Shoring and Bulk Excavation Plan

  Introduction 

The basement of the building has a capacity for parking about 93 cars over its two floors. For the design considerations, the excavation has to be ‘deep’ excavation. It is important that the initial surveys be carried out for many options of the walls and the support system to assess on the time and total cost of construction. Initial investigation should also be conducted to gauge on technical requirements of the proposed excavation for safety purposes. A deep analysis of the extent and possible effects of the influence of the whole size of excavation on adjacent structures has to be before conducted before the selection of the final option to produce a safe and economical design for excavation. 

For deep excavations, the design consideration is divided into five distinct parts, as explained below. 

Planning of Subsurface Investigation and Laboratory Testing 

Sufficient information should be obtained on the ground and ground water conditions together with the strength and deformation properties of the soils which will be retained (Burland, 2001) and the soil used to support the earth retaining structures, as the ‘code of practice for earth retaining structures’ BS8002:1994. Any source of knowledge should be consulted, together with the geological maps and handbooks of the local area. To produce a safe and economic design, a proper planning and supervision site investigation (S.I) and lab examination has to be conducted. For this particular site, the field test methods used in the formulation of design of the retaining walls of the basement is the use of boreholes (rotary wash boring) and Standard Penetration Tests (SPTs). For laboratory sampling, the most applicable method of field testing is the collection of distributed and undistributed soil samples for further testing in the laboratory (Ergun, 2008). 

In a deep excavation design, such as the one we have on hand, adequate knowledge of the ground water levels, seepage information and existing hydrostatic uplift pressure is essential (Gue, & Tan, 1998). The preliminary ground water conditions may be predicted using the local knowledge. To determine and confirm the existing round water conditions on site, standpipes and piezometers should be installed on the drilled borehole. It is important to seal the boreholes after completion of the exercise to prevent the soil from collapse and subsequent loosening of the subsoil. Boreholes in areas of potential ‘blow outs’ of ground water during excavation should be properly sealed. Grout is used to seal the holes, which pose a safety threat to animals and people as well. 

Evaluation of Foundation of Adjacent Properties and Existing Tolerances 

The impact of ground movements on the surrounding properties and utilities during excavation is an area of concern during planning and execution of excavation. The state of stresses on the ground mass around excavation changes, with the most common changes being the stress relieve on the excavation side resulting in horizontal ground movement, followed by vertical equilibrium, and increase in the vertical stress due to lowering of the water table, resulting in immediate and consolidation settlement of the ground. Ground movements which vary away from the excavation make the adjacent buildings, especially those with shallow foundations, to translate, deform, distort and finally sustain damage if the magnitude sustains tolerable damage. 

During the process of planning, it is important to carry out analyses to determine the magnitude and distribution of the ground movement due to the proposed excavation (Pearlman, et al., 2005). The tolerance of the structures and utilities to the deformations and distortions sustained as a result of the ground movements have to be evaluated. 

Building Damage Due to Ground Movement 

The empirical approach, usually very simple and conservative (Pearlman, et al., 2005), is used for the preliminary assessment for addressing the subject of building damage due to round movement. 

(b). Selection of the Type of Retaining Wall. 

The selection of the type of the retaining wall and its system of support is made on the basis of some of the following considerations. First, the subsoil conditions and ground water levels, the cost and time for the construction of each wall and availability of the local support in the construction of the said basement (Bryson & Kotheimer2010). Additionally, other considerations which have to be taken into account are strength and types of foundations for the adjacent buildings, work space requirements and site constraints, as well as designed limits on the wall and retained ground water movements. 

For the construction of this basement, the type of foundation that is most appropriate is the type of retaining wall for excavation is the secant pile system. They would be the most appropriate for the local soils, which have been found to be stiff, with the area generally having a low water table. For the purposes of excavation, the system used will incorporate the concept of permanent retained walls. The drilling conditions for the site should be conducive for our work, hence, the system will be fast, labor saving and efficient. The secant pile system has the ability to ensure obstructions like rock (Pearlman, et al., 2005), compared to any other available systems for excavations. The excavation system is also favored for its ability to ensure water tightness. The system consists of boring and concreting primary tiles, at the center with center spacing of less than twice the nominal pile diameter (Puller, 2003). The secondary bores are then bored at mid-distance between primary piles the before the concrete has achieved its full strength. The secant pile system is advantageous since it offers a form of full temporary protection in the face of the sensitive and collapsible soils Tanner (Blackburn & Finno, 2007), like the ones available on site, hence increasing the ease of coring into rock. 

