Soils And Site Investigation Construction Essay

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Settlement is caused by the weight of a new building structure. The weight on the ground can cause a little movement as the ground consolidates under the new load. This is the settlement. This usually happens early in the life of the structure; plus it rarely recurs. Thought there are exceptions for example in soft clay soils.

Subsidence results from external factors which cause disruption, displacement and contraction or distortion of the ground under or surrounding the structure. Common causes for example could be trees or drains to name a few.

Briefly explain the difference between cohesive and non-cohesive soil.

Cohesive soils such as soft and firm clay soils or soft silts contain higher water content plus a natural tendency to ball together when compressed. When under stress from loads these types of soil settle and consolidate over years.

Non-cohesive soils are coarse by nature and include compressed gravel, sand or rock soils. Under a load any settlement usually occurs reasonably fast.

Describe the three common basic types of foundation.

There are three basic type of foundation:

Strip Foundation- this is by far the most common form of foundation. Here there would be a strip of concrete underneath the load bearing walls. The width and depth of the strip would depend on the type of soil, ground conditions and building load.

Pile Foundation- Here long slender concrete members would be used to transfer the loads through weak or unstable soil passing through to ground with a higher load bearing capacity.

Raft Foundation- A concrete raft which spreads loads over the entire ground floor area.

What is the purpose of the Building Regulations?

The Regulations are legal requirements intended at achieving sufficient standards of building work for construction of commercial, domestic and industrial buildings. The purposes are to ensure the safety, health, welfare and convenience of people in and around buildings. They are also designed to further the conservation of fuel and energy.

What factors affect the depth of foundations?

Factors that affect the depth of foundations include the nature of the soil, and the building for example the weight. Furthermore any building regulations that are in force and finally the cost.

Foundations:

What determines the width of a simple strip foundation?

The width of a simple strip foundation would depend on three factors, the nature of the ground in question; in particular the grounds allowable bearing pressure.

The width also depends on the load of the building. The third factor is the need for working space. For example, trench width may be increased to provide space for the bricklayers.

When would you use piled foundations?

Piled Foundations would be used when good bearing ground can only be found at deeper levels or where physical restraints make different types of foundation impossible to use.

What are the advantages of trench fill foundations?

Trench fill foundations are quicker to set up than other forms of foundation such as deep strip foundations. This means that there is less disruption once the building work proceeds and not much labour time is necessary. Trench fill foundations also avoid the need for working space and trench support, with savings in cost of excavation and soil disposal. Subsidence is also less likely to occur.

When would you recommend the use of a raft foundation?

It would be recommended to use raft foundations when the ground is essentially stable but where the conditions underneath the surface, such as mining, may well resort to ground movement. Again best used on compressible ground such as soft clay, alluvial deposits and compressible fill materials.

What are brick footings?

Footings are the first part of the house which is built, they support the weight of the building; furthermore they prevent the building shifting from its intended position.

Walls:

What are the main disadvantages of "one brick" solid walls?

There are many serious disadvantages with one brick thick solid walls. A wall must be at least one brick thick, in order to maintain reasonable weather protection. This was the standard form of construction for most brick built buildings before the 1920's. Modern construction of houses etc do not use one brick thick walls, because weather protection is not good enough; now days there is an emphasis on sustainable houses. One brick thick solid walls are not sustainable due to damp penetration and issues of condensation. There are also problems of high heat loss, due to no cavity insulation. The relatively dense nature of the brick means heat is easily transferred through the wall.

Explain the difference between Flemish and English Bond?

Flemish Bond, this pattern is made from alternating headers and stretchers on every course. English Bond, this pattern is made from alternating courses of headers among courses of stretchers. Flemish Bond is the better looking of the two types of brickwork. English Bond on the other hand has greater strength value.

What sort of lintels would you expect to find in a typical Victorian town house?

A typical Victorian town house would usually have lintels made of a hard wood or steel. Concrete lintels did not become common practise until the second half of the 20th century.

What factors influence the strength of mortar?

