Question No. 226
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
- To know the history of the site
- 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.
Question No. 227
Did any of the Egyptian pyramids ever fall down?
Answer: Yes, many did. The great pyramid and others that have survived did so for a reason. Their shapes made them sturdier over time and were not the first design. Many steppe pyramids were built but were too steep and as the corners eroded, the whole pyramid fell under its own weight. They were built too steep and did not have the base structure as seen in the great pyramid.
Question No. 228
In designing mini-piles, should the strength of grout be neglected during assessment of loading carrying capacity?
Answer: In designing min-piles, there are two approaches available:
- In the first approach, the axial resistance provided by the grout is neglected and steel bars take up the design loads only. This approach is a conservative one which leads to the use of high strength bars e.g. Dywidag bar. One should note that bending moment is not designed to be taken up by min-piles because of its slender geometry.
- In the second approach, it involves loads to be taken up by both grout and steel bars together. In this way, strain compatibility requirement of grout and steel has to be satisfied.
Question No. 229
What are the functions of cap block, drive cap and pile cushion in driven piles?
Answer: Cap block is installed between the hammer end and the drive cap to control the hammer blow in order to protect both the hammer and the pile from damage. When the hammer hits the cap block, it compresses elastically and reduces the peak forces, thereby lengthening the time of hammer blow. Moreover, it should be capable of transmitting the hammer energy effectively to the piles.
Drive cap is inserted at hammer tip to enhance uniform distribution of hammer energy to the pile. Pile cushion is positioned between the drive cap and the pile top. It intends to protect the pile from driving stress induced during hammer blows. Moreover, it also serves to provide a uniform driving load on top of the pile.
Question No. 230
What are the methods to tackle negative skin friction?
- Use slender pile sections (e.g. H-pile or pre-cast pile) because smaller pile area when subject to the same working load would produce higher deformation, thus increasing the relative downward movement of piles.
- In a certain region of H-piles for ground water table fluctuation, painting is applied on the surface of H-piles because the rise and fall of water table contribute to the corrosion of H-piles. On the other hand, to reduce the effect of additional loads brought about by negative skin friction, bitumen is applied on the pile surface corresponding to the region of soils that has negative skin friction. However, bitumen should not be applied to the whole section of H-piles because it would be unable to derive the designed frictional reaction from soils.
- Design the piles as end-bearing so that they can take up more loads.
Question No. 231
How do you determine Specific gravity of cement?
Answer: Cement is usually purchased as a powdery substance that is mixed with sand, aggregate, gravel, and water to form concrete. Since the cement itself is usually a powder, it is hard to measure a standard value for its specific gravity. In addition, since cement is usually not used by itself, knowing its specific gravity is not particularly useful.
A more useful question is "What is the typical density of concrete?" A rule of thumb answer is that normal cured concrete has a density of about 150 pounds per cubic foot. This includes the weight of the cement, sand, aggregate, and that Part of the water that chemically binds with the cement to form the concrete. Since water weighs about 62.4 pounds per cubic feet, concrete is about 2.4 times as heavy. Thus, the specific gravity of concrete is about 2.4. If you took cement and mixed it with water, you would eventually have a hard lump of useless cement and it would also have a specific gravity of between 2 and 2.4.
Question No. 232
How and where are aqueducts built?
Answer: Aqueducts are built in areas where you have a bunch of motivated end users (like a town or group of farmers) at a low elevation in need of a more reliable source of water located somewhere fairly nearby at a higher elevation. The aqueduct builders construct a series of canals, elevated channels, and tunnels as required to get the water from the source to the end users. Some good examples:
- Roman engineers built aqueducts throughout Italy and France from mountain water sources to serve city dwellers
- Water-needy Southern California cities and farms are served by an aqueduct that brings them water from sources in Northern California
- New York City is supplied by an aqueduct and tunnel system from sources upstate.
- Inca farmers in coastal valleys built irrigation aqueducts from sources higher up in the Rockies
- Native American cultures in Phoenix area built irrigation canal systems that diverted water from sources at higher elevations to irrigate their crops.
Question No. 233
What is the meaning of a blue land surveyor’s flag?
