769

Geotechnical Engineering for the Preservation of Monuments and Historic Sites – Bilotta, Flora, Lirer & Viggiani (eds)

© 2013 Taylor & Francis Group, London, ISBN 978-1-138-00055-1

Use of jet grouting technique to realize substructures of historic buildings: The example of an apartment building in Warsaw

K. WanikSilesian University of Technology, Gliwice, Poland

ABSTRACT: The paper presents the application of jet grouting technology allowing to create a sub-structure of existing buildings. Neighbouring historic apartment buildings situated in the centre of Warsaw were used as an example. The ideas of the solution consisting in creation of basement walls together with new foundation, formed from cemented soil under existing continuous footings, were ana-lysed. The process of geotechnical engineering construction project implementation was surveyed in all stages of work, starting from preliminary concepts and the designing stage, through basic construction and finishing works. Suggestions have been included regarding the tests and observations required due to the exceptional characteristic of the facility and adopted solutions. Attention has been paid to the necessity of considering specific technological conditions. Additional difficulties related to the execution of works in the very centre of a large urban area and at substantially limited space of premises situated below the ground level have been determined. The issue of specific requirements and limitations resulting from the need to interfere into the structure of a historic building has been raised. The role of execution monitoring has been defined, suggesting the scope of required observations and measurements. Based on the experience gathered during the construction, attention has been paid to especially important issues of geotechnical engineering supervision.

Buildings were constructed using a traditional technology, with brickwork walls. Basement ceil-ings were constructed as ceramic barrel vaults and arched floors. Timber flooring systems were made

1 INTRodUCTIoN

Aleje Jerozolimskie is one of the most important and longest arteries of Warsaw, within the city nearly 12 kilometres long. It includes sections of national and regional roads.

At the turn of the 19th and 20th centuries, i.e. in the period of extensive development, the con-struction of two apartment buildings situated on Aleje Jerozolimskie was finished. These apartment buildings were part of frontage of monumental development, characteristic of the Warsaw city architecture at the turn of centuries (Fig. 1).

Now this frontage is one of few, which survived World War II. It shows the appearance of houses with a characteristic variety of building facades, each of which was designed in a slightly different style.

The described apartment buildings are situated in two fully developed and adjacent plots of shape close to a rectangle. The access to the courtyard was provided by two gateways. Courtyards creat-ing a “well” backyard (Fig. 2).

Buildings were constructed as four-storey base-ment houses, with habitable attic in the front part and non-habitable attic in outbuildings. Later on one storey and an external lift shaft were added.

Figure 1. Current state of apartment buildings on Aleje Jerozolimskie in Warsaw.

770

on higher storeys. Ceramic arched floors existed only in hygienic facilities. Roofs of timber structure were covered with roofing felt on a timber decking. Buildings were equipped with services, which were subject to numerous reconstructions with time.

In 2005 a discussion broken up on the future of both buildings situated on Aleje Jerozolimskie. There were opinions supporting a possibility of apartment buildings demolition due to their poor condition. However, finally after the intervention of monuments conservation officer a decision was made about a major renovation. A decision on reconstructing the destroyed front facades was also taken. Because of buildings condition and of requirements set the refurbishment works comprised numerous construction and conservation works. It included also geotechnical engineering works. The activities carried out were constrained by the need to maintain the historic nature of the building.

2 RENoVATIoN WoRKS

2.1 General assumptions

The basic objective of carried out works consisted of restoring the proper condition of buildings together with their adaptation to current require-ments. Because of historical value of both apart-ment buildings special care was devoted to the protection and conservation of preserved historic elements. The reconstruction of façade decora-tions was also included.

The adaptation works consisted in adapting the buildings to modern requirements set to service-office type buildings. This required performing a number of actions both in the architectural and structural field.

The necessity to equip the buildings with much more expanded building services forced placing

most of technical equipment in the reconstructed substructure. An additional space was obtained by building a technical room under the courtyard and connecting it with the existing basement premises (Fig. 3).

In general, the construction works consisted in strengthening, partial replacement and con-struction of new elements. Because of numerous cracks the lintels, piers between window openings and walls were strengthened. The timber floorings were replaced with reinforced concrete floors. Also brick floorings above the basement were partially replaced. The top storey was entirely reconstructed and a new roof structure was made. All the works were carried out without changes in external dimensions and maintaining the building’s historic nature.

