diaphragm wall

Process Description of a Diaphragm Wall made by Stein HT

[according to DIN EN 1538]

Preface/Fields of Application

A diaphragm wall is a vertically executed reinforced concrete wall. It is excavated by means of supporting suspension and has a sealing effect. Both, as a temporary and as a permanent structure, it can also assume load-bearing/load-transferring functions. Wall thicknesses typically range from 600 mm to 1500 mm and depths of max. 40 m are usual, although thicknesses of max. 2000 mm and considerably deeper walls are feasible.

A diaphragm wall is usually (but not exclusively) used for the following structures:

Tunnel and underground construction
Enclosure/stabilisation of building pits
Pipe jacking shafts
Underground car parks
Inner-city building pits
sluices
Landfill enclosures
As foundation element (barrettes)

The advantages of a diaphragm wall in comparison to other foundation processes are as follows:

Low-noise and low-vibration construction
Low deformation
In case of adequate statics, it can bear very high loads
Shorter construction period in comparison to e.g. bored pile walls
Sealing effect

Construction of a diaphragm wall

Site set-up

Site set-up constitutes the start of the building measure. The equipment to be supplied shall be selected in a way that the diaphragm wall to be built can be implemented in a technically perfect manner and according to the specified schedule. Attention also has to be paid to sufficient provision of supporting suspension (bentonite). The site equipment area is usually situated within the building plot or directly adjacent to it. Depending on the volume of bentonite to be provided, its surface ranges from 200/250 m² to 500 m².

The site equipment of Stein HT is typically made up as follows:

1 hydraulic crawler crane Liebherr HS8100HD/HS 8130HD or equivalent for diaphragm wall excavation (excavator)
1 hydraulic crawler crane Liebherr HS875HD or equivalent for incorporation of joints/reinforcement cages and the concreting process (service crane)
1 to 2 STEIN diaphragm-wall grab(s) of a width suited for the wall to be built (jaw width corresponding to the arrangement of the panels)
1 STEIN diaphragm-wall chisel of suitable width
1 STEIN diaphragm-wall plough
1 to 2 STEIN desanding plant(s) type II
1 STEIN container mixing plant 4.50 m³
1 silo including screw-conveyor for bentonite powder
Quantity of STEIN bentonite storage containers according to the size of the open panels (of 100 m³ capacity per set, each)
Quantity of STEIN stop-end elements of suitable width according to max. depth and number of starter panels
1 wheel loader Liebherr 526 or equivalent
1 STEIN intermediate skip
Quantity of tremie pipes and accessories according to the max. depth and number per panel
Quantity of Mohno bentonite pumps
Quantity of Varisco bentonite pumps
1 compressor (electric or diesel)
1 workshop container
1 laboratory container
Further accessories like excavator mats, bentonite refilling device (guard), guide wall protection etc.

Site Set-up

Sketch of a site set-up

Guide wall construction

Sketch for guide walls

Gui-wall-elements are availabfe
in length: 5,00m / 4,00m / 3,00m / 2,00m
in height 1,30m / 1,00m

width of diaphragm wall [m]	0,60	0,80	1,00	1,20	1,50
guide-wall width [m]	0,635	0,835	1,035	1,235	1,535

Guide wall construction forms part of the further preparation for the diaphragm wall works.

These guide wall has the following functions:

Position stability in the ground plan
Vertical guidance of the diaphragm wall grab
Prevention of scouring of the soil, as the bentonite level is always kept within the guide wall.

For this purpose, the guide wall will be designed as a double-sided R.C. angular retaining wall of a height of at least 1.00 m. At the latest during excavation of the guide wall trench, the real position of any existing supply and sewage lines will be ascertained. If any pipelines are installed in the area of influence of the diaphragm wall, they will have to be relocated.

On site, a trench will be excavated for construction of the guide wall. In this trench, the diaphragm-wall alignment will be adopted according to the panel plan and marked by means of a batter board. Then precast concrete elements will be placed along this alignment. These slabs are max. 5.00 m long and max. 1.30 m high and are provided with a projecting reinforcement in longitudinal direction and a projecting reinforcement for the guide wall base. The inner slab will be shored up against the outer slab. Then the reinforcement for the guide wall base will be completed and a base of 30 cm will be affixed by concreting. As a next step, the gaps between the individual slabs will be reinforced, encased, and concreted. After removal of formwork, the guide wall trench will be backfilled and compacted.

As dimensioning for the clearance between the guide wall sides, an additional trench width of 3.5 cm has proven. Thus, easy handling when immersing the diaphragm-wall grab into the open trench at simultaneous narrow guidance during the excavation is given.

