05-12-2012, 06:22 PM
EXCAVATION and BASEMENT CONSTRUCTION
1EXCAVATION and BASEMENT.pdf (Size: 578.47 KB / Downloads: 642)
Introduction
In general, excavation means to loosen and .take out materials leaving space above or below
ground. Sometimes in civil engineering term earthwork is used which include backfilling
with new or original materials to voids, spreading and levelling over an area. British Standard
CP6031 gives standards and recommendation to earthworks covering embarkment and
cuttings, levelling and compacting, and the use of earthmoving plants etc.
Brief description of plants
1. Face shovel excavators – This can be of cable or hydraulic operated, mounted on
wheel or track .They are fitted with μ±bucket which faces away from the machine. They are
used for loosening, excavating vertical or near-vertical soil above the machine base level.
They are not suitable for horizontal or below ground excavation.
2. Backactors (Backhoe) – They are used for below ground level excavation. The bucket
acts downwards and drag towards the machine and tilted upwards to hold the loads. They are
used mainly as trench or large scale open excavation, but sometimes they are also used as
loading machines.
Government Regulation in controlling excavation
Excavation work to8 certain extent is dangerous so government has regulation control over
works where large scale excavation is involved. Some of these regulations are in:
1. Building (Administration) Regulations
2. Building (Construction) Regulation
3. Relevant Practice Note for Authorized persons and Registered Engineers etc.
These requirements apply to excavation:
1. Deeper than 4.5m and exceed 5m in length (4.5m up from base)
2. Liable to affect any road, building, slope steeper than 30o or water main bigger than
75mm in diameter
3. Supporting proposal to be submitted and obtain consent before starting of excavation.
EXCAVATION
Excavation in most situations nowadays is done by mechanical means. However, the exact
method to be adopted still depends upon a number of factors:
1. Nature of subsoil – affect type of machine used and the necessity of soil protection.
2. Size of excavation – affect type of machine used and method to excavate.
3. Scale of work – large volume of excavation may involve complicated phasing
arrangement and work planning
4. Ground water condition – affect degree of protection (watertight sheet piling or
dewatering may required.)
5. Surrounding condition – impose certain restrictions and precautions (eg. diversion of
a government drain, or underpinning work to the nearby building foundation)
Using cofferdams
A cofferdam may be defined as a temporary box structure constructed in earth or water to
exclude soil or water from a construction area, such as for foundation or basement works.
Use of cofferdam suitable for excavation of larger scale can be of :
a) Sheet pile cofferdam – Also known as
single skin cofferdam. Interlocking type steel
sheet pile is used and can use for excavation up
to 15m. Sheet pile in this case acts as a
cantilever member to support the soil therefore
adequate depth of pi le or suitable toe treatment
may be required. In addition, cofferdams are
need to be braced and strutted or anchored using
tie rods or ground anchors.
Sheet Steel Piling
Steel, amongst other materials such as timber, is most effective to be used as sheet pile due to
its high tensile as well as their interlocking ability. It can be used as timbering to excavation
in soft and/or waterlogged soils especially in congested site where there is no enough space
for complicated shoring.
Steel sheet pi le can be of numerous shapes, thickness and sizes. Most of them can be watertighted
and for some heavy sections they can be driven down to 15m depth .To erect and
install a series of sheet piles and keep them vertical in all directions a guide frame may be
required. The piles are lifted by a crane, using the lifting holes near the top of each pile, and
positioning them between the guide walings of the guide. Powered hammer (fitted with a grip
to the pile) which are hanged by the crane is usually use to drive the pile. Sometimes
hydraulic hammer can be used to reduce noise.
Grouting
Grouting is often use to stop the penetration of water in sub-soil with high permeability, such
as in fissured and jointed rock strata. Row/s of holes are bored on the soil and, usually
cement grout, are injected under high pressure. The cement grout will penetrate into the voids
of the sub-soil and form somewhat an impermeable curtain vertically separating the ground
water.
Cement grout is usually a mixture of cement and water, or cement and sand under ratio
maximum 1:4. Sometimes chemical grout can be used to form a gel which can increase
strength and reduce permeability of soil. (eg. Sodium silicate + calcium chloride = calcium
silicate, which is a silica gel)
Waterproofing the basement
A water-tighted basement wall is an essential element to waterproof a basement. |However,
due to the basement walls are often constructed under complicated phases to match with the
excavation sequences and this may increase the possibility of leaking, therefore, careful
construction joining design is essential to ensure the basement structure is perfectly waterproved.
Very often the providing of water stops into these joints is helpful. However, the
most widely used method to water-proof a basement is to provide a cavity to the wall of the
basement (by building a skin wall to the sides). The ground water leaks into the basement can
then be collected through concealed channel to a sump pit and remove by pumps.