci]

As gokcealtun said, the ASTM standard provides good detail on this. You could also consider the ICE Specification for Ground Improvement, or the BRE guide Specifying Vibro Stone Columns.

In the UK it is common practice to test stone columns to between 1.5 to 3 times the working load, with maximum settlements of around 10-15mm usually acceptable. Maximum test loads are usually 8-11t, so your examples are much heavier foundations. Bear in mind that the results of small diameter plate load tests are only representative of immediate settlements unless the load is maintained for an extended period.

Normal piling plant is usually used as weight for the test, so the plate test is just jacked off the bottom of the rig. Typically you would only apply half the rig weight onto the plate otherwise the rig could become unstable.

Alternative testing within granular soils can use SPT, dynamic probes or geophyiscs, however these methods aren't very helpful in cohesive soils where there is no densification between column positions. The most effective test for cohesive soils is a maintainted load or 'zone' test (if time and money allows), which uses stell or concret ingots to apply load onto a larger plate to test a group of stone columns. This would normally be carried out over a period of days yo weeks depending on the situation.