Introduction
Geotechnical engineers are grappling with the challenges of geotechnical failures because they do not only cause loss of lives, but also great economic losses. A body of literature indicates that there are numerous causes of the geotechnical failures. According to Khan (2005, geotechnical failures can result from foundation factors such as change in water table, progressive soil erosion, soil heterogeneity, excavation activities, burrowing by animals, liquefaction of soil, and natural disasters such as earthquakes and landslides.
Herrero (1998) recommends that civil engineers should conduct investigations to determine risk factors that cause geotechnical failures and avert loss of lives and property. Other factors that contribute to geotechnical failures emanate from technical issues associated with engineering practices. Baars (2011) states that geotechnical failures occur due to large workload, calculation error, incompetence in design and unforeseen failing mechanism.
This shows that geotechnical engineers should have essential knowledge and skills for them to construct stable structures with reduced risk of geotechnical failures. Therefore, this report seeks to examine site investigation procedures and analyze case study of a geotechnical failure with view of providing recommendations.
Site Investigation Procedures
Before geotechnical engineers construct structures such as buildings, plants, roads, or bridges, they should conduct geotechnical site investigation. Geotechnical site investigation usually entails collection of information and assessing geological and environmental conditions relative to the nature of the structure that needs construction (Gao & Zhao 2013).
Collection of information and assessment of prevailing conditions help geotechnical engineers to design and construct stable structures that stand the test of time without incurring any geotechnical failures. Venktaramaiah (2007) defines site investigation as “the procedure of determining surface and subsurface conditions in the area of proposed construction” (p. 724).
Assessment of the surface and subsurface conditions is imperative because they are responsible for Geotechnical failures that result in the loss of lives and property in the construction industry. To prevent occurrence of geotechnical failures, geotechnical engineers usually follow engineering procedures as stipulated in the code of practice when conducting a site investigation.
Preliminary Study
The first step in geotechnical site investigation is a preliminary study of the area of interest. The purpose of the preliminary study is to examine suitability of the study for the proposed construction, economize the design, assess ground conditions, and evaluate impact on the pre-existing neighbouring structures.
Venktaramaiah (2007) argues that reconnaissance is one of the preliminary steps in geotechnical site that enables engineers to examine topographical features, which “yield useful information about the soil and ground water conditions and also help the engineer plan the programme of exploration” (p. 725).
Hence, during reconnaissance, engineers gain valuable information that influences their decisions regarding the nature, size, and design of the structure that requires construction in a given topographical site.
In the preliminary study, geotechnical engineers also require geological information that is in geological maps. The geological maps provide critical information about soil and rocks that underlie a given area of study. According to Venktaramaiah (2007), “data availability of natural resources such as oil, gas, and minerals will have to be considered carefully during the evaluation of the site,” (p. 725).
Lack of consideration of geological factors may lead to legal and technical issues that may hinder mining or construction activities in future. Geological study also entails seismic assessment of the site.
Seismic activity is a major factor that determines the size and design of structures depending on the vulnerability of the geographical area to earthquakes (Puzrin, Alonso, & Pinyol 2010). Therefore, technical engineers should consider seismic activity on the investigation site to avoid occurrence of geotechnical failures associated with earthquakes.
Aerial photography also provides critical information in the preliminary study of site investigation. When combined with geological information, aerial photography provides ample information about geology, hydrology, geomorphology, agricultural activities, and mining activities.
Simons, Menzies & Mathews (2002), ‘the bird’s eye view’ provided by the aerial photography allows the patterns made, say, vegetation or landforms to be recognized more easily and their engineering significance to be deduced” (p. 105).
Hence, aerial photography is important in the desk study of the investigation site. Additionally, aerial photographs have enhanced spectral sensitivity when compared to ground view, because they present information in details within the visible spectrum of the electromagnetism.
Ground Investigations
When preliminary study warrants site investigation to continue, geotechnical engineers proceed to ground investigations. In the ground investigation, geotechnical engineers utilize intrusive methods such as boreholes, trial pits, and hand auger to collect samples of soil for examination and further laboratory studies.
According to Shroff and Shah (2003), “the purpose of the detailed exploration is to determine shear strength and compressibility of all types of soils, density, density index, natural moisture content, and permeability” (p. 419). To achieve critical information, geotechnical engineers should collect and sample soil and rocks in a random manner to avoid biased results.
Poor sampling leads to poor results, which in turn cause errors during calculations and subsequent construction of structures (Pitts 1984). Significant number of geotechnical failures emanate from poor calculations and errors i structural design.
Use of geophysical methods is another part of the ground investigation. The main methods employed in geophysical investigations are seismic, gravity, electrical, and magnetic surveys. Shroff and Shah (2003) state that, “geophysical surveys make use of differences in physical properties such as electrical conductivity and elastic moduli, density, and magnetic susceptibility of geological formations in the area to ascertain the nature of the subsurface” (p. 419).
Seismic survey is the most dominant method because it detects refractive property of the soil and rocks, while the electrical conductivity follows due to its ability to detect soil profile. However, magnetic methods are applicable in detecting dykes, ridges, channels, and intrusions in the ground.
