Question No. 01
Can all utility detectors detect the depth of utilities?
Answer: In Hong Kong, underground utility detectors are normally divided into two types: electromagnetic detector and ground penetrating radar (GPR). For electromagnetic detector, it can detect the signals emitted by metallic utilities themselves by passive mode. While in active mode, the detector has to pick up the signals through a transmitter and sonda connected directly to the non-metallic utility. Both the alignment and depth can be found in active mode while only alignment can be found in passive mode. The electromagnetic detector available in market can detect utilities up to depth of 3m.
For ground penetrating radar, it sends radio waves into the ground and receives signals from reflections from utilities. It has the advantage of locating both the depth and alignment of utilities. More importantly, it can detect both metallic and non-metallic utilities. However, it suffers from the disadvantage that it is quite expensive and interpretation of data is not simple. Reference is made to LD, DSD (2000).
Question No. 02
In conducting triaxial test to determine shear strengths for soil samples, what consolidation pressures should be specified?
Answer: It appears that the selection of consolidation pressure is independent of in-situ soil stress theoretically. However, this may not be correct because the actual shear strength envelopes for soils are non-linear over a wide range of stresses. Therefore, consolidation pressure corresponding to the range of stresses relevant to site condition should be adopted.
Question No. 03
What is the difference between standpipe, standpipe piezometer and piezometer?
Answer: A standpipe normally contains plastic pipes with perforated holes at the base. The annular space between the perforated tube and casing is filled with gravel or sand backfill. Under such an arrangement, standpipe is used to measure water level of a certain region. A standpipe piezometer is a type of piezometer which measures pore water pressure at a certain level. It consists of plastic pipes without holes. The tip of the standpipe piezometer is perforated and the annular space between the tip of the piezometer and soil is filled with sand while the annular space between other parts of plastic tube and soil is filled with cement/bentonite grout to seal off water from entering the region of piezometer tip. This enables the pore water pressure in the region of piezometer tip to be measured. In essence, standpipe piezometers are installed to study the pore water pressure of a specified depth below ground. However, it suffers from the disadvantage that the response time is relatively slow in clayey soils. Reference is made to Marius Tremblay (1989).
Question No. 04
How does siphoning work?
Answer: Pretty good if the outside hose is a lot lower than the inside hose. The siphon works by a hose stuck in a fluid container higher than the receiving container. Suction is shortly placed on the lower side of the hose until the fluid is flowing down the down line. Gravity will cause a continued flow.
Question No. 05
In pumping stations one of the choices for the material of water tanks is fibre-reinforced plastic(FRP). What are the advantages associated with this kind of material?
Answer: There are two main advantages for FRP water tanks:
(i) It possesses high strength to weight ratio and this leads to the ease of site handling.
(ii) It is highly resistant to corrosion and thus it is more durable than steel water tank.
Question No. 06
For rigid pavement, what are the advantages of using lean concrete sub-base instead of traditional granular sub-base?
Answer: There are several shortcomings of using granular sub-base in concrete carriageway:
(i) Since sub-base is permeable, water can seep through sub-base and soil particles will be pumped out through contraction/expansion joints when subject to traffic load. Consequently, voids are formed underneath the pavement structure and the concrete pavement may crack under severe traffic loading.
(ii) Lean concrete increases the strength and renders the roads capable of carrying heavy traffic loads (David Croney and Paul Croney (1992).
(iii) Due to workmanship problem, it may have uneven distribution of sub-base and this result in cracking of concrete carriageway when subject to severe traffic loading.
Question No. 07
For marine pile type of steel tubular piles with reinforced concrete infill, minimum toe level is often specified in contract drawings. What is its purpose?
Answer: The purpose of minimum toe level is two-fold:
(i) In detailed design stage, ground investigation should be conducted and the approximate level of rock-head is known. Therefore, to avoid the marine piles to be founded prematurely on boulders, minimum toe levels of marine driven piles are specified in contract.
(ii) It provides sufficient length of soils for lateral and uplift resistance. Note: Minimum toe level refers to the minimum level that a marine driven pile should be driven into seabed.
Question No. 08
How do you compute volume of airflow?
Answer: Orifice meter is used for measuring flow rate air.
Question No. 09
What is the formula for figuring out how many gallons of water are in a pipe by the foot?
Answer: 0.61 × d², where d is the diameter of the pipe in inches.
Question No. 10
Soil nails are mainly designed for stabilization of major slips. How should designer cater for the stability of minor slips?
Answer: There are some methods to treat minor slips:
(I) Adoption of smaller diameter size bars at closer spacing;
(ii) Installation of tie beams at the same horizontal levels;
(iii) Provision of steel wire meshes in-between soil nails; and
(iv) Provision of short soil nails in combination of long soil nails.