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Ground Improvement Solutions to Mitigate Liquefaction: Case Studies

Abstract: There have been major advances occurred in the past in understanding as well as practicing of engineering treatment of seismic soil liquefaction and assessment of seismic site re-sponse. While research on liquefaction continues, the geotech-nical engineering practice has developed various techniques for site improvement that can mitigate the potential effects of liq-uefaction. The first part of this paper address soil liquefaction and second part concentrates on the case histories where ground improvement methods using vibro techniques were implemented to mitigate liquefaction-induced damages in major infrastruc-ture projects.

Liquefaction is defined as the transformation of a granu-lar material from a solid to a liquefied state as a consequence of increased pore-water pressure and reduced effective stress. Liquefaction is one of the critical problems in geotechnical engineering. High ground water levels and alluvial soils have a high potential risk for damage due to liquefaction, especially in seismically active regions. The most critical soil is fine sand with some silt content.

Evaluation of the Liquefaction Potential

A large part of India lies in potentially hazardous earth-quake prone zones. A large portion of eastern, western and northeastern part of the country comes under Zone V and Zone IV (Refer Fig. 1).

The simplest and probably most reliable method to eval-uate the soil liquefaction potential is statistical analyses from the past history. For this, the forces expected during a seismic event are compared with the forces that the subsoil under consideration can actually resist. Generally, the maximum surface acceleration on level ground is used as the charac-teristic value for the forces developed by an earthquake.

Estimation of two variables for evaluation of liquefaction resistance of soils is expressed in terms of Cyclic Stress Ratio (CSR), the seismic demand on a soil layer and Cyclic Resis-tance Ratio (CRR), the capacity of the soil to resist liquefac-tion. One of the most widely accepted and used SPT based correlations is the “deterministic” relationship proposed by Seed, et al (1984, 1985) represented in Fig. 2. Seed and Idriss (1971) formulated the following equation for calculation of the Cyclic Stress Ratio:

CSR = ( av/ vo’) = 0.65(amax/g)( vo/ vo’)rd

Madan Kumar Annam1, V. R. Raju2

1Technical Manager, Keller India2Managing Director, Keller Asia

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Fig. 1 Seismic Zone Map of India Fig. 2 SPT Clean-Sand Base Curve for Magnitude 7.5 Earthquakes

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Where, amax = peak horizontal acceleration at the ground surface generated by the earthquake (discussed later); g = acceleration of gravity; vo and vo’ are total and effective vertical over burden stresses, respectively; and rd = stress reduction coefficient which accounts for flexibility of the soil profile. Seed and Idriss (2001) used the approximated equation for CRR for clean-sands that used a base curve and fitting the following equation:

CRR7.5 = [1/ {34-(N1)60}] + [(N1)60/135] + [50/ (10(N1)60+45]Where (N1)60 is SPT N value projected for clean sand

obtained after corrections on measured field SPT N value. The above equation is valid for (N1)60< 30. For (N1)60>30, clean granular soils are too dense to liquefy and are classed as non-liquefiable (Refer Seed and Idriss 2001).

Ground Improvement Techniques

The liquefaction potential of weak deposits can be mit-igated with ground improvement techniques such as vibro replacement (vibro stone columns) and vibro compaction. These techniques use vibratory energy to densify loose soils at depth by backfilling. Principles of vibro replacement and vibro compaction techniques are discussed in this paper.

Deep Vibro Techniques

Vibro technique offers the weak deposits to get compac-tion, drainage and increase in shear resistance. Fig. 3 shows transition zone of soils tends to liquefiable and possible tech-niques of ground improvement with deep vibro compaction or replacement.

Vibration is achieved by means of powerful vibrator at deeper depths. The vibrator is connected to a source of elec-tric power and a high-pressure water pump. Extension tubes are added as necessary, depending on the treatment depth, and the whole assemblage is suspended from a crane. A Schematic showing Vibro Compaction technique is presented in Fig. 4.

