GROUND IMPROVEMENT INSTEAD OF PILING EFFECTIVE … · (stone columns), wet deep soil mixing and jet grouting. These methods were used in different projects and soil conditions, individually

GROUND IMPROVEMENT INSTEAD OF PILING – EFFECTIVE DESIGN

SOLUTIONS FOR HEAVILY LOADED STRUCTURES

Michał Topolnicki, Keller Polska, Gdynia, Poland, +48 58 7697540, [email protected]

ABSTRACT

It is shown that well designed ground improvement schemes may effectively replace conventional

foundation solutions for a wide range of applications involving heavy loads and structures sensitive to

settlement. This is achieved without compromising stringent design criteria. The presented case studies

illustrate the use of four ground improvement methods: deep vibro-compaction, vibro-replacement

(stone columns), wet deep soil mixing and jet grouting. These methods were used in different projects

and soil conditions, individually or in combination. Beneficial returns in terms of lowering construction

costs, shortening construction time and reducing CO2 emission are discussed where applicable. Keywords: ground improvement, vibro compaction, deep soil mixing, jet grouting, alternative design, CO2

INTRODUCTION

In standard design practice, a distinct gap exists between piling and ground improvement (GI) solutions

used for foundation support. Piling is typically adopted for heavily loaded structures, such as high-rise

buildings, bridges, large commercial centers, silos and other industrial objects, for which stringent

bearing capacity, stability and settlement criteria must be satisfied. Conversely, GI solutions are usually

considered to be of no or limited use for heavily loaded structures, mainly due to the perceived inability

to limit total and differential settlements to an acceptable level.

Undoubtedly, the predominant use of piling for foundation of heavily loaded structures is reasonable

and has a long tradition. It should be noted, however, there is also a range of applications where piling

and GI solutions may actually overlap. This is illustrated in Fig. 1, which shows tentative ranges of GI

and piling solutions in a schematic load vs. settlement graph. For an exemplary GI design, marked as

point 1, the associated settlement corresponds to a relatively susceptible foundation support. For the

same foundation and loading, a significantly stiffer GI design can be also applied, for example, by

increasing the area improvement ratio of the treatment. The resulting reduced settlement is represented

by point 2. Between points 1 and 2, other GI solutions exist for the same loading. Taking into account

different combinations of foundations, loads and GI methods a range of possible GI solutions can be

encircled by connecting a family of corresponding points ‘1’ and ‘2’. A complementary range of piling

solutions can be drawn in a similar way. The combined figure obtained reveals an overlapping area

where piling and GI solutions may both be applicable for a given range of loads without compromising

functional design criteria. In this respect, it should be noted that GI solutions usually offer shorter

execution times and lower construction costs, and are therefore attractive to clients and contractors. In

many situations they also reduce the project’s carbon footprint.

Fig. 1. Tentative ranges of piling and ground improvement solutions for foundation support

Subsequent parts present selected design-and-build project case studies to illustrate the

efficiency of tailored GI solutions compared to conventional piling designs. The ground treatment

technologies considered were vibro compaction and vibro replacement (bottom feed) processes, and

methods utilizing binders to stabilize soil, such as wet deep soil mixing and jet grouting (soilcrete).

Detailed descriptions of these technologies can be found in Kirsch and Bell, 2013. The underlying

projects involve different ground conditions, structures sensitive to total and differential settlements,

and heavily loaded foundations in the form of slabs and separated footings.

APPLICATION OF VIBRO COMPACTION (VC)

Densification of a loose cohesionless soil deposit by applying VC is one of the most effective GI

methods. Figure 2 illustrates the use of VC to support a high-rise building (Sea Towers) in Gdynia,

constructed behind a gravity-type quay wall. The area was a dredged zone of backfilled marine

sediments in a loose to medium dense state, composed of fine/medium/coarse sands with some silty

inclusions in deepest parts of the dredged zone. While the concentrated loads from both towers were

mainly taken by purposely located diaphragm walls, the design aimed to optimize the thickness of the

foundation slab utilizing subgrade reaction. To increase ground stiffness below the slab, VC was applied

to a depth of 5 to 8 m below the working level, and in two grids of 2 and 3m c/c spacing. A significant

improvement was achieved, allowing the use of an average constrained compression modulus Es of 160

MPa in the design. The GI design was based on FEM 3D soil-structure interaction analysis. There was

a good agreement between the observed and predicted settlements (Fig. 3). The VC solution was an

alternative to the original design, which comprised 1,312 stabilized columns 0.8 m in diameter and 3.5

m long, on average. The applied scheme was more than three times less expensive than the original

solution, and also shortened the construction schedule.

