STRENGTHENING THE RAFT FOUNDATION OF AN EXISTING RC BUILDING BY APPLICATION OF JET-GROUTING METHOD

A case study of soil-foundation system strengthening is presented in the paper. The studied building’s RC structure (columns and slabs for vertical loads and walls for seismic loads) has been designed in 2007 and planned to be realized in the seaside city of Burgas in Bulgaria. According to the original project the building consists of 14 levels as well as 5 underground levels. The execution process has started in 2008 and has been interrupted in 2010 as only the basement part of the building was constructed then. Due to investment intensions change it has been decided to construct the remaining superstructure and to extend it by 4 additional levels as well as to switch building’s function from office to residential. In order to do so a strengthening project has been prepared. The project includes a number of measures regarding the superstructure (reparation, RCjacketing, execution of new structural elements among others) as addition to the soil-foundation improvement.


INTRODUCTION
A case study of soil-foundation system strengthening is presented in the paper. The studied building's RC structure (columns and slabs for vertical loads and walls for seismic loads) has been designed in 2007 and planned to be realized in the seaside city of Burgas in Bulgaria. According to the original project the building consists of 14 levels as well as 5 underground levels. The execution process has started in 2008 and has been interrupted in 2010 as only the basement part of the building was constructed then. Due to investment intensions change it has been decided to construct the remaining superstructure and to extend it by 4 additional levels as well as to switch building's function from office to residential. In order to do so a strengthening project has been prepared. The project includes a number of measures regarding the superstructure (reparation, RCjacketing, execution of new structural elements among others) as addition to the soil-foundation improvement. The foundation of the existing part of the building consists of a raft. In order to reduce settlement due to the additional loads, [9], from the extension and for the sake of increasing the stiffness of the modulus of subgrade reaction in the numerical model it has been decided to execute jet-grouting as a hybrid soil improvement-structural strengthening measure - [18].
The operating conditions (height of 2.80 m in the basement) have made this solution as an only option.
Jet-grouting soil improvement technique (described in [1], [7] and [8]) has gained popularity, [6], during the last few decades. Its application range is wide and some typical examples include foundations, retaining structures, water barriers, tunnels among others. The jetgrouting process is recognized as a cement soil stabilization. With the aid of high pressure (400 bar) cutting jets of water or cement suspension having a nozzle exit velocity ≥100 m/sec eventually air-shrouded the soil around the borehole is eroded. The eroded soil is rearranged and mixed with the cement suspension. The soilcement mix is partly flushed out to the top of the borehole through the annular space between the jet grouting rods and the borehole. Single fluid version (described in [4]) of the jet-grouting technique has been adopted for the particular project. In the single fluid system, the water-cement grout is injected into the ground through one or more nozzles. In this case, soil remoulding and subsequent cementation are both caused by the same fluid.
The adopted configuration of the 206 jet-grouting columns having a diameter of 80 cm is given on Figure  2. The execution process consists of seven major steps as follows: 1) drilling the existing raft; 2) forming the jetgrouting columns (length of 7 m and 5 m) through highpressure injection of water-cement grout; 3) insertion of a steel pipes (•114.3x8, length of 5 m and 2.5 m) for load transfer from the raft to jet-grouting column and for the sake of increasing its compressive bearing capacity; 4) grouting the space between the raft and the pipe; 5) insertion of reinforcement in the pipe -the upper part of the reinforcement sticks out of the raft so that it could be linked to the reinforcement of the foundation top jacketing; 6) grouting the inner volume of the pipe and execution of a 15 centimeter RC strengthening (top jacketing) of the existing raft.

SOIL CONDITIONS AND VERIFICATION OF JET-GROUTING COLUMN PROPERTIES
The soil conditions on site are shown on Table 1. The foundation raft is located at level +9.05 meaning that it layes on saturated Layer 3 (Pliocene clays).
Usually in practice, it is necessary to correlate the jet grouting effects (i.e., column diameter and properties) to the original soil properties (i.e., grain size, shear strength) and to the treatment procedures (i.e., treatment parameters). However, because all soils are inherently heterogeneous, the mechanical and geometrical characteristics of the columns are usually variable.
In the presented project a simple approach for verification of the jet-grouting columns' diameter has been adopted - [19]. Three test columns (TC-A1, TC-A2 and TC-A3) have been executed by three different treatment procedures. Thereafter, boreholes have been drilled in the center and perifery (at distance 40 cm from the center) of all three columns. In order to prove that a diameter of at least 80 cm is ensured, a continous sample is taken through the whole length of the borehole - [12] and [14]. The judgment is made on the basis whether treated medium is observed through the whole sample or not. In the particular case study test columns TC-A1 and TC-A2 showed unsatisfactory results. In contrast, test column TC-A3 demonstrated a treated zone with the desired dimensions (Fig 3.).

