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Forensic analysis of a pile foundation failure
Engineering Failure Analysis 17 (2010) 486–497
Contents lists available at ScienceDirect
Engineering Failure Analysis journal homepage: www.elsevier.com/locate/engfailanal
Forensic analysis of a pile foundation failure F. López Gayarre a, C. González-Nicieza b,*, M.I. Alvarez-Fernández b, A.E. Álvarez-Vigil c a b c
Department of Construction and Manufacture Engineering, Engineering School of Gijon, University of Oviedo, Campus de Viesques, 33201 Asturias, Spain Department of Mining Engineering, Mining Engineering School, University of Oviedo, Independencia 13, 33004 Asturias, Spain Department of Mathematics, Mining Engineering School, University of Oviedo, Independencia 13, 33004 Asturias, Spain
a r t i c l e
i n f o
Article history: Received 15 June 2009 Accepted 6 September 2009 Available online 20 September 2009 Keywords: Forensic analysis Groundwater table Differential subsidence Wooden piles
a b s t r a c t This paper presents the results of the forensic analysis carried out on a set of buildings located on the same block in the center of Gijón (NW of Spain). The foundations of these buildings were laid between the years 1944 and 1945, using wooden piles. These wooden piles may have been affected by a subsequent reduction in the groundwater table, caused by the depletion of water that occurred during the construction of the basement of a building located on the adjacent block. Evidence of the first major damage to the building appeared upon initiating construction of the aforementioned basement. The situation was worsened by debris falling from one of the damaged buildings onto a public street below. Within a week, after the city authorities had reviewed the technical reports, they decided to proceed with the eviction of more than forty families in the affected buildings, thus triggering considerable social alarm throughout the region. This article provides a detailed description of the problem, analyzes the damage caused to the buildings, and based on such analysis and description, this article seeks to establish the different hypotheses in an effort to reveal what caused the problem. The focus of this analysis is to present the rotting of the head of the wooden piles of the foundation as the main cause responsible for the ruin of the affected buildings. The rotting was caused as a result of artificially lowering the groundwater table, which occurred during the construction of a building located in the vicinity. This study also analyzes the land subsidence and negative friction experienced by the piles as a result of artificially lowering the groundwater table. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction and background The first damage caused to a group of residential homes in Gijón occurred in 1987. The homes were located on the block between Avenida de la Costa, Santa Justa Street and Suerte Street (Figs. 1, 2 and 5). In November of 1989, it was found that debris had fallen from the building located on the road Avenida de la Costa No. 20, and had landed below on the public road. On that date, the part of the building adjacent to No. 22 was no longer habitable due to the appreciable damage that occurred as a result of major differential subsidence. After the technicians of the Gijón City Council had visited the area and viewed the damages, they ordered the eviction of the entire block, except for the building located on Avenida de la Costa No. 22, which intersects with Santa Justa Street, as no damage of any kind was detected. This decision triggered a major social alarm in the city, which was widely reported in the local and regional media at that time, particularly given that such decision impacted
* Corresponding author. Tel.: +34 985 10 42 66; fax: +34 985 10 42 67. E-mail address: [email protected] (C. González-Nicieza). 1350-6307/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.engfailanal.2009.09.008
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Fig. 1. Location.
Fig. 2. Block affected.
45 families. All the buildings that had been evacuated presented major cracks and collapses, with the most significant damage occurring in the building located on Avenida de la Costa No. 20. Once in-depth knowledge was acquired about the status of the buildings, the city decided to proceed with the demolition of all buildings on the block, except the one located on Avenida de la Costa No. 22. Fig. 2a shows a more detailed view of the damaged residential buildings. Fig. 2b shows the building located on Avenida de la Costa No. 22. In the same figure, the building located in the western area of the block is a new construction. The last building to be erected on the site of Fig. 2b is currently in its final stages of construction. This article presents the results obtained after conducting a detailed forensic analysis of the causes that may have led to the progressive deterioration of the buildings located in the area studied. Although the aforementioned buildings initially presented minor damage, the appearance of major cracks took place during the course of constructing the basement of the building on the adjacent block, located on the other side of Suerte Street, at No. 16 of Avenida de la Costa (see location in Fig. 2b). A failure in the foundation of the building was initially suspected, although a failure in the ground due to a subsidence phenomenon could not be ruled out [3]. For both hypotheses, the lowering of groundwater tables could have been what triggered the phenomenon. In any case, after expanding the radius of the study area, significant sinking and cracks were detected on both the sidewalks and the façades of other buildings. The problems of ground subsidence caused by excessive pumping of groundwater is a relatively common situation [1,2,5,6,8,9]. This study begins with an analysis of the geology and hydrogeology of the area. Section 3 contains a description of the foundation of the buildings affected, including some observations that may be related to the failure, and a description and analysis of the most significant damages detected. In Section 4, we discuss the various hypotheses of the failure; and finally, we present the conclusions derived from our study.
2. Geology, hydrogeology and geotechnics of the area The study area falls within the northern area of the city center, within the boundaries of the former wetlands that used to exist in Gijón (see Fig. 1).
