DIAPHRAGM WALL CONSTRUCTION

INTRODUCTIONThe purpose of this paper is to describe the application, construction process, and design methods for diaphragm walls, since this topic has not been addressed many conferences.Diaphragm walls are a method of creating a cast in-situ reinforced concrete retaining wall using the slurry supported trench method. As such, they are often known as slurry walls. However, the term “diaphragm walls” Concrete diaphragm slurry walls were first introduced in the United States in the 1960s, and have found a niche in urban environments such as Boston, New York City, and Washington, DC.

APPLICATIONS

Diaphragm walls are most commonly used :• in areas with dense and historic urban infrastructure,• where a very rigid earth retention system is required,• where noise and vibration must be limited,• where the geology and groundwater preclude the use of conventional earth retention systems• and/or where dewatering is not practical

Compared to other wall types, diaphragm walls are considered to be very stiff with respect toground movement control (Clough and O’Rourke, 1990).Diaphragm walls are often attractive in granular soils with a high groundwater level, especially when a low permeability layer underlies the granular soils. The diaphragm walls are typically terminated in the underlying low-permeability layer which can consist of soil or rock. Keying into this low permeability layer reduces groundwater seepage below the wall. (Pearlman, 2004) Projects that have used these walls include:• below grade parking/ deep basements• cut and cover subway tunnels• highways as cut and cover tunnel walls and for underpasses• shafts for deep sewers• dam appurtenances• landslides

BENEFITSDiaphragm walls can:• be formed to depths of several hundred feet, through virtually all soil types and throughrock, and with great control over geometry and continuity• facilitate excavations below groundwater while eliminating dewatering• provide fairly watertight walls• provide structural stiffness which reduces ground movements and adjacent settlementsduring excavation• be load bearing transferring loads to the underlying layer be reinforced to allowincorporation of many structural configurations,• accommodate connections to structures• be easily adapted to both anchors and internal structural bracing systems• be constructed in relatively low headroom (say 15 feet) and in areas of restricted access• be installed before excavation commences• provide economic solutions in cases where temporary and permanent support can be

integrated or redesigned into one retaining structure

Diaphragm walls combine into a single foundation unit the functions of temporary shoring,permanent basement walls, hydraulic (groundwater) cut-off, and vertical support elements. Because of this combination, they have proven to be an economical alternative in many circumstances (Pearlman, 2004).

CONSTRUCTION PROCESSOverviewThe trench excavation is performed using slurry for support. The slurry is typically bentonite and water or polymer and water.Diaphragm walls are constructed in the following steps:• pretrenching to remove obstructions• guidewall construction• panel (vertical segments) excavation• endstop placement• panel desanding• reinforcing cage placement• tremie concrete• end stop removal (if temporary)

Site Logisitics and Slurry Plant SetupIt is important to note that diaphragm wall installation requires sufficient work area to set up the slurry plant and to assemble the reinforcing cages prior to placement in the wall. This work may be difficult on congested sites. To reduce site area requirements, offsite cage fabrication is possible.

FIG-2 CAGE FABRICATION YARD

The slurry plant includes a slurry mixer, storage tanks, and desanding units.Sufficient storage tanks must be used for bentonite slurry hydration, several panels of bentonite,recycled bentonite.PretrenchingPretrenching is often performed to remove shallow obstructions and provide stable support for the guidewalls (next step). This pretrenching may be performed as open excavation backfilled with flowfill or excavated under self hardening slurry.Guidewall constructionGuidewalls provide a template for wall excavation and panel layout, support the top of the trench, restrain the endstops, serve as a platform to hang the reinforcement, provide a reference elevation for inserts ( anchors, slabs, etc.), support the tremie pipes, hold down the cage during concreting, and provide reaction for jacking out some types of endstops.Guidewalls are reinforced concrete typically four to five feet deep and constructed similar to the figure and photo below.The top of the guidewalls should be at least four feet above the groundwater table to allow for construction in the dry and to allow for slurry level to be three feet above groundwater table.

FIG-3 TYPICAL GUIDE WALL CONSTRUCTION

Panel (vertical segments) ExcavationSpecial clamshells also known as grabs or buckets are rectangular shaped (see photos) and used to excavate vertical slots known as panels. These clamshells may be cable hug or Kelly mounted, and the digging mechanics may be cable or hydraulic operated.

The excavation is performed in “panels” which are vertical slots. Trench stability is mostly provided by the fluid weight of the bentonite and the arching action of the soil around the trench. Calculations on trench stability often do not show that successfully excavated trenches should stay open which indicates conservatism and effects that have not been considered. The bentonite slurry is placed in the trench after a few buckets have been excavated and continuously added to maintain at least 3 feet above groundwater level and within 2 feet of the top of the guidewall. Panel lengths are typically 20 to 24 feet governed by the geometry of the project and the size of contractors special clamshells. The panel width is governed by the contractors clamshells. Various widths can be accommodated by reinforcing design including shear and bending reinforcement.Endstop PlacementEndstops are used to control the concrete placement so that adjacent secondary panels are notexcavating monolithic concrete. Endstops may be permanent or removed after concrete placement. Permanent endstops are typically wide flange shapes. Removable endstops can be pipe (Figure 1) or special keyway end stops (Photo below).

