REVIEW OF PROCEDURES TO OBTAIN
REMOULDED
REMOULDED
STRENGTHS OF VANE SHEAR TESTS
TABLE OF CONTENTS
1.0 INTRODUCTION
2.0 LOCAL PRACTICES
3.0 EFFECTS OF DIFFERENT METHODS OF SHEARING SOILS
4.0 SIGNIFICANCE OF SENSITIVITY
5.0 CONCLUSION AND RECOMMENDATIONS
6.0 REFERENCES
7.0 APPRECIATION
TECH 52
REVIEW OF PROCEDURES TO OBTAIN REMOULDED
STRENGTHS OF VANE SHEAR TESTS
By Wong King Ming
1.0 INTRODUCTION
It is a common practice to obtain in-situ shear strength and remoulded shear strength of fine soils using Vane Shear test, which can be in the borehole or push-in. Most specifications use BS Standard 1377 as a guideline to execute Vane Shear Tests. However, check on the Vane Shear test results performed by different Contractors, apparently gave different results. After personally observing some Vane Shear Tests being administered, it was found that different technicians used different procedures, particularly, in obtaining remoulded strengths. The remoulded strength is required to derive the sensitivity of the soft clay.
After some detailed desk studies, it was found that the procedures of obtaining remoulded strengths have not only varied from Contractor to Contractor, but also from one country to another country. A research study was therefore carried out to find out the difference by using two different procedures, i.e. six revolutions (BS Standard) and twenty-five Revolutions (Norway Standard and commonly practised locally) to remould the soil.
The average difference in results was found to be 37%, thereby confirming that remolded strengths and sensitivity values of the soils used in different books, Reports and Standards were not universal at all.
Local Vane Tests generally gave higher sensitivity values compared to the BS and US Standard.
The Author therefore wishes to highlight these discrepancies and recommend common “procedures” to be used so that “same mindset” can be reached when looking at the results.
2.0 LOCAL PRACTICES
The following procedures were used by the different Geotechnical Contractors to obtain remoulded strengths:
(1) Contractor A - Twenty Revolutions stated in Report, but
actually execute twenty-five complete
revolutions at site,
(2) Contractor B - Twenty-five revolutions stated in Report,
(3) Contractor C - Twenty-five revolutions stated in Report,
but actually execute ten Revolutions at site,
(4) Contractor D - Four to five revolutions stated in Report.
While BS 1377 does not mention the procedures in obtaining remoulded strength, BS5930: 1999 Clause 25.3 specifies using six complete revolutions and rest for five minutes before obtaining the remoulded shear strength. The Manufacturers of Acker Vane & Geonor Vanes, which are widely used, however, recommend twenty-five complete revolutions in the Manual for their equipments, therefore setting this local Standard.
A check with established books revealed wide variations of the procedures in obtaining remoulded strengths and are summarized in the following Table:
Table 1 – Comparison of Procedures
Item
|
Books \ Publication
|
Country
|
Procedures
|
1
|
Foundation Analysis and Design by Joseph Bowle
|
10-12 Revolutions and Rest a period
| |
2
|
Soil Mechanics in Engineering Practice by Karl Terzaghi / Ralph B Peck
|
Rotated Rapidly through several Revolutions
| |
3
|
Earth Manual by United States Department of Interior Bureau of Reclamation
|
Rotated through an angle 90 degree
| |
4
|
Site Investigation Practice by Mike Joyce
|
Six to twelve revolutions and wait for 5 minutes
| |
5
|
BS 5930 Site Investigation
|
Six complete Revolutions and rest for 5 minutes
| |
6
|
Acker Vane Manual
|
Twenty-five complete Revolutions – 2
| |
7
|
Soil Engineering Theory & Practice by Alam Singh / CR Chowdhary
|
Turn quickly at least 25 Revolutions
| |
8
|
Turn 5 complete Revolutions at 10 sec/rotation
| ||
9
|
Geonor Vane Manual
|
Twenty-five complete Revolutions.
