Under floor membranes

Polyolefin membranes should be placed beneath all slab-on-the-ground foundations, irrespective of site and ground conditions.

The membranes should have a thickness of not less than 0.25mm and should be placed beneath the slab and beam thickenings so that the bottom surface in entirely laid.

Where justified by appropriate geotechnical conditions, the membrane as in the case of slab-on-the-ground features, may be terminated at the face or edge of the internal beam.

Lapping of membranes at joints should not be less than 200mm.

Penetrations by pipes, plumbing fittings or punctures should be taped with a pressure sensitive adhesive tape.

In the case of floor tabs, the membranes should be turned up around the perimeter of the floor slab by at least the thickness of the slab.

Compaction

 The maximum height of fill beneath floor slabs and slab-on-the ground foundations measured at the lowest point should not exceed 400mm unless certified by the engineer.

Fill should be moistened prior to compaction so that a handful squeezed in the hand is firm but does not show signs of moisture. Fill should be placed in uncompacted layers not exceeding 100mm in respect of hand compaction or 150mm in respect of mechanical compaction.

Each uncompacted layer should be well compacted before additional fill material is added.

Compaction should be such that in excess of 3 blows of a dynamic cone penetometer are required to penetrate 100mm of the fill, provided that fills do not comprise more than 10% gravel size less than 10mm and contain no isolated boulders.

How to fill foundations on slope sites

The site should be cut and filled, or terraced, with engineered fill to receive a slab-on-the-ground foundation or a strip footing. The fill should continue past the edge of the foundation wall or edge beam by at least 1000 mm and have a batter of not more than one horizontal unit to one vertical unit. The fill should then be retained or battered beyond this point by a slope not steeper than two horizontal units to one vertical unit. The interior of the slab as well as the edge beam or strip footing will be set on natural soil or engineered soil.

Or a deepened edge beam or a masonry foundation wall supported on a strip footing set below the natural ground level shall be provided on the interior of the structure to retained controlled or engineered fill. Deepened edge beams and steps in the floor at any change in level of slab-on-the-ground may be accommodated.

What are the causes of defects in housing units?

 As per NHBRC, the causes of defects, excluding misuse and abuse of the consumer which affect strength, stability, durability and serviceability will fall in the following categories:

• Undue ground movement caused by unavoidable situations such as standing water including heave, subsidence, settlement and slip
• Inadequate structure strength, not caused by an abnormal event resulting from under-design/inadequate specification, poor construction, overloading and indirectly applied loads resulting from the deflection, deformation or shortening if interconnected structured elements
• Normal structured movement which has not been catered for, including shrinkage and creep of materials such as concrete, thermal movements, movements due to moisture changes, deflection of structural components and moisture expansion of burnt clay masonry units.
• The use of materials which are unsuitable with the Home Building Manual or inappropriate in the circumstances in which they are being used.
• The effects of chemical agents resulting in the rusting and corrosion of steel and other metals or the deterioration and disintegration of other non-metallic materials such as concrete masonry units, mortar, timber etc
• The effects of vegetation whether direct or indirect.
• Abnormal events such as high winds, floods, snow, hail and lightning, seismic events, fire and explosions
• Failure to undertake necessary maintenance and repairs timeously.

Site Preparation

Top soil containing grass roots should be removed from the area on which unreinforced or reinforced slabs are to rest.  Any loose or disturbed ground should be compacted.

The accuracy of the set up should be achieved by positive control measures; their relative location to site boundaries and adjacent structures should be verified.  Regular checks on the trench widths, trench lengths and the length of diagonals across external corners should be made.

On sloping, foundation trenches for strip footings may be stepped so that the required foundation depth is achieved.

Sites to receive slab-on-the-ground foundations should be leveled should be leveled.  The bases of edge beams should not be sloped more than 1:10.  Steps in slabs in excess of 400mm should only be permissable in approved by the engineer.

Steps in foundations should not be provided within 1.0 m from corners.

Excavations should be deepened locally where necessary to remove soft as well as hard spots.  Where soft spots/isolated boulders do not exceed 500mm in diameter, unreinforced strip foundations may be centrally reinforced with 2 No Y12 bars, extending a distance of not less than 1500mm beyond the face of such soft spots.

Excavation should be prodded with a 10 to 12 mm diameter bar immediately prior to the casting of concrete and uniform penetration should be maintained.  In the event of soft spots the appropriate controlled fill should be introduced.

Excessive foundation excavations should be avoided.

Any fill which edge beams of slab-on-the-ground foundations and strip footings are to be founded will be place under the supervision of an engineer and should be deepened to be founded on in situ material.  The controlled filled should continue past the edge of the foundation at least 1000 mm and be retained and battered beyond this point by a slope not steeper than 1:2 (vertical/horizontal).

Trenches should be kept free of surface water.

Where bottoms of foundations have dried out due to exposure or softened due to rain or ground water, the excavation should be rebottomed prior to concreting.

Trench bottoms should be horizontal or sloped with a gradient not exceeding 1:10

Except where founded on rock, the minimum founding depth below original ground level should be less than:

slab-on-the-ground foundations:         300mm

strip footings:                                     400mm

When a foundation is placed on rock, the bearing area should be cleaned and stepped or doweled as to prevent lateral movement of the foundation.

The role of the on-site project manager

Our project managers are a vital cog in the organization machinery. The project manager will be responsible for general liaison between the architect and owner, preparation of monthly statements for comparison with the budget, final check on monthly subcontractor requisitions, change orders and office correspondence regarding his projects and, of course, available at all times. He may even efficiently service two contracts concurrently with an assistant depending on the size and complexity of the work involved.

The project manager will probably be located in the field office. This would allow for a more unified control of the project and permits closer cooperation between the “inside” and “outside” and when the work is of sufficient proportion the full time service of an “inside man” will be required.

The project manager will be responsible for all activities on the site, starting with demolition, site engineering etc and continuing until the building is completed. Depending on the size of the project he will be aided by an assistant superintendent, one or more field engineers, one or more accountants and a paymaster, foreman, watchmen etc

The more important functions would be to check field engineering, monitoring time schedules, supervising all trades, checking quantity and quality of production, attending meetings with the architect and owner, collaborating with architect’s field representative, establishing sequence and coordination of operations and manning the project. He will also resolve disputes, supervise job meetings, maintain daily job diaries, complete insurance reports, check and approve sub contractors requisitions, order material and equipment and check equipment rentals.

In addition, he should maintain contact with the expenses department, keep a constant check on requirements of plans and specifications, check cost records and check and approve payrolls and guarding against construction accidents.