PERMANENT END STOP

Panel DesandingThe panel must be de-sanded to remove excess sand in the slurry and bottom of panel. The removalof sand from the slurry decreases the density of the slurry so that tremie concrete does not mix with the slurry or trap pockets of sand.Reinforcing Cage placementCarefully fabricated three-dimensional reinforcing cage are then inserted into the panel excavation.The reinforcing cage may also support future structural or utility connections using “knockouts” that are pre-set in the wall. Concrete is then placed around the reinforcing cage using tremie methods to form each concrete panel.

Tremie ConcreteTremie pipes are placed in the panel to within a foot of the bottom. Typically two tremie pipes are used for full size panels and one tremie pipe is used for single bite panels. Concrete with 8 to 10 inch slump is then tremied into the panel.The concrete mix is special to provide 4000 to 6000 psi strength with high slump and contains fairly high cement content, often other pozzolans, plasictizers and often other chemicals.

The concrete level is sounded after each load and records maintained on actual versus theoretical concrete take. Tremie pipe sections are removed as the concrete level rises but maintained 10 feet into the concrete. While the concrete is being placed, the bentonite slurry is pumped back to storage tanks for treatment and reuse.

End Stop Removal (if temporary)As the concrete is setting typically four hours after placement at a given depth, temporary endstops are removed by crane or jacks (see Photo of Special V Groove End Stop above). This often means late nights and overtime.

ABSTRACT: Inadequate space in urban settings has set forth a challenging trend togo deeper into the ground, and increase the space required for providing publicamenities, parking and for housing utilities. Closely spaced structures in the vicinityof excavation, soft and compressible landfills, presence of underground utilities, andrestriction of lateral ground movements have made the supporting systems a

formidable task to execute. The support systems commonly adopted include Bracedwalls, Sheet pile walls, Contiguous or Secant pile walls, Diaphragm walls and RCCretaining walls. This article aims to present constructional and design elements of theretaining systems very commonly adopted in cities of India, namely Diaphragm walls,Contiguous piles and Soldier pile system with wooden laggings. The experiences andfactors advocating selection of appropriate retaining system, determination of lateralearth and hydrostatic pressure distribution, constructional features, water relatedproblems and bottle-necks during execution are described herein.

2.HAZARDS OF DEEP FOUNDATION SYSTEM

Unsupported excavations pose several hazards, and the following list gives some ofthe important ones:(i) Very high risk potential of collapse or failure of excavation walls andconsequently posing hazard to workers and equipment(ii) Hazards during excavation due to presence of public utilities, suchas electricity, water, gas, or natural gases and oxygen deficientatmosphere(iii) Dewatering problems(iv) Wet, slushy ground conditions, causing slips, trips, or falls,complicated by limited spaces in which personnel work(v) Ground and/or ground water table changes affecting nearby structures.Support provision for excavation depends on the type of soil in the area, the depth ofthe excavation, the type of foundation being built, and the space around the excavation.

During excavation, some soil types pose greater problems than others. Sandy soil is always considered dangerous even when it is allowed to stand for a period of time after a vertical cut. The instability can be caused by moisture changes in the surrounding air or changes in the water table. Vibration from blasting, traffic and heavy machinery movement, and material loads near the cut can also cause earth to collapse in sandy soil. Clayey soils in general, present less risk than sand; however, soft clay can prove to be very treacherous. Silty soils are also unreliable and require the same precautions and support provision as sand.4. DESIGN PHILOSOPHY INVOLVING FLEXIBLE RETAININGSYSTEMSDiaphragm walls and Contiguous piles are commonly designed as flexible retaining

walls. Such retaining systems are considered to be vertical cantilever designed toresist lateral earth and ground water pressures, and to rotate about some point b belowthe dredge level (Fig. 1). The flexibility leads to development of passive pressure atthe toe in the backfill side of the wall. Blum’s simplification replaces the passivepressure behind the retaining wall with a force applied to the wall at some heightabove the toe (Fc in Fig.1B). The necessary depth of penetration is found by takingmoments about the replacement force position, C. Moment equilibrium gives therequired depth of penetration, provided that the net pressure diagram is calculatedincluding the effects of groundwater. The computed may be increased by 20 to 40%beyond the point required by equilibrium (Teng, 1962); or the effective horizontalpressure on the passive side may be reduced by applying a factor of safety of 1.5 to2.0 before the embedment depth of pile is computed. Unit length of diaphragm wall isconsidered for determining its reinforcement requirements, whilst for contiguouspiles, the c/c spacing is used for estimating reinforcement quantity.