(c). Selection of the Support System 

According to the form of support provided for the retaining walls, they are further categorized into three categories namely: strutted or braced wall, tied-back or anchored walls and for the shallow excavations, cantilevered or unbraced walls (Andromalos & Bahner 2003). The factors involved in the selection of a suitable support system are based on the provisions of Navfac Design Manual 7.2 by the United States Navy, 1982. For this particular assignment, some factors to be considered by the Engineer before adoption of the system are the problems of maintenance posed by the permanent ground anchors. Additionally, further precautions have to be undertaken to prevent leakages and loss of fine through the drill holes (Tomlinson & Boorman 2001). Approval of the owners of the adjacent property has to be obtained if it will involve encroachment of the ground anchors into the adjacent properties during excavation. Similarly, it is important to establish the removability of the temporary ground anchors especially if it is a requirement by the regulations of the local authorities, to avoid further problems. 

(d). Design of the Retaining Wall 

The ultimate limit states and serviceability limit states are considerations which must be taken into consideration when designing the earth retaining wall system. The short term behavior of the subsoil, which involves the undrained cohesive materials, and the long term subsoil behavior, relating to the drained cohesive soils, also have to be considered in the formulation of the retaining wall design for excavations of soils such as the one applicable for the proposed excavations ( Finno, et al., 2002). The serviceability limit state has to be considered in terms of wall and soil deformation at the rear end of the wall. There are empirical and semi-empirical methods which are used to predict the deformation of the wall and retained soil. Special attention should be given to ensure there is control of the ground water level in the retained ground for the design to prevent the lowering of the ground water, which would cause an increase in settlement and induce damages to the adjacent structures and services (Finno, et al., 2005). 

For the deep excavations, multi-levelled supporting walls are used instead of cantilevered or strongly supported walls (Shao, et al., 2005). The earth pressures which act on the multi-level strutted walls or the multi-level tied back walls depend on the wall stiffness relative to the soil, the support spacing and the pre-stress load. The method of construction for these walls is normally sequential, installing the wall and excavation in stages followed by the excavation in stages followed by the installation of support like anchor or prop at each installation stage (Tan & Chow 2008). The methods for the analysis can be categorized into three methods. The empirical methods based on the strut load envelopes for three types of soils, sands, soft to medium clays, and stiff clays (Tanner Blackburn, & Finno 2007), computer methods based on the Winkler Spring theory, also known as the beam-spring approaches, and the full, soil-structural interaction analysis, which employs the Finite Element Method, the FEM, the Boundary Element Method, or the Finite Differential Method. 

(e) Construction Considerations 

The complexity of the interactions between the ground and retaining structures for a deep excavation sometimes make it difficult to predict the behavior of the retaining wall accurately and in detail before the execution of the construction works (Tan & Chow, 2008). For the design engineer, their involvement continues even after the construction process has started, in a supervisory role of the activities of the construction works. The design engineer also gets involved in the review of the performance of the retaining structure and their comparison to the design requirements and actual predictions, taking if the necessary precautions to prevent the occurrence of the critical limit state like large displacement of the wall causing further displacement of the adjacent wall structures. The major considerations to be taken before the commencement of the construction costs cover the dilapidation survey of adjacent structures, instrumentation and monitoring system and supervision and control of the construction (Tan & Chow 2008). 

Dilapidation Survey of the Adjacent Structures 

The survey is necessary to avoid lawsuits or unnecessary contractual conflict (Tan & Chow 2008). It should be taken prior to the commencement of construction activities on site, and also forms part of the requirements by the local authorities. Carrying out the dilapidation survey also assists the developer, contractor and the consultants involved gain an insight on the present conditions of the adjacent structures. The survey also offers a reference point should the owners of the adjacent properties claim compensation for damages, if any. This is particularly useful since some of the old buildings adjacent to the proposed site may have suffered cracks and damages prior to the start of construction activities on site. 

