313 design concept of the soil improvement for road … · 2017-11-28 · gray silty clay with some...

DESIGN CONCEPT OF THE SOIL IMPROVEMENTFOR ROAD CONSTRUCION ON SOFT CLAY

Saravut JaritngamAssistant ProfessorDepartment of Civil EngineeringFaculty of EngineeringPrince of Songkla UniversityThailandFax: 001-66-74-212891Email: [email protected]

Abstract: This paper presents an innovative use of soil-cement mixing method using jetgrouting technique to improve the bearing capacity of subbase foundation for roadconstruction in Thailand. The construction sequences and the basic design example of jetgrouting for embankment works on soft clay are also given in this paper. The design conceptand method of analysis of jet grouting work using finite element technique, results of finiteelement analysis, the installation adopted and effectiveness of soil improvement system aregiven and discussed. It was found that the total settlement is reduced by the jet grouting. Acase history at Pak Phanang Diversion Dam project, Nakornsrithamarat, Thailand ispresented describing the engineering design and construction aspects of a successful project,which demonstrated the advantage of soil-cement mixing method over conventional piledriving for this site.

Key Words: finite element method, jet grouting, soil-cement mixing, soil improvement

1. INTRODUCTION

This paper presents an innovative use of soil-cement mixing method using jet groutingtechnique to crete a subbase foundation in soft clay is used. The conventional method of roadconstruction by driving concrete piles followed by stage of embankment construction isfound to be less cost-effective. Moreover, the problem of pile movements would imposepractical constraints to the pile driving works and hence slowed down the constructionprocess. A case history at Pak Phanang Diversion Dam project, Nakornsrithamarat, Thailandis presented. The design concept and method of analysis of jet grouting work using finiteelement technique, results of finite element analysis, the installation adopted andeffectiveness of jet grouting system are given and discussed. A total of approximately of144,600 m3 of jet grouting were installed. The jet grout diameter is about 1.6 m for jetgrouted piles and 3.0 m for sand-cement jet grouted. The top of the jet grouted piles isenlarged to nearly twice the average diameter and the enlarged section is extend over the top1 m of the column. The jet grouted was adopted to serves as a excellent subbase foundations,and resists settlement and stability during construction process. The benefits of the jetgrouted piles, cost-effectiveness and time saving are highlighted.

2. SOIL INVESTIGATION

The site is located at the south of Thailand. Based on a compilation of soil investigation results, the topsoil is essentially a very soft to soft silty clay of low plasticity with standard penetration test (SPT) N-Value of 0 blows/30 cm and permeability value (k) of 0 cm/s as well until about 18 m from the existing ground (upper soil layer). Following this is the brownish

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Proceedings of the Eastern Asia Society for Transportation Studies, Vol.4, October, 2003

gray silty clay with some gravel which is very stiff and of low plasticity (lower soil layer), with N-Values registered generally exceed 20 blows/30 cm.

For the simulation analysis, an idealized ground condition as stipulated in figure 1 is adopted. It is of interest noting that the idealized shear strength profile construed based on 5 times the N-value for the lower silty clay and 0.25 x effective overburden pressures for the upper silty clay assuming that it is normally consolidated, is generally consistent with the vane shear stength registered in the field. These soil parameters were used for without soil improvement by jet grouting methods. Subsequent parts will discuss the effectiveness of jet grouting method in reducing the total settlement.

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Figure 1 Soil profiles

3. INTRODUCTION OF JET GROUTING

Most embankment problems occur from low compressive strength subsoil, high void ratiosand unfavorable water content in the soil; therefore, basic concepts of soil improvementinclude densification, cementation, reinforcement, soil modification or replacement, drainage,and water content controls. Soil improvement modifies and increases the mechanicalcharacteristics of the improved soil, most notable its compressive strength, while decreases itspermeability. Jet grouting is one of the most successful techniques to achieve these results.This method can be applied to a wide range of soils from non-cohesive, poorly gradedgranular soils to cohesive plastic clays. Jet grouting used a high velocity jet stream to cut,remove and grout cement into the soil to form columns, panels or slabs. This method wasdeveloped in Japan in the 1960’s (Yahiro and Yoshida, 1972) before extending to Europe inthe late 1970s (Bell, 1993). The method is applicable across the whole spectrum of soilsfrom coarsest gravels to the finest clays. It can be conducted from a suitable access point,and can be terminated at any elevation, providing treatment only in the target zones. It can beconducted vertically or horizontally, above or below the water table. It has been successfullyused in construction of seepage barriers and bottom seals, construction of tunnel support,

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structural underpinning, construction of soilcrete struts/slaps prior to excavation, stabilisingslopes and containment of hazardous waste. Some of the published applications aresummarised by Jaritngam, S. (1996).

