GROUND IMPROVEMENT BY JET GROUTING TECHNIQUE FOR FOUNDATIONS OF A NATURAL GAS

COMBINED CYCLE POWER PLANT IN TURKEY

Alp Gokalp, MSc. C.E., Project Manager, Kasktas A.S., Istanbul, Turkey

Rasin Duzceer, MSc. C.E., Technical Manager, Kasktas A.S., Istanbul, Turkey

The aim of this article is to present an application of jet grouting technique in stiff clays for foundations of the natural gas combined cycle power plant, under construction in Aliaga, Turkey. The plant site is in the Aegean coastal region and 60 km north of Izmir. The foundation soil is composed of quaternary alluviums on top and tuff base rock. The thickness of the alluvium varies between 16 m to 30 m. Groundwater was observed 2.5 m below the ground level. At the design stage, settlement analyses were carried out for the various buildings and facilities in the power plant. Based on the analyses, it was predicted that total and differential settlements are not tolerable, thus jet grouting is implemented as a ground improvement under the majority of the foundations of the power plant. This paper also describes the selection of jet grouting technique, application type and details of the process parameters such as injection pressure, number and size of nozzles, lifting and rotation speeds. Prior to the commencement of the jet grouting works, a number of preliminary parameter tests and preliminary pull-out tests were carried out throughout the site in order to define and verify the jet grouting process parameters to achieve the required diameter of 600 mm and minimum jet grout strength of 3.2 MPa. Moreover as part of the comprehensive quality control and quality assurance program pursued in the project, a number of tests; such as continuous coring, integrity testing, installation of confirmatory jet grout columns, were carried out during the production of jet grout columns.

INTRODUCTION

This paper presents one of the largest Jet grouting application for the foundations of the natural gas combined cycle power plant, under construction in Aliaga, Turkey.

The power plant site is located near the town of Aliaga, in the Aegean coastal region and 60 km north of Izmir. The site is on a small plain surrounded by low hills. Jet grout columns were used as a ground improvement under the majority of the foundations. These include the power block structures, switchyard and transformers, demineralized water tanks, and selected footings in the water treatment area, cooling towers, plus several associated offices and services. Each power block consists of stacks, combustion turbines, steam turbines, transformers, an electric building and a water tank. Power plant lay out plan is presented in Figure 1.

Construction and operation of the 1523 MW natural gas combined cycle power plant is one of the most important build and operate type project in Turkey. The owner is InterGen-Enka and the main contractor of the project is Bechtel-Enka Joint Venture. The geotechnical subcontractor for site investigations and jet grouting is Kasktas A.S, The geotechnical designer was Enar Muhendislik Mimarlik Danismanlik Ltd. (Sağlamer et. al. 2001).

At the design stage, settlement analyses were carried out for the various buildings and facilities in the power plant. Based on the analyses, it was predicted that total and differential settlements are not tolerable, thus jet grouting is implemented as a ground improvement under the majority of the foundations of the power plant in order to limit settlement and increase the bearing capacity of the soils.

The details of the operational parameters of the jet grouting technique such as injection pressure, number and size of nozzles, lifting and rotation speeds and the pursued quality control program during the installations are also described.

SUBSOIL CONDITIONS AND GEOLOGY

Aliaga region is situated at the coastal part of Bakırcay and Gediz grabens at the western Aegean. Aegean region of Anatolia consists of a rise of metamorphic complex surrounded mostly Mesozoic aged marine foundations and Tertiary volcano sedimentary rock units. The plant site is situated at mid of Foca Depression and is considered as a former lagoon, filled by fine grained lacustrine, alluvial and colluvial sediments, connected to Nemrut Bay (Sağlamer 2000).

During the design phase, a total of thirty-four boreholes, with regular SPT’s, and twenty-seven CPT’s were carried out .

There are mainly two layers in the soil profile, quaternary alluviums on top and tuff base rock. The thickness of the alluvium varies between 16 m to 30 m. Groundwater was observed 2.5 m below the ground level. Soil layers encountered at the site are given below.

Figure 1 Site Plan

- Top Soil (Made Ground)

- Layer 1 : Alluvium: Dark gray, light brown-brown in color, contains gravels of limestone or tuff in the clay matrix. Upper layers of clay attain very stiff consistency with SPT blow counts in the range N=13-16.

- Layer 2 : Tuff Formation: It is green or grayish black colored. Upper layers are weathered and exhibit over-consolidated clay properties. Based on the RQD values it may be classified as “very poor rock”.

