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AN INNOVATIVE METHOD USED TO BUILD AN

UNDERGROUND CROSSING DOWNTOWN RIO DE JANEIRO,

BRASIL

Carlos E. M. Maffei1, Heloisa H. S. Gonalves

2, Maria C. Guazzelli

3

ABSTRACT

The intention of this paper is to present the construction method used to build the underground road

crossing, that is a part of the revamping project of "Praa XV", in the city of Rio de Janeiro, and discuss the

monitoring results obtained through instrumentation data. An alternative construction method was proposed

and successfully executed, allowing reduction of embedded lengths of diaphragm walls and avoiding the

need for temporary support. This innovative method distinguishes itself from the original design by using

jet-grouting and introduces the following features: smaller and thicker diaphragm walls, temporary supportelimination, no need of ground water level lowering and reduction of the bottom slab thickness. In this

alternative diaphragm walls constituted the retaining wall, while the columns of jet-grouting were disposed

as to form the support.

INTRODUCTION

To provide the revamping of "Praa XV", located in Rio de Janeiro downtown, it was built an

underground crossing for vehicle passage. The square area covering 50 thousand square meters of esplanade

was totally re-built in order to create a large playground for the population.

Basic design of the underground crossing set up the execution of a trench, with variable width as shown

in Figure 3 and 5.5m excavation executed through the inverted method, which consists of excavating under

the ground slab, that would support the retaining wall. Such diaphragm walls 60 cm thick, besides the fact

that they have been used to retain the soil, it would also be used as complementary foundation for ground

slab; barrettes would be used as central supports. The design, also provided internal water pumping of the

excavation, using a deep wells system in other to avoid base failure in the existing sand layer 16.5 meter

deep. In some regions, in view of existence of soft clay, water pumping would have caused consolidation

settlements and damage problems as the area is crowded of old buildings. Therefore, ground water had to be

kept at the original level by water re-injection. The bottom slab served as uplift slab supported by diaphragm

walls and barrettes.

As for the final design, the inverted method excavation was dismissed, having been substituted by the

conventional method, as in such a case, the temporary support was necessary. The length of the embedded

diaphragm wall was large, as the competent soil in this region would be only found at greater depths. As to

allow the reduction of diaphragm walls embedded lengths and avoid temporary support, it was proposed andsuccessfully executed an alternative construction method that differs from the original design by using short

columns of soil-cement performed by jet-grouting method and which presents the following features:

shorter and less thick diaphragm walls; temporary support elimination; elimination of ground water level lowering; reduction of bottom slab thickness;

1

Full Professor Escola Politcnica da Universidade de So Paulo, Carlos E. M. Maffei Engenharia S/C Ltda., So

Paulo, S.P. , Brasil2Phd, Professor Escola Politcnica da Universidade de So Paulo, So Paulo, S.P., Brasil

3MsC, Carlos E. M. Maffei Engenharia S/C Ltda., So Paulo, S.P., Brasil

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In this alternative, diaphragm walls constituted the retaining walls, while columns of jet-grouting formed

the support. Moreover, jet-grouting diaphragm had been made to avoid longitudinal water inflow. The

diaphragm walls that should reach the impermeable soil avoided the transverse water inflow.

Figure 1 : Comparison between the construction methods

DESCRIPTION OF THE CONSTRUCTION METHOD

This alternative design envisioned diaphragm walls that formed the retaining walls, while the support and

the bottom plug were formed using soil-cement short columns obtained from the jet-grouting method.

The diaphragm walls, besides the fact of having to confine earth fill, they also had the function of

providing a cut-off, which obliged water to percolate through the clay and silt layers, reducing the seepage

force on the excavation bottom, allowing the excavation to be done without risk of base failure or quicksand

occurrence. The embedded lengths of diaphragm walls in original design were enough to guarantee that these

functions would be attained.

Figure 2 : Geological profile

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It was idealized a support beneath the excavation bottom, previously executed, using soil cement columns

as consolidation, to allow the reduction of solicitation over the diaphragm walls. This soil-cement support

reduced the thickness of the diaphragm walls to 40cm and the embedded length from 15m to 6m as average.

It also allowed the elimination of the need of temporary support to reach the excavation bottom, thus

reducing excavation stated period. Furthermore, diaphragm walls behavior became less dependent of soil

properties in the embedded region, constituted by organic clay and loose sand, reflecting in more reliable

work regarding costs and time.

As it can be observed in the synthesized geological profile, presented in Figure 2, elaborated as per

drilling and tests disposable, the top of the clay could be found at variable depths. The diaphragm walls

embedded length almost all the time intercepted this layer, so that the water inflow would be very small

transversely. Meanwhile, close to picket 110 and 114 the small thickness of the clay layer would imply in

communication feasibility among deeper sands. So, a bottom plug was executed to ensure tightness and

feasibility of the excavation phase, avoiding water level lowering. This bottom plug was composed by two

parts, as shown above: a lower plug, impermeable, that annulled the water pressure, during temporary and

permanent phases and an upper plug, used as lateral support for the diaphragm wall, as shown in Figure 4c.

