Question

a Describe different type of forms of substructure commonly used for commercial and industrial civil engineering project. You must explain the use of piling or rafts (sheet, contiguous and secant) foundation and deep drainage techniques for use with service ducts in this foundation project.

b Describe how drainage is integrated into substructures.

c Describe how utilities are integrated into the substructures. This should include deep reinforced concrete culverts and plant rooms, service ducting, cable trenching and service ducts. This should build on the evidence for Task a and Task b.

d explain the hazards and risks associated with substructure activities. This should build on the evidence for Taska, b and c for this project.

Answer 1

1.Shallow foundations:-

Shallow foundations are used when the soil has sufficient strength within a short depth below the ground level. They need sufficient plan area to transfer the heavy loads to the base soil. Shallow foundation are usually loaded no more than 6ft below the lowest finised door.

Isolated Footing:-

Isolated footing under individual columns, these can be square, rectangular, or circular.

Wall Footing:- Wall footing is a continuous slab strip along the length of wall.

Combined Footing:- Combined footing support two or more columns. These can be rectangular or trapezoidal plan.

Strap Footing:- Strap footing are similar to combined footing when two isolated footing are combined by a beam with a view to sharing loads of both the column by the footing is known as strap footing and beam is called as strap beam

Mat & Raft Footing:- This is a large continous footing supporting all the columns of the structure. This is used when soil conditions are poor but piles are not used.

2. Deep Foundation:- The shallow foundation need more plan area and shallow foundation may not be economical or even possible when the soil bearing capacity near the surface is too low. In those cases deep foundation are used to transfer loads to a stronger layer of rock strata.

Basement foundation

These are hollow substructures designed to provide working or storage space below ground level. The structural design is governed by their functional requirements rather than from considerations of the most efficient method of resisting external earth and hydrostatic pressures. They are constructed in place in open excavations.

Buoyancy Rafts (Hollow Box Foundations)

Buoyancy rafts are hollow substructures designed to provide a buoyant or semi-buoyant substructure beneath which the net loading on the soil is reduced to the desired low intensity. Buoyancy rafts can be designed to be sunk as caissons, they can also be constructed in place in open excavations.

Caissons Foundations

Caissons are hollow substructures designed to be constructed on or near the surface and then sunk as a single unit to their required level.

Cylinders

Cylinders are small single-cell caissons.

Drilled Shaft foundations

Shaft foundations are constructed within deep excavations supported by lining constructed in place and subsequently filled with concrete or other pre-fabricated load-bearing units. Read more on Drilled Shaft foundations.

Pile foundations

Pile foundations are relatively long and slender members constructed by driving preformed units to the desired founding level, or by driving or drilling-in tubes to the required depth – the tubes being filled with concrete before or during withdrawal or by drilling unlined or wholly or partly lined boreholes which are then filled with concrete.

PART B

Subsoil drainage

Subsoil drainage can be used to improve ground stability, to lower the moisture content of a site, to enhance horticultural properties for landscaping and so on. It can be required to drain the whole site or to protect a particular part.

According to the Approved Document C of the Building Regulations, subsoil drainage should be provided to avoid the passage of ground moisture into a building’s interior or to prevent damage being caused to the building’s fabric.

Subsoil drainage is generally involves the use of pipes that are porous to allow subsoil water to pass through the pipe body, or pipes that are perforated with a series of holes in the lower half to allow subsoil water to rise into the pipe. Both types are generally laid dry jointed in a trench filled with rubble. A pervious membrane is generally placed on top of the rubble as a filter mat, then covered with normal backfill and topped with 150 mm of topsoil.

This type of groundwater control is only feasible up to a depth of 1.5 m, and any further lowering of the water table should be achieved by other methods. For more information, see Groundwater control in urban areas.

When drainage is used to protect a building’s substructure, a cut off drain is generally installed that intercepts the flow of water and diverts it away from the site.

PART D

Substructure Groundworks - Hazards

Substructures are construction activities which take place below ground level. You will learn how substructures are constructed safely, understand what is used to do this and why. Recognise hazards in the process. Sub-structure involves all the construction works below floor level, including the foundations and associated activities. A very important operation at this stage is excavation. This means digging the ground so that foundations can be constructed.

Control of Groundwater

Sub-soil water is the water present below ground. It is also known as groundwater. When designing and constructing sub-structure, by law sub-soil water must not be allowed to enter the building and damage it. There are two different ways of controlling water, one temporary and one permanent. Depending on the site, sub-soil water and surface water might just need to be controlled temporarily during excavation. This is called simple sump pumping , because the water collects in a sump or pool and is then pumped out. Some sites might need permanent control of sub-soil water. This is known as land drainage . There are several methods of land drainage.

Land Drainage

Land drainage involves burying a permeable pipe under gravel for water to enter it. This is then fed into a main drain or culvert for removal to a river. This is essential in areas built upon flood plains and areas with a high water table.

Earthwork Support

This is the support of the sides of excavation. There are different methods of earthwork support depending on the needs of the site and the type of soil you are excavating, including: Trench support Video Steel Trench Sheets Video Can be easily fitted and interlocked. They can also be easily removed and reused. Used extensively in underground constructions such as tunnelling. Can be cut to size in confined spaces.

Hydraulic Trench supports
Trench Cave-In Video Aluminium walling trench Supports/Boxes. A great website to look through. Trench Box construction digger using a trench box

Soil Types

Soil and stability Some soils are more stable than others. The type of soil is one of the factors that determine the chance that an excavation will cave in. There are three basic soil types that you may encounter • Type A – very stable. Clay is an example. • Type B – less stable than type A soil. Crushed rock, silt, and soils that contain an equal mixture of sand and silt are examples. • Type C – less stable than type B soil. Gravel and sand are examples. Soil has other qualities that affect its stability. These include granularity, saturation, cohesiveness, • Granularity refers to the size of the soil grains; the larger the grains, the less stable the soil. • Saturation means how much water soil will absorb. • Cohesiveness means how well soil holds together; clay is a cohesive soil.

Plant machinery close to an excavation
Excavated soil, called spoils, piled too close to the edge of an excavation can cause a cave-in. So can heavy equipment. Keep spoils and heavy equipment at least two feet from the edge; when possible, use vertical shores or shields that extend above the top of the excavation to restrain spoils. Spoils and heavy equipment that exert an excessive load on ground adjacent to the excavation could cause a cave-in. A dangerous spoil next to an excavation.

Safety in confined spaces
Working in confined spaces can be deadly. The common hazards present are: Gas – build up of gasses which lay in a tunnel or trench and are invisible. Water – sudden flooding from rain or a water main leak. Collapse of the excavation. There must be: Full training of the workers in safety procedures. Clear entrance and exits to the confined space Full risk assessment Full Personal Protective Equipment PPE

Overburden Avoiding Services
Overburden is an unwanted top layer of soil that must be stripped away to open access to useful construction materials buried beneath it. Overburden being bulldozed away from the excavation. The overburden must be stored safely so that it will not collapse. Avoiding Services Before excavations take place plans of current piping, drains, gas, electricity and water should be consulted and all contractors made aware of their locations.

1. List six hazard in excavations and explain the possible risks.
2. How are each of these risks best controlled to minimise injury? 3. You are on a site carrying out groundworks. You are told that there is groundwater in the trenches as well as a gas leak in the confined space of the excavation. a) Identify two risks for each of the two hazards mentioned above.

b) For each risk named in question 3a, list one control measure that could be used.
4. Explain how three kinds of earthwork supports work. 5. Explain how soil type can affect its stability in excavations. 6. Explain the dangers of plant and spoils near excavations