Mortar strength is decided by its mix and ratio of sand to cement. The more cement and less sand, the stronger it is. The more water added in comparison to cement will decrease the compressive strength of the mortar.

What is ashlar walling?

Ashlar is a style not a material, it is stone cut with high levels of accuracy and is laid on very thin mortar joints. Ashlar walling is one of the most expensive walling products; therefore usually it is only used as the facing material. This type of walling requires a structural backing of rubble or brickwork.

Cavity walls & Window Openings in Walls:

Describe three types of modern lintel commenting on their respective advantages and disadvantages.

Concrete Lintels- There are a lot of advantages in using concrete lintels, such as its high strength, durability and life span. Concrete is also an excellent fire resistant. Furthermore it is always available.

The only real disadvantage of using a concrete lintel is the shear weight of the material; the positives though defiantly out weight the disadvantages.

Steel Lintels- Like the concrete steel is very strong, plus it is a lot lighter. Though there are many disadvantages, steel lintels do not provide a very aesthetically satisfying appearance. There is also the issue of cost, steel is fairly expensive; and these types of lintels are not very durable and do not have a very good life span.

Timber Lintels: Timber on the other hand is very pleasing to the eye, making it more attractive to buyers. Timber is also a lighter weighted material and is more cost effective. Disadvantages though are that it is open to decay, so not very durable and has a lower life span. Furthermore it is not a strong as the other materials.

Why is a vertical DPC required when closing the cavity at the jambs of an opening?

Using a vertical Damp proof course when closing the cavity at the jambs of an opening is the most common way to prevent any damp from crossing the cavity and into the brickwork. The DPC should protect the cavity by about 25mm. Any water dripping off the side of the DPC cannot reach the inner leaf. If an insulated DPC is used it will also prevent Cold Bridging.

How can cold bridges be prevented when cavity is closed?

There are a number of options in order to prevent cold bridges. Firstly by installing an insulation board, fixed behind the plaster. Insulating the soffit of the lintel, jamb and cavity bellow the window board. This maintains a higher inner surface temperature.

Secondly, the frame is fixed deeper into the reveal and covers the cold bridge. This would require wider sills or the use of subsills, ensuring any drip is away from the brickwork.

Why do the Building Regulations require extra wall ties around openings in cavity walls?

Wall ties are formed to prevent water from passing across them and reaching the inner leaf. Many ties, especially the older ties corrode; so new stainless steal ties are needed. Openings in cavity walls, such as doors and windows, are week spots. Building regulations require additional tiles wires in order to strengthen the reveals and stop the water.

What is the function of a cavity tray and where is it necessary?

Cavity trays are inserted above doors and windows to deflect moisture that transfers across the outer skin of brick work, back out; this prevents damp.

Ground Floors:

What were the main problems of early raised timber floors?

There were and are many problems with early raised floors. Timber is a hygroscopic material, so it's very vulnerable to fungal and insect attack if the moisture content is above 20%. The air vents often block over the years, the job of the vent are to allow air to pass under and maintain a safe moisture content. Early vents were made of cast iron, over time they become especially easy to break; small animals and vermin have easy access to under floor space; in turn blocking and causing damage.

When should suspended concrete floors be used at ground floor level?

Suspended concrete floors should be used in situations where the hardcore is going to be above 500-600mm deep. They should also be used when there are aggressive chemicals in the ground, which could attack the concrete or hardcore. Or if the type of ground is for example shrinkable clay, the nature of the subsoil could well result in unstable ground; so suspended concrete floors would be needed.

Another reason is if the water table is too high, the damp proof membrane could not possibly protect against direct water pressure.

What are the advantages of a floating chipboard floor?

A floating chipboard floor can be laid onto any existing base; it reduces the weight of the building structure and provides better insulation against sound.

How should hardcore be laid and what precautions are necessary to protect a DPM below a concrete slab?

Sub-soil underneath the building is removed and replaced with the hardcore, reducing the thickness of the concrete. The hardcore must be laid and compressed in very thin layers no more than 600mm. The hardcore would have to be finished off with a blinding of sand for example, to a greatest thickness of 20mm. Though caution should be taken to ensure the blinding material is not too thick. Its job is only to protect the polythene DPM from being punctured by the sharp ends of the hardcore.

