Investigating the applicibality of lidar in the

detection of ground heave and settlements in

building foundations stressed by reactive black

shale Matthew J. Lato, Erik Endre Norwegian Geotechnical Institute, Oslo, Norway ABSTRACT A large number of buildings within the commercial district of downtown Oslo, Norway have built their foundations on black shale. The shale, when exposed to air and moisture can react in an aggressive manner. The result of the reaction can be in the form of the release of heavy metals/sulphur, the acidification of ground waters, and volumetric increases exerting large stresses on building walls and support columns. The impact will vary from shearing and cracking of walls, to settlements and sagging in ceilings, and zones of high compressions and tension around doorways. The use of lidar combined with image interpretation has enabled NGI to characterize the impact of black shale on building foundations very efficiently. The ability to quickly and accurately assess the deformations imposed from black shales has proven a valuable tool in the preliminary geotechnical investigation, for documentation of the condition of the constructions and for the design of the new foundation solutions. RÉSUMÉ A grand nombre de bâtiments commerciaux dans le centre-ville d’Oslo repose sur des schistes argileux. Lorsque les schistes argileux sont au contact de l’air, ils peuvent réagir de façon volatile. Cette réaction peut libérer des métaux lourds et du soufre, acidifier les eaux sous-terraines et générer un gonflement susceptible d’engendrer des ocontraintes importantes sur les murs des bâtiments et sur les pieux. Cela peut engendrer des fissures dans les murs, un affaissement des plafonds, et des zones de compressions ou d’extension au niveau des portes. L’utilisation du LIDAR a permis au NGI de caractériser l’impact des schistes argileux sur les bâtiments de façon très efficace. La possibilité d’évaluer avec rapidité et précision les déformations est un outil précieux pour les études géotechniques, la documentation sur les conditions des constructions et pour l’élaboration de solutions pour les nouvelles fondations. 1 INTRODUCTION Numerous buildings in Oslo’s main commercial and tourist district have strict laws regarding the renovation of buildings deemed to be of historic importance. The regulations state that exterior and main walls of historic buildings must be left unmodified. Construction and renovation of such buildings in which original support walls have to be left untouched has logistical and engineering related challenges. One commonly encountered challenge is the occurrence of black shale in the basement rocks. If black shale bedrock is encountered during a renovation project the impact can be quite severe if not quickly remedied.

The identification of black shale is critical when working in Central Oslo. Visual identification and laboratory testing is required to determine the argessiveness of the shale units as well as the potential impact on the building structure and environment.

Once black shale bedrock is encountered,

deformation and temporal modelling is a large challenge faced by property owners and construction managers. The use of Light Detection And Ranging (lidar) is one such method that has been tested by the Norwegian Geotechnical Institute (NGI) with promising results.

This paper briefly outlines the geological conditions and geochemical process that occurs which results in deformation of building foundations. As well, an explanation on how lidar data is manipulated to enable the calculation of deformation, as well as monitoring foundations and support walls. 2 GEOLOGICAL SETTING: BLACK SHALE The Oslo Region is located within a 45-75 km wide graben structure. The basement rocks are Precambrian in age and are overlain by Cambrian and Ordovician sediments. The units within the sedimentary sequence are sandstones, limestone, nodular limestone, and shale. The shale units comprise approximately 15% of the sedimentary sequence. The shale units are divided into three categories: grey shale, alum shale, and black shale.

The black shale units in the Oslo sedimentary

sequence were formed from organic rich sediments under heavily anoxic reducing conditions.

The black and alum shale units represent the greatest

risk in the Oslo area. The risk from black and Alum shale extends from human harm (to those who work in regions containing black shale), environmental harm, and

structural damage to buildings. Figure 1 illustrates excavated black shale in a building foundation that has undergone extensive weathering. The yellow rock contains ferric sulphates.

Figure 1. Highly weathered black shale excavated in a building foundation. Yellow material is ferric sulphates. 2.1 Geochemical Process The geochemical breakdown of black shale is due to sulphide to sulphate mineral reactions. These processes are governed by several types of acid producing bacteria and Fe3+ in the system. Access to oxygen either through air or through ground water enriched in oxygen by influx of surface waters are the main causes for triggering these acid producing reactions in the black shale. The acidic runoff from black shale interaction with groundwater can contain nickel, zinc and cadmium (Peterson and Grant, 2005; Druschel et al. 2004; Roden 2008)

The resultant geochemical processes produce gypsum minerals in between the shale layers. The gypsum formation results in the exertion of forces normal to the orientation of the bedding plane. The force exerted by the gypsum formation can be up to 2 MPa. 3 IMPACT ON BUILDING FOUNDATIONS Damage to foundations and basement floors due to ground heave (gypsum formation) are quite frequent in older buildings in Central Oslo. Typically problems are encountered through unexpected excavation of the shale units during renovation.

