Construction of secant pile wall is normally required when the building project is to be erected on soft ground or sandy soil. The purpose of this is to ensure that the strata behind the wall would remain undisturbed during construction and to ensure safety from the risk due to high lateral pressure.
So, a quality engineer is required to prepare a method statement for the construction of secant pile wall and shoring.
Method Statement for Building Secant Pile Wall and the Necessary Shoring
2. Scope of the Project
The purpose of building the secant pile wall is for a shoring system to support a 27-story apartment building to be constructed in (Location, City here).
The secant pile wall is to have the following features:
• Sturdy wall safely embedded in the soil well below excavation level to provide stability
• Wall must adapt ideally to the configuration of all required alignments
• On top there shall be low deflection
• Must be firm, tight and impervious to changes in soil structure.
To accommodate the type of soil and the requirements of the site, the secant was designed to have a diameter of 950mm, center-to-center pile distance, at 820mm held back with one layer of anchors in the ground to keep the depth of excavation at 12.75m, which reaches the level of formation at -11.5m National Country Datum (NCD).
3. Materials Required:
3.1 Materials for Secant Pile Wall
Primary piles – Feu of 20 N/mm2 (28 days).
Secondary piles – Feu of 40 N/mm2 (28 days).
High yield bars for strength – Fy of 460 N/mm2 Note: Should this be Fy or Feu?
3.2 Anchor Materials Required
These are summarized below:
1. One tendon anchor strands – Tendons are to be formed with 6-wire strand using relaxation class 2 and are to be in compliance with EN 1537 – Steel grade 1570/1770 N/mm2.
2. HDPE pipes to be used for grouting.
Cement is to be CEMI 32,5 or CEMI 42,5 with a cement/water ratio of grout to be between 0,4 and 0,55 so it reaches a compressive strength of 25-30 N/mm2.
To facilitate early hardening of the grout, additives may be added, which would depend on how the actual grout behaves.
4. Cement/grout mix is to be as follows:
Cement/grout mix (per batch) – 2 bags of OPC cement = 100 kg
55 liters of water = 55 kg
Water/cement ratio = 0.55
Compressive strength of grout to be designed at 28 days = 25 N/mm2
4. Manpower Required:
• Construction Manager
• QAQC Engineer
• Safety Officers
• 1 Mechanic
• 1 Welder
• Machine Operators
• Skilled Labor (riggers)
• Unskilled Labor
5. EQUIPMENT REQUIRED
5.1 Secant Pile Project
Our intention is to use type BG25 rotary drilling rigs (but you can use another type) to rotate the casings into the ground using a rotary powerhead. This equipment is specifically designed to create a minimum amount of disturbance to the surrounding soil and therefore allow this work to be done near existing structures.
5.2 Anchoring Project
Equipment required for installing the anchor and testing it is as follows:
6. Site Conditions: General & Specific
6.1 General Site Conditions
The development as proposed will have multilevel underground car parks to accommodate the cars of the occupants of the apartment buildings. The shoring structure being proposed will align with the plot line.
6.2 Geological and Soil Conditions
The subsurface is mainly made up of loose sand on the top layer, medium sand and a formation of calcarenite and sandstone below that. The secant piles have been designed to go to the depths needed to have the toe socketed into the hard calcarenite and sandstone layer so as to achieve a stronger interaction between the concrete and the ground.
The elevation of the groundwater table is determined to be -0.95m NCD. The water table level varies according to tidal and seasonal changes and any de-watering operations that may be occurring.
7. Construction Sequence
7.1 Elevation of Working Platform
It has been determined that the ground level is currently +2.0 m NCD. The elevation platform for doing the secant piling project will be at approximately 0.0 m NCD. A 2-meter pre-excavation will be needed to reach the desired platform level. The project’s working platform must be solidly firm, sturdy, leveled and dry.
A professional surveyor will handle everything required to complete the surveying works. He or she will mark the demarcation points with at least 2 points at each corner. The plotline, shoring line and the building line must be firmly established. The Engineer will be provided with the necessary facilities to check the points.
7.3 Guide Wall
A guide wall will be needed to maintain the verticality and alignment of the secant wall within tolerable limits. The pile alignment should not exceed 1% in each direction of the wall’s height after excavation at the level of excavation. A trench shall be excavated along the wall’s center line to a depth of 50 cm below the working platform. Concreting of the guide wall will done once the nominal reinforcement has been placed, as indicated on the drawing.
7.4 Construction of Secant Pile Borehole
A guide casing of the proper diameter shall be temporarily attached to the secant pile center through a slot in the guide wall. This temporary casing should be free from any major distortion and be uniform throughout the length of the cross-section. The length of the casing will depend on the requirements of the site.
Once the temporary casing has been installed the borehole shall be drilled with the proper drilling tools. As the casing is being pushed down, within the casing there will be simultaneous excavation in progress. An on-board computer system will be continually monitoring the depth of the drilling. This will be done manually at regular intervals using measuring tapes.
The bottom of the casing shall have cutting teeth fitted.
7.5 For Secondary Piles, A Reinforcement Cage Needs to be Placed
In preparation for placing the cage, the pile toe must be thoroughly cleaned using a cleaning bucket. The reinforcement cages must be built in advance in accordance with the approved design. The approved drawings/design will stipulate the number of and diameter of the primary bars. The spiral must be according to the approved drawings/design.
The length of the reinforcement cage must be in accordance with the pile toe level and cut off level provided in the drawing. A service crane will lower the pre-fabricated reinforcement cage into the pile shaft.