Vibro Replacement

The stabilization of weak deposits by displacing the soil radially with the help of a depth vibrator, refilling the result-ing space with granular material and compacting the same with the vibrator is referred to as Vibro Replacement.

The resulting matrix of compacted soil and stone columns has improved load bearing and settlement characteristics. A schematic showing the basic principle of the vibro replace-ment technique, explained in Fig. 5. Keeping the site conditions in view vibro stone columns can be installed either wet method (top feed) or dry method (bottom feed). Technically and func-tionally, vibro stone columns installed in both methods serve similar.

Fig. 3 Application ranges of the deep vibro techniques

Fig. 4 Schematic showing vibro compaction technique

Fig. 5: Schematic showing vibro replacement technique

Vibro Compaction

The basic principle behind the vibro compaction process is that particles of non-cohesive soils can be rearranged into a denser state by means of vibration.

The above ground improvement techniques were adopt-ed in various infrastructure projects to mitigate liquefaction potential across India.

Few case studies of the executed projects falls under seismic Zone IV and V as per IS 1893 Part 1 (2002) are dis-cussed in the following sections.

Case Studies

Power Plant at Goindwal Saheb, Punjab

A power plant of capacity 2 x 270 MW coal based Thermal Power Plant was built at Goindwal Sahib, near Amritsar, Punjab. Power plant structures such as Boiler, Electro Static Precipitator (ESP), Switch Yard, Power House Building, etc. were planned as part of development of power plant.

Soil at this project site is primarily sandy silt / silty sand to about 1.5m to 2m depth, followed by fine sands with fines content of about 6% to the final explored depth of about 30m. The average SPT N value is 10 up to a depth 4m to 6m from the existing ground level and SPT N value ranges from 15 to 25 to a depth of about 15m. Medium dense to dense sand layers were encountered beyond 15m depth with SPT N values are generally > 25.

The existing natural soils (fine sands) at the proposed site being loose were susceptible to liquefaction in an event of an earthquake. Hence, Ground Improvement by Vibro Compaction

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Technique was proposed to mitigate liquefaction and to en-hance the bearing capacity.

Vibro Compaction for main works has been carried out to a depth of 8m.

to liquefaction. Since the project location falls under Seismic Zone IV, the required field SPT (Standard Penetration Test) shall be more than 20 (performance criteria requirement by the designer), to mitigate liquefaction and to achieve bearing capacity.

Ground improvement technique using vibro compaction was proposed as treatment to mitigate liquefaction and to enhance the safe bearing capacity of the loose sand deposit till 9 m depth below the existing ground level.

About three boreholes prior to and two boreholes after the commencement of vibro compaction works were carried out in the project area to assess the effect of vibro compaction. It can be seen from Fig. 9 that the post treatment SPT N values (shown in discontinuous lines) are larger than the perfor-mance line confirming the effect of the ground improvement.

Fig. 6 Twin Vibrators in action.

Fig. 8 Pre and Post CPT results at ESP area

Fig. 9 Graph showing performance of vibro compaction

Fig. 7 The difference in levels achieved (about 1m) as a result of vibro compaction

Post Cone Penetration Tests (CPT) were conducted after completion of the vibro compaction for various structures, as part of QA/QC procedures. Based on the analysis of the post CPT results, a Relative Density of more than 70% was achieved. Pre and post cone resistance (Qc) values are shown in Fig. 8 in which the target relative density is also presented. The targe relative density is evaluated based on correlations proposed by Schmertmann with respect to Qc. In addition to the above, 2nos plate load tests (20 t/m2 & 40 t/m2) were con-ducted at cooling tower I & II area to assess the load vs set-tlement behavior of the improved ground for required safe bearing capacity of 10 t/m2. The plate load test results indi-cate that the settlements are less than 5mm in both cases.

A School Building, Noida

One of the reputed educational societies in Noida, Uttar Pradesh has proposed to build a school building. The soil in-vestigation at the proposed site revealed that loose to medium dense fine sand exists to a depth of 9m which is susceptible

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Fig. 10 Graph showing Load Intensity vs Settlement

Fig. 12 Load Intensity vs. Settlement curve at STP, Noida

Fig. 11 Installation of vibro stone columns at STP, Noida

Further, a field plate load test was also performed at site to assess bearing capacity of the treated ground. The observed settlements were within the acceptable limits for the applied load intensity as illustrated in Fig. 10.