Excavations that go beyond the ground water table risk the greatest consequences, as the influx of fresh water through the shale units will result in maximum reaction of the shale units. This can cause excavations to be local pollution sources and cause structural damage to surrounding buildings.

Shear cracking observed in a basement walls (Figure

2) is the most common and readily observed damage resulting from black shale heave.

As well, the impact from exposure of black shale can

include acidification of ground water, heavy metal leaching, and eradiation of nuclear and radon gas.

Figure 2. Shear cracking in basement walls as a result of differential uplift of the shale bedrock 4 LIDAR IMAGING Light Detection and Ranging (lidar) is a ranged-based imaging technique that enables the rapid development of highly accurate 3-dimensional surface models (Amman et al. 2001). The equipment used by NGI is a phase-based Faro Photon 120 scanner. The Faro Photon LS 120 measures at a rate of 1 000 000 pts/sec, with an

absolute accuracy of +/- 5 mm at 25 m, as of 2011 this is the fastest phase-based lidar scanner commercially available.

Using lidar is a non-destructive technique that involves minimal time and disturbance to active construction site. The data collection requires approximately 2.5 minutes per scanning location, which includes setting up and levelling the equipment. The main operational consideration is that the scanners line-of-sight to the target be clear and there be no major vibrations in the floor (all destructive construction must be stopped). The Faro Photon 120 scanner in use at an active construction site is displayed in Figure 3.

Figure 3. Scanning a support wall from one of six locations across two floors (site 1) 4.1 Scan Alignment The data alignment and post-processing is conducted using PolyWorks V11.0.27 (InnovMetric, 2011). The first stage in the data processing is combining the scans taken from different locations into one model. This is initially completed through a visual alignment, matching common points from both scans. When the models are visually aligned, a 3D iterative closest point algorithm is executed to conduct a best-fit analysis (Besl and McKay, 1989).

The result of the alignment process is a fully 3D surface model. Figure 4 illustrates an oblique view of a two story building scanned from six locations. The locations were selected to produce an unoccluded (a model with no shadow zones) model with sufficient overlap between the individual scans (Lato et al. 2010).

Figure 4. Oblique view of six lidar scans taken from two floors aligned into one surface model. The brown area is the wall under investigation. 4.2 Monitoring Possibilities The primary reason for using lidar is to assess deformation in support walls and ceilings. The analyses conducted through processing of the lidar data are limited in time to when the data is collected. It is impossible to assess what has happened before any data were collected.

There are two main analyses that are conducted at NGI to aid in the determination of building stability: deformation and temporal modelling, as will be discussed in Sections 4.2.1 and 4.2.2. 4.2.1 Deformation Monitoring The primary analysis executed at NGI is the calculation of minor deformation in walls and ceilings. Load bearing walls suspected of shearing or buckling as the result of differential uplift are scanned from multiple angles and combined into a single surface model. The data points that characterize the wall, typically 5,000 pts/m2, are best-fit to a numerically defined plane and then the original points are compared against the numerical plane.

This method of analysis enables the evaluating engineer to see zones in the wall that do not conform to a planar surface. Bucking of walls and sagging or uplift in ceilings is readily identified through visualizing of the surface model. 4.2.2 Temporal Monitoring The second data analysis procedure performed at NGI is temporal modelling. If lidar scan data is collected at the same site on multiple occasions the individual datasets can be compared against one another. This form of temporal modelling enables the evaluating engineer to model if regions of the wall that are continually deforming, the rate and magnitude of deformation, or if the region has stabilized.

Temporal modelling is a powerful tool to use when there is known deformations in a support wall. Specific areas can be closely monitored for movement. This measurement method has been widely used in natural environments for monitoring of erosion processes (Lato, et al. 2009). The accuracy and derived information from temporal modelling is dependent on the interval between scans as well as the total time elapsed between the first and last acquisition. 5 TEST SITES The application of lidar for modelling building foundations was tested at four sites in central Oslo, Norway. Sections 5.1 and 5.2 discuss and illustrate the results from two of the test sites. Both sites are currently under construction, and all detailed information regarding the locations of the sites cannot be disseminated. 5.1 Site 1 The first site is a multi-story building in the centre of Oslo. It was constructed in the early 1800’s, and partially rebuilt after a fire in the 1850’s. The current construction is to remove the inside of the building and completely rebuild the floors and interior walls. However for heritage reasons the outside and main support walls must be left intact.

The main support wall, as indicated by the brown highlights in Figure 3, is approximately 7 m tall. Below the visible section of the wall there was construction in the basement that excavated directly into a zone of reactive black shale. The pressure exerted by the shale was directly absorbed by this support wall. To assess possible deformations the ground floor and 1st floor of the building were scanned from six locations and combined into one 3D model. 5.1.1 Deformation Modelling The support wall, under normal loading from the weight of the building in combination with upward vertical stress from the reactive shale below resulted in the deformation visible in Figure 5 and 6.

