Unit 3

SUBSTRUCTURE DESIGN

Floors, walls and basements

Materials and durability

• Substructure construction is subjected to a less than ideal environment

• permanently wet, • yet needs to remain structurally sound• special care must be taken with density and porosity of

materials

• Substructure – wall, are usually, built in quality brick or dense block– Concrete work is generally dense and waterproofing

barriers

Damp proof courses and membranes

Those parts of the structure of a building in contact with the ground shall:

a) impermeable to moisture and so positioned as to prevent the passage of ground moisture.

OR

a) Be of such material and so constructed that ground moisture cannot penetrate to the inner surface of the building or to any part of the building that would be harmfully affected thereby.

The main forms of barrierThe two main forms of barrier are:

– Damp proof course (DPC)– Damp proof membrane (DPM)

Damp proof course (DPC)• provide a barrier to the passage of moisture from the ground to

levels in walls where damage can occur.

• Generally DPCs are placed in walls a minimum of 15Omm above ground level

• always be below the lowest element in the structure that can be damaged by moisture e.g. the underside of floor joists.

• used at any point in a wall where moisture is likely to enter the internal fabric of the building, e.g. around windows and doors.

• DPCs must be continuous,

• Materials used for DPCs should comply with BS 743, • bitumen, polythene and mastic asphalt.

Damp proof membrane (DPM)

• they are continuous sheets used under floors resting on the ground.

• Materials used are similar to DPCs • continuity is maintained over all the area of

the DPM.• Care where there are joints between

sheets, and where the DPC and DPM meet.

• services should not penetrate the DPM, but where this is essential special care must be taken in designing the detail.

Typical section

Penetration of the floor slab

Ground floor slabs

• The main component parts of a ground floor earth supported slab are:

• the sub- base, (sub-grade)

• the hardcore, (Sub-base)

• the blinding,• the DPM• the concrete bed

Sub-base

• Most ground will be uneven and it is unlikely that it will be at the correct level for the subsequent floor slab.

• Usually ground is covered with a top layer of organic soil (top soil) which is unsuitable for building on.

• initial work in is to remove the top soil• Reduce the ground to a flat level surface at the

correct height.

Hardcore

• Fill placed over the surface of the sub-base.• The purpose of this hardcore is to form a level and firm

base unto which the concrete slab can be placed. • It also acts against capillary action• Hardcore is normally laid in layers 100-150mm thick and

carefully compacted. • Materials used for hardcore should be:

– well graded – free from aggressive or organic substances.– Typical materials used are gravel, crushed rock, brick and

concrete rubble. • The last two layer must be carefully chosen.

Blinding

• Blinding is a layer of fine ash or sand placed on top of the hardcore

• Provide a crack free level surface unto which the DPM or concrete slab can be placed.

• Prevents fine particles in the concrete seeping into the hardcore and weakening the finished slab. (If DPM is not placed)

• prevents the DPM being pierced by sharp edges in the hardcore.

Retaining structures

• The basic function of a retaining wall is to retain soil held behind it.

• failure occur due to:• sliding,• overturning • overstressing of the wall itself.

Design Of retaining wall

• The toe is designed to prevent the wall slipping forward

• the weight of soil resting on the heel prevents the wall from overturning

• the wall buckling along its length can be overcome by either thickening the wall itself or by the use of counterforts (buttress)

Basements....Things that need to be considered.

• Can the basement floor transfer the loads from columns above safely to the soil below by itself, OR is there a need for additional foundation systems beneath some or all of the columns that come down to the level of the basement floor?

• Similarly, can the basement walls transfer the loads safely to the soil by themselves or will additional foundations be necessary?

• What method will be adopted for basement construction in view of the given soil conditions?

• What method will you use to prevent water ingress to the basement.

BASEMENTS

• Purpose of Basement– land is expensive,– limitations at to height of a building– Utilizing space below ground for services, storage and

car parking

• basement of a building can also act as a foundation, supporting all or part of the building above.

• With this type of basement the walls are designed to carry vertical loads.

Basements

waterproofing requirement

• little waterproofing is required for car park

• high quality of waterproofing is required for to store rooms or plant rooms .

• waterproofing is normally achieved through the use of asphalt tanking

• Tanking is a continuous waterproofing element that ‘wraps’ the building and eliminates the transmission of water from the wet ground to the dry building.

BASEMENT PERFORMANCE LEVELS

BS 8102:1990 (Code of Practice for Protection of Structures againstWater from the Ground) defines performance levels for the dryness of buildingsin four grades, as follows:

that the dryness level required for the Usage Grades could be summarized as follows

Tanking

Installing external tanking

• Basement volume is excavated

• Sub-base of concrete laid

• Horizontal coats of asphalt laid

• Concrete basement slab laid• Concrete basement walls

constructed

• Vertical coats of asphalt applied

• External built

• Excavation back filled

Integrity of tanking

• Waterproofing must be continuous and unbroken

• All items that penetrate the tanking is carefully designed ( e.g. services pass through basement walls)

Construction of Basement wall

1. Open excavation

2. Temporary earthwork support

3. Permanent earthwork support

Basements- open excavation

Temporary earthwork

Temporary earthwork support

1. Free standing, cantilevered steel sheet piles

2. Propped steel sheet piles

3. Anchored steel sheet piles

Free standing, cantilevered steel sheet pilesfor temporary support

Propped steel sheet piles for temporary support

Anchored steel sheet pilesfor temporary support

Ground water

Permanent earthwork

Permanent earthworksupport using basement walls.

Permanent earthworksupport using basement walls.

Permanent earthworksupport using basement walls.

Construction of Basement Permanent wall

Type of basement wall

Two main ways of constructing such walls are:

1. Diaphragm walls

2. Contiguous or Secant pilling

Diaphragm walls

Diaphragm walls

• Commonly used on clay and gravel sites

• Advantages:1. Free from vibration and excessive noise

2. Minimum disturbance to adjacent area

3. No temporary sheeting in needed

4. Wall are substantially watertight

Basement top-down construction using diaphragm walls

Excavation

Diaphragm wall reinforcement & concreting

The finished wall after excavation

Contiguous or Secant pilling

Contiguous pilling

• This technique is suitable to retain stiff and cohesive subsoils

• Commonly used in dry soil where ground water levels are below the eventual depth of excavation

• Bored piles are installed as close together as possible to form a permanent wall

• It is NOT a water tightness system

• More effort are required to provide an acceptable face finish to the walls

Contiguous pilling

Contiguous pilling

Secant pilling

• A series of adjacent piles

• There are two methods• Libore secant piling

( heavy duty secant method)

• Stent wall secant piling

• In both method:– Adjacent piles cut into each other

– Claimed to provide a water tight

– Libore secant is more expansive than other method.

Libore secant piling

• Only used for major engineering and heavy section and obstruction exist below ground

• More expensive than other method• Provide water tightness and

strength in difficult boring condition

Libore secant piling

Stent Secant Piling

Additional reading for various methods of waterproofing