Fig. 3. Ensuring mechanical properties and column dimensions by means of test columns
Probes have been extracted from the only test column with satisfactory dimensions -in this case TC-A3. The mechanical properties (unconfined compressive strength, ultimate axial strain and deformation modulus) of the jet-grouting columns have been evaluated in the laboratory. Due to soil's heterogeneity results show values of wide range as it could be seen on Table 2. The compressive strength varies from 3.25 MPa to 8.10 MPa - [10]. A characteristic value of 4.50 MPa has been adopted as input value for the design.

NUMERICAL ANALYSIS AND DESIGN
The "bed of springs" model has been adopted as an approach for consideration of the soil-structure interaction effect in numerical analysis. Soil (as physically and mechanically described medium in Table 1) has been modelled as a continuum and represented by the Mohr-Coulomb constitutive model in SAP2000 software for the sake of evaluating the modulus of subgrade reaction. Stress which has been obtained through the analysis has been divided by the calculated settlement for the sake of determining the springs' stiffness (Fig. 4).

Fig. 4. Evaluation of modulus of vertical subgrade reaction through a numerical solution
The modulus of subgrade reaction of the jet-grouting treated area has been evaluated on the basis of a loadsettlement relation which has been obtained through analytical procedures as well as a pile-test numerical FEM simulation as seen in [13] in the software PLAXIS 2D by using the Hardening-Soil (HS) constitutive model (explained in details in [17]) - Figure 5.

Fig. 5. Numerical FEM simulation of jet-grouting test in PLAXIS 2D
An overview of the adopted values for the modulus of subgrade reaction is given on Figure 6.
Furthermore, a 3D finite-element model which represents the superstructure in details has been developed in ETABS software. Elements from the program library have been adopted for the sake of representing the structural elements as the follows: frame elements for beams and columns, shell elements for walls, slabs and raft foundation. The soil has been modelled by area-spring elements. A comparison of the bending moments in the raft is made between a model with evenly distributed (same stiffness) springs (existing raft) and a model which considers the soil improvement (jet-grouting) by introducing zones with stiffer springs - Figure 7. A deterministic design approach (described in [5], [11], [15] and [20]) has been applied for the study. By means of such concept, which is typical in geotechnical engineering and suggested by Eurocode 7, uncertainties of the jet-grouting technique are considered by modifying actions on the structures, values of the material properties and overall bearing capacity by partial factors in order to obtain design values. For the material properties of jet-grouted elements, the characteristic values can be derived from the literature or, preferably, be taken from in-situ measurements.
Partial factors suggested by [2] and [5] are adopted in the presented study. A geometrical partial factor (γD) of 1.15 has been chosen on the basis of available experimental information (limited) and column performance (isolated) hence design diameter (Dd) of 0.70 m has been set: Dk / γD = 0.8 m / 1.15 = 0.7 m. A statistical analysis based on data from Table 2 has been adopted in order to set the characteristic value of unconfined compressive strength of the soil-grout column material (qu,k) to 4 500 kPa. By application of material partial factor ( γ M ) of 1.5 design value of unconfined compressive strength (qu,d) has been determined: qu,d = qu,k / γM = 4 500 kPa / 1.5 = 3 000 kPa.
Thereafter jet-grouting columns have been designed in a similar to piles matter. Naturally the treated zone has a remarkable bond with the surrounding soil due to the soil-mixing technique and consequently the geotechnical resistance (jet to soil failure), GEO Ultimate Limit State (ULS) according to Eurocode 7, is typically higher than the structural one (compressive strength of the column) -STR Ultimate Limit State (ULS) according to Eurocode 7.
Characteristic structural strength (bearing capacity) of the jet-grouting column, R  (Table 1) and soil type according to Figure 8 - [2].
According to [2] characteristic end-bearing resistance, Rb,k, is sanctioned depending on the method through which end-bearing, qb,k, is obtained. In the presented study end-bearing, qb,k, is evaluated on the basis of SPT results (Fig. 8) hence partial coefficient kSPT of 0.1 is adopted. Thereafter the characteristic endbearing resistance is related to the column crosssectional area as follows: x 2 000 kPa = 77 kN. Bearing capacity partial factor, γb, of 2.0 is used in order to modify the characteristic end-bearing resistance, Rb,k, and hence to obtain the design: Rb,k = Rb,k / γb = 77 kN / 2.0 = 38.5 kN.
The characteristic skin friction resistance, Rs,k, is determined on the basis of jet to soil contact surface (area) and skin friction, qs,k. Design skin friction resistance, Rs,d, is obtained by adopting a bearing capacity partial factor, γ