As water pumping was not used, settlement problems around the neighborhood and their effects to the

buildings and utilities were avoided.

Jet-grouting columns to function as support, were disposed transversely, as two rows of JSG (JumboSpecial Grouting) 1.20m diameter, spaced every 5m, coincident, preferably, with some diaphragm wall joint.

It was also made two rows of columns parallel to the diaphragm wall as to transfer the reaction from the

walls to the struts, as shown in Figure 3.

Figure 3 : Support scheme

Of all solutions using bottom plug, the simpler one consisted of using the weight to balance the uplift. Its

installation, from the excavation bottom was ideal, because besides other advantages, it enabled previous

support of the diaphragm wall beneath bottom excavation, that in this case was very important as the bearing

soil had poor strength and stiffness characteristics. Although, preliminary estimative that has been done

shows that plug thickness was substantial, making its use unfeasible (Figure 4a). So, it was decided to

execute a greater depth plug down the excavation bottom, using the weight of the existing natural soil

between the excavation bottom and the plug as weight over the plug (Figure 4b). Although, this solutiondemanded the use of a natural soil layer 4 meter thick; in such a case, an effective support and almost

without displacement in the treated layer, the diaphragm wall during the temporary phase, would not

mobilize significant passive pressure of the natural soil, so that the wall span between the support in the

ground slab and the plug would be too large, providing significant solicitations. Due to the same reason, the

height in which the wall works efficiently as a console would became limited, implying the use of temporary

support, to be removed only after slab execution. The solution shown in the Figure 4c, was created to better

use natural soil weight and reduce the solicitation over the diaphragm walls avoiding the temporary support.

As the diaphragm walls should balance at rest and hydrostatic pressure, considering superficial water level at

permanent phase, the solicitation on the temporary phase was not the most critical. The support granted by

treated upper layer almost coincident with the one given by the bottom slab and the loads were lower (active

and piezometric pressure with deeper water level, as shown in the drilling).

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Figure 4 : Bottom plug scheme

The uplift slab was not necessary because the predicted seepage quantity was very little. Such a fact

occurred, and it was compatible with the pumping capability and common seepage for underground

structures.

Great excavation depths between 106+10 and 120+15 picket limited the use of this alternative design. As

the diaphragm walls embedded length have reached the most impermeable layers in the ramp interval, two

cut-off of jet-grouting were set on the ends, avoiding water level lowering in this area; on the other hand, two

cut-off would be executed within the application limits of this alternative constructive method.

DESIGN CRITERIA

Bottom Plug

The bottom plug was designed to attain the following situations: permanent phase, at superficial water

level and at water level under normal condition; and temporary phase, at superficial water level as it was thecase of a great deal of rain along the construction period.

Safety admissible factor for floating verification purposes was the following: at the most critical case, it

corresponded to the permanent phase at superficial water level, the safety factor was equal to 1.1. Although,

in case of need during the construction period, predicted constructive procedures could have guaranteed

acceptable safety factor.

JSG Support

As well as JSG struts, JSG slab (upper plug), was designed to resist the normal strength, corresponding to

diaphragm walls support reactions, with adequate safety factor related to buckling and the one related to

compression failure.

It had been mostly noticed that JSG columns dimensions (diameter and length) had suffered changes

according to spotted conditions, which had also been taken into account for the calculation.

Diaphragm Walls

In the same way that was done to define bottom plug design criteria, considering temporary and

permanent phases. There were two types of structure to be analyzed in the permanent phase: walls in balance

(ramps) and supported walls (by ground slab, pergolas or footbridge). All the walls, either the ones of the

central part, were in balance before the concrete from the bottom slab had been done.

The walls had been represented by unitary width beans, tied by concrete slabs, pergolas and bottom slab,

considered as no displaceable supports; by jet-grouting and soil layers, considered as displaceable supports.

Taking into account the high stiffness of the first treated layer, in practice, the support works as a not

displaceable one.

In the permanent phase the walls were carried with at rest pressure and superficial water level, using the

safety factorf=1.4.

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The walls in balance were also designed at the water level under normal condition as to verify

displacements.

In the temporary phase all the walls, in balance, were carried at active pressure and water level under

normal conditions. It was chosen the case with the greater height, so it could verify the solicitations. The

calculation considered the geometrical and rheological compartment in the following step.

For all cases it was adopted conventional rheology of the materials: linear elasticity to reinforced concrete

and linear elasticity to the beams that represent the soil, either natural or treated, within their limits

corresponding to passive stress at 1.5 safety factor.