Fig. 2. Compaction of dredged soil with VC to enhance slab-subgrade interaction

In a similar project, VC was used to improve soil to exclusively support the foundation of a high-rise

building (Florido Tower) in Vienna, as presented in Fig. 4. Although below the planned foundation level

there was a competent subsoil, composed of gravel and stiff tertiary clay and capable of accommodating

a mean characteristic bearing pressure of 400 kPa, there was a risk of excessive foundation tilt because

Fig. 3. Predicted and observed settlement of the Sea Towers building foundation slab

of a highly varying thickness of the underlying gravel layer. Instead of a typical piling solution to

mitigate tilt, a GI scheme was implemented. The shallower parts of the tertiary clay were excavated and

replaced with loosely placed gravel to construct a ‘cushion’ of a uniform thickness under the entire

foundation slab. The gravel zone was subsequently intensively treated with VC to achieve a well

compacted and relatively uniform subbase layer, while in each treatment location the poker vibrator was

forced to penetrate as far as possible into the underlying clay layer to also assure ground improvement

in the transition zone between the gravel and stiff clay. Predicted maximum settlement was 60 mm,

while the measurements indicated 55 mm on average.

Fig. 4. Ground improvement solution adopted for the foundation of Florido Tower

APPLICATION OF VIBRO REPLACEMENT (VR)

A multistory building with a glass facade (LOT HQ in Warsaw) was built on a construction site with

varying soil conditions below the foundation level. Almost half of the foundation slab was located in a

soft clay area, whereas the remaining part was located in stiff sandy clay or a compacted sand zone (Fig.

5). Because of the sensitivity of the structure, there was a stringent design requirement to limit

settlements to 25 mm. Initial analyses predicted settlement of 29 to 56 mm for a shallow foundation on

the existing ground, indicating a tendency to tilt. Therefore a piling solution was initially considered by

the client, comprising of 720 CFA piles with a diameter of 0.5 m and a total length of 7,920 lm. Noting

that part of the ground was competent to withstand a shallow foundation, an alternative GI system was

proposed. The design included gravel columns with a total length of 3,590 lm, applied in the area of soft

clay and below heavily loaded pillars. The solution was 7 times less expensive and much faster than

piling. Monitoring validated the GI solution, showing uniform settlements less than 20 mm (Fig. 6).

Fig. 5. Gravel columns applied to control differential and total settlement of a sensitive building

Fig. 6. Measured settlement of the LOT building until the completion of construction works

APPLICATION OF WET DEEP SOIL MIXING (DSM)

The GI methods of soil stabilization with cementitious binders offer competitive solutions for a variety

of challenging applications. Figure 7 illustrates the use of DSM for the foundation of a large residential

and commercial complex in Warsaw, comprised of attached buildings of varying heights. The ground

below the foundation level consisted of medium and coarse sands to a depth of about 20 m, underlain

by stiff clay. The initial design adopted a shallow foundation system, with a continuous slab of consid-

erable thickness to accommodate highly concentrated loads. The 20-story buildings required the slab to

be 1.6 m thick. Aiming to reduce the volume of foundation concrete, an alternative GI system was

proposed with 705 DSM columns 1.2 m in diameter, strategically arranged in groups and rows to support

heavily loaded pillars and structural walls. The columns were 5 to 7.5 m long, and the total column

length was 4,350 lm, measured from the working platform level 0.6 m above the foundation level to

allow for column trimming. The adopted characteristic compressive strength of the stabilized soil was

5.5 MPa, and the partial safety factor for material strength was 2.5. The implemented GI solution

resulted in a reduction of the slab volume of about 4,000 m3 without compromising design requirements.

Fig. 7. Foundation solution with DSM columns for a large residential and commercial complex

The application of wet DSM for foundation of a large shopping center Rybnik PLAZA, supported on

186 pad foundations, is presented in Fig. 8. The characteristic bearing pressures varied between 330 to

550 kPa, resulting in a maximum foundation load of 14,420 kN. The subsoil consisted of a loose fill to

the depth of about 2 m, followed by a layer of soft silty clay, about 4.5 m thick, underlain by bearing

coarse sand and stiff clay deposits. The foundation level for a majority of the footings was at the depth

of 2, 2.1 and 2.3 m. Because of the presence of a soft clay layer below the foundation level, a conven-

tional piling solution was initially designed to control bearing capacity and differential settlements to

less than 1:550, i.e. to a maximum difference of 15 mm between adjacent pad foundations arranged in

a main grid of 8.6×8.6 m. The details of the original design with CFA piles are presented in Tab. 1.

