Topic
Understand foundation design and construction
Today’s Objective
1. Identify different types of site surveys
2. Understand the purpose of site investigation
Unit 6: Building Technology in Construction
Site Investigation
Three stages:1)A desk study.
2)A walk-over survey
3)A physical exploration and inspection, of the ground by means of
boreholes or trial pits. This third stage is sometimes called the ground
investigation.
Unit 6: Building Technology in Construction
Site Investigation: Desk Study
Stage 1
• The desk study is the first stage
in the site investigation.
• Essentially, it comprises the
collection and analysis of
existing information (records)
about the site.
• The information will come from a
variety of sources and, and,
once analysed, will form the
basis for the second stage, the
walk-over survey.
Unit 6: Building Technology in Construction
Desk Study
The desk study has two main objectives:
1) to determine the nature, past use, and condition of the site.
2) to determine whether this has any implications for the proposed
building and its foundations
It quickly ascertains whether the proposed structure can be built on the
site
In groups of 2/3 consider what sort of records would you be looking for?
Unit 6: Building Technology in Construction
Desk Study
Sources of Information
•British Geological Survey
•Ordnance Survey
•Landmark Information Group
•The Environment Agency
•The Local Authority
•County Records
•Utility Companies and the
Coal Authority
•Local History Library
•The site vendor
•Aerial Photographs
Unit 6: Building Technology in Construction
Desk Study
Outcome
1. A check list for
closer investigation
during the walk-over
survey
2. Identification of
potential hazards
that may require
specialist input
Unit 6: Building Technology in Construction
Site Investigation: Walk-over survey
This is the second stage in the site investigation. It's a
detailed site inspection which:
1. Enables much of the material discovered in the desk study to
be confirmed or further investigated
2. Identifies other potential hazards
3. Enables the surveyor to collect photographic records
4. Gives the surveyor/engineer the opportunity to create a site
survey; a detailed drawings of all those items (trees, existing
buildings, watercourses, topography, etc) which will have
implications for the building design
5. Obtains any additional information required to ensure that the
building can be constructed safely and within budget.
Unit 6: Building Technology in Construction
Walk-over survey
Unit 6: Building Technology in Construction
Site Investigation: Walk-over survey
The NHBC recommend that this stage is divided into 6
sections:
1.
2.
3.
4.
5.
6.
Topography
Soils and rocks
Surface water and ground water
Vegetation
Structural information
Local information
Unit 6: Building Technology in Construction
Site Investigation: Walk-over survey
Topography
Unit 6: Building Technology in Construction
Site Investigation: Walk-over survey
Soils and Rocks
Unit 6: Building Technology in Construction
Site Investigation: Walk-over survey
Surface Water and Ground Water
Site Investigation - Stage 2
Walk-over Survey-
Water
Unit 6: Building Technology in Construction
Site Investigation: Walk-over survey
Vegetation
Unit 6: Building Technology in Construction
Site Investigation: Walk-over survey
Structural and local information
Unit 6: Building Technology in Construction
Topic
Understand foundation design and construction
Today’s Objective
1. Understand the purpose of ground investigation
Unit 6: Building Technology in Construction
Site Investigation: Ground Investigation
A direct ground investigation is the third stage in the site investigation. It
provides detailed information on:• The nature and thickness of made up ground/top soil above the
subsoil.
• The nature, thickness and stratum depth of subsoil
• An assessment of allowable bearing pressure.
• Groundwater levels, chemicals in the ground etc.
• Existing structures or hazards in the ground.
Key term: Geotechnical properties
How soil is likely to perform when imposing loads on it or what will happen when
water is removed to allow work to take place.
Unit 6: Building Technology in Construction
Site Investigation: Ground Investigation
Ground investigations should provide the following:• Classification of soils
• Soil profile
• Soil parameters
Key term: Soil Parameters
How the soil will react to the building works and imposing loads.
The soil can be expected to carry a certain weight depending on its parameters
and its characteristics.
Unit 6: Building Technology in Construction
Site Investigation: Ground Investigation
Classification of soils
Soil is classified by:
1. Geological origin
2. Physical properties
3. Chemical composition
4. Particle size
The simplest classification used by
geologists is:• Rock
• Granular soils ( sands and gravel)
• Cohesive soils (clays)
• Organic soils ( peat and top soil)
• Fill or made up ground.
Unit 6: Building Technology in Construction
Site Investigation: Ground Investigation
Soil profile:• Identification of the layers of
soil below ground level,
depth and type.
• Boundaries between the
various layers/types are not
always distinct, which
means it is sometimes
difficult to tell what condition
will actually be found when
digging into the earth
Unit 6: Building Technology in Construction
Site Investigation: Subsoil survey
Soil profile:• Complete the worksheet
Unit 6: Building Technology in Construction
Site Investigation: Ground Investigation
Soil Parameters:The load imposed by a
building and the ability of
the ground to support this
needs to be quantified.
Eg. Approved Documents;
Part A, Table 10
Analysis of soil type and
loadings, giving
recommended foundation
widths.
Unit 6: Building Technology in Construction
Site Investigation: Ground Investigation
• Plan the direct methods required for site investigation. Identify the depths
of investigations required at different locations around the site.
• Boreholes should penetrate completely through made up ground or in fill.
• Identify suitable in-situ and lab testing methods.
• Decide on the number and location of bore holes, frequency of testing.
