Guidelines of the Swedish Weight Sounding Test-SWST in the Philippine Setting

International Symposium on a Robust and Resilient Society against Natural Hazards & Environmental Disasters and the third AUN/SEED-Net Regional Conference on Geo-Disaster Mitigation 392

Guidelines of the Swedish Weight Sounding Test (SWST) in the Philippine Setting

M. A. H. Zarco, D. C. Pekley Jr., & S. P. V. Tan Institute of Civil Engineering, University of the Philippines Diliman, Quezon City, Philippines

ABSTRACT: In this paper, the authors describe the use of the Swedish Weight Sounding Test (SWST) for subsurface investigation purposes in the Philippine setting. SWST tests were per-formed in parallel with Standard Penetration Tests (SPT) for purposes of assessing the appli-cability of existing correlations to local Philippine conditions. Data collected was used to de-velop an empirical relationship between SPT N-blow counts from SWST NSW half-turn count. The resulting empirical relationship closely resembles that proposed by Inada, indicating that other similar empirical relationships can also be used in the Philippines with minimal modifi-cation. Based on limited experience obtained from the application of the SWST in the Philip-pines, a set of guidelines is proposed by the authors for its use in the design of light structures, and preliminary geozahard assessment. As such, the SWST test provides an alternative to the widely used SPT, that is simpler, faster and less expensive to perform. This allows local communities the means to do subsurface investigations towards the goal of mitigating of geo-technical hazards.

1 INTRODUCTION

The importance of soil investigation in the construction industry, primarily to build safe and economical structures and assess geotechnical conditions that may lead to catastrophic foun-dation failures, cannot be over-emphasized. The single most important geotechnical informa-tion in the investigation is the determination of the soil shear strength. Overestimating the soil shear strength may lead to costly and irreparable damages, not to mention possible loss of lives. Underestimating on the other hand, would certainly lead to uneconomical design and costly construction. In developing countries, particularly in poor rural areas that are especially difficult to access, the need for portable, easy-to-use, manual and a reliable and cost-effective method of conducting soil investigation is apparent. This paper describes the adoption of the Swedish Weight Sounding Test in the Philippines, as an alternative to the more widely used Standard Penetration Test. The low cost for both fabricating the device as well as conducting the actual test, together with the portability of the device make it ideal for the Philippine set-ting. A correlation for the SWST penetration number with the SPT blow count is developed. The resulting correlation is similar to that existing correlations used in Japan. Consequently, it can be surmised that similar correlations for other geotechnical parameters can be used with little modification in the absence of local data.

21 EQUIPMENT AND METHODOLOGY

The Swedish Weight Sounding Test (SWST) was popularized and extensively used in Ja-pan and Sweden, and many parts of Europe. The test is a type of in-situ static penetration test used commonly in clay, peat and other organic soils and in loose to medium dense silt and sand. The Swedish cone consists of the following parts: screw, sets of 1m sounding rod, a to-tal of 100kgf (0.98kN) of dead weights and a rotating handle. (See Figure 1) The typical depth it can penetrate is up to 10 to 15 meters. This depth is sufficient for up to a 5 storey building. Kisojiban Consultants Co. Ltd. of Tokyo developed a portable version by eliminating the need to carry the set of dead weights. Instead of steel plates, the 100 kg dead weight is produced by


placing sand in a bag, which is collected at the test site. Since it is relatively portable, finan-cially impressive, and does not need electricity, fuel or drilling rig, it is ideal for remote or hard to reach areas and is applicable for small projects like construction of low-rise houses. Compare this to the Standard Penetration Test that requires a drilling rig, which has an inhe-rent limitation because of its bulkiness and financial cost that are usually expensive for small projects. Kisojiban Consultants Co. Ltd. of Tokyo developed a portable version by producing the 100kg dead weight by sand, which is collected at sites. This eliminates the need to carry the dead weight. Since it is relatively small and cheap, does not need electricity and borehole drilling, it is ideal for remote or hard to reach areas and is applicable for small projects like building a house. Compare this to the standard penetration test (SPT) that has limitations on location due to its bulk and may prove to be expensive if in small projects.

Similar to the standard penetration test, the Swedish weight sounding presumes the site to be on a flat semi-infinite ground. The site should have defined boundaries and should validate any effects of pre-stress around the area due to buildings, slopes, retaining wall or other ele-ments for evaluation purposes. After assembling the equipment, dead weight is added. Pene-tration without rotation may occur for extremely soft soils, the added dead weight of Wsw, less than or equal to 100kgf (0.98kN), may be sufficient enough for self-weight penetration. It is then recorded. If it does not self-penetrate, the handle is then rotated. The number of half rota-tion, 180 degrees, will then be counted per 2 meters of penetration and is called Nsw. It follows that the higher the Nsw, the harder the soil. The SPT-N value, bearing capacity and unconfined compressive stress may be obtained to this test. You may obtain the SPT-N value by a chart developed by M. Inada (1960) that correlates Nsw to the SPT-N value (Figure 1.). The follow-ing empirical formulas were developed:


Figure 1. Swedish Weight Sounding Method. Equipment, Principle and Presentation of Test Data. (Peckely et al., 2006.)