2.2 Soli profile characterization

All existing continuous footings had rested on stiff sandy clays prevail in the area in question. A uniform arrangement of layers forming the subsoil with parameters improving with increas-ing depth substantially simplified the analytical analyses (Table 1). No water level was measured in the investigations. only water infiltrations were recorded.

2.3 Geotechnical engineering works

Typical basements made during apartment build-ings construction feature usually low height, rang-ing from 1.7 to 2.5 m. The depth of continuous

Figure 2. The courtyard of apartment buildings in Aleje Jerozolimskie in Warsaw, creating a “well” back-yard, during the refurbishment works.

Figure 3. Excavation for the technical room under the courtyard on Aleje Jerozolimskie in Warsaw.

771

footing or base walls foundation is very shallow, between 0.2 m to 0.5 m. Willing to use fully the underground storey it becomes necessary to lower the floor level. It implies the need to perform new, much deeper foundations.

The underpinning must carry increased loads, which values include structural changes and the new way of premises use. Also it is necessary to consider local over excavations related to perform-ance of shaft pits and equipment, in particular technological tanks.

At first, the underpinning using classical meth-ods was considered, with performance of concrete in sections 1.0 to 1.4 m long under the existing continuous footings. This solution required appli-cation of reinforced concrete continuous footing 0.4 to 1.2 m wide.

The classical underpinning was related to the hazard of excessive cracks during the occurrence of temporary lack of support. The condition of walls selected for underpinning was variable, both due to their age and influences to which they were subject during the building existence (numerous reconstructions, war operations etc.). The neigh-bourhood of two apartment buildings was an addi-tional impediment for performance of foundations deepening using classical methods.

After a thorough analysis of the solution, and in particular related hazards, the search started for foundations deepening method, which could exclude or minimise the existing hazards. The pre-dicted long period of works execution, carried out in cramped and low premises, was also important.

At the stage of preliminary consultations a pos-sibility of jet grouting use to underpinning was considered. At a correct performance of works this solution would cancel hazards characteristic of a classical underpinning.

Advantages of the suggested solution con-tributed to performance of more detailed analy-ses, which ultimately resulted in jet grouting underpinning within a full scope. This allowed using entirely the existing space, connected with

a new technical room situated under the court-yard level (Fig. 3).

3 JET GRoUTING UNdERpINNING

3.1 Stages of designing works

preparation of design documents related to works described in this paper, requires planned and coor-dinated actions. They refer both to the stage of pre-design works and also to later designing and implementation of construction works.

The design works on geotechnical engineer-ing solutions should be started at the stage of concept development or ultimately at the stage of building permit design. It is important that persons responsible for the described issues get involved in the design team work already at that stage. Even working consultations create oppor-tunities to outline preliminary solutions and to determine the scope of required recognition. proper directing of actions and the awareness of hazards allows carrying out further work efficiently, coordinating activities of individual architect’s consultants and working out common, but primarily consistent, solutions (pieczyrak & Wanik, in press).

Adoption of a correct procedure simplifies the process of a construction project implementation. It allows avoiding unpredicted difficulties resulting from the lack of project coordination. In the case of necessity to introduce changes or of situation occurrence, which requires a quick response, it ena-bles effective activities.

In the considered case the work aimed at devel-oping a underpinning solution was started at the stage of preliminary design. Initially the adoption of a solution using classical technologies was con-sidered. only partial application of jet grouting was assumed, which would cover continuous footings of walls adjacent to the neighbouring buildings. Also the overall scope of foundations deepening was supposed to be much smaller, which made full utilisation of basement impossible.

The proposal to perform the works in a way limiting the hazards occurring at digging under the existing foundations as well as a possibility to include in them the whole basements was approved. A close cooperation of geotechnical engineer on the contractor side with investor’s design team started since that moment.

At the design stage, based on the provided guide-lines and approvals, an initial version of the design was prepared. After interdisciplinary coordination and minor corrections the documentation was sub-mitted for external assessment by an independent scientific worker.