7.6 Casting Concrete must be According to Specifications
Concrete must be mixed according to specifications established by the designers. Once the boreholes have been inspected and approved, the concrete shall be poured directly into the boreholes by way of a tremie tube from the mixer truck or fastened to a concrete pump. The concrete must be poured without any interruption to avoid previously poured concrete from hardening and a cold joint forming.
The tremie tube should be withdrawn gradually throughout the concreting process to ensure that the 2-3 meters at the bottom of the tremie continue to remain within the concrete previously placed.
There will be extra concrete remaining above your cut-off level, which must be chipped off prior to capping beam construction.
After concreting has been completed, the drilling rig shall withdraw the temporary casing. As the casing is being removed, it is vital to maintain a sufficient amount of concrete within it so that pressure from the external soil or water is exceeded. This will ensure that the pile is not contaminated or reduced in section.
During the process of extracting the casing, the level of concrete within the temporary cashing must be topped up when necessary.
7.7 Construction of Primary & Secondary Piles
There are primary and secondary piles that need to be constructed in an alternate sequence. A tentative sequence would go as follows: Day 1 – Primary pile; Day 2 – Primary pile; Day 3 – Secondary pile; Day 4 – Secondary pile
The difference between primary piles and secondary piles is that the primary ones are un-reinforced, whereas the secondary ones are reinforced. The concrete strength for constructing the primary piles is to be 20 N/mm2 (28 days).
But, the primary piles’ concrete compressive strength at 48 hours must not exceed 10 N/mm2. The secondary piles’ concrete characteristic strength is to be 40 N/mm2 (28 days).
7.8 Proper Disposal of Drilling Waste
Waste from drilling the boreholes is to be stored there on site and removed on a regular basis to a designated location.
7.9 Piling Reports Submitted Daily
A piling report must be prepared daily and submitted for approval to the Engineer.
8. Installing the Tempory Anchors
8.1 General Instructions
To diminish the bending moment and the amount of deflection occurring on the wall top, a single layer of tie back anchors must be installed. The design elaborates on this in terms of the loads on each anchor and the appropriate dimensioning.
The tie back anchors being installed for the secant wall are not to be de-stressed or taken from their position after construction of the basement wall. The anchor heads will be hidden in the wall, so there is no need to consider the location of the basement floor slabs.
8.2 Proper Installation Procedures
From the universal drilling wagon on the required platform level, UBW is to drill the anchor holes. Drilling rods are to be fastened to the UBW’s rotary drive, and are coring into the installed wall at the appropriate proposed locations.
The drilling rods are to be 133 mm in diameter and in steps of 2.0 m they are to be extended until they reach the approximate length of the anchor.
For the process to proceed, the first drilling rod must have the proper teeth and drill head needed to loosen the materials. Water is being pumped at the same time through the drilling rods to transport the loosened material. This mixture of water and loosened material will flush out alongside the drilling rods at the location being cored.
Then, the UBW is to be detached from the drilling rod so the bundled anchor can be inserted into the rods for the full length. Approximately 1.50 m of anchor will be exposed outside the secant pile wall.
After the final drilling depth has been reached and the tendon has been installed, the UBW is to be fastened to the rods and the rotary drive is to pump cement grout through the drill bit openings as the rods are raised. This fills the borehole up from bottom to top and replaces the water used in the drilling process. At the same time, the drilling rods are being extracted in stages of 2.0 m. While this is being done the void surrounding the anchor is being filled with grout.
The grout must be allowed to set, which will take approximately 3-4 hours. After that time, water at a pressure of about 10 bar is to be pumped into the HOPE grouting pipe, which is embedded. This will bring about the desired cracking of the grout body at the lower 8.0 m.
The next step in the process occurs 24 hours after completing the operations stated above. The post-pressure grouting going through the grout pipe will begin forming the resulting anchor grout body. This will create a huge structure that connects the grout, sandstone and anchor strands above the lower 8.0 meters.
8.3 Keeping Anchor Records
Records need to be kept on each anchor installation to include the following:
• Location, anchor type, date, time, installation and drilling procedures.
• Details to include the length and diameter of the drill hole, drilling method, type of drill bit used, how the hole was flushed and any peculiarities occurring during drilling.
• Along with the company control forms being submitted, forms must be submitted that discuss all tensioning results.
8.4 Anchor Testing, Stressing & Locking
Once the grout has been allowed to set so that reaches strengths of 10-15 N, at least 7 days afterwards, the testing, stressing and locking of the anchor is to proceed. Expect the 28 days strength to be around 35 N.
A HOP82 hydraulic jack is to be installed above the exposed anchor, pulling the overall anchor until the progression of the required design load.
Once the above procedure of testing and stressing is complete, the anchor wedge plate, including wedges, is to be fastened to the anchor. The constant force of pull in the anchor will prevent the wall from deflecting.
The anchor is to be tested to a load of a maximum of no more than 125% of the working load before being locked with the wedge plate, which shall be at 80% of the working load.
9. Health & Safety
The company is committed to complying with all Safety & Environmental Precautions required of the project. This includes keeping all related documents and paperwork, approved CEMP, Method Statements, Plans and Procedures for the entire life of the Piling Project as well as any temporary projects associated with it.
Protective clothing, hard hats, boots with steel toe caps are to be worn by everyone involved in the erection of a test frame and those involved in setting up the test equipment.
Hydraulic hoses, as well as any main leads being used, are not to be routed where any vehicles could run over them unless ducting buried under the ground would protect them.