Describe the various ways in which a timber ground floor can be effectively insulated?

Timber flooring is perceived as being warm, though it permits a lot of heat loss through flow of air underneath the joists.

Upper Floors:

What factors dictate the direction of floor joists and why are they normally spaced at 400 centres?

Joists are normally spaced at 400mm centres because joists set further apart need deeper sections since they are carrying a greater load; floor boards would also be required to be deeper due to their increased span. Joists which are closer together though use excessive amounts of timber.

How can external walls be adequately restrained at first floor level?

Building regulations require that walls are restrained; this is usually achieved by using restraint hangers. The joists can provide restraint at the first floor level, though in a lot of older buildings there is not adequate restraint where the joists run parallel with the wall. Here restraint is harder to achieve, this would require the use of galvanised or stainless steal straps. The steel strapping at 2m centres is fixed onto the inner block leaf and passes across three joists and is supported by timber blocking between the joists.

How are joists sized?

The joists are sized depending on the expected load which is going to be imposed onto the floor, the required span between two points of support and the spacing between each of the joists. If the joists are not deep enough they could bend and if they are not wide enough they will twist.

Give examples of typical situations using the appropriate table in the Building Regulations?

FLOOR JOISTS SUITABLE FOR DOMESTIC FLOOR LOADINGS

Maximum clear span in metres for joist spacing of

Size of joists mm 400mm 450mm 600mm

97 x 50 1.98 1.87 1.54

122 x 50 2.60 2.50 2.19

147 x 50 3.13 3.01 2.69

170 x 50 3.61 3.47 3.08

195 x 50 4.13 3.97 3.50

220 x 50 4.64 4.47 3.91

147 x 75 3.56 3.43 3.13

170 x 75 4.15 3.96 3.61

195 x 75 4.68 4.52 4.13

220 x 75 5.11 4.97 4.64

How and why is strutting provided in a suspended timber floor

Strutting is provided in order to stop joists from twisting or warping and even damage the ceiling finishes. You would usually find the strutting fixed on at right angles to the joists. The strutting also reduces bounce in the flooring. Staggered strutting is often used, using off cuts of joists. They are set in staggered formation so that they can be nailed.

Roof Structure:

What is the function of a wall plate and how is it held in position?

Most walls in modern times are built in cavity construction; it is regular practise to bed the wall plates in mortar onto the inner leaf. This in turn provides a steady fixing for the rafters, keeping them align. Often though in modern traditional construction the wall plates would be strapped to the inner leaf with galvanised straps. The function of this is also to stop the roof lifting in high winds.

Why is it usual to 'bird-mouth' the rafters over the wall plate?

This is a notch cut at the bottom part of the rafter which rests on the top of the wall. It is usual because without the bird mouth only the tip of the bottom edge of the rafter touching the top of the wall. The bird mouth allows a broader surface to nail the rafter to the top of the wall. The use of this joint reduces the chances of roof spread, they also help align the tops of rafters.

What is the function of a ridge board?

The ridge board runs along the top edge of a rafter of a roof and holds them in place. The ridge board makes it easier to build the roof on site. It helps to distribute wind loading on the brick gable ends into the roof structure; stopping any movement of the brickwork.

How are the ends of purlins usually supported?

The typical way of supporting the purlins is by use of a pair of timber struts situated opposite each other at ninety degrees to the common rafters. It can also be done by a truss rafter or being built into the masonry wall or gable. Though there are issues of safety, the best system is to butt the purlin against the wall instead of embedding it.

Some roofs contain hangers and binders. What are they for and how do they work?

The weight of the ceiling can sometimes cause the ceiling joist to sag; so the hangers and binders are normally required to give additional support to the ceiling joists. The end of the binder is often built into the gable.

Roof Structure (trussed rafters):

Briefly describe the advantages of trussed rafters when compared to more traditional forms of roof construction.