Jet grouting method involves injecting a continuous selected mix of cement grout at high pressure from a nozzle to cut into the surrounding subsoil, mixing and replacing it with cement slurry to form a hardened cemented mass. A typical set-up of jet grouting operation at the site is illustrated in figure 2. It is basically consists of a main plant, high pressure pumps and a specialized rig. High pressure hoses deliver adequate grout pressure from the plant to the jet grouting rig for the installation. The installation of jet grouting is presented in figure 3. This method starts with drilling a borehole, usually 60 to 160 mm in diameter, to the required depth of treatment. The borehole can be pre-drilled or self-drilled depending on the equipment used. After drilling, the next process is to erode the ground with jetting under high pressure. The soil is mixed with the cement grout. Excess soil-grout mixture is removed as sludge through the borehole by airlifting action as the grout rod is being withdrawn slowly. The treated soil will harden into a cylindrical soil-cement mass. The system used in this project is also known as double tube or double fluid system. The grout is injected through a nozzle under air pressure. The air jet helps to increase the cutting distance. The key properties of treated grounds are strength and stiffness. These properties are affected mainly by the type of soil being treated, the water-cement ratio, the rate of rotation and withdrawal and the overlap pattern. The resulting treated soil can harden into a cylindrical cemented mass. By installing jet grouting piles in closely spaced grids, a treated soil layer with improve strength and compressibility characteristics can be achieved.

COMPREESSOR

HYDRAULIC UNIT

HIGH PRESSURE PUMPGROUT MIXER

CEMENT SILO

WATER TANK

WATER PUMP

AGITATOR

JG MACHINE

DOUBLETUBE SWIVEL

Figure 2 The typical jet grouting plant set-up

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Step 1 Step 2 Step 3 Step 4 Step 5

Figure 3 The installation of jet grouted piles

Before the installation of jet grouting piles, the preliminary trial test jetting has been conducted on site to finalise the optimum values of grout mix and the operating parameters to achieve designed jet grouting piles diameter and strength parameters. In this project, the operation parameters used are given in table1.

Table 1 The optimum jet grouting operating parameters

Items Double tube system UnitJet grouting pressure 20 MPaGrout flow rate 60 litre/minAir pressure 700 kPaAir flow rate 3 m3/minWithdrawal rate 18 min/mRotation rate 10 rpmMix proportion 700 kg cement/ m3grout

4. METHOD OF ANALYSIS AND DESIGN

The jet grouted piles design was conducted in a trial and error approach using a finite element method. The series of analyses were conducted using a two-dimensional finite element software package, “EXCAV97” by the soil mechanics group of the Nanyang Technological University at Singapore. It used a 2-dimentional isoparametric element to model the soil medium. The soil behavior is modelled with the nonlinear hyperbolic model proposed by Duncan et al (1980). The hyperbolic model has been in use for about 30 years with reasonable success. The model requires only conventional triaxial conventional tests. It is basically a curve fitting model using an hyperbolic function. It obeys the generalized Hooke’s law of elastic deformations. The model can simulate the nonlinear, inelasic and stress-dependent behaviour of the soil. The short term undrained condition for embankment construction on saturated clay was modelled using the total stress analysis. The soft clay was assumed to be saturated and incompressible using a Poisson’s ratio of 0.49. The elastic modulus was computed based on Ei/cu ratio of 200, where Ei is the initial tangent undrained

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modulus. The recommendations which drawn from the published strength and modulus data are summarized and shown in table 2.

Table 2 The recommendations of strength and modulus

Soil types Unconfinedcompressive

strength (MPa)

Eu/qu Average Eu/qu Average Eu/cu

Gravel 10-30 300-1500 500 1000Sand 10-30 200-500 300 600

Silty soils 1-10 100-750 250 500Clay 2-6 50-250 100 200

A typical road embankment section is considered. The final finite element mesh for one-halfof a typical section in this study is used. Both with and without jet grouting cases have beenanalysed. In this study, the shear strength and modulus of the jet grout composite adopted are300 kPa (300 kN/m2), and 150 MPa (150,000 kN/m2), respectively. Ground water level istaken to be at the ground surface.