Sewage Treatment Plant, Noida

Greater Noida Development Authority was constructing a 137 MLD Sewage Treatment Plant in Greater Noida, Uttar Pradesh. Various structures such as Chlorination tanks, SBR basins, Air blower, Grit chambers etc. were proposed. The project location falls under seismic Zone IV, however as per the requirements of the Client the ground improvement techniques were adopted to satisfy liquefaction effects of Zone V conditions.

The soil profile comprise of silty sand layer of 3m to 4m thick with SPT N ranging from 6 to 8 followed by fine, loose to medium dense sand up to a depth of 10m below the existing ground level with SPT N ranging from 11 to 18. A clayey silt / silty clay layer was encountered up to 20 m depth with SPT N more than 20. A load intensity of 10 t/m2 to 15 t/m2 was anticipated due to various structures. The top soil layers up to 10m depth were susceptible to liquefaction.

Vibro Stone Columns were installed in triangular grids of different spacing under strip and raft foundation to a depth of 10m from existing ground level to mitigate the liquefaction and to enhance the bearing capacity.

A field plate load test was performed at site to assess bearing capacity of the treated ground. The observed settle-ments were found within the acceptable limits for the applied load intensity as shown in Fig. 12.

LNG Terminal, Hazira

Two liquefied natural gas storage tanks were construct-ed in Hazira LNG Terminal Project of each 84 m in diameter and with a filling level of approximately 35 m. The site is located at an estuary on the coast of the Khambhat Gulf in India.

The subsoil profile at the project site consists of loose to medium dense silty sand up to a depth of 16 m below the ex-isting ground level. The upper 4 to 5 m was recently reclaimed material. The fines content of the sand was in the range of 15% on average, sometimes slightly higher. Very dense sand with SPT > 50 was encountered below 16m from existing ground level.

A peak ground acceleration (PGA) of a = 0.24g confirming to seismic Zone IV conditions were assumed in the design of ground improvement system. Analysis and design was car-ried out using the method stipulated by Priebe (1998) for the initial in-situ density conditions. Ground improvement using vibro replacement technique was adopted to mitigate lique-faction.

Vibro stone columns of 16 m long were installed in a square grid pattern. Additional strips of stone columns were installed around the periphery of the tank to provide additional stabili-ty to the treatment area in case of a seismic event.

Pre and Post CPT at site on trial stone columns were carried out. Post treatment results showed a two-fold increase in the CPT values of the sand zones as shown in Fig. 13. Hydro tests were performed on the installed tanks and the expect-ed settlements were in the range of 120 mm, well within the limits at the centre of tank under full tank load of 23.0 t/m2.

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Fig. 13 Pre and Post CPT results

Table 1 Embodied Carbon Comparison

Fig. 14 Tanks commissioned after successful Hydro Test and in oper-ation for last 8 years

Power Plant in North Delhi

A gas based combined cycle power generating capacity of 108 MW was constructed at North Delhi. Main plant struc-tures were proposed to be built on deep foundations where-as lightly loaded ancillary structures such as clarifloccula-tor, storage tanks, switchyard etc. (loading intensity of about 10 t/m2) was proposed to be placed on shallow foundations.

The soil profile at the project site in general consists of loose to medium dense sandy soils with N values ranging between 5 and 10 to a depth of about 10 to 12m followed by dense silty sands / sandy silt (N > 15 to 30) to about 30m. This project site falls under Zone IV with peak ground accelera-tion of 0.24g. The native loose sandy soil deposits were sus-ceptible to liquefaction to a depth of about 10 to 12m below existing ground level.

Vibro replacement technique using dry vibro stone col-

umns (bottom-feed displacement method) was adopted to ensure required bearing capacity to eliminate pile foundations. In addition, the proposed ground improvement technique was designed to mitigate the liquefaction potential in the event of earthquake.