Fig. 8 Skin friction and end-bearing evaluation on the basis of soil type and SPT results -[2]
The smaller of the structural (STR) and geotechnical (GEO) bearing capacity is adopted as final design bearing capacity of the columns: R In other words, geotechnical failure has turned out to be critical for the shorter (5-meter) jet-grouting bodies whereas structural failure would be critical for the longer (7-meter) ones. Design forces from the analysis are evaluated as 820 kN and 1 200 kN for the 5-meter and 7-meter columns, respectively, which means that they are smaller than their design bearing capacity.
Eurocode 7 suggests that two out of three partial factor groups (actions group, material group and bearing capacity group) which have values higher than 1.0 ought to be combined and respectively applied depending on the adopted Design Approach (either DA1, DA2 or DA3). However, as seen in the above-described procedure, due to uncertainties in the hybrid soil-structure strengthening behaviour and according to provisions given in [2] and [5] partial factors larger than 1.0 for all three groups have been accepted in the presented study.

PROBLEMS AND SOLVATIONS
All foundation strengthening measures have been executed in limited operation space of 2.80 m - Figure 9. The extracted material during injection and soil-mixing (reflux) has been sucked out through a pump located on the ground surface. In order to avoid filling the existing foundation with reflux, caps have been plugged in the circular raft openings right after forming each consecutive column.
The execution process has been strictly monitored. The total injected grout volume has been tracked for each column in parallel with the Injection Pressure, Rotation and Flow, [16], by means of an Injection Diagram, [3], which has been obtained directly from the jet-grouting machine (MDT -Mc 80 B has been employed for the study) software. Volume of injection grout is expected to be similar for all jet-grouting bodies. If some deviation is observed then measures ought to be taken and the Designer is informed. The monitored parameters (Injection Pressure, Rotation and Flow) should be kept constant during the whole depth of treatment. Anomaly in the diagrams would mean that the soil has not been treated evenly and in such case variation in the jet-grouting column diameter might be expected. A typical Injection Diagram for Column No. 152 is shown on Figure 10.
During the execution of the jet-grouting columns a defect has been detected in about 90 of them. Although the injection procedure has been performed all the way to the top of the raft, settlement of the columns of about 70 cm below the bottom edge of the foundation has been observed the reason for which remains unknown. In order to solve the problem the following technology has been applied: 1) the affected zone between the raft and the jet has been flushed by water under pressure through a tube in order to liquefy the grout reflux in it; 2) expandable grout MAPEI Expanjet (up to 20% volume expansion and compressive strength of 10 MPa) has been injected at 5 bar pressure. In order to ensure a closed system all neighbouring openings (except for one for reflux excess) have been sealed with a packer. In the end 50 m 3 of grout has been injected additionally. The adopted approach is presented on Figure 11.

CONCLUSIONS
The adopted hybrid soil improvement-structural retrofitting approach by applying the jet-grouting technique has ensured an adequate performance of the structure during and after its extension. The strengthening measure has stiffened the soil-foundation zone below the high-rise part of the building which has influenced the redistribution of the bending moments in a favourable way as well as it has reduced the expected settlement significantly. Although some defects have been detected the reparation measures have guaranteed the undisturbed exploitation of the structure.