Field Investigations and Laboratory Tests
Field investigations involve the use of trial pits, boreholes, and hand auger. Trial pits are small holes dug in the ground that allows geotechnical engineers to examine soil profile and conditions of ground water. Atkinson (2004) recommends that the depth of trial pits should range from 3 metres to 2.5 metres. This type of field investigation is applicable in areas where soil structure is still intact and free from interference of human activities.
Hand auger is an important hand tool that enables engineers to collect soil samples up to 6 meters. Digging of boreholes is also part of field investigations that enable geotechnical engineers to examine and assess the nature of soil, rocks, and ascertain water level. Hence, these field activities form part of field investigations, which are imperative site investigation.
In the laboratory, geotechnical engineers perform tests such as plate bearing test, shear vane test, and standard penetration test. The plate-bearing test assesses the ability of different sizes of plates to bear the incremental load after every hour for a day. The shear vane test examines the shear strength of clay or silt obtained from boreholes. Comparatively, the standard penetration test is applicable to most soils, but not rocks.
International Navigation Association (2000) explains that, standard penetration test “provides quantitative data on the degree of compaction of granular soils and may also be used at an indicative level to measure the strength of cohesive soils and weak rocks” (p. 22). This test is applicable in rotary core boreholes because boulders and weak rocks, which could otherwise interfere with the test, are rare.
Site Investigation Report
After collecting ample information about the proposed site of construction, geotechnical engineers then produce site investigation report. Honjo, Kusakabe, Koda (2002) argue that the site investigation report is a legal document that guards geotechnical engineers in the event that unforeseeable events cause geotechnical failure.
The report should provide enough information regarding the ground conditions, give the results of all tests carried out, and offer recommendations. The information is helpful in enabling engineers to design appropriate structures and take necessary precautions to avert the occurrence of geotechnical failures.
Case study of Geotechnical Failure
The collapse of the Nicoll Highway in Singapore is an example of the geotechnical failure that occurred in 2004. On the fateful day, metro tunnel of the building pit collapsed after indicating some weaknesses during construction.
Although civil engineers were aware of the construction faults, they did nothing to prevent progression of the faults and occurrence of the geotechnical failure. According to Surhone, Timpledon and Marseken (2010), the collapse of the Nicoll Highway caused the death of four workers and three more sustained some injuries.
Moreover, the collapse caused about 100 metres of the Nicoll High to descend about 30 metres deep. Among other losses, the construction company lost two cranes and substantial steel beams were damaged. Hence, the case study shows that the collapse of the Nicoll Highway in 2004 led to the loss of lives and property.
Investigations conducted to determine the cause of the collapse indicated that it was a combination of many factors, which constitute geotechnical failure. According to Whittle and Davies (2006), the investigation report indicated that, “under-design of the diaphragm wall due to the method of analyzing the soil structure interaction and under-design of the wall in the strutting system” are two design errors of the earth support system that contributed to the collapse of the Nicoll Highway.
The findings indicate that the construction manager failed to design the Nicoll Highway well to prevent occurrence of the failure. Although the construction did show some faults, construction manager ignored them rather than initiating corrective measures.
The corrective measures would have helped in averting the geotechnical failure that led to collapse of the Nicoll Highway. Therefore, construction manager did not follow appropriate procedure that is critical in conducting geotechnical site investigation.
Discussion and Conclusion
Geotechnical failures usually occur due to a combination of factors such as construction design, geological conditions, construction problems and natural disasters. Although some unforeseen factors may contribute to the geological failure, they are not very significant. Given that code of engineering practice provides the procedure for geotechnical site investigation, engineers should adhere to it.
From the case study, it is evident that geotechnical failures are prone to occur if engineers do not follow appropriate procedures in their engineering profession. The case study of the Nicoll Highway indicates that the construction manager overlooked apparent faults during construction that had shown that there was an impending collapse.
Shimizu (2008) states that, following the collapse of the Nicoll Highway, “government lawyers wanted an inquiry panel to hold Nishimatsu’s three executive staff liable for the criminal charges for the collapse of the temporary support structures at the work site” (p.169).
Poor construction practices caused the Nicoll Highway to collapse, thus bringing into question the competency of the construction managers who were responsible for the construction.
In conclusion, geotechnical failure occurs due to a combination of factors that range from construction practices to natural disasters such as earthquakes. To prevent occurrence of geotechnical failures, engineering code of practice stipulates procedures that engineers should undertake when conducting a site investigation.
The procedures include preliminary study, ground investigation, field investigation, laboratory tests, and producing of the investigation report. The case study of the Nicoll Highway is an example of geotechnical failure that resulted from poor construction that failed to comply with the procedures of site investigation. Moreover, the investigations showed that under-design of the supporting structures caused Nicoll Highway to collapse.
Recommendations
- Due to the numerous causes, geotechnical engineers need to perform comprehensive site investigations by following all procedures to avert possible geotechnical failures in their structures.
- Since the case study of the Nicoll Highway shows that the poor construction procedure caused its collapse, construction companies should hire competent construction managers.
- Engineers should make appropriate design of a given structure because the nature of design determines the risk of geotechnical failure as in the case of the Nicoll Highway.
- Standard code of practice that provides for the corrective measures should be in place so that engineers can apply it in case there are progressive faults within and around structures during construction.
Reference List
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