This is a solution of civil engineering in which we focus on type of soils and how to make use out of it in better way
A detailed understanding of the nature of the ground is very important for developing a design for a structure which has to be economical as well as safe to construct. The durability and reduction of maintenance costs also depends on in depth understanding of ground investigation. This comprehension comes from an appreciation of the distribution of the materials in the ground, and their properties and behaviour under various influences and constraints during the construction and lifetime of the structure. An adequate and properly structured site investigation is therefore an essential part of any civil engineering or building project. Improper investigation of site is caused by the lack of awareness of the problems that are associated with the ground and its materials, inadequate focus of finance, less time and a lack of geo technical (or geo environmental) expertise. Hence it is absolutely necessary to perform a site investigation for every site, as without a properly procured, supervised and interpreted site investigation, hazards which lie in the ground beneath the site cannot be known.
Site investigation is a process where geo technical and soil data is acquired and processed to understand the problems those lie with that site prior to the construction of a major structure or a engineering project. There is a high degree of variability in soil and rock properties which may produce undesirable results for the desired structure. Thus, there is a need to determine the geo technical feasibility and the required parameters for the sustainability of the proposed structure. Coulomb’s paper presented in the year 1776, are recorded as first instances of advancements in soil mechanics. During the industrial revolution at the start of the twelfth century, techniques were developed, such as piling, compaction,, dewatering etc.; which are still used nowadays. However, major failures occurred in slope and dyke constructions and thus were felt the need of further research towards the betterment of existing geo technology methods.
Literature review and findings
Described below is a case history in which the replacement of a faulty piezometer without supervision by qualified geo technical engineer led to ingress of water at the bottom of a deep excavation. The excavation of interest was for research on construction of an underground station of the Taipei Rapid Transit Systems. It is located in central Taipei City. The subsoil distribution is typical of that in the Taipei Basin (Moh and Ou, 1979), with a thick layer of young sediments i.e. the so-called Sungshan Formation from the ground surface to a depth of about 48. The Sungshan Formation comprises 6 alternative layers of silty sands (SM/ML) and silty clays (CL/ML). Underneath the Sungshan Formation is a highly permeable gravel layer, i.e. the so-called Chingmei Gravels. This gravel layer was in artesian condition decades ago and the piezometric level in this layer had dropped to as low as RL 60m as result of excessive pumping before the 70s. The clayey sublayers in the Sungshan Formation are relatively impermeable and separate the entire subsurface soil stratum into three aquifers with different piezometric levels. The piezometric levels in the Sungshan Formation responded to the lowering and rising of the pressure heads in the Chingmei Gravels, there obviously are flows across neighbouring aquifers. A typical subsoil profile is shown in Fig. 1.
As shown in Figs. 1 and 2, the excavation was 23m in width and was carried out to a depth of 24.5m by using the bottom-up method. The pit was retained by diaphragm walls, 1.2m in thickness and 44m in length, and braced by 8 levels of temporary struts. The diaphragm walls toed in Sublayer II to provide a seepage cutoff. Sublayer II, which is an impervious layer consisting mainly of silty clay, essentially served as a seal at the bottom of soil plug which was enclosed by diaphragm walls on its four sides. With a length of soil plug of 20.5m, a factor of safety of 1.3 was obtained against blow-in for a piezometric head of 29.5m at the bottom of the plug. At the time when the incident occurred, the bottom of excavation had been reached and, except at the southern end where the incident occurred, base slab had already been cast.
The excavation was well instrumented with settlement markers, inclinometers, load cells, etc. Because blow-in was one of the major concerns, the piezometric levels in the Sungshan Formation and the Chingmei Gravels were closely monitored. One of the piezometers became faulty and the contractor attempted to replace it by a new one. At that time, the excavation at the southern end had already been completed and the bottom of excavation was protected by a layer of plain concrete. Drilling was carried out from the bottom of excavation at the location. As drilling reached RL 59.4m, water started to overflow from the borehole. Although various means have been tried, including extension of drilling casing, placing of sand bags on top of the borehole, grouting etc., the flow soon became uncontrollable and the pit had to be flooded to prevent the situation from deteriorating. As much as 70,000 tons of water was recharged to balance the hydrostatic pressure from the groundwater. It took 6 months to mend the damaged clay blanket under the bottom of excavation before the pit was drained and the works resumed. A total of about 3,000 cu m of LW (Labile Wasserglas) grout consisting of cement, sodiumsilicate and water and about 2,400 cu mof cement-bentonite grout was injected into the ground to fill up cavities made by the seepage flow. Figure 2 shows the settlement contour of the surrounding area. Effect of the incident extended to a distance more than 60m from the location of the borehole. Maximum ground settlement exceeded 250mm.
Analysis of findings
- The code requirements for site investigation, if any, usually stipulate the minimum amount of site work required.
- Site investigation is a specialized operation, requires specialized organizations and specialized personnel.
- Site investigation is the combined product from ground investigation contractor and geotechnical consultant. The contractor is responsible for obtaining reliable data. The geotechnical consultant is responsible for disaster planning and execution of the site investigation work, interpretation and analyses of data, recommendations of design and assumed professional responsibility.
- The extent and cost of site investigation should be such that the risk is at an established acceptable level to the designer and also comply to the accepted code of practice.
- The practice of recommending lowest tender as the main criteria for site investigation should not be preferred but be discouraged. Selection should be made on the basis of the geotechnical consultant’s competency and investigation contractors ability to provide reliable factual data.
Discussion of results
The analysis of the project shows that a good understanding of the site and careful investigation are factors highly detrimental to any construction. In this case, the drawings of the seawalls and knowledge of the excavation operation helped in gathering information for valuable ground investigation. The results of the ground investigation helped in developing the geological profile of the site, and the subsequent patterns in the properties of underlying soil strata. The ground conditions reported are fairly common and would even prove helpful for any future construction projects in the vicinity. Also, the ground investigation helped in preventing any disturbance to the foundations of any nearby existing structures. The data collected during investigation helped in the selection of efficient construction methods and equipments. The installation of sheetpiles through obstruction in areas of vibrations, and the use of penetration and grouting techniques can, in future, serve as a benchmark for a better understanding of different installation techniques for piled foundations under loose or soft ground. In this way, ground investigation of the site helped in selection of the most appropriate method for the particular ground conditions and thus, provided necessary precautionary and mitigation measures. Not only did proper site investigation outlined efficient guidelines and techniques for the desired construction, but also it eliminated all possibilities of unsuitable techniques prior to implementation. Thus saving both time and money.
Mitigation measures for structures showing movement:
- Commonly damage to structures facing high wind velocities result from roof damage. The roofs of such buildings should be inspected and retrofitted according to adequate standards.
- Storm shutters should be installed inside the structures.
- Structures in tectonically active areas should be inspected for weak spots, and retrofitted accordingly.
- Watercourses passing through areas of low soil settlement should be lined with concrete.
- Any construction adjacent to watercourses should be elevated by at least one meter to prevent potential flood inundation.
- Abbaszadeh Shahri A, Esfandiyari B, Hamzeloo H (2009). “Evaluation of a nonlinear seismic geotechnical site response analysis method subjected to earthquake vibrations (Case study: Kerman province, Iran)”, Arab. J. Geosci., Springer, accepted in 14 December, 2009, in print.
- Abbaszadeh Shahri A, Esfandiyari B, Behzadafshar K (2010). “A proposed procedure for nonlinear site response evaluation on strong ground motion during Ardabil earthquake (28 Feb. 1997) by using “Abbas Converter” computer code”, Journal of The Earth, ISSN 2008- 1499., 5(1):1-21.
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