Answer: If the flag was placed by Utility personnel responding to a "One-call" locate request, the blue flag indicates a buried water line. You see these marked when a contractor calls the "Call before you dig number" a couple of days prior to excavating. This is required by law in each state to reduce the likelihood of damaging underground utilities when excavating.
The standard color code used by almost all utility companies for Painting & flags is:
- White - Here is the area I plan on excavating!
- Blue - water line
- Red - electricity
- Yellow - natural gas
- Green - sewer
- Orange - telephone and/or fiber optic line
If the blue flagging was a fuzzy blue marker nailed to the top of a wood surveyor's stake, then it probably serves to indicate the top of the grade at which the engineer wants the earthmoving equipment to place fill dirt. These are called "blue-top" stakes.
Question No. 234
What are advancements in civil engineering?
Answer: Unlike other fields of engineering, the major advancement of the field has been in the early years of the century before the last century where the use of concrete technology is advanced. The use of cement as a construction material is since the turn of the last century, improvement in the field increase by the use of steel elements in the construction of buildings and bridges of various types. With the help of two, it was possible to do multi-story buildings in the world. Machineries were created to speed up the construction structures. The last century has also seen the advent of sophisticated design to withstand the effect of earthquake that was not possible before. With the use of computers, development of model and analysis of structures under the effect of loads was made possible. Before just two decades, it used to take months and months to design high-rise building and big bridges. Now it is a matter of hours.
Question No. 235
How did the Romans get water up hills using aqua ducts?
Answer: Technically, the Romans were not able to get water to move uphill in a general sense. All aqueducts move water from an elevated source (spring-fed streams in the mountains) to end-users at a lower elevation. The water flows almost entirely from the source to the end user. If the water needed to cross a valley, the Romans would build an arched structure with an elevated channel to cross the valley, but even this channel would have a very slight downhill gradient that allowed water to flow towards the end user. If a large hill was in their way, the Romans would either divert the channel around the hill, dig a trench through the hill, or dig a tunnel through the hill, all while maintaining a fairly constant, slight downhill gradient towards the end user.
The only exception to the rule of a generally constant downhill slope to the water channel is that specific tunnel segments, the Romans could build the tunnel as an inverted siphon (mentioned above) to cross a depression or valley and raise the water level on the downhill side almost to the level of the uphill side. To do this requires a well-sealed tunnel strong enough to withstand the increased water pressure within the siphon. Note, however, that except for gaining a little bit of elevation if you slow down fast-moving water, you normally cannot get water to flow out of the outlet at a higher elevation than the inlet. So technically, even the Romans were not able to get water to flow "up a hill".
Question No. 236
What is the difference between QA and QC?
Answer: Many people and organizations are confused about the difference between quality assurance (QA), quality control (QC), and testing. They are closely related, but they are different concepts. Since all three are necessary to manage the risks of developing and maintaining software, it is important for software managers to understand the differences. They are defined below:
- Quality Assurance: A set of activities designed to ensure that the development and/or maintenance process is adequate to ensure a system will meet its objectives.
- Quality Control: A set of activities designed to evaluate a developed work product.
- Testing is the process of executing a system with the intent of finding defects. (Note that the "process of executing a system" includes test planning prior to the execution of the test cases.)
Question No. 237
What is horizon or horizontal mining?
Answer: Horizon or horizontal mining can be applied to extraction of material from seams of any stratified mineral such as limestone or ironstone, but it is more usually associated with coal particularly where there are several seams that are inclined or folded and/or faulted. Horizon mining involves long level roadways (horizons) being driven from the shafts to the extremity of the area to be mined.
The levels of the horizons are chosen to intersect the maximum number of seams the maximum number of times. As the seams are intersected, headings will be driven into the seam so that the desired material may be extracted. This method of mining requires a thorough understanding of the geological structure of the area to be mined so that the level of the horizons can be chosen for optimum results. This method of mining is popular in modern coalmines with seams worked from several horizons. The considerable capital outlay of driving horizons before production can begin is recouped by the advantage of having long straight level roadways of generous dimensions unaffected by the crushing effect of nearby extraction of the mineral.
Question No. 238
What is the difference between absorption & adsorption and sorption?