Table 1. Geotechnical properties of soil layers.

depth below ground surface Soil layer

Geotechnical properties

0.0–0.5 Top soli –0.5–5.0 sandy clays

stiffcu = 32 kpa, Φ = 18°

M = 36 Mpa, γ = 21,0 kN/m3

5.0–12.0 Sandy clays very stiff

cu = 45 kpa, Φ = 23° M = 60 Mpa, γ = 21,5 kN/m3

772

The designed solutions obtained a positive opinion. Attention was drawn only to the need of current verification of the brickwork material condition in the bottom part of the foundation. Because there was a justified concern that locally it may have an insufficient strength.

Taking into account the recommendations, an assumption was made on taking core samples from drillings made through the foundation to form jet-grouted columns. Collected samples should be evaluated on a current basis in terms of material quality and strength.

The process of detailed design presented above integrates well with the model schemes. The coop-eration between individual members of the design team, comprising various disciplines, is extremely important. Mutual verification of adopted solu-tions allowed avoiding inconsistencies in the devel-oped documentation, which could result in further execution difficulties.

3.2 Description of adopted solutions

prepared final design assumed performance of foundations underpinning using columns formed in the jet grouting technology (Fig. 4–6). The idea of adopted solution consisted of production of a cemented soil body forming walls of the lowest sto-rey and a new foundation.

In the performed analytical calculations it has been assumed that the jet grouting underpinning will transfer the planned load on the load-bearing layers of the subsoil. An assumption was made that the underpinning would work like the continuous

footing without considering friction resistance on the petrified soil body side surface.

on the basis of adopted architectural solutions maximum differences in the bottom of existing

Figure 4. part of floor projection of jet grouting col-umns to strengthen and deepen foundations of apart-ment buildings on Aleje Jerozolimskie in Warsaw.

Figure 5. An example of trestle system of founda-tion underpinning in apartment buildings on Aleje Jerozolimskie in Warsaw.

Figure 6. An example of one-sided system of foun-dation underpinning in apartment buildings on Aleje Jerozolimskie in Warsaw.

773

foundations and the excavation bottom were deter-mined, ranging from 1.8 to 2.1 m. Locally there were over excavations of height differences reach-ing 3.25 m, related to the planned technological tank and the lift foundation. only in these sections jet grouting columns were reinforced using steel tubes 60.3 mm diameter.

Because of hazard connected with temporary lack of support a relatively small diameter of jet grouting columns, equal to 0.5 m, was adopted. Its selection was affected also by the type and condi-tion of soils occurring in the subsoil (Table 1).

Also the order of individual works performance was defined in the documentation. A possibility of forming on one day neighbouring elements of the underpinning was excluded. The necessity to form every third jet grouting column was imposed.

designing the works, because of adopted tech-nology and site specificity, slightly higher toler-ances were assumed. performance of works using small-dimension equipment is one of factors hav-ing an adverse impact on the achievable accuracy. A substantially lower possibility of accurate drill-ing tube rod running results in a greater sensitiv-ity to various obstacles existing in the subsoil. In the case of encountering boulders and stones, remnants of old buildings, bricks or timber, the drilling tube rod may be pushed away. This results in shifting the performed jet grouting element (Wanik, 2010).

Working levels, from which columns were formed, were assumed on ordinates corresponding to the level of premises finish floors of the low-est storey. Very shallow foundations (Fig. 5–6) excluded a possibility of preliminary lowering the working level. Combined with low premises it caused the need to demolish part of ground floor slabs before starting the works.

Small diameters of jet grouting elements and the requirement of forming a possibly uniform substi-tute foundation forced the necessity of forming inclined columns. drillings through brick foun-dations were made before starting injections. In accordance with guidelines there was an ongoing control of brickwork samples collected from the drill cores.

For safety reasons the performance of test holes was predicted prior to works start to localise unknown underground utilities.

Two types of jet grouting columns were adopted depending on the drilling locations accessibility. In premises, to which a two-side access was possible, trestle systems execution was planned. Columns were formed alternately on both sides of the foun-dation (Fig. 5). In the remaining cases petrifica-tion was carried out on one side, which required increased number of jet grouting elements to ensure a proper support of walls (Fig. 6).