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In the metropolitan area of Gijón, we must distinguish between two areas that are quite geotechnically distinct: The coastal area: mainly consisting of sediments caused by the fluvio-marine action and anthropogenic fillings. From a geotechnical point of view, all the coastal sediments are detritical materials, cohesionless, highly permeable and saturated with fresh water (except in the most shallow section), have low bearing capacity and are susceptible to processes of compressibility. The continental area: formed by rock materials and their corresponding products of alteration. As can be observed in Fig. 3, the study area is located in the coastal domain, consisting of sand that is interspersed with gravel and varying levels of peat. Based on various studies carried out in the area studied and it vicinity, four different types of materials have been identified: Anthropic fills: made up of clay, sand, gravel, pieces of debris, stones, bricks and some remains of peat. This material has a variable thickness between 0.6 and 1.8 m. Level of peat: it has an upper muddy area with a variable thickness between 0.6 and 2.3 m, followed by a level of peat, formed by mud, clay and fine sand, with debris, abundant organic matter and traces of decompositioned wood. The total thickness of this level fluctuates between 3.0 and 7.0 m, depending on the areas. As an example of the variability in the geotechnical behavior of these materials, Fig. 4 shows a penetrometer test that was conducted between Mieres Street and Schultz Street. It can be observed how in the levels of peat, the allowable bearing capacity is zero at depths between 1.6 and 4.8 m, and the value exceeds 1.3 MPa at greater depths. Also, the Standard Penetration Tests (SPT) carried out on the aforementioned materials corroborate these findings because they give NSPT between 2 and 5, which corresponds with allowable bearing capacity values of less than 0.03 MPa. Marly clays: located on the floor of the stretch of peat, there is a marly clays level that outcrops, with steely blue, gray, green and beige tones, and rock fragments. In general, it has a soft to medium consistency, with allowable bearing capacity values which vary between 0.06 and 2.24 MPa, based on penetrometer tests. Its thickness fluctuates between 0.3 and 1.4 m. Bedrock: It’s below the clay loam and is formed mainly of gray limestone, with small dark gray marl intercalations, and some levels of gray and beige dolomites, always presenting a high fracturation. An allowable bearing capacity of 0.3 MPa has been estimated for this level due to its degree of fracturation. Moreover, the study area is located in the Aquifer System No. 1, Mesozoic Unit of Gijón-Villaviciosa, which contains two distinct subsystems: Villaviciosa subsystem. Llantones subsystem.
Fig. 3. Geotechnical description of Gijón [4].
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Fig. 4. Dynamic penetration test.
Table 1 Variations in the groundwater table. Date of the study
Depth of the groundwater table (m)
March 1980 November 1989 October 1993 September 2005
0.4 0.9 1.1 5.0
More specifically, the study area is located in the Jurassic limestone aquifer, represented by the ‘‘Nodular Limestones of Gijón” and the ‘‘Magnesium Limestones of Gijón” of the Villaviciosa subsystem. From the hydrogeological aspect, the nature of the quaternary deposits that make up the first few meters of the subsoil in the study area ranges between permeable and semi-permeable, observing the existence of a water level associated with the level of peat, down confined by the level of marly clays. Moreover, the Jurassic materials that form the bedrock of the area are likely to contain water due to the karstification and the fracturation. According to various studies conducted in the analyzed area and its vicinity, there are high variations in the groundwater table. In this regard, a gradual increase is observed in the depth of the level of water, as is clear from the data shown in Table 1, which was derived from three studies conducted in the area.
3. Description of the problem and analysis of the damage to the buildings affected The buildings affected by cracks, subsidence, and collapses were built between the years 1944 and 1945, and their foundation was made using wooden piles and mass concrete blocks. Part of the building located on Avenida de la Costa No. 20 added three floors in 1956, although it was not possible to recover any technical documentation. Fig. 5 shows a layout plan of the buildings that were located on the block affected.
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Fig. 5. Plan view of the buildings on the block affected.
Marked in red1, Fig. 6a and b show the expansion that took place in the building located on Avenida de la Costa No. 20. It should be noted that no problems were found in the building within the 30 years following the addition of the three floors. Based on the original layout of the buildings affected, there were only measurements related to the foundation, which mention ‘‘foundation pits of 1.5 m 1.5 m 1 m, etc., each pit with six pine logs, and each log averaging 15 cm in diameter and 8 m deep.” In the excavation work carried out in 2008, for the construction of a new building located in the central area of the block (see Fig. 2b), pieces of wooden piles with diameters averaging between 11.5 cm and 17 cm were recovered. Fig. 7 shows a diagram of the concrete pile cap and the group of wooden piles, as well as some samples of the wooden piles which were obtained from the last excavation that was carried out. Based on the boreholes perforated, we know that the rock is located at an average depth of 6.50 m. Therefore, if the construction was completed according to the specifications provided, the wooden piles reached the rock. The building located on Avenida de la Costa No. 22, which is the only one that was not demolished, was built on concrete piles. One of the first observations that was made when analyzing the problem is that the pathologies extend beyond the block studied, given that in the past, some houses in the surrounding area that had been affected by cracks and major collapses ended up having to be demolished. Moreover, within a radius of approximately 300 m facing south–west, the sidewalks showed cracks and major subsidence. Today, even after the area has been remodeled, the sidewalks once again present major subsidence, as shown in Fig. 8. The surface of the entire block was leveled, in which, as shown in Fig. 9, it was found that there was a sinking of the land in a central strip of the block in the north–south direction. The level curves determine a subsidence trough whose axis passes approximately along the dividing wall of the buildings on Santa Justa Street No. 2 and No. 4, penetrating into the interior of the block and heading north, then turning toward the northeast and following the direction of the dividing wall of buildings No. 18 and No. 20 on Avenida de la Costa. The slope at the sides of this subsidence trough is at its maximum at the part of building No. 20 located on Avenue de la Costa, facing Santa Justa Street. This area is precisely where the greatest damage took place. The difference in the level between the highest point (corner of Avenida de la Costa and Santa Justa Street) and the lowest point (approximately at the dividing wall between No. 18 and No. 20 on Avenida de la Costa) is 0.5 m. Although this value includes the slope of the street (at least 0.15 m), corresponding to the subsidence that occurred. Today, in this very same place, the road Avenida de la Costa once again experiences a progressive sinking, as can be observed in Fig. 10. Due to the differential subsidence that occurred as a result of the decrease in the groundwater table, the following damages can be observed:
1
For interpretation of color in Fig. 6, the reader is referred to the web version of this article.
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a
b
Fig. 6. Expansion of the building located on Avenida de la Costa No. 20.
