BASEMENTS (mainly including waterproofing)

Question 1

What is the official term for a basement?

Answer 1

• The UK Building regulations classify a basement as storey with a floor which at some point is more than
  1200mm below the highest level of the ground adjacent to the outside wall.

Traditionally a basement is either partially or completely below ground level. Whilst traditionally used for (cold) storage they can be used for accommodation (originally servant spaces), car parking, a utility services space, recreation etc.

Question 2

What are basements used for?

Answer 2

• Nowadays Basements are considered to be a useable part of the dwelling i.e. a habitable room whereas Cellars are used for storage etc, other than habitation.
• Basements can increase the floor space without increasing the footprint of the property. They can be
  utilised to take advantage of sloping sites (split level construction) and soil provides enhanced thermal
  insulation.
• On the downside Basements are expensive (extra excavation, disposal of soil, waterproofing materials) and create safety risks (working below ground, confined working space etc). Above all they are difficult to keep dry from external and internal moisture (waterproofing can fail, so liability and insurance costs can create issues).

Question 3

What are the types of Basement?

Answer 3

Refurbishment, New garden basement, New basement under new housing, Retrofit and Deep basements.

Question 4

What is a Refurbishment?

Answer 4

Alterations to existing space below ground.

May require lowering the floor to increase head room; underpinning adjacent walls; improving ventilation and lighting. Typically involves improvements to, or new, waterproofing system; and new finishes and fixtures.

Question 5

What is a New garden basement?

Answer 5

New basement adjacent to existing property, usually in a garden space.

New structure below ground, with planted green roof or terrace at garden level.

Access from main house via new external covered staircase.

Question 6

What is a New basement under housing?

Answer 6

Basement space built as part of a new build development.

Arrangements of windows and internal and external access vary. Designed to suit current and future use requirements, site conditions, cost, and constructibility.

Question 7

What is a Retrofit?

Answer 7

New space created through excavation below ground floor of an existing property.

Allows creation of additional space below business or homes, which benefit from staying in the same location. Preservation of existing building possible (e.g. listed building).

Underpinning works required.

Question 8

What are Deep Basements?

Answer 8

Spaces below one storey deep.

Frequently used for car parking, plant services space, and storage below larger residential development and other uses including commercial, retail, or mixed use
schemes in urban areas.

Question 9

How is excavation conducted for the formation of basements?

Answer 9

• Using manual method, say, by wheel barrow.
• Using bucket and lift to ground level by crane.
• Using hoist rack (opening has to be provided in the basement/excavation pit first).
• Using gantry crane (opening has to be provided in the basement/excavation pit first).
• Using conveyor belt.
• Using excavating machine to removal spoil, may be in stepped position in case of very deep pit.
• Using dump truck but access provision has to be provided in advance (such as a temporary ramp
  or the permanent vehicular access into a basement).

Question 10

What are the three basement construction methods?

Answer 10

Open cut construction method, Vertical cut construction method and Top-down construction method.

Question 11

What is the Open cut construction method?

Answer 11

This method is applied for shallow basement which need to be excavated with sloping side.

The form is economical because foundation wall is not necessary in this case. The angle of slope and the area of ground will be distinguished based on design, analysis and evaluation result.

Question 12

What is the Vertical cut construction method?

Answer 12

Bracing excavation method is another name of this method. Unlike Open cut method, vertical cut requires perimeter wall around the area being excavated. On another hand, perimeter wall can replace by secant piling, diaphragm walling and steel piling.

During construction process, temporary braces and props must be a part of system to maintain the walls.

Question 13

What is the Top down construction method?

Answer 13

Construction of this begins from the top to the bottom of excavation and the first slab is completed before starting superstructure construction.

Constructing slab after each stage of excavation, hence, slabs are considered as strut in holding earth pressure.

The process of construction is: retaining wall construction and pile construction under column of superstructure; applying formwork and placing columns on piles for the 1st slab at the top, then,
other slabs would be built after each excavation.

Question 14

What are the advantages/disadvantages of the Open cut construction method?

