(Underpinning)

TOPIC : UNDERPINNING

BUILDING CONSTRUCTION

TECHNOLOGY

In construction, underpinning is the process of

strengthening and stabilizing the foundation of an existing building or other structure. (Wikipedia)

Underpinning is the process of modifying an existing foundation system by extending it to or into subsurface strata that is deeper and more stable than the near surface soil that support the existing foundation system.

DEFINITION

To transfer load carried by an existing foundation

from its present bearing level to a new level at a lower depth.

To replace an existing weak foundation.

To allow the adjacent ground level to be lowered.

To provide vertical support that is not present in the existing design.

To prevent/arrest the settlement of a foundation.

PURPOSES OF UNDERPINNING

Remedy for :

Uneven loading

Unequal existence of subsoil

Cohesive soil settlement

Deterioration of foundation concrete

FUNCTIONS OF UNDERPINNING

Deterioration of foundation concrete

Cohesive soil settlement

TYPES OF UNDERPINNING

TRADITIONAL UNDERPINNING

JACK PILE UNDERPINNING

MINI-PILING & NEEDLE

BEAMS

CANTILEVER NEEDLE BEAMS SINGLE PILE

CANTILEVER NEEDLE BEAMS

RAFT FOUNDATION

PILING

PERMEATION GROUTING

RESIN INJECTION

This is the process of digging out sections of the ground

and foundations external to the building and rebuilding the foundations underneath or backfilling with concrete.

This effectively rebuilds sections of the foundation in the hopes that those sections will be strong enough to shore up the whole building.

It is one of the cheapest methods but can be quite disruptive as mini-digger is at least needed on site.

Space is needed for this type of underpinning and it is only suitable if the load bearing strata can be found a maximum of 1.5m below the existing foundations.

METHODS OF UNDERPINNING

1. TRADITIONAL UNDERPINNING

Similar to traditional underpinning, it required to dig out

sections of the existing foundations and backfill with concrete to reach solid footing.

A hydraulic jack is then inserted between the concrete pile and the existing foundations (or walls) and it expands to breach the space between.

The steel bars and concrete pile are then installed in this space before the hydraulic jack is removed.

2. JACK PILE UNDERPINNING

This method carries all the

same issues as traditional underpinning but the walls must be strong enough to span over the heads of the concrete pile caps which are cast onto the jack pile heads after the hydraulic jacks have been removed.

It is used for depths deeper than 1.5m.

This is where bores are drilled down into the

foundation and floors and a liquid or grout is pressure pumped into the ground.

The grout fills all voids and spaces underground and makes the ground more solid and closer to solid footing which means that the foundations are more solid.

This is a cheaper way to underpin and has been extremely successful, again it is highly useful where the site is difficult to access or has limited space for maneuver.

3. PERMEATION GROUTING

Pit Underpinning.mp4

Underpinning, Residential Division.mp4

Depends on the method used in underpinning process.

The selection of underpinning method are based on the problem of the foundation.

Mass pour method : Excavate sections in sequence to a pre-established depth below the footing and place concrete on each pit.

Screw piles and brackets : Some building might require excavating to great depth or maybe is unfeasible to used a piling rig and the screw piles and brackets method is selected. The screw piles and brackets can be installed by only two man crew by hand or using small equipment such as a mini excavator

MATERIALS USED IN UNDERPINNING

Pile and beam : Using this system requires that a min-pile must

be installed on either side of the affected wall. After the piles have been installed, then brickwork is removed below the wall and reinforced concrete needle beam is used to connect the piles and support the wall. Augered piles or case driven piles can be used with this method of underpinning

Piled raft : It is used when the whole structure need to be underpinned. Piles are placed at determined locations by loading conditions, then pocket below footings are broken, and reinforced needle beam are placed to bear the wall’s load. A ring beam is then built to link all needles and the structure is poured with concrete.

Factors in selecting plants & equipments.

• Scope of work : Big or small area to underpin

• Type of underpinning works

• Method of underpinning works

PLANTS & EQUIPMENTS

EXAMPLES

Pit underpinning and soil nailing to allow excavation for a new hospital addition in Rochester. - Hand excavation

Large scope using mini excavator to back fill.

Flat jack plants : pump, gauge, steel plate

Jacked pile : pump , packing and hydraulic jack

Pynford stools Stool – support the existing building

MEASUREMENT

Section H H.1. GENERAL 1. Given as a section in BQ 2. Location and work extent and particulars of existing structure to be

underpinned - Shown on drawing under clause B.3.1 - Shown on further drawings which shall accompany BQ 3. Information of the ground, given in accordance with D.1 4. Should state: - limit of length in operation - number of sections contractor may be permitted to undertake at one time. 5. Given separately and so described: - If carried out from both sides OR - If carried out from one side only OR - If carried out from within an existing building 6. Curved underpinning shall be so described

SMM2

WORKS H.2 EXCAVATION 1. Item, stating any particulars: temporary supports to work to be underpinned 2. Excavation, accordance with clauses D.9-10, subject to: - excavate prelim, trenches down to the level of the base of existing

foundations -excavate below the level of the base of the existing foundation shall be so described 3. If cutting away projecting foundations, it shall be in metres, stating: - number of courses of footings - dimension of concrete to be removed 4. If prepare the underside of existing work to receive the pinning up of the

new, it shall be in metres, stating: -width of existing work

H.3.DISPOSAL OF WATER

- Given accordance with clauses D.15-16

H.4.EARTHWORK SUPPORT

1. Given in accordance with clauses D.13-D.14 subject to :

So described

- Earthwork support to preliminary trenches - Earthwork support below the level of existing foundations base

H.5.CONCRETE, BRICKWORK AND ASPHALT

-Accordance with the rules of the applicable work sections.

SPECIFICATION

2.0 Material s used : shall comply with Malaysian Standard (MS) and with the approval of S.O

3.0 Classification of concrete mixes

- Concrete mix ≈ 3.1, 3.2

- Concrete Grade ≈ 3.3.2

4.0 Concrete Compressive Strength Tests and Compliance

- All sampling, curing and testing of concrete, fresh or hardened, shall be carried out in accordance with MS 26 and the relevant Parts of BS 1881 ≈ 4.1

5.0 Production of concrete (workmanship)

6.0 Construction of concrete

- In situ /precast

7.0 Steel Reinforcement

8.0 Formwork

- Design and construction (workmanship)

- Classes of formwork ≈ 8.2.2, 8.2.3

CONCRETE

Piling shall fulfil General Requirements as per section B.1 (PWD Specs)

State the type of pile used, all piles shall comply with S.O approval and MS or BS.

Workmanship, and the production of piles shall be state.

Elements that are being stressed for piling are :

- Marking, Handling and storage of piles

- Tolerance in Pile Dimensions

- Length Of piles

- Pitching and Driving of Piles

- Lengthening Piles

PILING

Pile shall be tested in accordance to SECTION B.5, fulfilling the

General requirement of the PILE TESTING :

- Type of test necessary ≈ 1.1

- Safety Precautions ≈ 1.2

Together with the other relevant steps to be taken such as :

- Supervision ≈ 3.0

- Reaction system ≈ 4.0

- Testing equipment ≈ 5.0

ESTIMATING