| |
10
|
ASTM D 2573-72
|
Shear soils rapidly 10 Revolutions and measure within 1 minute
|
The following books do not specify how to obtain the remoulded strength and sensitivity, but the parameters are often mentioned:
(a) Soil Mechanics in Engineering Practice by Karl Terzaghi/Ralph & Peck , USA
(b) Soil Mechanics by Lambe/William, USA
(c) Essentials of Soil Mechanics by David McCarthy, USA
3.0 EFFECTS OF DIFFERENT METHODS OF SHEARING SOILS
A series of vane tests were executed at two opposite banks of Undup River
Table 2: Comparison of Sensitivity Using Different Number of Shearing Revolutions
Depth
(m)
|
(kPa)
|
Remoulded Strengths (kPa)
|
Sensitivity
|
Difference
In sensitivity
| ||
6 Rev
|
25 Rev
|
6 Rev
|
25 Rev
| |||
VT1
1.5
2.5
3.5
4.5
5.5
6.5
7.5
8.5
9.5
10.5
|
18.4
21.6
11.5
16.6
15.6
15.2
17.5
23.9
24.9
22.1
|
3.2
4.6
1.8
4.6
3.7
3.7
4.6
5.5
5.1
5.5
|
2.7
2.8
1.4
4.6
2.7
3.2
3.6
5.1
4.2
4.6
|
5.7
4.7
6.3
3.6
4.3
4.1
3.7
4.2
4.8
4.0
|
6.7
7.8
8.3
3.6
5.7
4.7
4.8
4.7
6.0
4.8
|
+18%
+65%
+32%
0
+33%
+15%
+26%
+9%
+22%
+20%
|
VT 2
1.5
2.5
3.5
4.5
5.5
6.5
7.5
8.5
|
19.8
22.1
32.2
48.8
45.1
58.4
45.1
64.9
|
6.0
9.2
10.1
15.6
14.7
19.3
15.6
22.1
|
4.6
5.0
6.8
8.3
10.5
11.5
11.9
16.6
|
3.3
2.4
3.2
3.1
3.1
3.0
2.9
2.9
|
4.3
4.4
4.7
5.9
5.9
5.1
3.8
3.9
|
+30%
+83%
+47%
+90%
+39%
+70%
+31%
+34%
|
It can be concluded that the difference in sensitivity value for twenty-five Revolutions compared with six revolutions, can vary from 0% to +90%, with an average of +37% for the eighteen tests done.
Variations are greater for VT2 which have higher vane shear peak strengths.
Lower remoulded strengths can be expected for higher number of revolutions, as lower revolutions may only partially remould the soil.
Most of the riverine soft soils are in flocculated state and with more number of revolutions, soils will become more dispersed which leads to lower strengths.
Besides the number of revolutions, the BS Standard also requires five minutes rest before measuring the remoulded strength. This actually allows thixotropic effect to step in, i.e. recovery of some strength. The US Standard, however, requires remoulded strength to be measured within one minute. Therefore the US standard tends to give a lower remoulded strength and thus a higher sensitivity.
From above evaluations, it can be concluded that the remoulded strengths obtained all these years by the local Contractors are not only varied but also do not follow BS or US Standards.
4.0 SIGNIFICANCE OF SENSITIVITY
Sensitivity of soils is defined as the ratio of peak strength to remoulded strength. High sensitivity of soil generally means the soil easily loses strength upon disturbance, such as vibrations caused by piling and heavy machinery movements. For a sensitivity of 7, the strength of a soil can be 35 KPa normally, but reduces to 5 kPa upon extreme disturbance. Therefore, high sensitivity often results in sudden slope stability failures for filled embankment and pile failure (friction pile) even with partial loss of strength, when disturbed. Such failures had been encountered near Simunjan town where average sensitivity can range from 2 to 56, with an average of 12 (based on 38 Vane Shear Tests). High sensitivity of soil is normally caused by leaching of salt from the pore liquid. The leaching of salt is due to underground water movement. For Simunjan site, the adjacent high hill and large water catchments area results in the continued underground water movements from the hill towards the river, thereby causing leaching of salt from the soils.
Based on a USA published Book, the following Table indicates the various conditions of soils with different sensitivity values:
Table 3 : Sensitivity of Soils (USA
(Assuming using ASTM standard)
Description
|
Sensitivity
St
|
State
|
Most clay
Sensitive Clay
Problematic soils
|
4 < = St
4 < St < = 8
St > 8
|
Insensitive
Sensitive
Extra Sensitive
|
However, the procedures to obtain the remoulded strengths were not mentioned. If it is based on 10 Revolutions (ASTM standard), then the remoulded strength is much higher and the sensitivity value is much lower compared to 25 revolutions procedures. Then the above table has to be adjusted (assuming +29% differences for 10 Revolutions) as follow before reaching the same mindset:
Table 4 : Sensitivity of Soils (Undup River
(Using Norway
Description
|
Sensitivity
St
|
State
|
Most clay
Sensitive Clay
Problematic soils
|
5.2 < = St
5.2 < St < = 10.3
St > 10.3
|
Insensitive
Sensitive
Extra Sensitive
|
5.0 CONCLUSION AND RECOMMENDATIONS
The use of various procedures (different number of Revolutions to shear the soils) had created some doubts of the “actual” values of remoulded strength and sensitivity value.
