Vincent T. H. CHU

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).

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

The common practice is to apply jointing sand at the first place followed by subsequent compaction. However, there are several potential problems associated with this method. Firstly, the presence of sand provides support to paving blocks leading to inadequate compaction. Secondly, the excess jointing sand may be crushed during compaction and leaves stains on the surface of paving blocks. Thirdly, damaged paving blocks appear to be difficult for removal owing to frictional grip by jointing sand.

The alternative method is to carry out compaction firstly and then followed by application of joint sand and then re-compaction is carried out again. This method eliminates all the shortcomings of the first method described above. However, it suffers from the demerit of two passes of compaction instead of a single stage of compaction is required. Moreover, the compaction operation tends to be noisier because of the absence of the infilling sand which helps to reduce noise level. The direct contact between individual paving blocks is more vulnerable to spalling during compaction.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

The principal function of edge courses is to form a retaining structure with concrete bedding and haunching to resist sideway movement arising from traffic loads. Other functions of edge courses include:

(i) It serves as level guides when carrying out screeding of sand bedding.
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In pavement made up of paving blocks, the edge courses are normally designed to sit on concrete bedding and haunching. The reason of such provision is to prevent lateral movement of paving blocks when subjected to traffic loads. In essence, the edges formed by edge courses tie the body of the paving blocks as a single unit and they have to rely on concrete bedding and haunching to resist the sideway forces generated by moving traffic.

The concrete bed serves to keep the edge courses in position for surface level and it provides the dead loads in the retaining structure. On the other hand, concrete haunching also holds the edge courses in position from the point of view of lateral movement.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

Cement sand is a mixture of cement and sand and it acts as a cohesive mass once mixed. Normally, a 20mm to 30mm sand layer is laid underneath precast paving block units. However, in locations of steep gradients where it stands a high possibility that rain runoff will wash out infilling sand and sand layers, cement sand should be sued instead. Similarly, when high pressure jetting is anticipated to be employed frequently in routine maintenance, sand layers beneath precast paving block units is not preferable owing to the reason of potential washing out of sand.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

In the paving block pavement, it normally consists of the following main elements: sub-base, sand bedding and paving blocks. Sub-base is the main structural element to take up traffic load and spread it into larger area so that the traffic stress is small enough for subgrade to sustain. Sand bedding is used for providing correct line and level for paving blocks to lie on. To achieve this, screeding of bedding layer is implemented so that the paving blocks could be laid directly on it without the need of further leveling.
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Normally after the laying of sub-base layer of the paving slab/interlocking blocks, a 30mm thick sand bedding is screeded and tamped over the pavement area. Then paving slabs are laid horizontally with joints of 2-3mm wide and are laid in uphill direction. After completing the laying of paving slabs, sand used for filling joints is spread over the surface of the units and brushed into the joints such that all joints are completely filled. The paving slabs are then bedded into final position by using plate vibrators.

The sand layer serves the same purpose of normal blinding layer under concrete structure:

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In Hong Kong, footway is normally designed with paving slabs/interlocking blocks instead of concrete because the extensive use of concrete in pavement is boring. Moreover, poor appearance will result in concrete pavement due to frequently trench openings for utility work. In addition, owing to the prolonged setting and curing time of concrete, the use of concrete pavement inevitably generates disturbance to the general public.
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The paving for concrete blocks consists of closely packed paving blocks in pre-determined patterns and the tiny joint spaces between individual blocks are filled with sand. The presence of sand avoids the displacement of a single block unit from the remaining blocks. Moreover, the horizontal interlocking provided by the arrangement of paving blocks in special patterns (e.g. herringbone pattern) prevents any single block from moving relative to one another. For instance, vertical loads acting directly on one concrete paving block are not only resisted by the block itself, but also by the blocks adjacent to it.
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Concrete road belongs to rigid pavement and they do not deflect under traffic loads. On the contrary, bituminous pavement deflects when subjected to vehicular load.

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