Arrangement of panels

After completion of the guide wall, its position will be re-examined by the surveyor and approved by the local site management. Afterwards the positions of the joints of the panels of the diaphragm wall are marked on the guide wall by means of steel nails or signal colour. Finally, every panel is marked with the corresponding panel number.

Panel-arrangement

Principle

Suspension-supported excavation is carried out by means of cable-guided, mechanical STEIN diaphragm-wall grab.

For technical reasons, the panels are classified into:

Starter panels (with two inserted stop-end-elements)
Intermediate panels (with one inserted stop-endelements)
Closing panels (without inserted stop-end-elements)

First, the starter panels are made. Then the panels on the left and right of the starter follow in order to close the diaphragm-wall line between them.

Herstellung einer Schlitzwand

Excavation

The process is the same for every panel of the diaphragm wall. First the diaphragm wall grab is set up for the panel. According to the excavation progress, the excavated soil is continuously replaced by bentonite as supporting liquid. In this context, attention is paid to the fact that the suspension level must not fall below 30 cm, measured at the upper edge of the guide wall. The excavated soil is temporarily stored on site behind the excavator for subsequent disposal or thrown into an intermediate skip and loaded on trucks by means of wheeled loaders.

Abb.7 -Aushub2

At regular intervals, the depth of the diaphragm wall is measured by means of a plumb. At the same time, the excavated soil layers are recorded. The local site management is responsible for acceptance of the final depth of the panels of the diaphragm wall. Together with other information on the dimensions of the panels, bentonite, reinforcement and concreting process, these conditions are recorded in a diaphragm-wall report per panel.

After reaching the final depth and in case of work interruptions, the open trench will be covered. The employment of staff, use of equipment, development of the diaphragm wall works and peculiarities are documented in complementary daily reports. Depending on the point of time of reaching the final depth, the trench will still be concreted on the same day.

Abb.7 -Aushub3

In case the panel should not reach the final depth before noon, the trench shall remain open until the next day, however, by keeping the bentonite level within the guide wall. If too much bentonite diffuses into the soil, bentonite will be refilled by the diaphragm wall guard; this operation is either carried out by staff or by means of an automatic refilling device (guard).

Abb.7 -Aushub4

Proof of verticality

The verticality measurement of the panel is qualitatively carried out by means of the grab cables. The excavator operator can identify any deviations of the grab along the diaphragm wall alignment by means of two alignment bars in the grab area. His assistant at the trench observes any deviations of the diaphragm wall grab in transversal direction to the diaphragm wall alignment. As long as the grab is visible, both orient themselves by the grab frame. During the progress of excavation, the grab is no longer visible, and both orient themselves by the hoisting cable of the grab. In case of a deviation of the grab, the hoisting cable accordingly moves out of the middle of the guide wall. Moreover, the verticality of the panel of the diaphragm wall is quantitatively measured by means of an inclinometer made by Jean Lutz and via the Liebherr measuring system PDR2. The data measured by the inclinometer mounted in the grab are transmitted by radio to the driver cabin of the excavator for every excavation stage. The excavator driver can thus track deviations of the diaphragm wall grab in the x- and y-axis, as well as the twisting. The results of the measurement of the respective panel are transmitted by the PDR2 system to a Liebherr server, where they can be retrieved. Illustration are made via diagrams with the respective deviations depending on the depth. In case a too large deviation from the vertical is ascertained, the deviation will be removed by means of the diaphragm wall plough. It is hence guaranteed that the deviation of 1 % as defined in DIN EN 1538 is not exceeded.

Excerpt from the proof of verticality

Cleaning of the joint

Every concreting operation starts with cleaning of the adjacent diaphragm wall joints from soil residues and excess concrete. To this end, the diaphragm wall grab is used. Then the trench bottom is cleaned from mud by means of the diaphragm wall grab.

A bentonite sample is taken from the panel and examined with regard to various parameters. If the permissible sand content according to DIN 4126 is exceeded, the bentonite will be regenerated. For this purpose, a submersible pump will be installed at the bottom of the panel in order to pass the bentonite to the desanding plant, where the sand floating in the suspension is separated
and removed. Thereafter the cleaned bentonite is fed to the panel again.

In case further limit values defined in DIN 4126 should be exceeded, the suspension remaining in the trench will be replaced by fresh suspension.

Installation of the stop-end-elements

For designing the joints of panels, we use the STEIN CWS flat stop-end system. During this process, flat stop-end elements of steel and adequate width are installed into the excavated panel before concreting so as to create a vertical delimitation of the panels.