Fig. 7. Type of foundation and sections of piles used in the buildings affected.
Fig. 8. Sidewalk subsidence in the vicinity of the affected area.
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Fig. 9. Leveling in the block affected.
Fig. 10. Subsidence of Avenida de la Costa at near the block affected.
Fig. 11a shows some of the major cracks that were produced in the building located on Avenida de la Costa No. 20, particularly to the interior courtyard that it shares with the building on Santa Justa Street No. 4.
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Fig. 11. Cracks observed in the interior courtyard, extending from Santa Justa No. 4 to the building located at Avenida de la Costa No. 20.
Fig. 11b shows how the lintel of the window has lost its perpendicularity with the jamb due to the angular distortion caused by the subsidence. Fig. 12 once again shows the rupture caused in the wall of the lower floor of the building located at Santa Justa Street No. 4. Fig. 13 shows one of the cracks that occurred in the interior partitions of the building located at Santa Justa Street No. 2. The rupture can be seen by following the joints of the bricks, where the mortar shows less resistance. Of the various buildings that were located on the block, the building located on Avenida de la Costa No. 20 was undoubtedly the one that suffered the most damage, especially due to the cracks caused by differential subsidence [1]. This subsidence may have been caused as a result of having dividing walls that are shared with the building located at No. 22 of the same avenue. However, as above mentioned, the building located at No. 22 of the same avenue was built on mass concrete piles and hardly suffered any damage. Another important observation is that the damage that occurred in the block affected coincides with a major remodeling of the neighborhood, which includes buildings that were constructed around 1940 and had not shown previous signs of pathologies. As part of the construction carried out, coinciding with the onset of the damage, we must highlight the building located on the block adjacent to the one studied, facing Suerte Street, Avenida de la Costa No. 16 and Schultz Avenue. Its location can be observed in Fig. 14. To accommodate the basement of this building, an excavation was carried out, digging
Fig. 12. Rupture and displacement of the wall due to ground subsidence.
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Fig. 13. Cracks in the walls due to differential subsidence.
Fig. 14. Front elevation in the block affected.
6 m deep and constructing traditional concrete walls, thereby lowering the groundwater table. Pumping then took place over the course of several months. Because it was not possible to achieve complete watertightness with the construction of the wall, intermittent pumping continued once construction of the building had been finished. We were able to verify that the pumping still continues to take place to this day. The intensive pumping of water that occurred while the basement was being built, as well as the subsequent intermittent pumping, may have contributed to lowering the groundwater table in the block affected and may be what was responsible for causing the damage. Based on the documentation reviewed for this study, it is worth noting a report issued in 1976 by the architect Jorge Hevia. In the report, which was issued for another building located in the vicinity, the architect states ‘‘... the need to avoid laying foundations for new buildings in the vicinity which may require the groundwater table to be lowered...”
4. Hypothesis of the failure As previously discussed, the intensive pumping of water causes an overall lowering of the groundwater table in the surrounding area (Fig. 15). The degree to which the groundwater table is lowered depends, among other factors, on the permeability of the ground and the depth at which the pumping is carried out.
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Fig. 15. Lowering of the groundwater table due to the pumping of water in the adjacent building.
In the study area, the groundwater table was 0.90 m deep in 1989; although the survey that was conducted on Suerte Street 24 h later, showed that the groundwater table had decreased to 1.80 m. This fact can be explained either by a slow recovery in the survey, or it may be because the groundwater table in that area is lower than in the rest of the areas. In this case, the only possible explanation is due to pumping or drainage in the vicinity. Taking into account the characteristics of the layers of subsoil and the existence of a predominantly horizontal flow, we can estimate that due to the pumping carried out, the lowering of the groundwater table has a radius of effective influence of approximately 40 m. This radius covers the majority of the block affected, estimating that in the center of this radius, the average reduction of the groundwater level may have reached as much as 30 cm. Given that the pumping occurred over a prolonged period of time, such decrease in the groundwater table could have reached 1 m during dry periods, therefore also increasing its radius of influence. The main effects that cause these decreases in the groundwater table are attributed to: – Overall settling in the surrounding area (subsidence): the immediate effect of lowering the groundwater table causes an increase in the actual weight of the soil that is above ground as well as that which was once submerged. This increased weight causes a compression in the lower soft stratum, mainly peat, and thus causes the surface of the ground to sink. Such subsidence is clearly apparent in the leveling that was carried out. It is not exaggerated to estimate subsidence of 20 cm as a result of a decrease in the groundwater table by 1 m. – Deterioration of the wooden piles: as is logical, when the groundwater table is lowered, this leaves part of the wooden piles exposed to the air, causing them to rot due to the onset of fungus and microorganisms. If the foundation of the building was constructed with piles measuring 8 m, the base of the concrete block coincided with the groundwater table in 1989, which presumably was somewhat lower than in 1944. This situation enables us to assert that a small decrease in the groundwater table, by 20 cm or 30 cm, could have been enough to leave the heads the wooden piles exposed, thus causing them to rot and decay. Intense rotting can occur in pine wood after only a few months of being exposed to air [7]. – Negative friction on the piles: negative friction is set off by even very minor subsidence; therefore, it is clear that this phenomenon also had to play a role. Such friction causes an excessive load on the pile. Based on the effect caused by the lowering of the groundwater table, we can establish that, although the wooden piles were subjected to an increase in load due to negative friction, the damage to the buildings can be better explained by the decay of the head of the piles and by the land subsidence. Both causes are responsible for the major differential subsidence that was detected. Yet, there is a special aspect of the problem that lies in determining why the building located on Avenida de la Costa No. 20 was affected before the other buildings and why it is that it suffered more significant damage. This may be due to the following circumstances:
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Fig. 16. Area of transition of the paleochannel.
