Question

Explain a detailed step-by-step method to install a Slurry Wall SOE.

Assumptions and given data:

1. The SOE is constructed in an urban setting with an excavation depth of 60ft and it will be a permanent structure.
2. The water table is located 20ft below the ground surface.
3. Topmost 40ft of excavation is entirely sand (N value < 30)
4. Below this sand is a soft-to-medium clay to a depth of 60ft and below it is the Bedrock.
5. The external groundwater level must be maintained.

Provide the following in the explanation:

1. Equipment used to install the slurry wall.
2. Procedure for excavating downstage after the slurry walls are installed.
3. Give details about the concerned construction risks and advantages of slurry wall construction.

Answer 1

1. Procedure:

i). Step 1: Guide Walls

Slurry walls are typically constructed by starting with a set of guide walls, typically 1 metre (3 ft 3 in) deep and 0.5 metres (1 ft 8 in) thick. The guide walls are constructed on the ground surface to outline the desired slurry trench and guide the excavation machinery.

ii). Step 2: Excavation

While a trench is excavated to create a form for a wall, it is simultaneously filled with slurry (usually a mixture of bentonite and water). The fresh slurry is added as required during the excavation to maintain a constant level of slurry near the top for the stability of the trench. The water from the slurry suspension bleeds into the sides of the trench and leaves behind a thin densely packed layer of colloidal particles that acts as a membrane, commonly referred to as a “filter cake.” The hydrostatic force of the slurry, acting in combination with the filter cake, provides stability to the sides of the trench. The excavator digs down to design depth (or bedrock) for the first wall segment.

iii). Step 3: Panel Construction Method

The excavator is then lifted and moved along the trench guide walls to continue the trench with successive cuts as needed. The trench is at all times kept filled with slurry to prevent its collapse, but the liquid filling allows the excavation machinery and excavation spoil to be moved without hindrance.

iv) Step 4: Placing Reinforcement and concreting

Once the excavated depth is reached, the reinforcing steel cage is lowered into the panel and the slurry is displaced with concrete placed through a tremie pipe from the bottom up. The heavier concrete displaces the bentonite slurry, which is pumped out, filtered, and stored in tanks for use in the next wall segment, or recycled. A series of panels are connected together by overlapping the panels or by the use of end stops to form a continuous rigid wall. When the concrete has hardened, excavation within the now concrete-wall-enclosed area can proceed.

2. Equipment:

Generally, the excavation is done using a special clamshell-shaped digger (for soft to medium ground conditions) or a Hydromill trench cutter (for hard ground conditions such as rocks), suspended from a crane.

Since the mentioned ground are relative soft ground conditions, we will proceed with the clamshell-shapped digger or, in general terms, a grabber. As the slurry walls are installed with a grab, a two-jaw slurry wall grab suspended on a duty cycle crawler crane excavates the slurry wall panels.

Slurry wall grabs are of 2 types:

i). Mechanical Slurry Wall Grab

ii). Hydraulic Slurry wall grab.

Mechanical grabs are opened and closed via rope, hydraulic grabs via hydraulic cylinders. Hydraulic grabs can additionally be fitted with flexible guiding strips on the grab frame which allow aligning the grab within the trench.

3. Downstage Excavation Sequence:

As the slurry is has finished construction, excavation to downstage commences. The excavation is typically done in different stages. The depth of excavation of each stage depends on a variety of factors. One of the major factor is the temporary support systems such as tiebacks or internal crossbeams are which installed to prevent the slurry wall from collapsing during downstage excavation. When completed, the structure built within the walled-off area supports the wall, so that tiebacks or other temporary bracing may be removed. Usually, in case of temporary support, the depth of excavation in each stage is kept slightly above or below the consecutive tie installation level. Once the ties are installed using local excavation, the corresponding stage is completely excavated.

The stage depth also depends on groundwater conditions, since GWT is at a depth of 20 ft below Existing Ground Level, till 20ft the stage depth can be more and thereafter it will depend on the dewatering capacity at the site. Each stage has to be completely dewatered before proceeding further. That is, if the pump capacity is such that it can dewater, say 2m, in 4 hrs but the excavators can dig upto 4m in 4 hrs, then the stage depth should be limited to 2m.

Another contributing factor would be muck removal capacity. The faster the muck can be removed and placed, more can be the stage depth and vice versa.

Taking account of all the factors generally, a stage depth of 8ft to 10 ft is adopted.

4. Construction Risks:

Principal unique hazards associated with the slurry walls include:

a. Physical Hazards

(i) Physical injury of workers due to site open excavations, equipment movements etc.

(ii) Noise hazard to workers due to excessive noise during operations.

(ii) Fire, electrocution, or explosion hazards may exist during installation of the slurry wall, should a backhoe rupture an underground utility, such as sewers, pipelines, or electrical or gas lines.

(iii) During mixing operations, workers may be exposed to inhalation/ingestion/dermal hazards from airborne dusts, volatile organic compounds (VOCs), or metals from soil/bentonite mixtures and waste materials. Eye exposure may occur resulting in scratching and scarring of eyes.

(iv) The heavy equipment (small and large) used for site operations may roll over on steep slopes or unstable ground, seriously injuring the operator. Trucks loaded with backfill can back up too far and become stuck in the trench.

b. Chemical Hazards

During the excavation/installation activities, workers may be exposed to caustic irritants such as Portland cement. This material may become airborne during application and can cause skin burns and act as a lung irritant. Other agents such as bentonite used in slurry walls may contain free silica. Workers may also be exposed to waste materials such as organics and heavy metals. These materials may become airborne during excavation and expose workers via ingestion/inhalation/dermal contact routes.

c. Radiological Hazards

Radiological materials may have been buried, or naturally occurring radioactive material (NORM) may be present in soils, sludge and groundwater. Some radioactive materials may present an external hazard. All radioactive materials may present an internal exposure hazard through inhalation or ingestion. It should be noted that this may be a rare hazard to encounter using this remediation technology.

d. Biological Hazards

At those sites involving medical wastes or sewage sludge, microorganisms in the soil may pose exposure hazards during the soil mixing and waste stabilization activities. Workers may be exposed to inhalation/ingestion/dermal contact with pathogens, such as Coccidioides sp., Histoplasma sp., and Mycobacterium sp. if contaminated specks of dust become airborne.

e. Geological Factors:

If the soil profile varies considerably along a short length the design may have to be revised to incorporate any excess loads coming onto the slurry wall. This can risk the integrity of the wall as a continuous rigid member.

5. Advantages:

Slurry walls offer the following advantages:

i. Low cost

ii. High productivity

ii. Very low permeability

iii. Verifiable continuity and depth

iv. Excellent resistance to contaminated groundwater

v. Ability to easily flex with ground movements, even some earthquakes

vi. The slurry remains fluid, allowing time for penetrating difficult layers or obstacles

vii. Re-use of most of the excavated materials