The 500,000 data points defining the wall (Figure 5) were compared with a planar geometric surface. The individual point deviations from this surface are coloured by magnitude. The red to yellow bulge in the lower right side of the wall illustrate a potential buckling instability.

The capacity to determine this potential instability was

determined through visual investigation of the wall. However, only with the addition of lidar data was the deformation magnitude calculated.

Figure 5. Displacement observed in the wall with respect to a true planar best-fit surface. Red zones represent maximum offset. The zoomed in section of the wall, in Figure 6, clearly illustrates the prominent zone of deformation. This model has been used for preliminary assessment and will be used as a baseline for further analyses if the suspected deformations continue and temporal modelling is required.

Figure 6. Zoomed in region of Figure 5. The red region represents the maximum deformation and likely instability of the wall. Monitoring this region is critical to understanding the stability of the wall. 5.2 Site 2 The second site is a multi-story building in the centre of Oslo. It was originally constructed in (please fill in ERIK). During a recent renovation project the basement was excavated to convert the space to usable commercial real estate. During the excavation and deepening of the basement black shale was encountered.

Similarly to the other projects the original evaluation was visually conduced and a detailed lidar evaluation was requested to confirm and identify specific zones of deformation. 5.2.1 Deformation Modelling The primary analysis conducted was determination of deformation on a load bearing wall situated directly above reactive black shale. The scan data were compared to a planar best-fit geometric surface. The deformations calculated in the wall demonstrate a large amount of bucking on the lower right corner of the wall (Figure 7) resulting from the uplifting in the shale bedrock.

Figure 7. Deformations in the support wall calculated from a true vertical surface. The purple region in the bottom right corner of the image represents a bulge, likely resulting from the bucking of the wall due to high vertical stress in the upward direction from black shale below.

The uplift in the bedrock is also visible in the ceiling as illustrated in Figure 8. The red and yellow zone is approximately 3 cm higher than all other sections of the ceiling. The maximum horizontal deformations as calculated in the wall directly correspond to the maximum deformation in the ceiling.

Figure 8. Deformations in the ceiling calculated from a horizontal plane. The area of maximum uplift (red zone) directly corresponds to the maximum region of deformation in the wall (Figure 6).

Another analysis tool through manipulation of lidar data is the ability to generate cross sections at virtually any location and orientation. Figure 9 illustrates a cross section of the support wall and ceiling at location A-A’-A’’ as seen in Figures 7 and 8. The cross section visually demonstrates the lateral displacements and unstable region in the support wall. The section also illustrates the portion of the wall where the deformation is focused as well as where it begins.

Figure 9. Cross section line A-A’, as seen in Figure 7 and 8. The curvature at the base of the wall again illustrates the displacement of the wall. 5.2.2 Temporal Modelling The main support wall at Site 2 was scanned in October of 2010 due to evidence of deformations. As was confirmed through lidar scanning (Figures 7-9) the wall

was again scanned in January of 2011. During this time an additional support wall was constructed to absorb the load of the building.

The comparison of the two datasets was ideal as the site conditions and scanning locations were identical. The results of the comparison computed negligible displacement during the time period, as plotted in Figure 10. The displacements observed were less than 3 mm, which is comparable to the accuracy of the system.

Optimally the scanning will be carried out at regular

intervals and deformations observed over a much longer time period can be analysed.

Figure 10. Comparative displacements observed between October 2010 and January 2011. The colours indicate there has been less than 3 mm of displacement (which is equivalent to the accuracy of the system) 6 DISCUSION The applications of lidar are a continually expanding field, one that has seen extreme growth in the past ten years. The monitoring of construction sites using non-destructive technologies has typically been restricted to repeated measurements of precise locations through the use of dedicated monitoring systems. Or installation of onsite sensors installed directly into the foundation. Both of which are time consuming and expensive methods of deformation measurement. The ability to use lidar represents a breakthrough in terms of efficient data collection, rapid generation of results, and visually interpretable results.

As the methods discussed within this paper continue to develop the use of lidar scanning during construction will become a more reliable solution. Currently there a few software programs developed for this type of data processing as well as little user experience in this field. Through applied research such as conducted for the above examples, workflows and guidelines for data collection and processing will be established to optimize the accuracy of the results as well as a deeper understanding of what the data can be used for in future projects.

The analysis techniques presented in this paper illustrated potentially valuable tools to be used for

construction management when working in regions that contain black shale. 7 CONCLUSIONS The use of lidar to monitor building foundations exposed to the differential uplift of black shale is a new methodology employed at NGI. The results presented in this paper represent preliminary attempts to use lidar for explicit deformation monitoring as well as temporal modelling.

The research efforts on this topic as well as many other engineering applications of lidar are underway at NGI. The data collection speed, accuracy of the data, and versatility of the data procession options have proven lidar to be an invaluable tool for remote evaluation of both natural and manmade environments.

The engineering applications of lidar for construction

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