For both apartment buildings performance of in total 2472 jet grouting columns was designed, of overall length of 9921 m. A detailed specification of designed elements is presented in the table 2.

4 SpECIFIC NATURE oF CARRIEd oUT WoRKS

4.1 Jet grouting

Jet grouting consists in destructing the structure of soil forming the subsoil within the range of injection stream action and in its mixing and par-tial replacement with a grout. destruction of soil structure and its mixing are obtained as a result of action of a high-energy injection stream of liq-uid, which usually is also a grout. Elements formed during injection are called jet grouting columns (Fig. 7) or jet grouting walls, depending on their shape. The jet grouting may be performed in a sin-gle, double or triple fluid system. These systems differ between themselves in the number of media used for soil loosening and mixing. The most com-monly used single fluid system uses one medium, which usually is a cement grout.

The range of injection stream depends on many factors, including the type and condition of sub-soil ground and technological parameters of col-umns forming (Bzówka 2009, Flora et al. in press, Modoni et. al. 2006).

Table 2. Specification of the number of planned columns, Aleje Jerozolimskie in Warsaw.

Column length

Number of columns Total length

m units running metres

Aleje Jerozolimskie 611.0 152 1522.0 111 2223.5 64 2244.0 27 1085.0 878 4390

Total 1232 pcs 5096 m

Aleje Jerozolimskie 631.0 189 1891.5 20 304.0 718 28724.5 8 365.0 132 6606.0 173 1038

Total 1240 pcs 4825 m

774

Technological effluents originate during col umns drilling and forming, being an excess of injection grout mixture with soil. These are wastes flowing out to the ground surface through the free space between the drilling rod tube and the injection hole wall.

In the initial stage of columns forming a hole is drilled to the target depth using drilling rod tube equipped with an injector. The borehole diameter is much smaller than the column diameter and ranges between 90 and 150 mm. This stage is used to reach the required depth, from which column forming starts and possibly to preliminary loosen the soil structure. The soil loosening is aimed at increas-ing the range of the injection stream forming the column, which ultimately translates to its higher diameter. Then the proper column forming starts from the borehole bottom. The energy of injection stream getting out through injection nozzles situ-ated at the end of drilling rod tube destroys the soil structure and forming a cylindrical column. The column material is the injected grout mixed with the soil, containing in its volume everything, which in the considered subsoil was within its range of action (Wanik, 2010).

The technological line used in the single fluid system comprises: a set of mixers, an high pres-sure injection pump, a drilling rig with a drilling-injection rod tube equipped with an injector, and a set of tubing connecting individual elements of the system. depending on the specific nature of drill-ing and injection this set may be expanded with additional elements.

The use of drilling diameter much smaller than the target diameter of jet grouting elements is one of advantages of described technology (Fig. 8). It translates to a smaller interference in the existing elements, which has a substantial importance for the works carried out within historic buildings.

The lack of vibrations at the execution of drilling and injection work related to columns forming is another important engineering aspect worth notic-ing. It is particularly important, when we consider the earlier mentioned and frequently existing poor condition of strengthened structures. The need of vibrations introduction may be related only to the use of a hammer drill to pass through obstacles, such as boulders, concrete elements etc. However, even in this case the generated vibrations do not threat the structure, which has been confirmed by numerous implementations in recent years (Wanik, 2009).

In the considered case the main advantage of the technology adopted was a possibility to apply a small-dimension, hydraulically driven drilling rig. The use of a separate power pack and the location of remaining technological elements, i.e. the set of mixers and pump outside the building, had a sub-stantial importance at the limited inside space.

4.2 Work within the historic building

performance of works within historic build-ings is related to the necessity of fulfilling much wider scope of requirements. The necessity to per-form the works in a way that does not change the

Figure 7. Interpenetrating jet grouting columns of the trestle system of apartment buildings in Aleje Jerozolimskie in Warsaw.

Figure 8. Jet grouting underpinning foundation of the apartment buildings on Aleje Jerozolimskie in Warsaw with removed offsets. Visible places of drilling through brick continuous footings.

775

building or maintains the historic elements ranks first. It makes that the process of individual design and material solutions approval turns into labori-ous negotiations. The presented proposals need approval and the process of decision making is usually long.