There are many advantages of using trussed rafters when compared to traditional roofing methods. It is cheaper and is a lot faster to construct, this is largely because of the pre-fabricated components; this in turn saves space on site since there is no need for timber storage. Furthermore reduces site waste and loss of materials. Trussed rafters use around 40% less timber than traditionally built roofs.

There are also other advantages such as skilled labour is not needed, unlike more complicated methods. Finally trussed roofs do not require internal support from load bearing partitions.

What is the purpose of the bracing in trussed rafter roofs?

Longitudinal and diagonal bracing is essential to produce structural unity between the individual trusses; strapping is necessary at the gable ends. Wind loads can cause racking, toppling of the rafters, diagonal bracing is a necessity to prevent this.

Why should upper floor partitions be fixed after a roof is covered?

The roof should always be covered before the upper floor partitions are fixed, because if they are done before the roof is complete the partitions may well buckle.

Describe three methods of providing adequate ventilation to the eaves of a modern roof.

For normal double-pitch roofs there should be ventilation openings at the eaves level to produce cross ventilation. The ordinary way of providing this ventilation is by the use of specially manufactured soffit boards, plastic ventilating strips, or by keeping a space between the soffit and the wall.

Eaves to eaves ventilation is suggested by the building regulations, but; it is not always that effective compared to eaves to ridge ventilation. This form of ventilation ventilates the whole of the roof void; plus it functions even without wind.

Why and how should modern roofs be strapped to the gable walls?

It is a requirement of the building regulations that the gable walls are tied back to the roof structure at spaces of 2m maximum. The restraining straps are installed to transmit wind loads on the walls into the roof structure; preventing movement of the gable.

Typically this would be achieved by using 20 x 5 mm thick galvanised steel straps fixed to at least 3 trusses with 3.35 x 65mm long corrosion restraint nails. On steep roofs strapping could be necessary at joist level.

Roof Coverings:

Explain the difference between single and double lap pitched roof coverings?

Single lap coverings are where each one of the tiles laps apart of the tile below it. Where as double lap pitched roof coverings means that each tile laps not just the one underneath it; but the one underneath that as well.

Double lap pitched roofing is a more expensive method of covering, plus there is a need for slightly higher skilled labour; adding to cost.

What is the function of underfelt and how should it be fixed?

Underfelt provides insulation and a barrier for any moisture which gets under the roof covering. It should be first nailed to the rafters and then to the wooden battens.

What is a dry verge?

The dry verge is a means of using interlocking covers which fit over the edge of the roof tiles and cover up all the cracked unsightly mortar. Some manufactures produce special dry verge tiles which can be used on plain tile roofs, this is a fairly fast construction method; and is also very durable.

What is an undercloak?

An undercloak is a course of plain tiles used under the first course at the eaves. Shingles are a roof covering with overlapping tiles. The undercloak provides a small overhang and support for the mortar pointing which will fill up the space between undercloak and the tiles.

Services:

Explain the difference between direct and indirect cold water supply

Direct cold water supply is far more frequently used now days and does not require storage. Indirect water supply requires a water cistern at high level. Diagrams below illustrate the differences-

Provide a section through a cold water storage cistern

How should cold water storage cistern's be supported?

By British standard BS4213:2004 - cisterns for domestic use, quotes;

'the base of the cistern should be fully supported over its whole area by a durable, rigid, flat and level platform…' 

The base should be supported by a platform and fixed across the joists, then securely nailed to the joists.

Explain the difference between direct and indirect hot water cylinders

What is a 'Fortic' hot water storage cylinder?

Fortic is a type of central heating boiler, the domestic hot water and central heating water share the same tank, rather than separate ones. Fortic cylinders have as a feature CRT technology; this saves costs of installation etc by eliminating the need for a plumber. Which other storage systems would need.

What is he difference between hard and soft water?

Water hardness relates to the amount of dissolved salts. Hard water is alkali, causing a high deposition of calcium carbonate in installations; mainly where water is heated. There are two types of hardness permanent and temporary, temporary can be removed by boiling.

Soft waters are acidic and therefore corrosive. This can cause major problems when pipes are made of lead.

