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COMBINING TOTAL GEOLOGICAL HISTORY AND OTHER APPROACHES IN RESOLVING CONSTRUCTION DISPUTE

BY: MOHAMAD FARUQ SYAHMI MD ARIPIN

INTRODUCTION

Total geological history: a model approach to understanding site conditions is an approach developed by Fookes et. al. (2000) which stated that the ground conditions at any site are a product of its total geological and geomorphological history (generally abbreviated to “total geological history”) which includes the stratigraphy, the structure and the past and present geomorphological processes and climatic conditions. The total geological history is responsible for the mass and material characteristics of the ground. To understand this history, the development of a site-specific geological model is required based on consideration of the regional and local geological and geomorphological history and the current ground surface conditions (Fookes, 1997).  

GMT Group were approached to help in resolving construction dispute by producing a geological report which includes back analysis and modelling. Total geological history approach is the best approach for this report. The dispute involves in differences in soil and rock volume during the preliminaries estimation and post-excavation works. 

RESULT

GEOLOGICAL BACKGROUND

The area is located within metasedimentary rock of Kenny Hill Formation composed of mainly quartzite and phyllite which is a deep-seated depositional environment as suggested by Lee (2001). Three block models were reconstructed where each model specified to reconstructing geological process, to visualized weathering process, and to recreating current geological condition.  

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Figure 1: Reconstructing geological process
Figure 1: Reconstructing geological process
Figure 2: Understanding weathering process
Figure 2: Understanding weathering process
Figure 3: Recreating current geological condition
Figure 3: Recreating current geological condition

Based on the models, preliminary conclusions were outlined as below:  

  • The rock at site location mainly is thick quartzite (>5m) interbedded with very thin phyllite (<1m) 
  • The rock bed is dipping between 50° and 60° with dip direction of between N280°E to N290°E. 
  • The weathering of bedded rock has developed variably strength of subsurface earth material, hence giving variable SPT value. 
  • The quartzite is not abrasive enough due to low grade of metamorphism and weathering, enabling rock to be drilled without changing to drilling technique to rock coring where this change is normally used as an indication of a rockhead is encountered. In this quartzite, drilling method in used soil still can be advanced with core bits or tricone bits. High SPT value is expected.  
  • Based on rock thickness ratio of 5m quartzite to 1m phyllite, it is to be noted that 80% of the rock at site location is medium strong quartzite with the strength of more than 20MPa that complied to contract specification.  

 

CORRELATING SPT N-VALUE

Based on the SI borehole logs and report, the SPT N-value for each layers were correlated with Unconfined Compressive Strength (UCS) based on recommendation by Look (2004).  

Table 1: Relationship between SPT N-value and UCS (modified from Look, 2004)
Table 1: Relationship between SPT N-value and UCS (modified from Look, 2004)

With the information in hand, cross-section of subsurface prior to construction were interpolated to estimate the depth of soil and rock. 

Figure 4: Cross-sections of rock level based on SPT N-value
Figure 4: Cross-sections of rock level based on SPT N-value

CALCULATING VOLUME OF EXCAVATED ROCK

The calculation of excavated rock volume was done using GIS software. The volume differences of the excavated material were first calculated based on difference from initial topography and excavated finished level. The volume difference is then correlated with the rock head isopach map to calculate the cut and fill volume of rock. 

Based on the initial estimation (quartzite:phyllite = 4:1), the actual volume of the ‘rock’ excavated should be 80% of the total cut volume calculated from GIS calculation. 

Figure 5: Initial thematic map generated for volume differences calculation.
Figure 5: Initial thematic map generated for volume differences calculation.
Figure 6: Actual rock volume is estimated by 80% of total cut volume.
Figure 6: Actual rock volume is estimated by 80% of total cut volume.

CONCLUSION 

  1. Geology in study area comprises of quartzite and phyllite of varying degree of weathering. 
  2. Quartzite represent approximately 80% of total volume of rocks that complied to contract specification. Quartzite is medium strong (based on ISRM suggested method) and require blasting, hacking and/or ripping.  
  3. The thickness and dipping of quartzite beds together with mineral content, arrangement and cementation of mainly quartz in quartzite made it not so abrasive and less compact. Hence, it is should able to advance the borehole without changing the drilling technique from rotary wash boring to diamond rock core drilling. The borehole may still be advanced using core bits or tricone bits should penetration is slower. 
  4. Based on calculations, the total volume of 88,620.039m3 of rock is excavated at site location. 80% of it (quartzite) complied to contract specification for rock excavation which yield a volume of 70,896.031 m3 – contractor manage to claim the sum for rock excavation. 
  5. Combination of multi-disciplinary geoprofessional approach is proven can help to solve geo-engineering issues 

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