Specific requirements force the designer and contractor to look for new and unconventional solutions. performance of simple tasks requires frequently application of incommensurably com-plex actions. Limitations to technology and mate-rials adoption possibility substantially make the works more difficult or longer. They can have a significant impact on the increase in investment implementation costs.

With respect to the described deepening and strengthening of foundations the conservation requirements focused on ensuring as high as pos-sible safety of the building. It was understood as limiting to minimum the possibilities of creating new or development of the existing damages. This forced searching for new alternative solutions to the initially proposed classical underpinning. The adoption of solution based on jet grouting ensured the required safety level. However, it required allowing demolition of the ground floor slabs and of part of basement walls.

4.3 Protection works

Works related to formation of a new foundation are related to transferring the load onto deeper sit-uated layers of subsoil. The process of loads trans-ferring is accompanied by a change of the stress state, which leads to origination of settlements. When using the jet grouting technology, at prop-erly carried out works, the settlements are minute and do not cause origination or development of the existing damages.

In the case of buildings in poor condition, despite limitation of hazard it becomes neces-sary to undertake additional protective measures. damaged walls are usually repaired through their rebuilding of masonry or stitching in places of the existing cracks.

older buildings do not have ring beams and the timber floorings poorly stiffen the structure. Ceiling beams initially supported in shallow holes after a few decades of use do not provide a suffi-cient guarantee of proper support. In most cases, prior to repair works start, it is recommended to clamp building walls on the floors level. Both steel bars and reinforced concrete perimeter beams are used.

Attention should be paid also to the condi-tion of window and door lintels. Cracks existing within them create a substantial threat for the safety of building and people working inside it.

The best solution is to make new elements prior starting the basic renovation works. Frequently, to increase the spatial stiffness, openings are tempo-rarily walled over.

Numerous cracks of walls and piers existing in described buildings and a poor condition of timber floors forced proactive performance of strengthen-ing works. At first, lintels were repaired and piers between window openings were strengthened. Repairs covered mainly walls, on the courtyard side, which were in the worst condition. Rebuild-ing of masonry and additionally stitching the walls using steel bars were locally performed.

The necessity of ground floor slabs demolish-ing, required due to making premises available to carry out underpinning, forced a proactive replace-ment of floors on higher storeys. Works staging was assumed so that demolition works would not be performed in adjacent rooms. New floors were bolted to walls.

drilling and injection works were started once the strengthening was performed and assuming that above the room, where the works are carried out, reinforced concrete floors have been made.

4.4 Works in the city centre

When performing construction works in the centre of a big city it is necessary to take into account additional difficulties, characteristic of urban areas. They result from a dense development limiting the usable space and from significant impediments related to equipment and materials transport.

The location of described works in the very city centre of Warsaw, at one of main arteries, was a major logistic challenge. The equipment was transported and unloaded in early morning hours. Because of lack of pedestrian pavement possession possibility, the site facilities for injection works were situated on the apartment buildings court-yard. The delivery of high-pressure injection pump created the greatest difficulties. Existing gateways did not allow entering the courtyard by the trans-port (Fig. 9). The pump casing had to be removed and the pump rolled in to the target location using special rollers and a block and tackle.

After performance of the first stage it was nec-essary to move the grouting point to the courtyard of the neighbouring house, not covered by the ren-ovation works. The transport issues were resolved in a similar way. However, a problem appeared related to the works performance in the surround-ing of residential and office premises. To reduce the nuisance caused by the equipment operation, the possessed area was fenced, performing also a tem-porary canopy roof. Soundproofing was installed on the pump and the exhaust gas was discharged by a duct above houses roofs.

776

The aforementioned impediments together with limitation of permitted working hours during the daytime to a large extent contributed to extension of the execution.

4.5 Injection grout punching

A problem of injection grout punching to premises not related with the performed works occurred during the injection works. In the basement of the adjacent apartment building there was an outflow of the cement grout. This situation did not create a major hazard and the originated damages were immediately removed.