– All of the affected buildings were located on a paleochnnel in which the lowering of the groundwater table must have been greater in comparison to the buildings located outside of the paleochannel. This paleochannel may contain softer and more compressible peat. The existence of this possible channel is deduced based on the morphology of the subsidence observed in Fig. 9. A follow up study can be conducted, considering the areas most affected by subsidence in the block studied. The existence of this channel is also based on the information obtained from the subsidence that is occurring on the sidewalks and buildings in the immediate surrounding area. In particular, as shown in Fig. 16, the sedimentary structure of the water flow may have followed the direction of Mieres Street and then turned north and penetrated through the center of the block affected, possibly leading out into the former wetland area in Plaza de Europa, currently the lowest point in Gijón. The same figure also shows the damaged sidewalks in the transition area of the old underground channel. The most distinctive landmarks of the path are the major subsidences that have occurred on the road and sidewalks in the vicinity of the block affected. Despite the remodeling of the area, these subsidences have reappeared, as can be observed in
Fig. 17. Subsidences in the surrounding area following the course of the paleochannel.
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Fig. 17, which also shows that some buildings located on Mieres Street have suffered damage due to subsidence. Fig. 10 shows the significant subsidence that currently exists on the road of Avenida de la Costa, in the area of the block affected. – In addition, the existence of a common dividing wall with the building located on Avenida de la Costa No. 22 of the same street, built on concrete piles, which hardly experienced any settling, is what led to the differential subsidence and significantly more damage that was caused to the building located at No. 20. It is clear that as the foundation of building No. 20 failed, it was being sustained by the adjacent buildings, which transferred considerable loads upon it. The fact that the buildings not adjacent to No. 20 on Avenida de la Costa are also damaged, indicates that the general problem that exists is due to the subsoil of the block. – The fact that the foundation of the building has resisted so well after many years suggests that the foundation was properly designed and constructed despite the three floors that were added to building No. 20 (Fig. 4).
5. Conclusions We can rule out a poor design of the foundation, given that it has behaved correctly for more than 40 years. The three floors that were added to the building had no bearing on the failure of the foundation. The excavations carried out which led to the lowering of the groundwater table and the subsequent intermittent pumping that still continues in the adjacent block has caused decreases in the groundwater level by more than 30 cm in the block affected. These decreases have been enough to expose the top part of the wooden piles connected to the concrete pile caps, and are responsible for causing the wooden piles to rot. There are strong indications and geotechnical justifications for attributing the major subsidences endured by the building on Avenida de la Costa No. 20 to the failure of its foundation, which has been determined to be due to the rotting of the wooden piles as a result of the lowering of the groundwater table. The problem may have been aggravated by other phenomena associated with the lowering of the groundwater table, such as the widespread land subsidence, given its highly compressible nature, and also the negative friction of the piles. Differences of 50 cm in the groundwater table have been measured in the block affected, 15 cm of which is attributed to subsidence phenomena that occurred after construction of the building. This subsidence has occurred due to compression of the existing peat in the subsoil, which is a consequence of the lowering of the groundwater table. In comparison to the adjacent buildings, the greatest damage caused to the building on Avenida de la Costa No. 20 can be attributed to its location on a paleochannel or area of wetlands that is very compressible and covered with a layer that is either more permeable or contains more sand than the rest of the areas upon which the buildings have been constructed. Consequently, the groundwater table is notably lower and there are more significant subsidences in these areas. The fact that the building on Avenida de la Costa No. 20 suffered the most significant damage in comparison to the adjacent buildings may be attributed to the differential nature of the subsidences, given that this building has a common dividing wall with the adjacent building No. 22 on the same street. The foundation of this adjacent building was constructed on concrete piles and did not suffer any damage. To avoid similar problems in other areas of Gijón, as a result of the lowering of the groundwater table, free or sloping excavations, or excavation below the groundwater table should be limited when the depth of such excavations is located at a distance of more than 1 m from the groundwater table. In these cases, the continuous shield technique shall be used for the excavations. Acknowledgement The authors want to knowledge to D. Gonzalo Cueto-Felgueroso, to Construcciones Fercavia S.A, to D. Laudelino Villa and to D.L. Jorge Noval the collaboration and information contributed for the development of this paper. References [1] Doolin DM, Wells DL, Williams PL. Assessment of fault-creep deformation at memorial stadium, University of California, Berkeley, California. Environ Eng Geosci 2005;11(2):125–39. [2] Duong TT. Initial study on Hanoi land subsidence. Master Thesis. Asian Institute of Technology, Bangkok, Thailand; 2005. [3] Gonzalez-Nicieza C, Alvarez-Fernandez MI, Alvarez-Vigil AE, et al. Forensic analysis of foundation failure in gypsiferous ground. Eng Failure Anal 2008;15(6):736–54. [4] Gutierrez Claverol, Torres Alonso, Luque Cabal. The subsoil of Gijón. In: Licer CQ, Oviedo SL, editors. Geological aspects; 2002. [5] Li CJ, Tang XM, Ma TH. Land subsidence caused by groundwater exploitation in the Hangzhou-Jiaxing-Huzhou Plain, China. Hydrogeol J 2006;14(8):1652–65. [6] Noe DC. Case history of damage to a school building caused by differentially heaving bedrock. J Perform Constr Facil 2003;17(2):97–105. [7] Peek RD, Willeitner H. Accelerated fixation of chromate-containing wood-preservatives by superheated steam. 1. Effect of different heat-treatments on the leaching of preservatives. Holz als Roh-und Werkstoff 1981;39(12):495–502. [8] Phien-wej N, Giao PH, Nutalaya P. Land subsidence in Bangkok. Thailand Eng Geol 2006;82(4):187–201. [9] Yong RN, Maathuis H, Turcott E. Groundwater abstraction-induced land subsidence prediction: Bangkok and Jakarta case studies. In: Proceedings of the fifth international symposium on land subsidence. FISOLS-95, The Hague, Netherlands; 1995.