Actual tests in an alluvial soil at Undup River
The Author wishes to highlight that sensitivity and the remoulded strengths obtained all these years by the local Contractors are not only varied, did not follow exactly the BS or US Standards, and a few did not even follow the Manufacturer’ recommendations. Local Engineers who used these values and refer to USA or British books /research papers for interpretations, therefore would have mis-interpreted the conditions of the soils. It is, therefore of utmost importance to use same standard procedures from now on, so that a “Universal” value can be obtained.
Since Malaysia generally adopts BS Standard, six complete revolutions with five minutes rest are recommended for the remoulded tests, but in order to co-relate to 25 revolutions which many Geotechnical Contractors had been using for years, it is also recommended to obtain another set of remoulded strengths based on 25 complete revolutions, so that past Reports can still be co-related and used. After-all, extra remoulded test only requires another 15 minutes.
Lastly, the use of soil parameters obtained locally and then referring foreign publications for direct interpretation shall be done with caution. The Engineer must find out whether the procedures used in these publications are similar before concluding and adopting design parameters. It is also hoped that all the Engineers in other countries can also adopt the same procedures so that a common “language” can be reached without the need to “adjust”.
6.0 REFERENCES
(a) Bowles J.E. (1988), “Foundation Analysis and Design”, 4th Ed., McGraw-Hill Book Co., pp 153-157, pp 88.
(b) Bowles J.E. (1985), “Physical and Geotechnical Properties of Soils”, 2nd Ed., McGraw-Hill Book Co., pp 455-465.
(c) Terzaghi K. and Peck R.B. (1967), “Soil Mechanics in Engineering Practice”, 2ndEd., John Wiley & Sons, pp 29-31, pp 324-326.
(d) Holtz R.D. & Kovacs W.D. (1981), “An Introduction to Geotechnical Engineering”, 1st Ed., Prentice-Hall Inc., pp 570 – 587.
(e) U.S Department of the Interior Bureau of Reclamation (1965), “Earth Manual”, 1st Indian Ed., pp 562-574.
(f) McCarthy D.F. (1998), “Essential of Soil Mechanics and Foundations”, 5th Ed., Prentice-Hall, Inc., pp 370-371, pp 103-104.
(g) Koerner R.M. (1985), “Construction and Geotechnical Methods in Foundation Engineering”, 1st Ed. 1985, McGraw-Hill Book Co., pp 402-404.
(h) Dunn I.S. Anderson L.R. & Kiefer F.W. (1980), “Fundamentals of Geotechnical Analysis”, 1st Ed., John Wiley & Sons Inc., pp 30-32, pp 162-163, pp 373.
(i) Teng W.C. (1980), “Foundation Design”, Prentice Hall of India, pp 42, pp 17.
(j) Lambe T.W & Whitman R.V. (1969), “Soil Mechanics”, John Wiley & Son Inc., pp 79-81.
(k) Acker Vane Shear Instruction Manual.
(l) Geonor Vane Manual.
(m) BS 5930:1999 “Code of Practice for Site Investigations”, pp 47-48.
(n) BS 1377-Part 9: 1990 “Soils for Civil Engineering Purposes”, pp 33-34, pp 53-54, pp 62.
(o) ASTM – D 4048: “Standard Test Method for Field Vane Shear Test of Cohesive Soil”.
(p) Joyce M.D. (1982), “Site Investigation Practice”, E. & F.N. Spoon Ltd., pp 187-189.
7.0 APPRECIATION
The Author wishes to thank Timur Drilling Sdn Bhd for executing the two Push-in Vane Shear Tests at Undup River 11/10/2005 to 13/10/2005 , to make this Research Study possible.
BIOGRAPHY
Wong KM, B.E. (Hons), P. Eng, MIEM, and former C. Eng (UK), MICE (UK), MIStructE (UK), M.I.H.T. (UK), MASCE (USA), MIE (Aust), CPEng (Aust), is a Director of a Consulting Engineering Firm, with working experiences of 34 years. He had been extensively involved in the investigation, design, research and construction of Civil, Structural and Geotechnical Engineering.
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