Injection rout punching cases are pretty com-mon and difficult to avoid. They may be caused by a lower level of adjacent premises floor finish, local loosening of the soil and too long period of upper column part forming directly under the founda-tion. Also the subsoil fracturing due to a tempo-rary loss of the injection opening flow capacity cannot be excluded.

4.6 Obstacles in the subsoil

When carrying out drilling works it is necessary to consider a possibility of encountering in the sub-soil of obstacles, difficult to drill through.

An especially high probability exists in a devel-oped area and when the works are performed in the place, where earlier development existed. In areas covered with postglacial formations boulders exist in the subsoil.

during the work under foundations erratic boulders of rock material were encountered twice (Fig. 9). performance of the drilling, due to a small force of drilling rig thrust, required using a dawn the hole drilling hammer.

Apart from the obstacles mentioned above in Warsaw there is a high probability of encountering duds dating back to World War II. They create a hazard for people and buildings safety. If the work is carried out in an area of especially high hazard, usually UXo monitoring is present on the site.

4.7 Geotechnical engineering monitoring and supervision

performance of measurements and observation and also an ongoing verification of assumptions made at the design stage becomes highly important.

The strengthening works, carried out within foundations, create a hazard of originating exces-sive, but primarily uneven, settlements. If paral-lel to that wall and header cracks occur, there are no bond beams and floors, which could bind the structure ensuring its spatial stiffness, the hazard scale increases.

The design regarding the apartment buildings underpinning contained a provision imposing the necessity to perform land surveying and observa-tions of existing damages development. The meas-urements should have been initiated before starting the works, carried out during their performance and continued till the completion of works com-prising the underground storey.

The soil conditions were verified on a current basis during the drilling works. Also the wall struc-ture condition was assessed, which has been previ-ously mentioned.

Special attention was paid to ongoing assess-ment of carried out land surveying. during the works performance local increases in settlement, reaching a few millimetres, were observed twice. As it has been shown by later thorough analyses, the reason was in damages of walls structure and in performed demolition of floors.

The jet grouting underpinning did not represent displacements of higher parts of the wall.

Knowing the scale of hazards and being aware of importance of the tasks performed in the case of large and complicated investment projects it is nec-essary to appoint an experts team. Specialists in the field of geotechnical engineering cannot be missing in the group of persons supervising the course of the construction (pieczyrak & Wanik, in press).

5 SUMMARY

on the basis of the presented implementation an attempt was made to introduce the possibility

Figure 9. Erratic boulder in the underpinning struc-ture in the apartment buildings on Aleje Jerozolimskie in Warsaw.

777

of jet grouting use to create an substructure of a historic building. Important issues related to the construction process, comprising implementation of complex geotechnical engineering works, have been discussed. In particular the importance of proper order of actions has been emphasised.

Attention has been paid to the existing dif-ficulties and to sources of possible threats. The role of strengthening works, performed prior to start of underpinning, has been emphasised, as well as of the later execution supervision.

The presented procedure allowed a safe perform-ance of work. Concerns related to the occurrence of excessive settlements have not been reflected in the actual performance. It is possible to state that the application of larger diameters of jet grouting columns would not have an adverse impact on the building condition. The reduction of jet grouting elements number obtained this way would sub-stantially accelerate the works.

REFERENCES

Bzówka, J. 2009. Interaction of jet grouting columns with subsoil. Monograph. Gliwice: Silesian University of Technology publishers (in polish).

Flora A., Modoni G., Lirer S. & Croce p. 2012. The diameter of single, double and triple fluid jet grout-ing columns: prediction method and field trial results. Geotechnique in press.

Modoni G., Croce p. & Mongiovi L. 2006. Theoretical modelling of jet grouting. Geotechnique 56, No. 5, 335–347.

pieczyrak J. & Wanik K. Geotechnical engineering requirements for deeply founded investments in devel-oped areas. XXVIII polish Workshops of Structure designer Work, Wisła: 2013 pZITB. In press. (in polish).

Wanik K. 2009. Application of jet grouting to protect and strengthen foundations of close-to-shaft surface facilities. Wiadomości Górnicze 7-8/2009: 443-449 (in polish).

Wanik K. 2010. Selected design and technological conditions of jet grouting application. Inżynieria i Budownictwo 2/2010: 68–70 (in polish).