CONTROL METHODOLOGY

It used to be a control methodology to guarantee the hypothesis admitting for the design, including

instrumentation, with pore pressure measure in the lower base contact of jet-grouting and additional

geotechnical investigation. Besides that, percolation parametric calculations were done of different

geotechnical profiles to permit defining execution controls.

Map

The region map was carried out by drilling before starting the works and taking sampling during the jet-grouting and diaphragm walls execution.

Additionally, existing drilling SP01 up to SP09 was took before carrying out nineteen new SPT

determination drilling and many others without SPT determination. This new drilling was taken to ensure a

better knowledge of subsoil layers. All drilling done at site was sealed with soilcement columns hindering

the communication among different water level inside the sand.

The subsoil was constituted of the embankment layer, above sand and organic clay layers up to 12m; at

some places this layers is up to 15m deep. Under this depth there was a silt clay containing much sand in

some places.

There is sand silt close of the ramps where the jet-grouting was not utilized.

Subsoil good knowledge allowed to locate the plugs in places that could create communication risks

among sand layers or base failure.

Nevertheless, there had been the reduction of the uncertainties due to drillings, during the jet-groutingexecution, were extrudes sampling utilizing the same equipment to complete the region map.

Control of the penetration of the diaphragm wall into the clay

Diaphragm wall total penetrations were guaranteed due to visual and tactile observation during wall

excavation and the results of the slurry tests. Parametric studies demonstrated that the quantity of flow

variation due to diaphragm wall penetration into impervious layer was little; for this reason it was defined a

minimal depth into impervious layer equals to 1.5m.

So, the diaphragm wall was executed up to design deep only 1.5m into the clay layer; in the other case,

the diaphragm wall embedded length would be completed with increased wall length or jet-grouting

columns. An example of this solution is in the Figure 3.

Control of the pore pressureThe pore pressure control was performed by piezometers installed in every place. But, in dangerous zones

there were more piezometers having the purpose of guaranteeing safety during the construction.

Control of the quantify of flow

During construction, the flow quantity would be controlled and the greatest quantity of water after the

construction would be smaller than 1.5l/h x m of trench with gradient close zero.

To control flow quantity its measuring was done during the excavations at each front.

Control of jet- grouting characteristics

Design considered unconfined compression strength value of 2500kN/m2

for jet-grouting columns.

During the works samples were extracted to test, but it is important to remember that, regarding consolidated

soil, it is important strength and stiffness as a whole. The tests results of the small samples could be lower

due to contamination or spotted cracking.

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Control of the horizontal displacements of the diaphragm walls

During excavation measurings of horizontal displacement from the top of diaphragm walls, it was done

and confirmed jet-grouting support efficiency, as the excavation was done in one phase only, without any

other support installation, except for the historical buildings which the top of the diaphragm walls were

supported before reaching the excavation bottom.

Executive Sequence

The excavation and bottom slab concrete execution were done by service fronts.The excavation of each

front was done only after the diaphragm walls and the jet-grouting services had reached a 15m distance from

the front end, in a way that it was feasible to execute a frontal protection slope. Two cut-off of jet-grouting

columns were made to avoid longitudinal water inflow.

INSTRUMENTATION ANALYSIS

It was installed 10 piezometers, 3 inclinometers, immediately behind the diaphragm walls, 12 settlement

indicators on the viaduct piers and 3 settlement indicators on buildings.

The piezometers accused a water level variation from +0.40m to 0.50m, during the construction,indicating that there was initially an increase of water level in the region, with a posterior level lowering, as

expected, due to small water inflow inside the excavation.

Settlement indicators placed in the buildings, as well as in the viaduct piers, did not indicate settlements

occurrence. There had been a variation from +0.5mm to 0.5mm in the measurements, values that were

within the accuracy of the apparatus used to observe the settlements.

The inclinometers accused horizontal displacements of diaphragm walls of few millimeters; the highest

value on the top of the diaphragm wall was 6 mm. In the region of the jet-grouting struts the highest

displacement was close to 2mm.

CONCLUSIONS

Jet-grouting columns as the unique support for diaphragm walls executed in the construction of theunderground crossing at "Praa XV", in Rio de Janeiro, were used successfully. This design innovative

conception brought significant economy in terms of costs and time, without damaging its performance. On

the other hand, the fact that water lowering was not necessary in a region with organic clay avoided the

settlements it could had been caused.

ACKNOWLEDGMENT

Our special thanks to RioUrbe, its counselor, Valdir Mello and Constructor Queiroz Galvo due to their

confidence in executing an innovative design. We also thank Novatecna, Este, Geoprojetos and Sondotcnica

which were part of the execution of the construction and technical discussions.