The existing ground level varies between + 3.52 m DMD to + 4.29 m DMD. The working platform level for Double U wall is considered at the average ground level. A Double U channel beam wall with one layer of tie-back anchors shoring system is proposed along the four sides of the plot for a maximum excavation depth of –9.55 m below the existing ground level.
In order to avoid un-allowed influence on adjacent buildings or roads, the double U beam will be located into pre-drilled boreholes. For this purpose, a temporary casing of min. 2.0 m length will be rotary into the soil. The borehole will be drilled by rotary drilling rig using bentonite slurry to stabilize the borehole.
The length of the embedment of the double U. beam depends on the particular statical requirements and will be defined in the structural analysis. Up to the proposed excavation level, the borehole will be filled with gravel.
Above the proposed excavation level, the temporary casing will be filled with granular soil or excavation materials (if suitable) and the temporary casing will be withdrawn by our drilling rig.
PREPARATION OF DOUBLE U BEAM
Double H. beam will be fabricated by connecting 2 U beams of suitable size back to back. The grade of steel shall confirm to B.S. 4, part 1, 1993 of grade 43A. The space between the U beams will be maintained minimum 12 cm in order to accommodate the anchor.
INSTALLATION OF TIE BACK ANCHORS
Tie back anchors shall be installed for the proposed Double U beam in order to reduce the bending moment deflection on the H. beam wall and thereby reduced deflection on the wall top. Loads on each anchor and its dimensioning is made as elaborated in the design.
INSTALLATION PROCEDURE
A powerful drill Wagon shall be used to install the tie back anchors, following sequence shall be adopted for the installation of the anchors.
- Install the drill Wagon at approximately the same level of the anchor head. Adjust the swing mast to the required inclination to meet the design/site requirement. Note: The mast drill Wagon can be rotated in all directions to accommodate the site requirements.
- Temporary steel casing of 114 mm dia shall be drilled through the soil with the help of its water flush system. In order to prevent any escape of soil while drilling, the casing shall be provided with an end plug. Note: Extreme care shall be taken to avoid any damage of underground structures while installing the anchor. In case of existence of any obstructions such as pile foundations, the direction of the anchors can be shifted to any adjustable mast if drill Wagon or the position of the anchor itself can be relocated. The drilling system provided on drill Wagon allows the operator to observe any obstruction.
- After reaching the required anchor length, ready-made anchor with 2 Nos. grout pipe at the middle will be inserted into the casing. The temporary casing shall be withdrawn whilst filling the bore with primary cement grout. The grout will carry on until the grout has replaced the water in hole completely.
POST GROUTING
After approximately 4 to 6 hours of anchor installation bottom 5.0 m of the anchor shall be injected with water pressure in order to create cracks in the hardened grout then pressure grout will take place to perform the bond length of the anchor (bottom 6.0 m).
The cement grout mix design will be as follows:
– Cement grout mixture (per batch) : 2 bags cement = 100 kg
55 liters water = 55 kg
– W/C ratio = 0.55
– Design compressive strength of the grout at 28 days = 25 N/mm2
The equipment used for grout mixture:
– Grout mixer
– Pressure pump
ANCHOR STRESSING
7 days after post grouting, the anchors shall be stressed and locked off to the required load, after grout compressive strength has achieved 60 to 70% of the design strength.
INSTALLATION OF CONCRETE PANELS
For the stability of the Double U. beam, it is important that the ledger will be installed in steps directly after the excavation (high free stands of the soil especially over a longer period must be avoided). The ledger must be fixed tight to the Double U. beam and pressed against the undisturbed soil. Any soil movements behind the Double U-channel wall from over excavation or too high free stands must be avoided. In already excavated areas, the soil must be refilled in compacted layers.
Dewatering is to be done 1.0 m below the excavation level outside the shoring system. Care will be taken to avoid any loss of fines while dewatering.