5. ANALYSIS FOR GROUND SETTLEMENT

The objective of this part is to assess the effect and sensitivity of various parameters in the design of jet grouting for road construction on soft clay. The parameters considered are the effect of with and without jet grouting. The variable adopted for comparison purpose are maximum ground settlement for each cases.

In an investigation of the behaviour of embankments constructed on a thick layer with soft clay, it is important to analyze first the total settlement without cement columns. Factors such as the shear strength and the spacing of the columns, the thickness of the treated clay layer, and the thickness of the stiff crust will influence the total settlements.

The two analyses compare the results of two systems with and without jet grouting. The design configuration of a 8 m wide and 2 m high embankment fill constructed on a deep layer with uniform soft normally consolidated clay with stiff crust at the surface has been analyzed as shown in figure 4. Below the soft clay is a hard incompressible stratum. The unit weight of the soft clay is 16 kN/m3 while the unit weight of the stiff crust and cement columns is 18 kN/m3. The undrained shear strength of the soft clay and of the stiff crust is 15 kPa and 50 kPa respectively. The poisson’s ratio, � is 0.49. The undrained Young’s modulus, Eu of the soil has been taken as 200cu. The diameter of the cement columns is 1.6 m. The spacing of the column is 2.5 m. The column area ratio, the ratio of the total area of the columns and the total stabilized area, is the main factor to be consider in the analysis. It should be noted that the column area ratio increased when the spacing of the cement columns is reduced. This will improve the effectiveness of the cement columns. The strength of the cement columns will influence the total settlements of the embankments. The load carried by the cement columns will increase with increasing strength of the cement columns since the cement columns and the soft clay settle together. The undrained shear strength of the stabilized clay in the cement columns was 300 kPa. This strength is not difficult to achieve. The length of the cements is another important parameter to consider in the analysis. Since the total shaft resistance of a cement column depends on the length of the cement columns, the length will affect the

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settlements. It should be noted that the ratio of the length of the cement columns and the total thickness of the soft clay is defined as the relative column length. A relative column length ratio of 0.22 was chosen in the analysis. The rigidity of the stiff crust above the soft clay affects the settlements. A 1 m thick stiff crust has been used in the analysis. The stiff crust is underlain by 18 m of soft normally consolidated clay. The stiff crust behaves essentially like a raft and the surcharge load will be distributed over a relatively large area, there by reducing the stress increase in the soft soil below the embankment, and thus the total settlements. The foundation stabilized which cement columns will then settle as a block, thus reducing the differential settlements. The cement columns area ratio is an important factor in the cement column method. The area ratio when the 1.6 m diameter columns are spaced 2.5 m center to center is 0.32.

It is interesting to note that the maximum ground settlement is reduced with the case of usingjet grouting method. The computed maximum ground settlement in the case of no jet groutingand with jet grouting, the computed maximum ground settlement is 355 and 112 mm,respectively. The ground settlement are reduced by 68%. From these results, the designconfiguration as shown in figure 4 is used under the consideration of construction cost.

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6. ANALYSIS FOR SLOPE STABILITY ANALYSIS

The procedure for the design concepts of jet grouted piles consist of the successive repetitionof two steps. First step is to select the geometry of the jet grouted piles such as pile length,pile diameter and pile spacing. The next step is to compute the stability of the slope. If theresults of the following calculations is unsatisfactory, the new geometry is altered and a newanalysis is made.

For this project, slope stability analysis was carried out using the modified Bishop method.The jet grouted piles design was conducted in a trial and error approach using a computerprogram for the slope stability analysis. The analysis was carried out by means of theconventional circular sliding calculation. A typical section for the embankment section wasconsidered. The final computer section for one-half of a typical section in this study wasused. Both with and without jet grouting cases have been analysed. The strength andmodulus parameters of the jet grouted columns assumed in the study are given in table 2. Jetgrouting can be used to strengthen the soft soil as a stiff clay layer. It has been successfullyused on this projects. Jet grouting can bring the slope failure circle deeper down, so thefactor of safety became increase.