What materials are commonly used for both hot and cold water distribution within a house?

Most commonly used materials in both hot and cold water distribution within a house are; copper, the best and most common used type of pipe in modern day. Also Galvanised steep pipes (used to be galvanised iron). PB (polybutene) and PEX (cross linked polyrthylene). Also Lead was very common traditionally, but not today due to the potential health risks.

Explain the difference between a traditional vented gas fired central heating and hot water boiler and a non-vented combination boiler?

Combination boiler works similarly to system boilers though expansion vessels and safety characteristics are inside the boiler. This system heats the water for washing instantly. It provides a permanent flow of hot water, though lower rate than most other hot water systems.

In vented systems they are open vented, these requires a feed and expansion cistern.

Explain the key controls required for a modern gas fired central heating system

There are many key controls required for modern gas fired heating systems, such as the need for programmable room thermostat with hot water timing, so if the temperature rises over a preferred temperature it will turn off. There is also TRV's on all the radiators except those rooms with a thermostat. Other requirements include cylinder thermostat, boiler interlock and the automatic bypass valve.

Explain the difference between a 'two pipe' and 'single stack' above ground drainage

In 1900 two pipe above ground drainage was the standard form of drainage. The Soli and vent pipe (SVP) is not trapped, but the waste stack is trapped. The traps also fitted to individual appliances.

In Single stack, all sanitary appliances were discharged into the SVP. The SVP is not trapped; it relies on the reliability of individual traps.

What are the advantages and disadvantages of a separate underground drainage system?

In this system the foul water and rainwater flow in separate drainpipes, which in turn are connected to separate sewage systems. There are many advantages and disadvantages of separate underground drainage systems. An advantage is that there is less workload needed from the sewage treatment plant.

Disadvantages are that there is a lot more pipe work involved compared to combined systems. Separate systems are also more expensive.

What is a Back Inlet Gully?

Back inlet gullies are found on buildings with internal downspouts. The downspout is on the inside of the external wall and the gully on the outside.

Why is access to underground drainage so important and when might this be required?

Building regulations require frequent access fittings; via man holes, inspection chambers and rodding eyes. Access to underground drainage is very important as if there is not sufficient drainage, there can be serious flooding when there is a heavy downpour.

What is the difference between a radial circuit and a ring main?

Firstly the CPC and neutral go back to the consumer board with the ring main, this is not true with the radial.

Also the ring main uses a particular sized cable in order to carry more current than the radial circuit. This is done because of the savings in copper and cost. Finally the ring main has near to no voltage drop; it is used usually in buildings bigger than 100 metres squared.

What does the consumer unit do?

The consumer unit, or fuse box; contains the mains switch which if switched can isolate all of the circuits in the house. It also contains the fuses and sometimes circuit breakers. Within the consumer unit individual circuits providing the lights and power are provided. Often fuses trip within households, fixing it would only take a flick of the switch to return the power.

Explain the difference between an MCB and an RCD

Miniature circuit breaker (MCB) and residual current devises (RCD) are similar in there responses. The MCB's switch turns off when a fault occurs, isolating the circuit. An example is if inside a house hold there are to many appliances on at one time. When the fault is recognised the switch is turned back on.

The RCD's job is to prevent dangers such as electric shocks, by switching off the flow of electricity when an imbalance is detected between live conductors. RCD's are use on circuits which have socket outlets such as a hover or powered garden tools.

A big difference between the two is that, if the entire system was protected by an RCD then all the power will switch off; including the lighting. This could not be done as no lighting would not be particle and dangerous itself. MCB's on the other hand does not switch the light off.

What is bonding and why is it so important?

If any electrical leakages occur in the bathroom the leaking current will try and find a passage to earth. This current will travel down anything damp, metal or any other conductors; a damp human for example. It is therefore extremely important that safety measures are taken; otherwise it can be fatal. This is the bonding.

All metal objects such as copper pipe work may come into contact with an electrical current, so bonding using earth clamps should insure the copper is connected firmly to the main earth terminal.

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