For sand,

0.02 sw8 W= (1)

2 0.067 sw8 8= + (2)

For sand,

0.03 sw8 W= (3)

2 0.05 sw8 8= + (4)

Inada suggests a safety factor of ±1.5-4 for sandy soils and 3 for cohesive materials due to

the various types of soil and other factors. Inada’s empirical equation for the soil’s unconfined compression strength is as follows:

0.045 0.75 u sw swq W 8= + (5)

where uq = unconfined compression strength in kPa. Based from the Japan MLIT Notification No.1113 (2002), the equation below is used for getting the bearing capacity or allowable unit stress (Qa) for a long period, and doubled when getting for a transient or short period.


( )2 30 0.6 kN / ma swQ 8 ′= + (6)

For the bearing capacity, SW8 ′ is taken as the mean equivalent rotation 2 meter beneath the

foundation bed and that the maximum SW8 ′ is 150. Getting a value higher than the maxi-

mum SW8 would mean that the soil is too hard for the device.

For loam,

( ) ( ) 1/ 9 for 90sw sw8 8 8= < (7)

For sand and gravel,

( ) 1/12 sw8 8= (8)

On the other hand, Ueda (1957) came up with a general nonlinear formula for clay, sand and gravel,

0.755

0.318 sw8 8= (9)

22 DESCRIPTION OF THE STUDY AREA The test area is in Bagong Nayong Pilipino at Manila Bay Tourism City located in Pasay

City. The City of Pasay is located at the southern end of Luzon on the natural harbor provided by Manila Bay. Pasay is surrounded by neighboring cities like Manila, the Philippines’ capital found from Pasay’s north, Makati City from its northwest, Taguig City from its east and Pa-rañaque City from its south. The project area is popularly known to have undertaken massive land reclamation and soil stabilization. Since it is a reclamation site, no related literature was found for its geological or topographic history. However, it can be apprehended for reclama-tion areas that silty sand and clayey silt will be found in the soil substructure. Based on Tan (1983), the soil profile illustrated by Figure 2 can be expected. A tentative guideline in using the SWST was proposed by the author by collecting related literatures that can form a guide-line in using the SWST. The SWST equipment was provided by PHILVOCS. The screw was shipped from Japan while the rest of the materials were fabricated in the Philippines. The au-thors performed the SWST method in Bagong Nayong Pilipino at Manila Bay Tourism City. Prior to the first day of SWST testing, the SPT was conducted months before by AGES. The SWST and SPT were conducted on the same borehole test points. The SWST test point was situated one meter away from the conducted SPT borehole to avoid soil disturbances caused by the hammer of the SPT. After setting up the SWST apparatus. The data gathered were the depth location and the equivalent number of turns of the SWST. After collecting both SWST and parallel SPT data, the analysis followed by correlating the N-values for both SPT and SWST tests and by getting the empirical formula of the test site data to check the validity of Inada’s equation in Philippine soil. A conclusion was drawn from the data to finish the objec-tive of the paper—this is to make a guideline for the SWST.


(a) Location of Study Area

(b) Manila Soil Profile (Tan, R. C., 1983).

Figure 2. Description of Study Area

23 RESULTS AND DISCUSSION

After five (5) days of fieldwork for the SWST, 39 holes were finished each having a depth of approximately 8.5 meters. The table below shows the work completed in detail. After ga-thering and compiling 208 N-value data points each from the SWST and SPT testing, the data is plotted in two ways: one is by charting SPT N-value vs SWST N-value equivalent and the latter is getting the empirical conversion equation of Nsw to SPT N-value. Along the process, it was later realized that for a certain depth interval, say 1-1.45 meters, there is one SPT N-value and three SWST-N values. This is because since the SWST is a small device, it can measure N-values in a more precise interval, say per 0.25m, as contrast to the SPT that has an interval of 1.45m. To normalize the situation of comparison, the author opted to get the high-est, average and lowest value of the three data available from the SWST and compare each of them to the SPT. This method was also done in computing for the empirical conversion equa-tion of the study area. Shown below is a sample data table that was used in getting graphical interpretations.


Table 7 umber of Completed Boreholes

DAY umber of Boreholes Com-

pleted

Oct. 16, 2009 5

Oct. 17, 2009 7

Oct. 19, 2009 9

Oct. 20, 2009 10

Oct. 21, 2009 8

TOTAL 39

Table 8 Sample data analysis

Figure 3. SPT-N Value vs. SWST-N Value Equivalent

Statistical analysis using Spearman r shows that the highest (r = 0.47), average (r = 0.47)

and lowest (r = 0.45) SWST N-value with regards to the SPT N-value shows that both values tend to increase or decrease together. Spearman correlation formula was used because the data was inconclusive for a normal or Guassian distribution. Spearman assumes no data distribu-tion. The P value of the graph (P<0.0001) implies a significant correlation. For the next analy-sis, it is assumed that Inada’s formula is not conclusive for Philippine soil. The graph plotting

sw8 against SPT-N value would then provide as a proof if Inada’s formula would be suitable

SWST -value Equivalent Rotation for 1-m Penetration (sw)

Depth (m)

SPT -Value Highest Average Lowest Highest Average Lowest

1.0-1.45 6 6 5 4 64 43 20


in Philippine conditions. The Figure 4 through 6 below show the empirical equations of each set of Nsw vs. SPT-N

values corresponding to the lowest, average and highest value of sw8 obtained for the equiv-

alent SPT-N value.