The alternative GI solution, shown in Fig. 8, featured DSM columns 0.9 m in diameter, arranged

tangentially in groups of 3 to 36 to support nine types of foundation pads, F1 to F9. For the most

unfavorable combination of characteristic loads, the calculated maximum compressive stress in a DSM

column was 764 kPa. In relation to the adopted characteristic strength of stabilized soil equal 2.2 MPa,

the resulting partial safety factor for material strength was 2.88. Settlement calculations, which included

different combinations of adjacent foundation pads and soil conditions, determined based on the nearest

soil investigation profile, revealed that total and differential settlements were within the prescribed

limits. An example result is shown in Fig. 8 for three selected pads, type F7, F6 and F3, with significantly

different geometries, loads and DSM column patterns.

Fig. 8. Shopping center PLAZA founded on 205 pad foundations supported on DSM columns

Compared to the original piling design, the alternative proposal offered significant overall cost savings

of about 45% (Tab. 1), arising from a lower unit rate for DSM columns (despite an increase in total

length), a reduction in the total volume of foundation pads, and less steel reinforcement due to the more

uniform support of pad foundations. During construction works a further optimization of the foundation

design took place. A cut off wall, composed of overlapping DSM columns 0.8 m in diameter in an axial

spacing of 0.6 m, was constructed around a fire protection water tank planned at the depth of 6 m (Fig.

8). The cut off wall made it possible to excavate the soil in a dry pit, exchange the remaining soft clay

below the foundation level, and to construct the tank. The tank structure was subsequently used to

underpin several pillars initially founded on DSM supported pad foundations. This illustrates the

flexibility of DSM technology, which can be used for different purposes.

Table 1. Comparison of piling and DSM foundation solutions for the PLAZA center

The next example demonstrates the combined use of VR and DSM to support a large silo in the CHP

Siekierki power plant in Warsaw, with a height of 70 m and an ash storage capacity of 20,000 m3 (Fig.

9). The silo was constructed on a ring foundation, with an axial diameter of 23.6 m and a width of 5 m,

exerting a mean characteristic bearing pressure of 1,125 kPa (or 5,625 kN/m) when loaded to capacity,

or 324 kPa (1,620 kN/m) when empty. To limit settlements to 200 mm during operations, it was

necessary to apply standard piles or design and execute a tailored GI solution. The GI works were

conducted in two stages (Fig. 10). In the first stage, 279 sand/gravel columns were installed in a circular

pattern to the depth of approximately 12 m below the working level. The purpose of the VR treatment

was to increase the bearing capacity and to equalize the susceptibility of the sand layer under the silo to

reach an equivalent minimum constrained compression modulus of Es= 80 MPa (cf. Fig. 9).

DSM cut off Water tank

Fig. 9. Ash container in CHP Siekierki and the adopted GI scheme using combined VR and DSM

In the second stage, 216 DSM columns 1.2 m in diameter were installed to support the ring foundation.

The maximum design column load was 2,774 kN, resulting in a design compressive stress of 2.45 MPa.

FEM analysis revealed ring settlement of 245 mm when the silo was filled and 50 mm when empty,

indicating a settlement of 195 mm during operations, which was within the allowable design limit.

Fig. 10. The arrangement of VR and DSM columns, executed in two consecutive stages

APPLICATION OF JET GROUTING (JG or Soilcrete)

A high-rise building Olivia Star in Gdańsk, 156 m high and almost rectangular in plan above the ground

level, was constructed on a site constrained within the existing development (Fig. 11). Since the ground

Fig. 11. Olivia Star building, founded on a slab-soil interaction system with JG columns

consisted of competent fine sand with a relative density of 0.5 to 0.7, an optimized GI foundation system

was proposed instead of conventional piling. The design utilized 124 soilcrete columns 1.8 m in diam-

eter, purposely arranged below the foundation slab to effectively support heavily loaded pillars and

structural walls, and the soil-slab interaction to transfer part of the load to the subsoil. The columns

extended 5, 11 and 13 m below the foundation level at -14 m, and were executed from a working

platform at -11.4 m inside the construction pit (Fig. 12). The JG technology was advantageous because

a smaller rig could be used in the excavation, which was also needed to construct underpinning soilcrete

columns along a short sidewall of the adjacent building, supported on a strip foundation at the depth of

-12.9 m. Furthermore, overlapping soilcrete columns 1.8 and 2.5 m in diameter were also used to con-

struct a deep shaft in the central part of the building, comprising a peripheral wall and a bottom plug to

facilitate excavation in dry conditions. Ten columns along the shaft wall were reinforced with soldier

profiles IPE500, installed in fresh elements to increase bending resistance.