Contents lists available at ScienceDirect
Engineering Failure Analysis journal homepage: www.elsevier.com/locate/engfailanal
Forensic analysis of a pile foundation failure F. López Gayarre a, C. González-Nicieza b,*, M.I. Alvarez-Fernández b, A.E. Álvarez-Vigil c a b c
Department of Construction and Manufacture Engineering, Engineering School of Gijon, University of Oviedo, Campus de Viesques, 33201 Asturias, Spain Department of Mining Engineering, Mining Engineering School, University of Oviedo, Independencia 13, 33004 Asturias, Spain Department of Mathematics, Mining Engineering School, University of Oviedo, Independencia 13, 33004 Asturias, Spain
a r t i c l e
i n f o
Article history: Received 15 June 2009 Accepted 6 September 2009 Available online 20 September 2009 Keywords: Forensic analysis Groundwater table Differential subsidence Wooden piles
a b s t r a c t This paper presents the results of the forensic analysis carried out on a set of buildings located on the same block in the center of Gijón (NW of Spain). The foundations of these buildings were laid between the years 1944 and 1945, using wooden piles. These wooden piles may have been affected by a subsequent reduction in the groundwater table, caused by the depletion of water that occurred during the construction of the basement of a building located on the adjacent block. Evidence of the first major damage to the building appeared upon initiating construction of the aforementioned basement. The situation was worsened by debris falling from one of the damaged buildings onto a public street below. Within a week, after the city authorities had reviewed the technical reports, they decided to proceed with the eviction of more than forty families in the affected buildings, thus triggering considerable social alarm throughout the region. This article provides a detailed description of the problem, analyzes the damage caused to the buildings, and based on such analysis and description, this article seeks to establish the different hypotheses in an effort to reveal what caused the problem. The focus of this analysis is to present the rotting of the head of the wooden piles of the foundation as the main cause responsible for the ruin of the affected buildings. The rotting was caused as a result of artificially lowering the groundwater table, which occurred during the construction of a building located in the vicinity. This study also analyzes the land subsidence and negative friction experienced by the piles as a result of artificially lowering the groundwater table. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction and background The first damage caused to a group of residential homes in Gijón occurred in 1987. The homes were located on the block between Avenida de la Costa, Santa Justa Street and Suerte Street (Figs. 1, 2 and 5). In November of 1989, it was found that debris had fallen from the building located on the road Avenida de la Costa No. 20, and had landed below on the public road. On that date, the part of the building adjacent to No. 22 was no longer habitable due to the appreciable damage that occurred as a result of major differential subsidence. After the technicians of the Gijón City Council had visited the area and viewed the damages, they ordered the eviction of the entire block, except for the building located on Avenida de la Costa No. 22, which intersects with Santa Justa Street, as no damage of any kind was detected. This decision triggered a major social alarm in the city, which was widely reported in the local and regional media at that time, particularly given that such decision impacted
* Corresponding author. Tel.: +34 985 10 42 66; fax: +34 985 10 42 67. E-mail address: [email protected] (C. González-Nicieza). 1350-6307/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.engfailanal.2009.09.008
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Fig. 1. Location.
Fig. 2. Block affected.
45 families. All the buildings that had been evacuated presented major cracks and collapses, with the most significant damage occurring in the building located on Avenida de la Costa No. 20. Once in-depth knowledge was acquired about the status of the buildings, the city decided to proceed with the demolition of all buildings on the block, except the one located on Avenida de la Costa No. 22. Fig. 2a shows a more detailed view of the damaged residential buildings. Fig. 2b shows the building located on Avenida de la Costa No. 22. In the same figure, the building located in the western area of the block is a new construction. The last building to be erected on the site of Fig. 2b is currently in its final stages of construction. This article presents the results obtained after conducting a detailed forensic analysis of the causes that may have led to the progressive deterioration of the buildings located in the area studied. Although the aforementioned buildings initially presented minor damage, the appearance of major cracks took place during the course of constructing the basement of the building on the adjacent block, located on the other side of Suerte Street, at No. 16 of Avenida de la Costa (see location in Fig. 2b). A failure in the foundation of the building was initially suspected, although a failure in the ground due to a subsidence phenomenon could not be ruled out [3]. For both hypotheses, the lowering of groundwater tables could have been what triggered the phenomenon. In any case, after expanding the radius of the study area, significant sinking and cracks were detected on both the sidewalks and the façades of other buildings. The problems of ground subsidence caused by excessive pumping of groundwater is a relatively common situation [1,2,5,6,8,9]. This study begins with an analysis of the geology and hydrogeology of the area. Section 3 contains a description of the foundation of the buildings affected, including some observations that may be related to the failure, and a description and analysis of the most significant damages detected. In Section 4, we discuss the various hypotheses of the failure; and finally, we present the conclusions derived from our study.
2. Geology, hydrogeology and geotechnics of the area The study area falls within the northern area of the city center, within the boundaries of the former wetlands that used to exist in Gijón (see Fig. 1).
