TECHNIQUE SHEET

Plunge columns are the key to Top-down construction which allows simultaneoussuperstructure construction and basement excavation, significantly improving projectprogrammes.

BASIC TECHNIQUE

Balfour Beatty Ground Engineering (BBGE) can installPlunge Columns into rotary bored piles to allow for a‘top-down’ construction sequence, 'launching'structures before the basement is excavated. Topdown construction can significantly reduce the overallconstruction programme resulting in earlier projectcompletion dates. Plunge columns can either beinstalled with a hydraulic or fixed frame and havebecome popular in the construction industry as theyfacilitate deeper basements and taller structures.

CONSTRUCTION PROCESS

Generally the construction process is as follows; arotary bored pile is excavated to the required depth, areinforcement cage is then installed and then the voidis filled with concrete stopping the required distancebelow ground level. Whilst the concrete is still wet ahydraulic plunging frame is installed within thetemporary casing and then the plunge column isinstalled to structural tolerances. The concrete is thenleft to set in the pile and once this is complete theempty bore above is backfilled. When all the plungecolumns are installed the construction of thestructural frame above ground level can begin.

INSTALLATION OF PLUNGE COLUMNS

TECHNIQUE SHEET

EXPLAINING THE PROCESS…

Stage 2Insert plunge column into frame usinglifting head or follower if required.

Stage 4 Add lowering frame if required and plungeinto wet concrete.

Stage 5Trap off at correct level and complete a finalcheck on position. Leave over night toallow concrete to set. Next day removeframe and brace and fill annulus with peagravel, to support column.

Stage 1Once the pile is dug to toe level andconcrete is placed, install plunge columnframe into casing and brace off. Accuratelylevel and plumb the frame using lasers.

Stage 3Trap off plunge column and remove liftinghead. Check column for level andverticality.

TECHNIQUE SHEET

THE BENEFITS

Plunge columns have a number of advantages infoundation construction, some of which areoutlined below:

ü Frame accommodates a wide range of columnsizes from 350mm to 650mm square.

ü Frame is compatible with a range of pilediameters from 1500mm to 2500mm.

ü Segmental plunge frame allows for a widerange of depths to cut off level.

ü Very high levels accuracy for both position (+/-10mm in plan and level) and verticality (1:400).

ü Frame can be centred independently of thepile position allowing the difference betweenthe pile tolerance and the column to beneutralised. It is also possible to accommodateplunge columns designed to be located off thepile centre, subject to pile diameter.

HYDRAULIC VS. FIXED FRAME

Plunge columns can be installed using one of twomethods; hydraulic frame or fixed frame. The fixedframe method involves creating a bespoke frameto fit the column size whereas the column on thehydraulic frame is trapped by rollers. Both are prelocated on the casing and checked for positionprior to concreting and then repositioned at theend of the pour with the column then beingplunged through the frame. There are advantagesand limitations to both methods which are outlinedbelow.

Fixed+ Plunge columns installed in 1.2m dia. piles+ Simple method+ Modifiable system for bespoke uses – can

accommodate protrusions and differentsection sizes , good for high cut offs

- Typical maximum column weight of 15T,though heavier columns can beaccommodated

- Requires welded spacers

Hydraulic

+ The flexibility of this frame allows for easyplunging of columns with additional plates, finsor shear studs welded to the sides.

+ Frame length can be adjusted+ Very large columns can be plunged - 600 x

600, Up to 50t, Length unlimited (withinreason!)

- Requires a minimum pile 1,800mm diameter.- Cannot accommodate small columns (less than

300mm sq)- Min depth to cut off >6m

PLUNGE COLUMN EMBEDDED IN CONCRETE AT DEPTH COLUMN AND FRAME ARRANGEMENT – BORED PILING

LASER SYSTEM ENSURES HIGH LEVEL OF VERTICAL ACCURACY

TECHNIQUE SHEET

CONTACT USBalfour Beatty Ground Engineering Pavilion B, Ashwood Park, Ashwood Way, Basingstoke RG23 8BGT: 01256 400400 | W: www.bbge.com | E: info@bbge.com

PREVIOUS EXPERIENCE

BBGE have designed and installed plunge columnson many different projects, including most recentlyat Nova Victoria, 100 Bishopgate, Heathrow T2B,The Shard London, Coleman Street and OxfordUniversity.

THE SHARD CASE STUDY

The Shard’s 87 stories, including a three levelbasement are supported by BBGE Plunge Columnfoundations. The project called for 9 differenttypes of Plunge Column in piles of 1,800mmdiameter. The use of plunge columns was selectedby the project team in order to reduce programmeas they allow for a top down constructionsequence. The scope involved installing 67no. 17tplunge columns that were up to 25m long into1800mm diameter piles that were up to 55m deep.

Each of the Plunged Column piles was designed tocarry a maximum working load of 24,000kN.

To achieve the specified high constructiontolerances and to accommodate the multiple sizedplunge columns with external protrusions BBGEdeveloped a bespoke hydraulic plunge columnframe. All columns were installed to better thanthe specified vertical tolerance of 1 in 400, with anaverage tolerance of 1 in 800 being achieved. Theworks on The Shard were completed on time andto budget.

INSPIRING INNOVATION IN CONSTRUCTION

BBGE constantly strive to make improvements toour processes and systems so when evaluating theplunge column system it was decided that therewas room for advancement. Plunge Columnstypically need to be supported inside theexcavation after the Frame has been removed. Toachieve this support gravel is typically addedaround the column. It is imperative that the gravelexerts an even pressure around the column so itdoes not push the column out of position.

The initial method was to loosely tip gravel aroundthe plunge column however this method couldcause a gravel build-up on one side of the column,potentially causing the column to lean. ThereforeBBGE developed the gravel spreader whichguaranteed that gravel would build evenly aroundthe column; thus exerting an even load on thePlunge Column and vastly reducing the risk ofleaning columns.

Funnel used to place pea gravel evenly aroundthe plunge columns

PLUNGE COLUMNS AT THE SHARD (FOLLOWING BASEMENT EXCAVATION)