Generalized soil profile and geotechnical properties of clay and sand layers for East Cooling lower, Heat Recycle Steam Generators (HRSG) 1, HRSG 2, HRSG 4, Demineralized Water Storage Tank, Fire/Filtered Storage Tank and Ware House Building are given in “Figure 2”.

FOUNDATION DESIGN

The facilities to be placed in the İzmir Aliağa Power Plant

are shown in “Figure 1”. Dimensions and foundation depths of some of these facilities are given in “Table 1”.

-12.00

Light Brown Gravelly CLAY

+7.50~+8.00

-3.00

+9.50

+8.50TOP SOIL

Clayey Gravelly SAND Medium Dense

= 10.0 MPa ,

Brown Gravelly Sandy CLAY Very Stiff

Es

Brown Slightly Gravelly Sandy CLAY

= 13-16 = 1.20-1.40 MPa

Nqc

GWL

-0.50

-9.00

Stiff

McCu

= 5.0 MPa= 50 kPa

n = 18 kN/m3

= 25 ,N = 6.0 MPa ,qc Ø = 34 °= 18 kN/mn 3

= 2.90 MPa= 26

qcN = 110 kPa

= 12.0 MPa= 19 kN/mn

CuMc

3

Clayey SAND and GRAVEL Dense-Very Dense

= 15.0 - 20.0 MPa Es= 30 - 45 N

= 20 kN/mnØ

3= 40 °

Hard, Weathered TUFF> 40N= 20.0 MPaMc

= 200 kPaCu= 20 kN/mn 3

Figure 2. Generalized Soil Profile and Geotechnical Properties of soil layers for East Cooling Tower, HRSG 1, HRSG 2 and HRSG 4

Table 1 Dimensions and depth of foundations of Power Plant Facilities

Structure Length (m)

Width (m)

Df (m)

Cooling Towers 113.4 30.60 0.45 Heat Recycle Steam Generator 39.50 17.00 2.10 Combustion Turbines 35.00 17.00 1.35 Steam Turbines 40.00 20.00 2.85 Main Set-up Transformer 13.00 10.00 2.35 Auxiliary Transformer 13.10

8.70 10.00 8.00

2.35 1.05

Waste Neut. Tank D= 8.00 D= 8.00 1.00 Fire Water Storage Tank D=12.20 D=12.20 1.00 Demin. Water Storage Tank D= 9.15 D= 9.15 1.00

All of the facilities to be built within the Aliağa Power Plant will have foundations resting on stiff gravelly clay layers. The average allowable bearing capacity for the shallow foundations of the facilities was calculated as qall = 100 kPa.

Foundations of the facilities to be constructed will exert soil pressures between 50 kPa and 127,5 kPa. The base pressure transferred from the Fire/Filtered Water Storage Tank to the soil is determined as 180 kPa.

In the facilities other than the Steam Turbine Generator (STG) and Fire/Filtered Water Storage Tank, bearing capacity problems are not expected.

Settlement analyses were carried out for the various

buildings and facilities in the Power Plant. Base pressures, predicted total and differential settlements for the facilities are summarized in Table 2.

Since the predicted total and differential settlements are not tolerable, foundation of the structures were proposed to rest on shallow foundations after the soil improvement. Jet grout columns were selected mainly for supporting the raft foundations and single footings of the structures to transfer the base loads of 50 to 180 kPa to denser strata, thus controlling settlements and improving the compressibility characteristics of the soil.

Table 2 Predicted Settlements of Facilities Resting on Shallow Foundations

Structure Base

Pressure (kPa)

Total Settl. (cm)

Diff. Settl. (cm)

Heat Recycle Steam Generator 1 100 22.4 11.5 Heat Recycle Steam Generator 2 100 26.2 13.6 Heat Recycle Steam Generator 3 100 21.4 11.2 Heat Recycle Steam Generator 4 100 19.9 10.4 Combustion Turbine 1 52.1 13.4 7.0 Combustion Turbine 2 52.1 15.9 8.6 Combustion Turbine 3 52.1 14.9 9.2 Combustion Turbine 4 52.1 10.8 5.7 Steam Turbine 1 127.5 28.2 14.4 Steam Turbine 2 127.5 28.9 14.6 Transformer 1 100 18.9 8.7 Transformer 2 100 19.9 8.9 Transformer 3 100 17.3 8.3 Transformer 4 100 18.3 8.2 Transformer 5 100 16.9 6.5 Transformer 6 100 16.8 6.6 Waste Neut. Tank 80 11.6 4.1 Fire/Filtered Water Storage Tank 180 20,6 7,9 Demin. Water Storage Tank (1) 110 16,2 6,7 Demin. Water Storage Tank (2) 110 16,1 6,7 West Cooling Tower 50 11,5 7,7 East Cooling Tower 50 12,8 8,5 Miscellaneous Footings (2.5 m x 2.5 m)