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In the metropolitan area of Gijón, we must distinguish between two areas that are quite geotechnically distinct: The coastal area: mainly consisting of sediments caused by the fluvio-marine action and anthropogenic fillings. From a geotechnical point of view, all the coastal sediments are detritical materials, cohesionless, highly permeable and saturated with fresh water (except in the most shallow section), have low bearing capacity and are susceptible to processes of compressibility. The continental area: formed by rock materials and their corresponding products of alteration. As can be observed in Fig. 3, the study area is located in the coastal domain, consisting of sand that is interspersed with gravel and varying levels of peat. Based on various studies carried out in the area studied and it vicinity, four different types of materials have been identified: Anthropic fills: made up of clay, sand, gravel, pieces of debris, stones, bricks and some remains of peat. This material has a variable thickness between 0.6 and 1.8 m. Level of peat: it has an upper muddy area with a variable thickness between 0.6 and 2.3 m, followed by a level of peat, formed by mud, clay and fine sand, with debris, abundant organic matter and traces of decompositioned wood. The total thickness of this level fluctuates between 3.0 and 7.0 m, depending on the areas. As an example of the variability in the geotechnical behavior of these materials, Fig. 4 shows a penetrometer test that was conducted between Mieres Street and Schultz Street. It can be observed how in the levels of peat, the allowable bearing capacity is zero at depths between 1.6 and 4.8 m, and the value exceeds 1.3 MPa at greater depths. Also, the Standard Penetration Tests (SPT) carried out on the aforementioned materials corroborate these findings because they give NSPT between 2 and 5, which corresponds with allowable bearing capacity values of less than 0.03 MPa. Marly clays: located on the floor of the stretch of peat, there is a marly clays level that outcrops, with steely blue, gray, green and beige tones, and rock fragments. In general, it has a soft to medium consistency, with allowable bearing capacity values which vary between 0.06 and 2.24 MPa, based on penetrometer tests. Its thickness fluctuates between 0.3 and 1.4 m. Bedrock: It’s below the clay loam and is formed mainly of gray limestone, with small dark gray marl intercalations, and some levels of gray and beige dolomites, always presenting a high fracturation. An allowable bearing capacity of 0.3 MPa has been estimated for this level due to its degree of fracturation. Moreover, the study area is located in the Aquifer System No. 1, Mesozoic Unit of Gijón-Villaviciosa, which contains two distinct subsystems: Villaviciosa subsystem. Llantones subsystem.
Fig. 3. Geotechnical description of Gijón [4].
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Fig. 4. Dynamic penetration test.
Table 1 Variations in the groundwater table. Date of the study
Depth of the groundwater table (m)
March 1980 November 1989 October 1993 September 2005
0.4 0.9 1.1 5.0
More specifically, the study area is located in the Jurassic limestone aquifer, represented by the ‘‘Nodular Limestones of Gijón” and the ‘‘Magnesium Limestones of Gijón” of the Villaviciosa subsystem. From the hydrogeological aspect, the nature of the quaternary deposits that make up the first few meters of the subsoil in the study area ranges between permeable and semi-permeable, observing the existence of a water level associated with the level of peat, down confined by the level of marly clays. Moreover, the Jurassic materials that form the bedrock of the area are likely to contain water due to the karstification and the fracturation. According to various studies conducted in the analyzed area and its vicinity, there are high variations in the groundwater table. In this regard, a gradual increase is observed in the depth of the level of water, as is clear from the data shown in Table 1, which was derived from three studies conducted in the area.
3. Description of the problem and analysis of the damage to the buildings affected The buildings affected by cracks, subsidence, and collapses were built between the years 1944 and 1945, and their foundation was made using wooden piles and mass concrete blocks. Part of the building located on Avenida de la Costa No. 20 added three floors in 1956, although it was not possible to recover any technical documentation. Fig. 5 shows a layout plan of the buildings that were located on the block affected.
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Fig. 5. Plan view of the buildings on the block affected.
Marked in red1, Fig. 6a and b show the expansion that took place in the building located on Avenida de la Costa No. 20. It should be noted that no problems were found in the building within the 30 years following the addition of the three floors. Based on the original layout of the buildings affected, there were only measurements related to the foundation, which mention ‘‘foundation pits of 1.5 m 1.5 m 1 m, etc., each pit with six pine logs, and each log averaging 15 cm in diameter and 8 m deep.” In the excavation work carried out in 2008, for the construction of a new building located in the central area of the block (see Fig. 2b), pieces of wooden piles with diameters averaging between 11.5 cm and 17 cm were recovered. Fig. 7 shows a diagram of the concrete pile cap and the group of wooden piles, as well as some samples of the wooden piles which were obtained from the last excavation that was carried out. Based on the boreholes perforated, we know that the rock is located at an average depth of 6.50 m. Therefore, if the construction was completed according to the specifications provided, the wooden piles reached the rock. The building located on Avenida de la Costa No. 22, which is the only one that was not demolished, was built on concrete piles. One of the first observations that was made when analyzing the problem is that the pathologies extend beyond the block studied, given that in the past, some houses in the surrounding area that had been affected by cracks and major collapses ended up having to be demolished. Moreover, within a radius of approximately 300 m facing south–west, the sidewalks showed cracks and major subsidence. Today, even after the area has been remodeled, the sidewalks once again present major subsidence, as shown in Fig. 8. The surface of the entire block was leveled, in which, as shown in Fig. 9, it was found that there was a sinking of the land in a central strip of the block in the north–south direction. The level curves determine a subsidence trough whose axis passes approximately along the dividing wall of the buildings on Santa Justa Street No. 2 and No. 4, penetrating into the interior of the block and heading north, then turning toward the northeast and following the direction of the dividing wall of buildings No. 18 and No. 20 on Avenida de la Costa. The slope at the sides of this subsidence trough is at its maximum at the part of building No. 20 located on Avenue de la Costa, facing Santa Justa Street. This area is precisely where the greatest damage took place. The difference in the level between the highest point (corner of Avenida de la Costa and Santa Justa Street) and the lowest point (approximately at the dividing wall between No. 18 and No. 20 on Avenida de la Costa) is 0.5 m. Although this value includes the slope of the street (at least 0.15 m), corresponding to the subsidence that occurred. Today, in this very same place, the road Avenida de la Costa once again experiences a progressive sinking, as can be observed in Fig. 10. Due to the differential subsidence that occurred as a result of the decrease in the groundwater table, the following damages can be observed:
1
For interpretation of color in Fig. 6, the reader is referred to the web version of this article.
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a
b
Fig. 6. Expansion of the building located on Avenida de la Costa No. 20.