Answer: Absorption generally refers to two phenomena, which are largely unrelated. In one case, it refers to when atoms, molecules, or ions enter some bulk phase gas, liquid or solid material. For instance, a sponge absorbs water when it is dry.
Absorption also refers to the process by which the energy of a photon is taken up by another entity, for example, by an atom whose valence electrons make transition between two electronic energy levels. The photon is destroyed in the process. The absorbed energy may be reemitted as radiant energy or transformed into heat energy. The absorption of light during wave propagation is often called attenuation. The tools of spectroscopy in chemistry are based on the absorption of photons by atoms and molecules.
Adsorption is similar, but refers to a surface rather than a volume: adsorption is a process that occurs when a gas or liquid solute accumulates on the surface of a solid or, more rarely, a liquid (adsorbent), forming a molecular or atomic film (the adsorb-ate). It is different from absorption, in which a substance diffuses into a liquid or solid to form a solution.
Question No. 239
Will Water damage concrete?
Answer: As far as only concrete is concerned i.e. plain concrete, the effect of water seepage is very little (depending upon the grade of concrete) whereas for RCC (reinforced cement concrete) water that seeps in corrodes the reinforcement and thus reduces the life of the structure. The defects that water seepage induces in concrete are as follows:
- Induces capillary formation (due to the detiorating characteristics of water)
- With these capillaries the concrete starts spalling out; i.e. the places where capillaries are formed, with even slight amount of stress that portion comes out and exposes the steel to the atmosphere
- Concrete has a pH of about 12 - 13. It also reduces the pH of the concrete when in salty water (or) when exposed to marshy areas.
- Reduces the overall strength of concrete
- Reduces durability
- Reduces permeability to further water see Page
- Results in ageing of structures
Question No. 240
Should air test or water tests be selected to test the leakage of constructed gravity pipelines?
Answer: For gravity pipes, air tests or water tests are carried out after completion of laying and jointing of the pipes. These tests are conducted to check the water-tightness of joints and to ensure the pipelines are free from damage where leakage may occur.
Air test has the advantage that the test itself is simple and faster to be carried out. It does not require the disposal of significant quantities of water used in the test which is a mandatory requirement for water test. However, in case leakage exists in the constructed segment of gravity pipelines, the position of leakage can hardly be located in air test. Moreover, the rate of water leakage cannot be determined from air tests. In addition, air test is readily affected by atmospheric condition because air has a relatively high coefficient of thermal expansion. The test is also influenced by the moisture condition of the test pipelines because it affects the passage of air through the pipelines.
For water test, though it is comparatively slow, it can detect the location of water leakage. However, the leakage rate results from water test may not truly reflect its actual leakage because pipeline materials like concrete and clay are porous and would absorb water during the test.
Question No. 241
What is “preset” during installation of bridge bearings?
Answer: “Preset” is a method to reduce the size of upper plates of sliding bearings in order to save the material cost. The normal length of a upper bearing plate should be composed of the following components: length of bearing + (2 × irreversible movement) + (2 × reversible movement). Initially the bearing is placed at the mid-point of the upper bearing plate without considering the directional effect of irreversible movement. However, as irreversible movement normally takes place at one direction only, the bearing is displaced/pre-setted a distance of (irreversible movement/2) from the mid-point of bearing in which the length of upper plate length is equal to the length of bearing + irreversible movement + (2 × reversible movement). In this arrangement, the size of upper plate is minimized in which irreversible movement takes place in one direction only and there is no need to include the component of two irreversible movements in the upper plate.
Note: “Preset” refers to the displacement of a certain distance of sliding bearings with respect to upper bearing plates during installation of bearings.
Question No. 242
What is the function of water-stops in joints of box culverts and drainage channels?
Answer: The principal function of water-stops is to prevent liquids (e.g. water), water-borne materials and solids to pass through concrete joints. In essence, it aims at providing water-tightness to the drainage channel. Besides, water-stops in drainage channels or box culverts can also serve two other purposes:
- To avoid water contacting joints’ dowel bars and causing corrosion.