Answer 14

Advantages:
* Low budget
* Save construction time for shallow basement
* High quality
* No lateral support

Disadvantages:
* Not able to use for deep basement.
* Require large space during construction process
* High risk of collapsing in high moisture content or
flowing water

Question 15

What are the advantages/disadvantages of the Vertical cut construction method?

Answer 15

Advantages:
* Minimize the excavation area
* Keep the side of deep excavation stable
* The effect of the flow groundwater is reduced

Disadvantages:
* Not able to use for large scale of basement
* Require temporary system
* Require supporting perimeter wall
* High budget

Question 16

What are the advantages/disadvantages of the Top down construction method?

Answer 16

Advantages:
Short time for construction
Construction area safer as slabs are
stronger than struts
Low risk of ground movement
Reduce the effect of bad weather

Disadvantages:
High budget
Require complex support construction
Require lighting and ventilation
Restrict the access of material handling.

Question 17

What is the name of the wall that supports the basement?

Answer 17

Diaphragm Walling

In straightforward terminology a diaphragm wall (or slurry wall) is a reinforced concrete retaining wall that is cast in a very deep trench.

The trench is excavated to the required depth in alternate sections of approximately 1.50m to 6.0m wide depending upon the soil type and the soil conditions.

Diaphragm walling refers to the in-situ construction of vertical walls by means of deep trench excavations.

Stability of the excavation is maintained by the use of a drilling fluid, usually a bentonite suspension.

The walls are constructed in discrete panel lengths ranging typically between 2.5m and 7.0m using purpose built grabs or, in appropriate circumstances, milling machines (hydro-mills).

Question 18

How do lattice beams support Diaphragm walls?

Answer 18

1. Use of lattice beams

A series of lattice beams steel trusses or flying shores are installed so that they span between the top of opposite diaphragm walls enabling them to act as
propped cantilevers.

The trusses can be removed after the internal floors have been constructed and receiving all the lateral forces from soil.

Question 19

How do ground anchors support Diaphragm walls?

Answer 19

2. Use of Ground Anchors

Diaphragm walls are exposed by carrying out the excavation in stages and ground anchors are provided to stabilise the walls as the works proceeds.

This method is most effective for basement of very large span or without intermediate floors as lateral support.

Question 20

How is the floor constructed in the top down method?

Answer 20

3. Construct floor slab as support (top-down method)

After the perimeter diaphragm walls have been
constructed, the ground floor slab and beams are cast
providing tip edge lateral support to the walls. An
opening is left in the slab for labours, material or
plant as access to continue excavation to the lower
stages. This is repeated until the required depth is
reached.

Question 21

How is the slab casted for basements in construction?

Answer 21

4. cast the centre basement slab to support struts
   Centre area between the diaphragm walls can be
   excavated leaving an earth berm around the
   perimeter to support the walls whilst the lowest
   basement floor in centre can be constructed.
   Slots to accommodate raking struts acting between
   the wall face and the floor slab are cut into the berm.
   Final excavation and construction of the remaining of
   the basement can take place in stages around the
   raking struts.

Question 22

How many waterproofing methods are there?

Answer 22

Type A (Barrier) system, Type B (Structurally Integral) system and Type C (Drained) system.

Question 23

What are the names of the materials used in waterproofing?

Answer 23

Category 1: Bonded sheet membranes.

Category 2: Cavity drain membranes.

Category 3: Bentonite clay active membranes.

Category 4: Liquid-applied membranes.

Category 5: Mastic asphalt membranes.

Category 6: Cementitious crystallisation active systems.

Category 7: Proprietary cementitious multi-coat renders, and coatings.

Question 24

What foundations are generally used for basements?

Answer 24

• Piles are more commonly used for deep basement construction, rather than domestic situations and come in various forms, but can be useful for
  the creation of retaining walls to facilitate excavation in areas of restricted access or close to site boundaries.
• Secant or fair-faced contiguous piles can effectively become the outer wall of a Type C construction, or be faced with concrete or waterproofed to provide Type B or Type A protection.
• Piles can also be used to support the basement floor slab if required.
• Underpinning