Lesson Note On Foundation Construction

Foundation Construction

1 Site exploration and preparation

Present economics have imposed a high degree of
mechanization on IS 4991 foundation construction methods.  This demands an
efficient system of temporary 

roads and good site drainage to maintain a high rate
of work in all weathers.  Site preparation must include tracing and clear
marking of all underground services, 

power, telephones, gas, water and sewers. 
Accidental cutting of electric cables and gas mains can kill people, as well as
result in claims for damages if supply is

cut off from e.g. a factory.  Repairing damaged
telephone trunk lines can cost thousands of rupees.  Burst water mains
will stop potable water supply to the towns

/ villages and can cause flooding and collapse of
incomplete excavations with disastrous results.  Collaborate with the
Supply Authorities on safety measures 

whenever the site is approached closely or crossed by
overhead power cables or underground services.

2 Dilapidation and condition survey of adjacent properties 

Jointly with the owners, occupiers and their
professional advisers if appointed, record and agree the condition of their
properties before our operations start.  This 

record will be invaluable in the event of any
subsequent damage claims and can provide information to protect.  Once
fixed it will enable all movements to be

monitored.  This information assists in agreeing
the causes of movement.

Record in detail the decorative and structural condition. Photograph all
significant points.  Fix tell-tales across cracks and arrange to both
parties to record

readings at appropriate intervals during the progress
of the works.

Proprietary graduated Perspex tell-tales are available. Corner, floor and
displacement types are available.  They can be read to 0.25 mm by
eye.  For more 

sensitive reading or less noticeable tell-tales,
small discs set in each side of a crack can be monitored using vernier gauges.

Extensive and continuous monitoring can use
electronic transducers, recording meters and printers providing automatic
records.  This system is useful when 

regular access to locations requiring monitoring is
difficult or expensive.

Where necessary establish and agree levelling points and record in a similar
manner.

Surveys also include condition of roads, paving
etc.  This is particularly important where deep excavation, blasting, or
dynamic compaction or piling works are 

involved.

Vibration monitoring, where appropriate can also be undertaken by specialists.

3 Foundation depths 

If the foundations are designed by in-house or
outside Structural Consultant, they must be insisted that the drawings shall
show the Site Investigation Report

reference and state the allowable net ground bearing
pressure used for the design of foundations, the depth at which foundations are
to be placed and the soils

expected at this level.  Concrete mix details
will take into account any aggressive chemical conditions discovered by the
Site Investigation.

No variation must be made to the specified depths of foundations unless first
agreed with the Client’s Engineer / Project-in-charge / Architect as
appropriate.

In case of doubt, or if ground conditions differ from
those expected, notify the Client’s Engineer / Project-in-charge / Architect
and obtain his instructions in writing.

If there is any likelihood of adjacent foundations being at all affected, site
management must obtain written instructions from the Client’s Engineer /
Project-in-

charge / Architect before proceeding with the work.

4 Safety in excavations IS 3764

It is necessary to properly support the sides of all
excavations to ensure stability and safety in all weather and ground water
conditions.

See Section 4 (Earthworks and excavation) of this manual and the Company Safety
Instructions (if available) or refer the matter to experts on Site Safety.

See also the legal requirements relating to confined spaces and toxic
atmospheres:  The Construction (General Provisions) Regulations
1961.  Any underground

space is a potential death trap due to possible
poisonous, asphyxiating and/or explosive atmospheres.  Before entering
such spaces test the air, and assistance

and breathing apparatus must be immediately to hand.

REMEMBER: BY LAW, ALL EXCAVATIONS DEEPER THAN 1.2 m MUST HAVE  SIDE
SUPPORT OR EXCAVATED TO A SAFE BATTER.

5 Excavation for foundations and preparation of formations

Any soft spots encountered at formation level are
normally replaced with lean mix concrete.  See the specified requirements
in the contract.

Clays are highly susceptible to softening when in contact with water. 
Subject to the approvals referred to in 4.8, clay formations should be
protected with blinding

concrete or with the foundation concrete as soon as
possible after completion of the excavation.  Formations in granular soils
will usually have been loosened by

the excavation process and should be compacted using
suitable vibratory plant prior to placing blinding concrete.

Surfaces of rock are required to be sound, completely exposed, and generally
normal to the direction of load and of a capacity required in the design. 
Soft rock

surfaces (eg chalk) are swept clean of loose debris
then blinded with 50 mm of 10 N/sq. mm concrete to prevent softening by
rain.  Hard rock formations are

preferably cleaned with water or air-water jet,
followed by an air jet to remove excess water.  Remove any standing pools
of water.  Keying of footings into the rock

may be required.  Study the specification and
drawings thoroughly.