- To avoid water seeping in from the underside of drainage channels or box culverts, thereby washing in soil particles and causing voids underneath these structures and finally leading to their failure.
To serve the second purpose, obviously only one water-stop is required at any depth location.
To serve the first purpose, a water-stop has to be installed on top of dowel bars to prevent water from drainage channels from leaking through. On the other hand, a water-stop has to be provided below dowel bars to avoid underground water from surging upwards.
In fact, the other way out to serve the first purpose is by using corrosion resistant bars.
Question No. 243
Is stainless steel really stainless in construction application?
Answer: Stainless steel refers to alloy steels with more than 10.5% of chromium and consists of several groups like austenitic, ferritic, martenistic etc. Austenitic stainless steel is normally used in structural applications because of its high corrosion resistance. Austenitic and ferritic types of stainless steel cover about 95% of stainless steel applications. Stainless steel is not stainless although it is corrosion resistant under a wide range of conditions.
A passive layer of chromium oxide is formed on stainless steels surface which renders it corrosion resistant. This chromium oxide layer acts as a stiff physical barrier to guard against corrosion and makes it chemically stable. Moreover, when this layer is damaged, it can perform self repairing where there is a sufficient supply of oxygen. However, stainless steel will still corrode by pitting in marine environment where chloride attack occurs. Therefore, appropriate grades and types of stainless steel have to be selected in polluted and marine environment to minimize the problem of corrosion. Reference is made to Euro Inox and the Steel Construction Institute (2002).
Question No. 244
What is the function of drilling fluid in rotary drilling in site investigation?
Answer: Drilling fluid in rotary serves two main purposes:
- Facilitate the rotation of drilling tube during rotary drilling;
- Act as a cooling agent to cool down heat generated during drilling operation.
Traditionally, water is normally employed as drilling fluid. However, it suffers from the following drawbacks:
- It affects the stability of nearby ground with the introduction of water into the borehole (borehole for soil; drill-hole for rock);
- It affects the quality of sample by changing the water content of soil samples collected from the borehole/drill-hole.
Substitutes are available in market to replace water as drilling fluid (e.g. white foam).
Question No. 245
Does the presence of rust have adverse impact to the bond performance of bar reinforcement?
Answer: In fact, the presence of rust in bars may not have adverse impact to the bond performance and it depends on the types of bar reinforcement under consideration.
For plain round bars, the rust on bars improves the bond performance by the formation of rough surfaces which increases the friction between steel and concrete.
However, for deformed bars, the same theory cannot apply. The presence of rust impairs the bond strength because corrosion occurs at the raised ribs and subsequently fills the gap between ribs, thus evening out the original deformed shape. In essence, the bond between concrete and deformed bars originates from the mechanical lock between the raised ribs and concrete. On the contrary, the bond between concrete and plain round bars derives from the adhesion and interface friction. With such differences in mechanism in bonding, the behaviour of bond between deformed bars and plain round bars in the presence of rust varies. Reference is made to CIRIA Report 147.
Question No. 246
In the design of corbel beams in a pumping station, why are shear links designed in the top 2/3 of the section? What is the general advice on the design?
Answer: Corbel beams are defined as z/d < 0.6 where z is the distance of bearing load to the beams’ fixed end (or called shear span) and d is depth of beams. The design philosophy is based on strut and tie system. To establish the design model, it is firstly assumed the failure surface, i.e. shear cracks extending to 2/3 of depth of beam. Experiment results verified that the failure cracks extended only to 2/3 of beam while the remaining 1/3 depth of concrete contributed as concrete strut to provide compressive strut force to the bearing loading.
Horizontal links are normally provided to corbel beams because experimental results indicated that horizontal links were more effective than vertical links when shear span/depth is less than 0.6. For shear span/depth>0.6, it should be not considered as corbel beams but as cantilevers.
In designing corbel beams, care should be taken to avoid bearing load to extend beyond the straight portion of tie bars, otherwise the corners of corbel beams are likely to shear off. Reference is made to L. A. Clark (1983).
Question No. 247
Should stiff or soft fenders be designed for berthing in piers?
