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    JET GROUTED COLUMNS-GROUND IMPROVEMENT FOR LIQUEFACTION RESISTANCE – A CASE STUDY Jibi C Yohannan, Va Tech Wabag, India, Email: c_jibiyohannan@wabag.in Viswanathan N, Va Tech Wabag, India, Email: n_viswanathan@wabag.in Miguel Dimadura, SEGCON, Philippines, Email: dimaduramiguel@gmail.com ABSTRACT This paper presents a case study of soil treatment carried out to mitigate liquefaction potential of the site for a Sewage Treatment Plant at Tunasan, Muntinlupa City, Philippines. The assessment was carried out using LiquefyPro software to determine the liquefaction potential and estimated settlement of soil deposit due to seismic loads of PGA 0.71g. Design approach using Baez (Advances in the Design of Vibro Systems for the Improvement of Liquefaction Resistance) was followed using the stress concentration criteria to eventually co-relate to a new factor of safety for the improved site with jet grouted columns. After installation, using high pressure jet injection system, cores were taken to evaluate the diameter formed and the compressive strength of the soil cement columns to validate the design. Keywords: Liquefaction, Ground improvement, Jet Grout columns, INTRODUCTION The site for proposed sewage treatment plant is situated south of Magdaong River and adjacent to Muntinlupa Science High School, Tunasan, Philippines. ( Fig-1) The city, in general, is underlain by alluvium (Holocene), marine and terrestrial sediments ( Pilocene-Pleistocene), and volcanic piedmont deposits (Pliocene- Quartenary) Alluvium deposits on the east side of the area are basically from deposition of sediments as the river streams following into Laguna Lake. Nearest seismic source is the West Valley Fault (WVF), a type A source, located well with in 5km distance, and to the east of the  project (Fig-2). Fig-1. Site Location Fig-2. West Valley Fault A Seismic hazard assessment study was conducted to establish design acceleration parameters corresponding to the Maximum Considered Earthquake (MCE) as defined in ACI 350.3.06 by GEOSEED. Results of the site spectra were then compared with code (NSCP-2010) spectra and concluded a high PGA 0.71g.   © 2018 Deep Foundations Institute 329    The results of the geotechnical investigation shows that the site is underlain by loose sands and silts, interspersed with medium stiff to stiff clay. Considering the location of the area near the lake shore, and the proximity of a seismic source capable of generating high-magnitude earthquakes, it is evident that the loose to medium dense sands (SPT N-value = 4 to 27) within the site subsurface are susceptible to liquefaction. Liquefaction analysis considering SPT data was undertaken using LiquefyPro software. This is based on the most recent methods recommended by the National Center for Earthquake Engineering Research (NCEER) Workshop on Liquefaction and Special Publication 117 (Guidelines in Analyzing and Mitigating Liquefaction in California). PRE-TREATMENT AND POST-TREATMENT LIQUEFACTION ANALYSES The objective is to determine the extent of material prone to strength loss with seismic loading, and to estimate the induced settlement in the event of an earthquake. The analysis is directed to implement the findings from soil investigations conducted and the subsequent Probabilistic Seismic Hazard Analysis. To expedite the calculations, LiquefyPro v.5 (Civil Tech) was used. The program determines the liquefaction  potential and estimated settlement of soil deposit due to seismic loads. The evaluation requires the input mainly of CPT, BPT or SPT data, design earthquake parameters, groundwater location, and soil classification. The PGA value for liquefaction potential is calculated using 100% code-based response spectra (NSCP 2010). The calculated PGA value of 0.5676g, which is based on the zero (0) time period, is then multiplied by an importance factor of 1.25 resulting to a PGA value of 0.71g. The Factor of Safety (FS) for liquefaction potential is calculated as the ratio of the Cyclic Resistance Ratio (CRR) to the Cyclic Stress Ratio (CSR). Liquefaction-induced settlements are based on the Ishihara and Yoshimine (1990). (FS = CRR M / CSR fs). SOIL PROFILE The soil profile for borehole BH-3 as presented hereunder (Fig-3b). The first 9m is composed of sand. Except for the 1.5m intervening sand layer, the material from Elev. -9.0m to -22.5m is composed of silt. Its consistency varies from stiff to very stiff whereby the PI is larger than 18 up to around 18m. The lower layering is a sequence of alternating layers of sand and silt. Groundwater table is measured at around 0.3m below the natural grade line. LIQUEFACTION ANALYSES Liquefaction analyses are typically done on the upper 15m strata as per earlier methods by Seed and Idriss (1971), with the procedure even extendable to 20m depth. In practice, liquefaction investigation beyond 24m are generally waived due to limitations of both empirical and detailed modelling methods, known decreased impact with depth and difficulties of mitigating deep liquefaction. For this study, the depth of analysis was limited to 20m.   For the required program input (LiquefyPro v.5), the following information, including the document sourced, are as follow: 1. SPT N values, Groundwater Location and Soil Classification from Geotechnical Investigation Report Proposed Tunasan STP Project 2. Peak Ground Acceleration - 0.71g 3. Moment Magnitude – 7.2 by the West Valley Fault 4. Overburden pressure – given by H of fill, with the net foundation load converted to an equivalent soil height. 5. Soil Unit Weight – total unit weight of soil. To facilitate data entry, an average value of 19 kN/m3 was used globally. 330    6. Fines % - percentage of soil passing No. 200 sieve (Grain Size Analysis, Final Report). This value is used to correct the cyclic resistance of the soil known to increases due to the presence of fine grained material. Fig. 3 (a)  Pre Treatment Liquefaction Analysis Graph, (b)  Bore log of BH-3 331    LIQUFACTION ANALYSIS FINDINGS Based on the parameters and soil information used, the location of liquefiable material and the estimated induced settlement are as plotted in Fig. 3a for BH number 3. Areas with Factor of Safety less than 1.0 are deemed liquefiable in the event of an M7.2 earthquake by the West Valley Fault. For BH3, liquefiable material is generally confined between -3.0m to -20.0m. However, since the induced settlement of the stretch from -9.0m and below is only 2.5cm and its effect at the upper layer could be curbed by arching effect and distance, this lower liquefiable layer shall be left untreated. Further, if PI>18 is invoked as non-liquefiable (Bray), the layers from -9.0m to -15.0m would not yield to any settlement. As such, the liquefiable layer that needs soil treatment is from -3.m to -9.0m. The summary of liquefaction analysis and computed settlement (averaged values 7.52 to 19.36cm) of various boreholes are tabulated below. (Table-1). Table 1. Average computed settlement for various boreholes OPTIONS STUDIED TO MITIGATE LIQUEFACTION Following three options were considered for mitigation- -   Stone columns -   Pile -   Jet grouted column or Deep cement mixing Based on economy of the method, Stone columns were found the most economical and Piles of high cost. The final option was guided by: -   Soil profile -   Local equipment availability, -   Pre and post installation and test regime procedures for similar ground improvement techniques, -   Economic considerations. Depending on the specific site condition and method of installation few options were more influential than others. In the case of low permeability silty soils not densifiable by the action of vibrations from the stone, the stress concentration criterion would be more relevant for a viable solution. Considering the above Jet grout columns were finally chosen. 332
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