Abstract:
“Prediction of Side Friction of Drilled Shafts in Very Dense Fine SANDâ€
UAE - Case Study
By: Emad Sharif
GTC Lab - Director
Emad.sharif@gtc-lab.com
+971 50 456 6264
Abstract:
A project in Ras Al Khaima Emirate in UAE, includes nine (9) large diameter (84m) water reservoirs. Drilled piles were selected for the foundation by the consultant in the tender documents.
The subsurface materials were explored with many deep boreholes, CPT tests and down hole seismic logging, and was found to be composed of very dense fine to medium sand. The SPT (Standard penetration tests) blow counts ranged from >50 to 100 blows. Ground water table is found at more than 60m depth.
Theoretical prediction of side friction of drilled piles in sand may be done by different published procedures. The most recent empirical ? - Method is widely used. In this method, the maximum skin friction is only dependent on the overburden effective pressure, as: fsz = ? ?'z , and ? is only dependent on the depth z below ground surface. The initial relative density (RD), or likely over-consolidation (OC) of the sand is not an input parameter because, according to this empirical method, the soil will be disturbed during pile boring and installation, and the lateral pressure between the soil - shaft interface is controlled by concrete fluidity.
Application of the ? - Method to predict the side friction for the case of very dense sand has resulted in very long piles to achieve the design capacity given in the tender.
No time was allowed to conduct trial test piles at the site, and the contractor was required to submit his design of the piles based on the site investigation results.
Noting the large number of piles in the project, therefore, prediction of side friction with other method which accounts for likely over-consolidated nature of very dense sand became necessary.
An appropriate method which can be used for the assessment of the side friction of the OC soil with SPT >50 is the IGM3 Model, presented by “FHWA-RD-95-172 - Load Transfer for Drilled Shafts in Intermediate Geomaterialsâ€, which presents a method for predicting the performance of drilled shafts in IGM (Intermediate geomaterials) with IGM3 representing materials sampled with SPT having N>50 and <100.
This method predicts the OCR (Over consolidation ratio) and predicts the maximum side friction with N (SPT) as input parameter, and gives results much higher than the ? - Method.
The optimized pile design was approved subject to verification with site - specific static loading tests.
During construction stage, two static loading tests were conducted on sacrificial piles (750 and 900mm diameter) up to 250% of the adopted allowable axial capacity (Preliminary test piles).
Further, several static pile loading tests were conducted on the working piles (1%) up to 150% of allowable axial capacity, in addition to many dynamic loading tests. The tests results provided great substantiation to the adopted pile designs, and has generally indicated that the adopted maximum side friction ranges could have been further increased.
The paper will highlight the nature of the very dense fine sand formation which forms a major formation in UAE and Arabian Gulf Area, and will focus on the use of the IGM-3 formula for predicting reliable ranges of side friction. The results of the pile loading tests will be presented and analyzed to illustrate the technical feasibility of the approach, which has achieved significant savings.
Key words: Over consolidation, Relative Density, Side Friction, pile loading tests.