To increase imperviousness between the rectangular foundation elements, a Siteg flat joint strip is installed in the flat stop-end element before placing the joint. 10 cm of this strip will then be concreted in the panel to be concreted. After excavation of the neighbouring panel and removal of the STEIN CWS flat stop-end element, the other half of the joint strip becomes visible and will be concreted during concreting of the neighbouring panel. In this way, an utmost tightness of the joint is guaranteed.

Installation of reinforcement

Abb.12 -Einbau der Bewehrung1

Thereafter, the diaphragm wall reinforcement will be incorporated. This reinforcement is supplied to the site in ready plaited cages according to the reinforcement drawings. The local site management is responsible for acceptance of the reinforcement.

Abb.12 -Einbau der Bewehrung2

The service crane is required for picking up the cages. The head of the reinforcement is picked up by the hook of the excavator. The service crane picks up the reinforcement cage at the lower third point. Then the excavator pulls the head of the reinforcement towards the tip of the boom. During this operation, the service crane prevents that the bottom of the reinforcing cage touches the ground. In case of larger panel depths, it is customary that the reinforcing cages are split lengthwise. In this case, they consist of bottom and top or of bottom, centre and top.

One part is let down each time and supported on the guide wall by means of steel beams. Then the following part is placed and inserted by using the overlapping length. The respective cage components are linked to each other by means of corresponding cable clamps. After reaching the final length, the reinforcing cage is lowered to the required target position.

Abb.12 -Einbau der Bewehrung3

When lowering the cages in the panel, the flat steel spacers mounted on the cage ensure sufficient concrete panelcover of the diaphragm wall reinforcement. The individual reinforcement cage is then braced by means of holding brackets and the beams placed across the guide wall. Then the height and position of the reinforcing cages is checked and corrected if need be.

Abb.12 -Einbau der Bewehrung4

Concreting of the panel

Abb.13 -Skizze für die Betonieren des Schlitzes1

After reinforcement, the tremie pipes will be installed up to approx. 50 cm above the diaphragm wall base. These concreting pipes have an interior diameter of 250 mm. On the top, the tremie is mounted last.

The number of tremie pipes depends on the length of the panel. In most cases, one concreting pipe is mounted per reinforcing cage. Before pouring the first concrete, the tremie pipe is provided with a separation plug in a suitable manner, in order to avoid mixing of the bentonite with concrete in the tremie pipe when pouring the first concrete.

The diaphragm wall concrete is supplied from the concrete plant by lorry mixers in a flowable consistency class. From the lorry mixer, the diaphragm wall concrete passes the tremie and the tremie pipes and is thus installed in the panel. At the same time, the bentonite displaced by the concrete poured at the bottom is pumped out at the upper edge of the guide wall and conveyed to the storage containers, where it passes the desanding plant, in order to remain in the storage containers until the next use.

The rate at which the concrete rises must not remain below a value of 3 m/h according to DIN 4126. According to the concreting progress, the tremie pipes will be pulled out, but at the same time ensuring that the tremie pipes must still remain immersed at least 2 m in the fresh concrete. The upper edge of the concrete is continuously measured by means of a plumb. When the required upper edge of the diaphragm wall is reached, concreting is stopped. The incorporated concrete volume is recorded in the diaphragm wall report.

Abb.13 -Skizze für die Betonieren des Schlitzes3

For technical reasons, it is not possible to interrupt the concreting process. Thus, a mandatory workflow has to be observed. The diaphragm wall excavation as such must be finished in the evening. Next morning, the panel of the diaphragm wall will be cleaned, and the stopend-elements and reinforcement cages will be installed. At noon, the concreting process as such must start at the latest.

Abb.13 -Skizze für die Betonieren des Schlitzes2

Quality management

For quality assurance, a person on the site will be entrusted with the execution of the following inspections and their documentation in the diaphragm wall reports and daily reports,

Trench depth
Verticality of the trench
Rate at which the concrete rises and concrete consumption
Suspension monitoring

On the site, the necessary equipment will be provided in the site laboratory in order to check the following suspension properties every day,

Weight per unit volume [kN/m³]
Liquid limit [N/m²]
Marsh viscosity [sec]
Sand content [%]
pH – value [-]
Filtrate water [ml]

Site clearance

After concreting of the last panel, the diaphragm wall works as such are completed. The facilities will be dismantled, and all equipment removed from the site. As soon as the site is cleared, excavation of the building pit can start. During this excavation, the guide walls will also be removed.

Baustellenräumung