100 8.5 3.5

As per the design, jet grout columns of 600 mm diameter, 12 m effective length, with a service load of 450 kN, were selected. Distribution and spacing of jet grout columns at the plant site are summarized in “Table 3”.

Based on the design the total soil improvement work consists of 78.000 m of jet grout columns of 600 mm diameter.

TEST JET GROUT COLUMNS AND PROCESS PARAMETERS

Thirteen 6 m long trial jet grout columns were installed without pre-jetting in May 2000, using a water cement ratio of 1:1. The top 2 to 3 m of each of the trial columns were excavated and exposed so that the variation in shape and diameter could be measured. Twelve 12 m long test jet grout columns were than installed using the optimum parameters derived from those trial columns. All of these trial and test columns used a water cement ratio of 1:1. Six of the test columns were cored to measure their continuity and strength, while reinforcing

bars were installed in the other six and pullout tests were performed to twice the design load (900 kN) in accordance with ASTM D 3689.

Table 3. Jet Grout Column Distribution for Power Plant Facilities

Jet Grout Columns Structure

Foundation Dimensions

(mxm) Spacing (mxm)

Number

Cooling Towers 113.4x30.60 2.5 x 2.5 996 Heat Rycyle Steam Generator

39.50x17.0 2.0x2.0 1364

Combustion Turbines 3.50x17.0 2.5x2.5 996 Steam Turbines 40.0x20.0 1.8x1.8 1269 Main-Auxiliary Transformers

13.00x10.00 2.0x2.0 414

Demin. Water Storage Tank

D= 9.15 2.0x2.0 90

Electric Buildings 11.2x20.40 2.0x2.0 119 Switchyard -- 2.0x2.0 809 Water Treatment Plant 53x24 2.5x2.5 396 RMS Facility -- 2.0x2.0 45

Steel bars of 32 mm diameter were grouted into the center core hole of the column for the column length. Maximum and residual displacements of trial jet grout columns are presented in “Table 4”.

Table 4. Results of Pull-out Tests Performed on Trial Jet Grout Columns

Displacement Jet Grout No.

Test Load (kN) Max.

(mm) Res. (mm)

Trial – 1 900 1.82 0.70 Trial – 2 900 2.86 0.60 Trial – 3 900 3.29 0.53 Trial – 4 900 3.23 0.77 Trial – 5 900 2.66 0.36 Trial – 6 900 3.97 0.36

The majority of the test columns were excavated and exposed to 3 to 4 m depth, while two were excavated to around 7 m depth. The conclusions reached from the test columns were that the pullout and strength capacity were more than adequate while the diameter did not generally reach the required 60 cm or minimum of 55 cm.

A further seventeen 12 m long trial jet grout columns were installed with pre-jetting in July 2000, six using a water cement ratio of 1:1, while the remaining 11 used a water cement ratio of 1.25:1. Again, the diameters of the excavated and exposed columns did not generally reach the required average of 60 cm nor minimum of 55 cm. An additional ten 6 m long trial jet grout columns were then installed in pairs at 5 locations throughout the site in late July 2000, using higher water and grout pressures. All of these columns satisfied the diameter requirements. The parameters listed in “Table 5” were selected based on the results of these trial jet grout columns for the production jet grout columns.

All of the trial jet grout columns used SoilMec mixing and pumping equipment and Casagrande drilling equipment with 90 mm diameter rods and monitor with 2 nozzles

each 1.8 mm diameter and a drill bit diameter of about 120 mm.

Table 5. Process parameters

Jet Grout Column Diameter mm 600 Injection Pressure MPa 55 Number of Nozzles no 2 Nozzle Diameter mm 1.8 Rotation Speed of Rods rpm 20 Lifting Speed of Rods mm/min 350 Cement Consumption kg/m3 630 – 710 Water / Cement Ratio w / c 1 : 1 Cement Type - OPC 42.5

Jet grouting is an eroding process, and therefore both displacement as well as relaxation of the soil may occur during grouting. Single fluid system combined with pre-cutting has been used for the stiff cohesive soil layers to ensure that a jet grout column of 600 mm or larger average diameter is achieved.