Figure 4. Highest Nsw vs. SPT-N

Figure 5. Average Nsw vs. SPT-N

Figure 6. Lowest Nsw vs. SPT-N


Comparing the three empirical equations to Inada’s equation, it can be said that the findings

are analogous.

Table 3 Comparison of Empirical Formulas

Inada Y = 3 + 0.05X

Highest Nsw Y = 3 + 0.02X

Average Nsw Y = 3 + 0.04X

Lowest Nsw Y = 3 + 0.09X

It shows that since the slope is positive, it means that the x value increases as the y value increases. Spearman r values for the graphs would also point out that the two variables tend to increase or decrease together. The results from the data collected proved to be a satisfactory sign to move on to the main objective of this paper, which is to be able to write a technical guideline for the use of the SWST in Philippine setting.

24 CONCLUSIONS AND RECOMMENDATIONS

(ア) Conclusions

The results of the data collected in the fieldwork showed that the applicability of the SWST in the Philippines is plausible. Minor adjustments were made from the tentative guideline. Furthermore, the empirical equations used from Inada proved to be suitable in the Philippines. However, in any data population, aberrations are clearly seen in the manner where data points were scattered in the graphs plotted in Graph 1, 2, 3 and 4. The author was able to investigate the possible sources of deviations. The main errata may be attributed to human error since as earlier stated, with the use of a SWST as well as other penetration methods such as the SPT, it is emphasized that the accuracy, is largely dependent on the experience and care of the opera-tor. Since the operators of the SWST were new to the device, errors may have occurred. The failure to conduct elevation survey of the SPT borehole location with regards to the SWST test points may prove to have a deviation of depth comparison on the data analysis. Aside from that, since the SWST was conducted later than the SPT, heavy construction equipments (ie. backhoe) may have ran to and fro the test points and may have caused a significant com-paction on the test point’s soil layer. To normalize the possible error, the author did not corre-late the first 1 meter interval of the SWST N-value data to the SPT N-value. To see how the error points affected the resulting correlation and empirical formula, the author disregard the obvious scattered points that deviated from the linear regression line generated in each graph. It resulted to negligible change from the previous results of the graphs. Weather condition could have also played a role for error. On the first day of field work, October 16, 2009, rain occurred around the area. The data collected in that day showed bigger deviations as com-pared to the days that did not rain. However, the author has yet to quantify the effect of rain or other weather conditions with regards to the relationship of the SPT and SWST N-values.


(イ) Recommendations

The repeatability and reliability of the Swedish Weight Sounding Test in the data shown above based on Philippine soil has proved to be encouraging. However, being the first correla-tion and test done in Philippine soil, this paper is admittedly an exploratory path. Improve-ments can be further attained with the following recommendations:

Investigate the equation for sand by M. Inada in Philippine soil. Since the author assumed the test site’s soil profile is cohesive, Inada’s empirical equation for sand was not tested. Fur-ther research can be done by getting the SPT’s data on the soil profile found for each test point. By knowing what depth of the soil strata is cohesive or sand, you can convert the respective Nsw to its specific Inada formula. The correlations may become more exact. Investigate on the variance distribution of the graphs presented. By using statistical methods, it is possible to get a percent variation of the graphs presented in this paper. The finding may lead to discover the maximum tolerable number of turns, Na, in Philippine soil. Correlate the SWST data to a cor-rected SPT N-value using the blow count correction factor. As previously stated in the RRL, the SPT N-value itself may have deviations in energy loss, overburden pressure, borehole site that can be accounted by equation 10. It would be interesting to find out if a stronger correla-tion will be extracted from correcting the SPT N-value. Apprehend deviations and account as factors to be included in the affected computations. REFERENCES

Inada, M. (1960). Use of Swedish weight sounding tests. Tsuchi-to-Kiso, Japanese Geotech-nical Society, 8(1), 11-13 (in Japanese).

Ministry of Land, Infrastructure and Transport of Japan (2002). Notification No.1113.

Peckley Jr., Daniel. (2006) Uchimura, Taro and Towhata, Ikuo. Damage Inspection and Swedish Weight Penetration Tests Paracelis Poblacion Landslide at Paracelis, Mt. Prov-ince, Philippines. Technical Report Submitted to the PCASTRD-DOST

Tan, R.C. (1983) Engineering Properties of Manila Subsoils. University of the Philippines, Diliman, Master's Thesis. Master of Science in Civil Engineering. 80 pp.

Ueda, Y. (1957). On the Swedish weight sounding rod, Tsuchi-to-Kiso, Japanese Geotechnical Society, 5(5), 9-12 (in Japanese).

Guidelines of the Swedish Weight Sounding Test-SWST in the Philippine Setting

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