Fig. 12. Arrangement of JG columns below the foundation slab and around the central shaft

Advanced design analyses were conducted to predict foundation settlement which had to be limited to

50 mm. Example results are presented in Fig. 13. The foundation stiffness was derived from FEM

calculations with PLAXIS, and was subsequently transformed to a simplified ‘stiffness map’ which was

used in the structural design of the foundation slab. The calculations revealed that the overall load was

distributed between the JG columns and the subsoil in a ratio of about 60 to 40% (the ground surface

below the foundation slab was re-compacted before pouring lean concrete). The maximum characteristic

compressive force acting on a single soilcrete column was found to be 6,800 kN.

Fig. 13. Results of analyses carried out for the soil-structure interaction model with JG columns

To verify the stiffness of soilcrete columns adopted in the design, as well as confirm the quality of the

installed columns, a loading test was conducted on site to a high load reaching 150% of the maximum

column load (Fig. 14). The observed settlement corresponding to the maximum load was only 5 mm,

exceeding expectations. The soilcrete material was able to withstand a compressive stress of 3.9 MPa

during the maximum loading stage without noticeable creep deformation.

Fig. 14. Static loading test of JG column up to 10 MN

CARBON FOOTPRINT OF FOUNDATION SOLUTIONS

The LOT building and the PLAZA shopping center examples can be utilized for a comparison of carbon

dioxide emissions associated with applying different geotechnical approaches to an individual project.

The calculations of CO2 emissions were conducted using a standardized Excel spreadsheet tool (Carbon

Calculator v.3), developed jointly by the EFFC and DFI. The emissions related to the materials and

energy consumed, freight and personnel transportation were calculated based on project specific input

data, whereas the emissions linked to mobilization/demobilization, assets and wastes produced were

estimated using default settings. All calculations were based on the application of CEM III/A slag

cement for the construction of piles and foundation caps as well as for mixing of soil, with a default

GGbs content of 51%, while a default factor of 41% was used for recycled steel. The results are

presented in Fig. 14.

Fig. 14. Carbone dioxide emission associated with piling and GI designs for two selected projects, namely the LOT building and the PLAZA shopping center

In the LOT building example, there is a considerable difference in CO2 output in favor of the GI solution,

arising from a combination of an environmentally friendly technology (VR) with an optimized GI

design, applied only under a portion of the original foundation slab. The PLAZA center example is more

complex. In this case the comparison of CO2 emission includes not only alternate designs using DSM

columns vs. CFA piles, but it also takes into account the associated differences in the overall foundation

volume and reinforcement amount, which have a significant impact on the combined emission output.

The GI solution likewise offers a distinct reduction of carbon dioxide emission as compared to a standard

piling design (-16%).

CONCLUSIONS

It has been demonstrated based on practical design-and-build experience that creative geotechnical

designs involving the use of contemporary ground improvement technologies, applied individually or

in combination, offer attractive alternative solutions to a wide range of foundation problems associated

with heavily loaded and settlement sensitive structures, and are therefore appealing to clients and

contractors. The benefits of GI solutions typically comprise shorter construction times and lower

construction costs, but may also potentially include significant reduction of project related carbon

dioxide emissions. It is therefore expected that the role of unconventional GI solutions in civil

engineering practice will grow in the future, offering novel and innovative possibilities for researchers,

geotechnical designers and specialized contractors.

REFERENCES

Carbon Calculator Tool v3.0, EFFC and DFI, http://www.geotechnicalcarboncalculator.com/de

Kirsch, K. and A. Bell (editors), 2013. Ground Improvement. CRC Press, London.

0

500

1000

LOT - VR LOT - CFA

37

854

0

2000

4000

PLAZA - DSM PLAZA - CFA

3249

3852

Materials Energy Freight Mob/demobilisation Peopel's transportation Assets Waste

tCO2e tCO2e