QUALITY CONTROL TEST ON PRODUCTION JET GROUT COLUMNS

As part of a comprehensive quality control program pursued in the project, a number of quality control tests were carried out during the production of jet grout columns.

Coring and Strength Testing

Coring was performed one in every 50 production jet grout columns, three samples from each cored column were selected for strength tests 21 days after installation. Each core was drilled about 10 cm off center. Core drilling was performed using double-tube, ball bearing swivel type core barrels. Each core run was 3 m in length. Unconfined compression tests were performed on the core samples taken from the jet grout columns, in accordance wirh ASTM C42. Unconfined compressive strength values of qu= 3.6 MPa to 20.4 MPa were observed, with an average of 6.8 MPa (Table 6).

Table 6. The results of uniaxial compression tests carried out on samples taken from jet grout columns.

Area Coring (no)

Uniaxial Test (no)

Mean qu

(MPa) Power Block 1 38 114 6.7 Power Block 2 25 75 6.8

Cooling Tower 1 3 9 7.3

Cooling Tower 2 3 9 6.2 Switchyard 7 21 6.4

Water Treatment Plant 3 9 8.5

Total : 79 237 6.8

Visual Inspection

A series of confirmatory jet grout columns were installed close to production column area. During the execution of works a total twenty nine 600 mm diameter, 7 m long confirmatory jet grout columns were installed in each of the 5 areas of the plant. Figure 3 shows the exhumation of the jet grouting column for visual inspection.

All the test columns were excavated down to 3.5 m from the working platform level. The columns were inspected and the diameters of the column were measured at every 500 mm. The followings are encountered :

• The diameter of column varies between 600mm to 850 mm.

• The average diameter at the top 3.5 m of the column is greater than 650 mm diameter.

• A slight reduction in the diameter at around ground water level was observed.

Figure 3 – Visual Inspection of Confirmatory Jet Grout Columns

Integrity Testing

Integrity tests were performed in every 20 production jet grout columns to assess the length and the integrity of the columns. PDI pile integrity tester was used for this purpose (Figure 4).

However, in the interpretation of the integrity test results, it was observed that there is a difficulty in receiving clear toe reflections since the modulus of elasticity of a jet grout columns is very close to that of the soil layers (Figure 5).

Figure 4. Execution of integrity test on a jet grout column

Figure 5. PIT Velocity Signal and Shaft Impedance Profile for the tested Jet Grout Column

CONCLUSIONS

One of the largest jet grouting applications in Turkey is described and discussed in this paper. The following conclusions can be drawn from the results if this case study:

1. 78.000 m jet grouting installation was completed within a period of six months with close cooperation among the general contractor, designer and jet grouting contractor.

2. Pre-production test grout column installation assisst in evaluating the effectiveness of the equipment to be used and selection of the appropriate and optimum injection parameters.

3. A comprehensive quality control and verification testing program were incorporated in the project. The properly planned and executed quality control program resulted in early identification of potential problems and allowed the contractor to make necessary adjustment and/or modifications to solve these problems.

4. The unconfined compressive strength of the jet grouted soil ranged from 3.6 to 20.4 Mpa. The test results exceeded the minimum specified strength of 3.2 Mpa. The results of strength tests reveal that the measured unconfined compressive strength of jet grouted soils are in conformity with the values obtained in similar soil conditions.

5. Based on the trial test results performed at site, an additional step was added to the basic procedure to ensure that jet grout column of 600 mm or greater diameter is achieved. Prior to jet grouting sequence, a pre-jetting sequence was incorporated where high pressure water is jetted through the nozzles, with rotation and lifting in a similar manner to that used during jet grouting.

6. The quantity of cement per cubic meter of treated ground in stiff clay varies between 630-710 kg/m3.

REFERENCES

Saglamer, A., (2000). “Geotechnical Report on Izmir Aliağa Combined Cycle Power Plant”. Faculty of Civ. Eng., Geotechnical Engineering Department, Istanbul Technical University.

Saglamer, A., Duzceer, R., Gokalp, A., Yilmaz, E. (2001). “Recent Applications of Jet Grouting for Soil Improvement in Turkey” Proc. 15th International Conference on Soil Mechanics and Geotechnical Engineering, ISSMGE Istanbul, vol. 3, 1839-1842.