If blasting is used, this must be carried out by blasting specialists who must
control charges to avoid damage to other foundations or finished work.

6 Avoidance of surcharge on adjacent excavations etc.

It is essential that foundations do not surcharge
existing drains, trenches, retaining walls and other excavations.  To
achieve this, take undersides of new
foundations down so that a line, at 45 degrees from
the nearest bottom edge of the new foundation, passes under the drainage,
trenches etc.  And conversely to

prevent new excavations from undermining adjacent
foundations, excavations must not be taken down below the 45 degrees line.

On similar lines, care shall be taken to avoid any surcharge between the
adjoining footings within a new construction, and any such condition in the
substructure

shall be discussed and a confirmation on the adequacy
obtained in writing from the consultant, Architect and Engineer in
charge.

 7 Records of obstructions

Ensure that all obstructions are recorded
precisely.  Records must show nature, location, depth and dimensions of
obstructions, and be similar to but separate

from the record of foundations.  The Engineer
must be promptly told about such obstructions so that any significance these
may have on design may be

investigated.

8     Bottom of excavations
(formation level)

Bottoms of excavations (formation level) are to be inspected and passed by the
Client’s Engineer / Project-in-charge / Architect / Statutory Authority as
required by 

the Contract.  Immediately after approval all
formation levels other than hard rock are to be protected normally by blinding
with concrete for protection against the 

weather and to provide a firm working surface for
subsequent steelfixing operations if needed.

9     Foundation in shrinkable
clay

The specified formation level will normally be below
the zone of cyclical shrinkage and swelling due to weather changes, and taking
into account the significant 

effects of trees and hedges.

Site survey drawings normally indicate all past, (if stumps are visible) and
existing trees and hedgerows.  Account of these will have been taken in
the design of 

foundations.

Trees, stumps and root systems of dead trees can occur outside the site
boundary, but close enough to affect new foundations. If old stumps or root
systems are 

discovered or if there are any doubts about the
possible effects of trees and hedgerows over the site boundary, contact the
Client’s Engineer / Project-in-charge / 

Architect  before work proceeds.

For detailed information on heights and limiting
distances for trees see BRE Digest No. 298 or any similar publication.

Even small ornamental and fruit trees can make clay
movement problems worse when grown close to shallow strip foundations.

Protect formation surfaces in clay from the softening effects of rain and
drying shrinkage until inspected, then immediately blind with 50 mm of 10N/sq
mm 

concrete, or otherwise cover in accordance with the
specified requirements.  Remove any soil inadvertently softened before
blinding. 

DELAY IN CONCRETING A STRIP FOUNDATION MAY CAUSE PROBLEMS DUE TO SHRINKAGE OR
SWELLING , PARTICULARLY IN CLAY as given below:

(a)    Sun and wind will drive the moisture out and shrink the
exposed clay.  Once placed, the foundation may suffer heavy movement later
when the clay takes up 

moisture and swells.

(b)   Similarly, a foundation placed on a wet, swollen clay base will
settle later as the clay compresses under the wall load.

10 Concreting
foundations 

Before placing concrete ensure that formation
surfaces are clean and trimmed and that any side formwork is securely strutted
with all gaps at joints between

shutter panels packed to prevent grout leakage. 
If this occurs and the concrete contains reinforcement, make good the
honeycombed concrete before backfilling

to prevent ingress of ground water to the
reinforcement.

Struts bearing against the earth sides of excavations must bear on adequate
spreader plates or timbers.

The materials, the mix, the water content and the methods of mixing,
transporting, placing and curing the concrete are to fully conform with the
specification.

Surfaces against which concrete is to be placed must be firm and free from
loose material.  If concrete is to be placed upon or against, and or, is
required to bound

with, old concrete surfaces, clean the surface
of  the previously cast concrete of oil, grease, or other foreign matter
and laitance, preferably by wet sand-blasting. 

Ultra high pressure water blasting is also an
appropriate method.  Surfaces sometimes require roughening. 
Roughening for its own sake is not necessary to obtain

bond if a thoroughly clean surface, comparable in
cleanliness to a fresh break, is obtained.  Such a clean joint surface
approaching dryness without free water is

best for bond strength.

Avoid damaging the formation if sand or water blasting methods are used.

Give all reinforcement the specified amount of  cover of concrete. 
See Engineer’s drawings for details. Use adequate spacer blocks and supports.

When concreting foundations, arrange for competent and continuous supervision
of this operations until completion, then check and record the finished
concrete

levels.  Comply with the procedures in your
Company Quality Systems and Procedures or Standing Instructions or Project
Instructions.