4.1 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.
4.2 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
Vibration monitoring, where appropriate can also be undertaken by specialists.
4.3 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.
4.4 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.
4.5 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.
4.6 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.
4.7 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
4.8 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.
4.9 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
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.
4.10 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.