Instrumentation and Monitoring System 

The instrumentation and monitoring program for carrying out the excavation of basements of the project on hand will have to be effective to ensure the safety of the surrounding structures. The instrumentation and monitoring scheme also allows for the validation of the design requirements and allows the professional to identify any remedial measures needed or an alteration to the sequence of construction before serviceability of the current structures or surrounding buildings and services affected (Tan & Chow, 2008). Commonly used instruments of deep excavation include inclinometers, which allow for the measure of deformation of a retaining wall and piezometers, which allow the changes to the subsoil to be measured. Settlement markers, which are crucial for establishment of ground profiles using leveling. The tilt meters, load cells and deep extensometers are also useful instruments deep excavations. The monitoring of the instruments is usually made on a weekly basis for deep excavations such as the site in question (Tan & Chow 2008). However, should there be discrepancies noted in terms of the results on the ground, the frequency of the monitoring scheme should be increased to daily, until the causes of the discrepancies are identified and remedial measures carried out. 

Supervision and construction control 

Construction and workmanship should be closely supervised to ensure the quality of the works and safety of the retaining walls (Tan & Chow 2008). The construction should also be closely supervised to ensure that it complies with the specifications, drawings and use of the approved methods of statement, in adherence to the provisions of uniform building bylaws of 1984. 

Conclusion 

The success of the design and construction of a deep excavation begins from a well-planned and closely supervised subsurface investigation works including investigation works both in the field and from the laboratory tests. The design of the retaining wall and support system has to follow the appropriate guidelines, standards and good practices. Intimate input from the design engineer at every stage of the construction procedure, from setting out to the establishment of the monitoring equipment is essential. The construction also has to follow the approved method of mission statement and have a checklist on the supervision to prevent mistakes and carelessness in the execution of the works highlighted. 

References 

Andromalos, K. B., & Bahner, E. W. (2003). The application of various deep mixing methods for excavation support systems. In  Grouting and Ground Treatment  (pp. 515-526). 

Bryson, L. S., & Kotheimer, M. J. (2010). Cracking in walls of a building adjacent to a deep excavation.  Journal of Performance of Constructed Facilities ,  25 (6), 491-503. 

Burland, J. B. (2001). Assessment methods used in design.  Building response to tunnelling, London ,  1 , 23-43. 

Ergun, M. U. (2008). Deep excavations.  Electronic Journal of Geotechnical Engineering, Available at: www. ejge. com/Bouquet08/UfukErgun_ppr. pdf . 

Gue, S. S., & Tan, Y. C. (1998). Design and construction considerations for deep excavation.  Lecture. IEM Northern Branch ,  26 . 

Finno, R. J., Bryson, S., & Calvello, M. (2002). Performance of a stiff support system in soft clay.  Journal of Geotechnical and Geoenvironmental Engineering ,  128 (8), 660-671. 

Finno, R. J., Voss Jr, F. T., Rossow, E., & Blackburn, J. T. (2005). Evaluating damage potential in buildings affected by excavations.  Journal of geotechnical and geoenvironmental engineering ,  131 (10), 1199-1210. 

Shao, Y., Macari, E. J., & Cai, W. (2005). Compound deep soil mixing columns for retaining structures in excavations.  Journal of geotechnical and geoenvironmental engineering ,  131 (11), 1370-1377. 

Tan, Y. C., & Chow, C. M. (2008, March). Design of retaining wall and support systems for deep basement construction–a Malaysian experience. In  Seminar on “Deep Excavation and Retaining Walls”, Jointly orgnaised by IEM-HKIE, Malaysia (Vol. 24). 

Tanner Blackburn, J., & Finno, R. J. (2007). Three-dimensional responses observed in an internally braced excavation in soft clay.  Journal of Geotechnical and Geoenvironmental Engineering ,  133 (11), 1364-1373. 

Tomlinson, M. J., & Boorman, R. (2001).  Foundation design and construction . Pearson education. 

Pearlman, S. L., Walker, M. P., & Boscardin, M. D. (2004). Deep underground basements for major urban building construction. In  GeoSupport 2004: Drilled Shafts, Micropiling, Deep Mixing, Remedial Methods, and Specialty Foundation Systems  (pp. 545-560). 

Puller, M. (2003).  Deep excavations: a practical manual . Thomas Telford