Fig. 7. Type of foundation and sections of piles used in the buildings affected.
Fig. 8. Sidewalk subsidence in the vicinity of the affected area.
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Fig. 9. Leveling in the block affected.
Fig. 10. Subsidence of Avenida de la Costa at near the block affected.
Fig. 11a shows some of the major cracks that were produced in the building located on Avenida de la Costa No. 20, particularly to the interior courtyard that it shares with the building on Santa Justa Street No. 4.
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Fig. 11. Cracks observed in the interior courtyard, extending from Santa Justa No. 4 to the building located at Avenida de la Costa No. 20.
Fig. 11b shows how the lintel of the window has lost its perpendicularity with the jamb due to the angular distortion caused by the subsidence. Fig. 12 once again shows the rupture caused in the wall of the lower floor of the building located at Santa Justa Street No. 4. Fig. 13 shows one of the cracks that occurred in the interior partitions of the building located at Santa Justa Street No. 2. The rupture can be seen by following the joints of the bricks, where the mortar shows less resistance. Of the various buildings that were located on the block, the building located on Avenida de la Costa No. 20 was undoubtedly the one that suffered the most damage, especially due to the cracks caused by differential subsidence [1]. This subsidence may have been caused as a result of having dividing walls that are shared with the building located at No. 22 of the same avenue. However, as above mentioned, the building located at No. 22 of the same avenue was built on mass concrete piles and hardly suffered any damage. Another important observation is that the damage that occurred in the block affected coincides with a major remodeling of the neighborhood, which includes buildings that were constructed around 1940 and had not shown previous signs of pathologies. As part of the construction carried out, coinciding with the onset of the damage, we must highlight the building located on the block adjacent to the one studied, facing Suerte Street, Avenida de la Costa No. 16 and Schultz Avenue. Its location can be observed in Fig. 14. To accommodate the basement of this building, an excavation was carried out, digging
Fig. 12. Rupture and displacement of the wall due to ground subsidence.
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Fig. 13. Cracks in the walls due to differential subsidence.
Fig. 14. Front elevation in the block affected.
6 m deep and constructing traditional concrete walls, thereby lowering the groundwater table. Pumping then took place over the course of several months. Because it was not possible to achieve complete watertightness with the construction of the wall, intermittent pumping continued once construction of the building had been finished. We were able to verify that the pumping still continues to take place to this day. The intensive pumping of water that occurred while the basement was being built, as well as the subsequent intermittent pumping, may have contributed to lowering the groundwater table in the block affected and may be what was responsible for causing the damage. Based on the documentation reviewed for this study, it is worth noting a report issued in 1976 by the architect Jorge Hevia. In the report, which was issued for another building located in the vicinity, the architect states ‘‘... the need to avoid laying foundations for new buildings in the vicinity which may require the groundwater table to be lowered...”
4. Hypothesis of the failure As previously discussed, the intensive pumping of water causes an overall lowering of the groundwater table in the surrounding area (Fig. 15). The degree to which the groundwater table is lowered depends, among other factors, on the permeability of the ground and the depth at which the pumping is carried out.
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Fig. 15. Lowering of the groundwater table due to the pumping of water in the adjacent building.
In the study area, the groundwater table was 0.90 m deep in 1989; although the survey that was conducted on Suerte Street 24 h later, showed that the groundwater table had decreased to 1.80 m. This fact can be explained either by a slow recovery in the survey, or it may be because the groundwater table in that area is lower than in the rest of the areas. In this case, the only possible explanation is due to pumping or drainage in the vicinity. Taking into account the characteristics of the layers of subsoil and the existence of a predominantly horizontal flow, we can estimate that due to the pumping carried out, the lowering of the groundwater table has a radius of effective influence of approximately 40 m. This radius covers the majority of the block affected, estimating that in the center of this radius, the average reduction of the groundwater level may have reached as much as 30 cm. Given that the pumping occurred over a prolonged period of time, such decrease in the groundwater table could have reached 1 m during dry periods, therefore also increasing its radius of influence. The main effects that cause these decreases in the groundwater table are attributed to: – Overall settling in the surrounding area (subsidence): the immediate effect of lowering the groundwater table causes an increase in the actual weight of the soil that is above ground as well as that which was once submerged. This increased weight causes a compression in the lower soft stratum, mainly peat, and thus causes the surface of the ground to sink. Such subsidence is clearly apparent in the leveling that was carried out. It is not exaggerated to estimate subsidence of 20 cm as a result of a decrease in the groundwater table by 1 m. – Deterioration of the wooden piles: as is logical, when the groundwater table is lowered, this leaves part of the wooden piles exposed to the air, causing them to rot due to the onset of fungus and microorganisms. If the foundation of the building was constructed with piles measuring 8 m, the base of the concrete block coincided with the groundwater table in 1989, which presumably was somewhat lower than in 1944. This situation enables us to assert that a small decrease in the groundwater table, by 20 cm or 30 cm, could have been enough to leave the heads the wooden piles exposed, thus causing them to rot and decay. Intense rotting can occur in pine wood after only a few months of being exposed to air [7]. – Negative friction on the piles: negative friction is set off by even very minor subsidence; therefore, it is clear that this phenomenon also had to play a role. Such friction causes an excessive load on the pile. Based on the effect caused by the lowering of the groundwater table, we can establish that, although the wooden piles were subjected to an increase in load due to negative friction, the damage to the buildings can be better explained by the decay of the head of the piles and by the land subsidence. Both causes are responsible for the major differential subsidence that was detected. Yet, there is a special aspect of the problem that lies in determining why the building located on Avenida de la Costa No. 20 was affected before the other buildings and why it is that it suffered more significant damage. This may be due to the following circumstances:
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Fig. 16. Area of transition of the paleochannel.
