Question No. 01
Which one is a better choice, a large diameter piles or a system of several smaller piles with the same load capacity?
Answer: The choice of a large diameter pile suffers from the disadvantage that serious consequences would occur in case there is setting out error of the pile. Moreover, in terms of cost consideration, for the same load capacity the cost of a group of small diameter piles is generally lower than that of a large diameter pile. On the other hand, for small diameter piles i.e. mini-piles, they are advantageous in site locations with limited headroom and space. In addition, in some structures with only a few piles, it is uneconomic because of its high mobilization cost. Reference ismade to Dr. Edmund C Hambly (1979).
Question No. 02
In some codes, they limit the ratio of weight of hammer to weight of pile for pile driving. What is the reason behind this?
Answer: When a hammer with initial motion collides with a stationary pile, the transfer of energy is most efficient when the two masses are comparable. That is the reason why some codes limit the ratio of weight of hammer to the weight of pile to be more than 0.5. If the weight of hammer is too low, most of energy during hammer driving is distributed to the hammer and this causes tension induced in hammer and results in inefficient transfer of energy.
Question No. 03
What is the significance of driving sequence of driven piles?
Answer: For basement construction, if piles are driven from the centre to the perimeter, there is a tendency of soils to move outwards. Such lateral movement of soil may cause damage to nearby structures and utilities.
However, if piles are driven from the outside perimeter inwards, there are little soil lateral movements. This results in a well-compacted centre with an excess pore water pressure built up to resist the loading of piles. Consequently, shorter pile lengths than the original designed ones may result. However, some time after the pile driving operation, the excess pore water pressure is dissipated and the shorter driven piles may not be able to take up the original design loads. In this situation re-driving is required to drive the piles to deeper depths after dissipation of excess pore water pressure.
Question No. 04
During concreting of diaphragm walls, three tremie pipes are used in one time. However, only one concrete truck is available. How should the concreting works be carried out?
Answer: The most ideal situation is to supply each tremie pipe with a single concrete truck. However, if only one concrete truck is available, all the fresh concrete in the truck should not be placed in one single tremie pipe. With all fresh concrete placed in one single tremie pipe while the others left void, then due to the huge supply of concrete to the tremie pipe, a small concrete hump may form at the base of the tremie pipe and it is likely that it may collapse and trap the slurry inside the diaphragm walls. Therefore, the fresh concrete should be evenly shared among the tremie pipes to avoid such occurrence.
Question No. 05
For a rigid pile cap with vertical piles at the middle and raking piles at the sides, what is the pattern of load distribution of piles in such arrangement?
Answer: Due to the effect of interaction of individual piles, the central piles tend to settle more than the edge piles when the pile cap is under a uniform load. Therefore, raking piles at the edge take up a higher fraction of total loads and are subject to higher axial and bending loads in case the pile cap is stiff. In the extreme case, failure of these raking edge piles may occur.
Question No. 06
What is the difference between direct circulation drilling and reverse circulation drilling?
Answer: For direct circulation drilling and reverse circulation drilling, the major difference in drilling method is related to the direction of movement of drilling fluid. For direct circulation drilling, the drilling fluid is circulated from the drill stem and then flows up the annul us between the outside of the drill stem and borehole wall. The drilling fluid that carries the drill cuttings flows to the surface and the subsequent settlement pits. Pumps are employed to lift the cuttings free fluid back to the drill stem.
For reverse circulation drilling, the direction of flow of drilling flow is opposite to that of direct circulation drilling. Drilling fluid flows from the annul us between the drill stem and hole wall to the drill stem. The drilling fluid is pumped to a nearby sump pits where cuttings are dropped and settled.
Question No. 07
Should engineer relay on Hiley’s formulas in the design of H – Piles?
Answer: About 90% of H- piles adopted hiley’s formula for design. However, this formula is only applicable to pile lengths less than 30m and is suitable for course-grained materials (not suitable to fine-piles after the hammer impact, the pile capacity could be readily obtained from the response of the impacting force. Therefore, the individual pile capacity could be obtained by this dynamic method.
However, in normal foundation, groups of H-piles are present and the soil foundation may not be able to support these H-piles simultaneously even though individual piles are proven to have sufficient capacity by using dynamic method. In this case, static method should be employed to ascertain if the soil foundation could support these H-piles.
Question No. 08
What is the difference between routing maintenance and major maintenance in school facilities?
Answer: The routine maintenance is the minor and consistent repairs that are engaged in the school premises. Examples could include Painting, replacement of fixtures, louver blades, furniture repairs, and patching cracks in the school buildings.The major maintenance could also be a total rehabilitation of the school, either school buildings orany other project within the school community. It could be total restructuring of the lighting system in the school or sanitary system.
Question No. 09
What are the uses of alloys in daily life and how are alloys made?
Answer: Alloying is not always done to produce a ‘superior’ material, but to produce material shaving a desired requirement in the industry or elsewhere. A classic case is of lead solder (containing lead & tin), in which the melting point of the constituent elements are lowered, not necessarily a desirable property. Alloying can be carried out using hot press method (a sheet of material is sandwiched with alloying material), rolling the heated base metal in a drum full of alloying metal powder, using hot spraying, galvanizing (dipping the base in a molten solution of alloying material) etc. Sometimes the alloying material is added in small proportions to the molten base metal (e.g., in production of different types of steel).
Question No. 10
What are the reasons for geotechnical site investigations in Uganda?
Answer: To know the soil properties, in so doing be in position to determine whether the particular site is suitable for the purpose intended
(i) To know the history of the site
(ii) To know what remedies need to be put in place before construction can start Based on the soil properties that can be determined on site and in the lab, design the appropriate foundation for the structure.