• Plan and Make allowances for any unforeseen ground and groundwater
conditions on site.
Key term: Borehole
Holes sunk into the ground to extract soil samples at differing levels.
The information is recorded as the holes are drilled so that the design engineer
discovers at what depth each soil is found.
Key term: Made Ground
Any ground that has been artificially made from previous works, e.g. layers of
stone compacted and laid to form a level surface ready for construction work.
Unit 6: Building Technology in Construction
Site Investigation: Ground Investigation
Stage 4: Keep Records
Unit 6: Building Technology in Construction
Site Investigation: Ground Investigation
Techniques: Trial pits (Max 5m deep)
Trial pits are particularly useful in the
investigation of sites intended for low – rise
construction
Records need to be kept as a ‘log’ with the
profile recorded as the hole is dug.
Trial pits are relatively cheap but care must
be taken with exposed excavations, and
some may need shoring with timber or
hydraulic earth supports. These also need
to be well identified as open excavation
can also be dangerous to any person on
site.
Unit 6: Building Technology in Construction
Site Investigation: Ground Investigation
Unit 6: Building Technology in Construction
Site Investigation: Ground Investigation
Trial pits clip
Unit 6: Building Technology in Construction
Site Investigation: Ground Investigation
Trial holes
List the advantages and disadvantages
Unit 6: Building Technology in Construction
Site Investigation: Ground Investigation
Techniques: Boreholes (105-200mm)
Light percussion equipment is used to
drive a hollow tube in to the soil
In clay soils a ‘clay cutter’ is used
In granular soils a ‘flap valve’ is used
The main advantage of light percussion
drilling is its ability to make deeper
holes in a wide range of ground
conditions.
This method is considerably more
expensive than shallow trial pits and
auger holes.
Unit 6: Building Technology in Construction
Site Investigation: Ground Investigation
Bore hole clip
Unit 6: Building Technology in Construction
Site Investigation: Ground Investigation
Boreholes
List the advantages and disadvantages
Unit 6: Building Technology in Construction
Site Investigation: Ground Investigation
Exploratory holes: How deep?
Depth depends on ‘stress distribution’ under the foundation.
Boreholes should penetrate all deposits unsuitable for foundation purposes
such as unconsolidated fill or materials that need compacting e.g. peat,
organic silts, and very soft compressible clays.
Depth requirements should be reconsidered when the results of the first tests
have become available, as it may be possible to reduce the depth of the
subsequent borings.
Key term: Stress Distribution
How the foundation distributes the load of a building. A very wide, flat foundation
will support more load than a narrow thin strip.
Unit 6: Building Technology in Construction
Site Investigation: Ground Investigation
Exploratory holes: How many?
The maximum number of boreholes will depend on the complexity of the local
geology and construction project.
The number can change as more information becomes available from earlier
investigations.
Exploratory holes: Where?
The location of boreholes is dependent on the nature of the site and proposed
construction. Additional boreholes are drilled at problem areas and near the
site of the proposed structure.
Where practicable boreholes should be located along ‘grid lines’ at regular
intervals to enable section drawing to be produced across indicated planes.
Unit 6: Building Technology in Construction
Site Investigation: Radon Gas
• Radon is a naturally radioactive gas and
is found in many areas within the UK.
• It has no taste, colour or smell, it can
gather in concentrated levels below
construction projects.
• Building regulations require that
buildings and extensions constructed
after 2000 in RADON affected areas
have protective measurers installed
during construction.
• To prevent the build up of radon gas,
barriers are built into the structure. This
barrier must cover the total area of the
foundation footprint of the structure.
Unit 6: Building Technology in Construction
Site Investigation: Groundwater Conditions
Ground water is found below
the ground in the spaces and
cracks between soil, sand
and rock.
The level of water in the
ground is known as the water
table.
Unit 6: Building Technology in Construction
Site Investigation: Groundwater Conditions
Why is it important to know the water table
level?
• Any design and operations will be affected by
the presence of water.
• Digging into the ground can cause ground water
to fill some trenches and excavations with water.
• Construction companies have to plan how to
remove or deal with any water during
construction; ground water conditions are
significant because they can affect construction
in a number of ways.
Unit 6: Building Technology in Construction
Site Investigation: Groundwater Conditions
What is the impact on construction of a high water table?
• Extra costs – from dewatering techniques, increased trench
support and ground stabilisation requirements all making
construction more difficult, longer and increasing costs.
• The presence of chemicals in any water such as acids can lead
to damage of foundations and other materials used below ground
level.
• A high ground water table implies that pore water pressure in the
soil is high (gaps between soils), which usually means that the
soil is weaker. As well as influencing foundations, high pore water
pressure will and can affect the stability of slopes (landslides?)
and the pressure on retaining structures.
Unit 6: Building Technology in Construction
Site Investigation: Groundwater Conditions
What is the significance of this information on the building
contractor?
•
•
•
•
•
•
•
Viability of site
Cost of ground works
Type of foundations
Amount of labour required
Skill of labour required
Requirement of engineered foundations
Any contamination – remediation?
Unit 6: Building Technology in Construction
Site Investigation
Remember!
The cost of a typical site investigation is
around 0.5 – 1% of the capital cost of a
project.
It is considered good practice to
investigate adequately at this stage.
Key term: Capital Cost
The total cost of all the equipment and necessary expense to complete the works.
Unit 6: Building Technology in Construction