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Based on slope stability analysis and 2-dimentional finite element analysis, the cross sections of jet grouting block are to be modified to improve the factor of safety of the embankment against sliding and reduce the settlement of the embankment surface.

7. CONSTRUCTION PROCESS

The jet grouted piles in the form of subbase foundation is an effective way to reducesettlement and prevent sliding of soft soil. If the conventional method using driven pilesmethod was adopted, the pile length has to take into account the depth of 12.0 m and theproblem of pile movement during installation process may be happened. The system wasconstructed with the jet grouted piles of diameter 1.6 m at centre-to-centre of 2.5 m atappropriate depth. A soil improvement scheme in the form of jet grouting was adopted tostabilize a 4.0 m depth of soft clay for this project. The jet grouted piles increase the bearingcapacity of the subbase, which allowed civil engineer to design embankment and still meetsafety requirements. It has been successfully used in several projects in Thailand.

The jet grouted piles at the site were installed using a track mounted jet grouted rig. Thereafter, the jet grouted piles forming the improve subbase were installed using a total of 3rigs on a 20 m truss girder spanning across the existing excess road as shown in figure 5. The truss girder connected together by bolts and nuts and it sat on a railing system at each end. On top of the girder, a railing system was specifically designed to allow free manoeuvre of the three rigs. The use of a girder ensures the existing road functions as normal during the progress of construction work. The jet grouted piles could be installed easily and accurately. Figure 6 shows jet grouting works at the site. The concept of the soil improvement using jet grouted piles beneath the road embankment with a movable girder for installation purpose is not only cost-effective but also safe, reliable and time-saving with the success of the project.

Figure 5 Truss girder for installation of the jet grouted piles

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Figure 6 Jet grouting method in construction site

Experience indicates that the strength and modulus of the grouted soil mass were approaching that of a lean concrete.

8. RESULT OF WORK

After the jet grouting operation, various quality control measure were taken to ensure the quantity of forming piles. The diameter of each columns was visually checked after installation process. It was apparent from figure 7 that the jet grouted piles provides an excellent basement foundation. Coring of the jet grouting mass shall be conduced as shown in figure 8. The coring sampling was focussed on the location between 1/6 to 2/6 times diameter of the center pile. The continuous cored samples were stored in box and sent for unconfined compressive strength tests as shown in figure 9. These strength tests shall be carried out in the specified intervals for quality control tests. It is worth noting that the 28 days unconfined compressive strength test results of core samples from the jet grouted piles exceed the design target value of 600 kPa.

Figure 7 Columns’ diameter of jet grouted piles after installation

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Figure 8 Coring of jet grouted piles

Figure 9 Unconfined compression test of jet grouted piles

9. CONCLUSIONS AND RECOMMENDATIONS

Conventional pile driving methods are very time consuming, costly and may cause significant delay of the project. The application of jet grouting technique, as a soil stabilization method was applied. Jet grouted piles become a suitable choice under these conditions. The concept of the soil improvement using jet grouted piles beneath the road embankment with a movable girder for installation purpose is not only cost-effective but also safe, reliable and time-saving

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with the success of the project. The advantage of using jet grouting as a subbase foundations is a simple for road construction works on soft clay.

Based on the analysis of the jet grouted piles, the reduction of the total settlement was found to increase with increasing column area ratio, with increasing column length ratio, and with increasing shear strength of the jet grouted columns.

The mixing of the cement with the soft soil is important. The effect of the mixing on the strength and the compressibility should be investigated in order to predict more accurately the performance of the columns in the field.

REFERENCES

Duncan, J.M., Byrne, P., Wong, K.S. and Mabry, P.N. 1980. Strength, stress-strain and bulkmodulus parameters for finite element analyses of stresses and movements in soil masses,Geotechnical engineering research report, No. VCB/GT/80-01, University of California,Berkeley.

Duncan, J.M. and Chang , C.S. 1977. EXCAV: A computer program for analyses stresses andmovements in Excavations, Geotechnical engineering research report, No. TE 77-4,University of California, Berkeley.

Jaritngam, S. 1996. Method of reducing soil movements for deep excavations in soft clay,M.Eng. Thesis, Nanyang Technological University, Singapore.

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313 design concept of the soil improvement for road … · 2017-11-28 · gray silty clay with some...

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