foundation system of burj al arab

Lateral Load Testing for Micropile

We know different sources of lateral loads are applied in deep foundation and we consider deep foundation is capable of taking these loads. In this regard some tests should be conducted along with axial load testing.

When owner side think lateral testing is essential, this testing will be conducted before axial load testing and should comply with ASTM D3966.

ASTM D3966-“Standaard test methods for deep foundation under lateral loads”-this testing method in field provides us most acceptable relationship between lateral load to which deep foundation subjected and lateral movement as a result of such loading.

This test results also provide information that can be used to determined lateral resistance distribution along pile shaft and also long-term behavior of load-deflection.With this test results foundation engineer can determine ultimate capacity of pile under lateral load and deflection under service load after using proper factor of safety of pile/pile group.

Soil-pile interaction properties can be determined using appropriate instrumentation; like co-efficient of subgrade reaction in horizontal direction which helps designer to calculate lateral deflecting resulting from bending stress over entire length of pile.

The quality of test result depends on


• Efficiency of personal involved in this test
• Suitability of equipment
• And used facilities 


Sometimes piles are installed in inclined direction; if foundation engineer considers testing pile for lateral loading in inclined pile is impractical, lateral load test results for a neighbor vertical pile can be used to interpret lateral capacity of inclined pile.

If considered feasible, keeping loading within safe structural load on pile/piles or pile cap, maximum load should be applied to reach failure load, which help engineer to determine ultimate capacity under lateral load of pile. With failure load, foundation designer may improve efficiency of foundation applying less factor of safety which will reduce pile quantity/size and length too.



Preparation of test area:


Test area should be filled or if necessary excavated to reach final grade before commencing test of micropile or group as a whole. The test area is considered within an area of 6 m radius from test micropiles or their groups. When necessary pile head should be extended or cut off to make pile butt or butts sufficiently above adjoining ground surface to facilitate

• Installation of load applying apparatus
• Placement of required instrumentation and testing equipment
• Observation of various instrumentations.

Any space in upper annulus of test pile or piles should be backfilled with suitable materials like sand and identical method and materials of backfilling should be applied for all other production piles. Loading selected for lateral load testing should be applied at elevation of pile cut-off level. 



Calibration:


Testing equipment should be calibration before each test or tests in a testing program.Hydraulic cylinder should be calibrated under load testing equipment having hydraulic cylinders for complete range of traveling piston to increase or decrease applied loads. Double action hydraulic cylinder should be calibrated for both push & pull modes. Calibration reports should contain temperature at which calibration was conducted. The equipment which need to be calibrated should defined before commencing calibration. 


Except pulling load, lateral load should be applied by one or sometimes more hydraulic cylinder, having spherical bearings. If more than one hydraulic cylinders are required to exert test loading, all should have identical piston diameter, should be connected to common manifold & pressure gauge and one hydraulic pump should be used for operation.

Total jacking system including valves, hydraulic pump, hydraulic cylinder and pressure gage should be treated as a unit while calibration; the recommended accuracy should not be less than 5 percent of applied load. Load cells and similar devices should be calibrated to accuracy ≥ 2% of applied load and should have spherical bearings.

If pulling force is applied to exert lateral loading, the equipment should be capable to apply constant steady forces within expected range of loading. In-line load indicative devices like dynamometer should be calibrated with an accuracy of not less than 10 percent of applied load. 



Test plate, Bearing plates, Struts and Blocking

Test plate or plates are set vertical side of test pile at application point or points of load and normal to line or lines of loading. Test plate should be of steel having sufficient stiffness to have resistant against bending under applied load,but at least 50 mm (2”) thickness is essential.

Test plate should have adequate size to accommodate hydraulic cylinder; the dimension of horizontal side of plate should be at least on half of pile diameter or receiving side dimension of test pile or piles but not more than pile diameter or dimension of receiving side of pile or piles.

When tests conducted on single piles except square piles, the pile head can be capped to facilitate placement of test plate against a vertical plane bearing surface or it can be set by high-strength grout or can be welded adequately to side of pile using appropriate filler material to establish complete bearing against projected area of test pile. If test plate is/are supported independently of test pile while assembling testing apparatus, temporary supports should be removed while applying test loads.

Bearing plates should be of steel and of adequate size to accommodate

• Hydraulic jacks
• Bearings
• Load cells
• Struts

The main objective is not to be overstressed to transmit applied loads. Sufficient thickness of bearing plates should be ensured to resist bending under lateral load but must be at least 50 mm thick.

Struts should be of steel having stiffness and size to transmit applied loads without buckling or bending. Blocking is used between hydraulic cylinder or reaction piles and reaction system, as shown in Fig. blocking should have sufficient strength and size to resist crushing or any type of distortion under test loading.


Reaction system

In applying test load in lateral direction, one reaction system is required to support hydraulic cylinder or cylinders. This should provide greater resistance than expected maximum test load and can be placed in any suitable distance from test pile. Hydraulic cylinder (s) with or without load cells should be set in horizontal position along the line of lateral loading and fixed against test plate. Between base of cylinder and reaction system, steel strut or other suitable blocking should be placed steel bearing plates as discussed above are set with both end of strut or blocking.

Steel struts should be placed horizontally along line of applying load and suitable bracing should be ensured so that they don’t shift while loading. When two hydraulic cylinders are required, both cells (with load cells, if used) and struts/blocking should be placed at same level and placed at equal distant from line of load application i.e. placed at opposite side of each other.This line is considered passing through center of test group. Necessary support should be provided to jack, struts, bearing plates and cribbing/blocking for stability.

Reaction piles:

The hydraulic cylinder, load cells, struts or blocking are transferring system for lateral loading, but supports are required to hold them. Reaction piles offer us such supports; usually two or more piles are installed vertically but batter piles sometimes in combination with vertical or only batters are used. They offer required reaction capacity to support maximum expected lateral loads chosen to testing. Reaction piles are capped by steel, timber or reinforced concrete. Reaction piles as reaction system

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