– All of the affected buildings were located on a paleochnnel in which the lowering of the groundwater table must have been greater in comparison to the buildings located outside of the paleochannel. This paleochannel may contain softer and more compressible peat. The existence of this possible channel is deduced based on the morphology of the subsidence observed in Fig. 9. A follow up study can be conducted, considering the areas most affected by subsidence in the block studied. The existence of this channel is also based on the information obtained from the subsidence that is occurring on the sidewalks and buildings in the immediate surrounding area. In particular, as shown in Fig. 16, the sedimentary structure of the water flow may have followed the direction of Mieres Street and then turned north and penetrated through the center of the block affected, possibly leading out into the former wetland area in Plaza de Europa, currently the lowest point in Gijón. The same figure also shows the damaged sidewalks in the transition area of the old underground channel. The most distinctive landmarks of the path are the major subsidences that have occurred on the road and sidewalks in the vicinity of the block affected. Despite the remodeling of the area, these subsidences have reappeared, as can be observed in
Fig. 17. Subsidences in the surrounding area following the course of the paleochannel.
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Fig. 17, which also shows that some buildings located on Mieres Street have suffered damage due to subsidence. Fig. 10 shows the significant subsidence that currently exists on the road of Avenida de la Costa, in the area of the block affected. – In addition, the existence of a common dividing wall with the building located on Avenida de la Costa No. 22 of the same street, built on concrete piles, which hardly experienced any settling, is what led to the differential subsidence and significantly more damage that was caused to the building located at No. 20. It is clear that as the foundation of building No. 20 failed, it was being sustained by the adjacent buildings, which transferred considerable loads upon it. The fact that the buildings not adjacent to No. 20 on Avenida de la Costa are also damaged, indicates that the general problem that exists is due to the subsoil of the block. – The fact that the foundation of the building has resisted so well after many years suggests that the foundation was properly designed and constructed despite the three floors that were added to building No. 20 (Fig. 4).
5. Conclusions We can rule out a poor design of the foundation, given that it has behaved correctly for more than 40 years. The three floors that were added to the building had no bearing on the failure of the foundation. The excavations carried out which led to the lowering of the groundwater table and the subsequent intermittent pumping that still continues in the adjacent block has caused decreases in the groundwater level by more than 30 cm in the block affected. These decreases have been enough to expose the top part of the wooden piles connected to the concrete pile caps, and are responsible for causing the wooden piles to rot. There are strong indications and geotechnical justifications for attributing the major subsidences endured by the building on Avenida de la Costa No. 20 to the failure of its foundation, which has been determined to be due to the rotting of the wooden piles as a result of the lowering of the groundwater table. The problem may have been aggravated by other phenomena associated with the lowering of the groundwater table, such as the widespread land subsidence, given its highly compressible nature, and also the negative friction of the piles. Differences of 50 cm in the groundwater table have been measured in the block affected, 15 cm of which is attributed to subsidence phenomena that occurred after construction of the building. This subsidence has occurred due to compression of the existing peat in the subsoil, which is a consequence of the lowering of the groundwater table. In comparison to the adjacent buildings, the greatest damage caused to the building on Avenida de la Costa No. 20 can be attributed to its location on a paleochannel or area of wetlands that is very compressible and covered with a layer that is either more permeable or contains more sand than the rest of the areas upon which the buildings have been constructed. Consequently, the groundwater table is notably lower and there are more significant subsidences in these areas. The fact that the building on Avenida de la Costa No. 20 suffered the most significant damage in comparison to the adjacent buildings may be attributed to the differential nature of the subsidences, given that this building has a common dividing wall with the adjacent building No. 22 on the same street. The foundation of this adjacent building was constructed on concrete piles and did not suffer any damage. To avoid similar problems in other areas of Gijón, as a result of the lowering of the groundwater table, free or sloping excavations, or excavation below the groundwater table should be limited when the depth of such excavations is located at a distance of more than 1 m from the groundwater table. In these cases, the continuous shield technique shall be used for the excavations. Acknowledgement The authors want to knowledge to D. Gonzalo Cueto-Felgueroso, to Construcciones Fercavia S.A, to D. Laudelino Villa and to D.L. Jorge Noval the collaboration and information contributed for the development of this paper. References [1] Doolin DM, Wells DL, Williams PL. Assessment of fault-creep deformation at memorial stadium, University of California, Berkeley, California. Environ Eng Geosci 2005;11(2):125–39. [2] Duong TT. Initial study on Hanoi land subsidence. Master Thesis. Asian Institute of Technology, Bangkok, Thailand; 2005. [3] Gonzalez-Nicieza C, Alvarez-Fernandez MI, Alvarez-Vigil AE, et al. Forensic analysis of foundation failure in gypsiferous ground. Eng Failure Anal 2008;15(6):736–54. [4] Gutierrez Claverol, Torres Alonso, Luque Cabal. The subsoil of Gijón. In: Licer CQ, Oviedo SL, editors. Geological aspects; 2002. [5] Li CJ, Tang XM, Ma TH. Land subsidence caused by groundwater exploitation in the Hangzhou-Jiaxing-Huzhou Plain, China. Hydrogeol J 2006;14(8):1652–65. [6] Noe DC. Case history of damage to a school building caused by differentially heaving bedrock. J Perform Constr Facil 2003;17(2):97–105. [7] Peek RD, Willeitner H. Accelerated fixation of chromate-containing wood-preservatives by superheated steam. 1. Effect of different heat-treatments on the leaching of preservatives. Holz als Roh-und Werkstoff 1981;39(12):495–502. [8] Phien-wej N, Giao PH, Nutalaya P. Land subsidence in Bangkok. Thailand Eng Geol 2006;82(4):187–201. [9] Yong RN, Maathuis H, Turcott E. Groundwater abstraction-induced land subsidence prediction: Bangkok and Jakarta case studies. In: Proceedings of the fifth international symposium on land subsidence. FISOLS-95, The Hague, Netherlands; 1995.