For computation of the earth pressure, classical earth pressure theory is used. Thepressure distribution shall depend on the nature of backfill, which is often observed tobe heterogeneous at site. Diaphragm walls account for hydrostatic pressures from theback side. While in the contiguous piles, on account of provision of clear spacebetween pile faces, water table is assumed to be at the dredge level. The hydrostaticpressure below the dredge level is assumed to cancel out.

A generalized equation for active and passive earth pressure computation is statedbelow:pa = (q+ h)Kγ a – 2c Ka1/2 ---- (1)

pp = (q+ h)Kγ p + 2c Kp1/2 ---- (2)For any height of water column h, the hydrostatic pressure is computed as γw h.Where,pa and pp are Active and Passive earth pressure intensity,Ka and Kp are the coefficients of earth pressures for active and passive states,respectivelyq = surcharge load intensityc = unit soil cohesion = unit weight of soilγ

h = depth under consideration.Typical earth pressure diagrams for diaphragm walls computed on the basis ofRankine’e earth pressure theory in sandy, clayey and stratified soils can be seen in

5. DIAPHRAGM WALLS5.1 GeneralDiaphragm walling is a technique of constructing a continuous underground wallfrom the ground level. Diaphragm walls provide structural support and watertightness. These reinforced concrete diaphragm walls are also called Slurry trenchwalls due to the reference given to the construction technique where excavation ismade possible by filling and keeping the wall cavity full with bentonite-water mixtureduring excavation to prevent collapse of vertical excavated surfaces. These retainingstructures find following applications: earth retention walls for deep excavations;basements, and tunnels; High capacity vertical foundation elements; Retaining wallfoundations; water control. These are also used as a permanent basement walls forfacilitating Top-down construction method.Typical wall thickness varies between 0.6 to 1.1m. The wall is constructed panel bypanel in full depth. Panel width varies from 2.5m to about 6m. Short widths of 2.5mare selected in less stable soils, under very high surcharge or for very deep walls.Different panel shapes other than the conventional straight section like T, L are7

possible to form and used for special purposes. Traditionally, panel excavation iscarried out using cable supported Grab. Hydraulic grabs with Kelley arrangementhave recently been introduced in India on large Infrastructural projects. More recentlydeveloped hydraulic cutter type machines are not being used in India hence have notbeen discussed here. Slurry wall technique is a specialized technique and apart fromthe crane mounted Grab, other equipment involved are cranes, pumps, tanks, desandingequipment, air lifts, mixers etc.Steps involved in the construction of diaphragm wall can be broadly listed as follows:(i) Guide wall construction along alignment(ii) Trenching by crane operated Grab/ hydraulic grab(iii) Bentonite flushing(iv) Lowering reinforcement cage(v) Concreting using tremieThe sequence of construction of diaphragm wall panel has been schematicallyillustrated in Fig. 3. It must be remembered that Diaphragm walls are constructed as aseries of alternating primary and secondary panels. Alternate primary panels areconstructed first which are restrained on either side by stop-end pipes. Before theintermediate secondary panel excavation is taken up, the pipes are removed and thepanel is cast against two primary panels on either side to maintain continuity. Waterstoppers are sometimes used in the construction joints between adjacent panels toprevent seepage of ground water.5.2 Merits and DemeritsDiaphragm wall construction is relatively quiet, and minimum noise and vibrationlevels make it suitable for construction in urban areas. The water tight walls formedcan be used as permanent structural walls and are most economical when used in thismanner. The finished structural wall formed prior to excavation allows subsequentconstruction of the basement in a water tight and clean environment. Once thediaphragm walls are constructed, work can be planned to proceed simultaneously

above and below the ground level. There is a minimum of space wasted. Work maybe carried out right against existing structures and the line of wall may be adjusted toany shape in plan.

Diaphragm walls however, require the use of heavy construction equipment thatrequires reasonable headroom, site area, and considerable mobilization costs. Inlimited headroom conditions, smaller cranes can be used though this couldcompromise efficiency. They are not considered efficient means in hard and rockygrounds, where the conventional grabs are undeployable.

Diaphragm Wall Construction at BC-24 Stretch of DelhiMetro Corridor

Diaphragm wall at Jungpura StationParadip Port Harbour Works-Wet Basin Area

REFERENCESClayton, C.R.I., Milititsky, J., Woods, R.I. (1993). Earth Pressure and EarthRetaining Structures. Blackie Academic & Professional, London.Teng, W.C. (1962). Foundation Design, Prentice Hall International.Winterkon, H.F, Fang Hsai-Yang (1975). Foundation Engineering Handbook, VanNostrand Reinhold Company, New York.