Answer: The elasticity of fenders is related to the ability to release the stored energy during berthing of vessels. However, it has no effect on the reaction force and the deflection of fender system. The amount of energy that a fender can absorb is dependent on the reaction-deflection curve and is represented by the area under the curve. The higher is the reaction force, the higher amount of energy would be absorbed by the fender provided that the resistance of ships’ hull is sufficient to withstand the force without permanent deformations. Although stiff and soft fender may have the same deflection under the same maximum reaction force acting on the berthing vessel, the amount of energy absorbed by stiff fenders is much higher than that of soft fenders. Consequently, stiff fenders should be employed for berthing purpose.
On the other hand, in mooring operations where vessels are constantly subject to wave action, it is desirable to keep the tension force on the rope to a low value. In this connection, it is recommended to use soft fenders.
Question No. 248
In soil compaction test, if a test result exceeds 100%, should engineers accept the result?
Answer: Soil compaction is the process of increasing the soil density by reducing the volume of air within the soil mass.
Soil compaction depends mainly on the degree of compaction and the amount of water present for lubrication. Normally 2.5 kg rammers and 4.5 kg rammers are available for compaction in laboratories and the maximum dry densities produced by these rammers cover the range of dry density obtained by in-situ compaction plant.
Regarding the second factor of water content, it affects the compaction in the following ways. In low water content, the soils are difficult to be compacted. When water content is increased gradually, water will lubricate the soils and this facilitates the compaction operation. However, at high water content, as an increasing proportion of soils is occupied by water, the dry density decreases with an increase in water content.
For soil compaction tests, the dry density obtained from compaction carried out in-situ by vibrating roller/vibrating plate is compared with the maximum dry density conducted in laboratories using 2.5 kg rammer of compaction with similar soils. In essence, the in-situ compaction is compared with the compacting effort of using 2.5 kg (or 4.5 kg) rammer in laboratories. In case the compaction test results indicate values exceeding 100%, it only means that the in-situ compaction is more than that being carried out in laboratories which is treated as the basic criterion for satisfactory degree of soil compaction. Therefore, the soil results are acceptable in case compaction test results are over 100%. However, excessive compaction poses a risk of fracturing granular soils resulting in the reduction of soil strength parameters.
Question No. 249
What is the function of longitudinal joints in concrete road pavements?
Answer: A longitudinal joint consists of a tie bar placed at the mid-depth of a concrete pavement and it is not intended for joint lateral movement. Then one may doubt the reasons of placing longitudinal joints in concrete pavements. In fact, longitudinal joints are normally designed at a regular spacing e.g. 4.5 m to accommodate the effect of differential settlement of pavement foundation. When uneven settlement occurs, the tie bars in longitudinal joints perform as hinges (Ministry of Transport (1955)) which allow for the settlement of concrete carriageway. Moreover, it also serves to cater for the effect of warping of concrete due to moisture and temperature gradients by permission of a small amount of angular movement to occur so that stresses induced by restrained warping can be avoided.
Dowel bars are provided in longitudinal joints for the following reasons:
- In case of the occurrence of uneven settlement between adjacent panels, it helps to maintain a level surface by transfer of loads through dowel bars.
- Keep the longitudinal joints close.
Question No. 250
What is the problem in traditional marine piling system of steel tubular pile with concrete infill and what are the possible remedial measures?
Answer: In the design of marine piles of steel tubular piles with concrete infill, loads from pier deck are taken up by steel tubular piles before the occurrence of corrosion of steel piles above seabed. In fact, it is assumed that steel piles above seabed level will all be corroded after a certain year. The load transfer mechanism after complete corrosion of steel pile above seabed is as follows: loads from pier deck are taken up by concrete infill above the seabed level. Below the seabed level, loads would be transferred to steel piles through frictional forces between concrete infill and steel casings.
However, substantial radial shrinkage and contraction occurs after concreting of concrete infill and this will hinder the load transfer from the concrete infill to steel piles because the bond may be ruptured by radial shrinkage. It is in doubt if frictional forces can be properly developed in this situation. To solve this problem, shear keys could be installed at regular spacing inside steel piles to ensure their rigid connection with concrete infill. Alternatively, expanding agents may be adopted in concrete mixes to ensure that there is no shrinkage after the concreting process.