The proposed technique allowed fast construction in a congested site avoiding usage of water and subsequent muck removal. Single column plate load test was conducted on the installed dry vibro stone columns to a maximum load intensi-ty 50 t/m2. Settlement under the ultimate test load was ob-served to be less than 16mm (Refer Fig. 15).

In addition to the technical performance and commercial benefits, an embodied CO2 calculation showed an environ-mental benefit (lower greenhouse gas emissions) of the Vibro stone column solution as shown in Table 1.

Sl. No. Item Embodied CO2 [kg]

BCIS Piles Stone Col-umns

1 Concrete (incl. wastage) 667,516 -

2 Stone aggregates (incl. wastage) - 42,412

3 Reinforcement Steel 382,373 -

4 Fuel consumption for installation 199,354 118,350

Total 1,249 T 161 T

Product Packaging Unit at Babrala, Uttar Pradesh

Expansion of the existing Product Packaging Unit was under development at Babrala, Uttar Pradesh. As part of ex-pansion, various structures such as Conveyor Belts, MCC cum Control room and the Wagon Loading Platform were to be constructed.

The soils at the project site consisted of loose to medium dense sand with fines less than 10% to about 12 m depth from EGL with top 3.0 to 4.0m of clayey silt. The ground water table was encountered at a depth of 3.0 m from EGL. The site

Fig. 15 Load Intensity vs. Settlement Curve

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was prone to liquefaction during an event of earthquake as it falls under seismic Zone IV.

Ground improvement using vibro compaction/vibro re-placement technique was proposed to mitigate the liquefac-tion potential of the soil to enhance safe bearing capacity of soil and reduce the estimated total and differential settle-ment of the soil to a depth of 12 m below EGL.

Vibro stone columns were installed for conveyor belt foundations and the transfer towers. A combination vibro stone columns and vibro compaction has been used to sup-port the foundations of MCC room which is the most inter-esting aspect of the project.

The vibro stone columns were constructed in the top 4m followed by vibro compaction up to 12m. An illustrative sketch is shown in Fig. 17.

Fig. 16 Typical borehole log at Babrala

Fig. 18 Pre and Post SPT comparison

Fig. 17 Sketch showing ground improvement with combination of vibro techniques at MCC Room of the plant

Plate load tests were performed over the treated area and results confirmed that the settlements at design loads were within the allowable limits.

Post treatment CPTs’ were also executed which proved an average improvement of two folds in the Qc values and of 70% relative density was achieved. Also, post treatment SPTs showed a considerable improvement in SPT N values to about 2 to 3 times in the treated area as shown below in Fig. 18.

Conclusions

The presence of liquefaction soil does not mean that one has to abandon the site or to install deep foundations. In seismic zones with liquefiable soils, ground improvement technique provides technically sound and cost effective solu-tions. Efficient and economic solutions to problems caused by soil conditions require a thorough evaluation of project conditions, project needs, method capabilities and a field test program.

Six projects have been described in which vibro meth-ods have been successfully used to accomplish the required ground improvement to mitigate liquefaction and to en-chance bearing capacity requirements. The success is mea-sured in terms of either load tests or by the comparison of results of pre and post soil investigation done at particular sites.

References

1. Seed H.B., and Idriss, I.M (2001). Simplified procedure for evaluating soil liquefaction potential, J. Geotech Engineering. Div., ASCE 97 (9), 1249-1273

2. Seed et. al (2003). Recent advances in soil liquefaction engineering: A unified and consistent framework.

3. Priebe, H.J (1995), The Design of Vibro Replacement, Ground Engi-neering, December 1995

4. Priebe, H.J (1998), Vibro Replacement to Prevent Earthquake In-duced Liquefaction, Ground Engineering, September 1998

5. Raju et. al. (2010). Some Environmental Benefits of Dry Vibro Stone Columns in a Gas Based Power Plant Project, Indian Geotechnical Conference, December 2010. w

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