This past Eid-al-Adha holiday that is during15 thru 20 November-2010, we (I and a couple of friends and our families) planned a visit to Hufuf for the sole mission of sight-seeing at the famous Cave site.

This Cave site is officially known as Jabal Al-Qarah (pronounced as Garah) – or Mount Qarah in English. It is also known as the “Ali Baba Cave” - after the famous character in the classical 1001 Arabian Nights Fairy Tales of Ali Baba and His Forty Thieves. It is located about 13 Km east of Hufuf city.

Hufuf was the capital of the Eastern Province until 1953. Many parts of the old town still exhibit evidence of the Ottoman rule era – including an old fort. The agricultural date farms make Hufuf the largest oasis in Saudi Arabia. Geologically speaking, these are an intricate cave system developed in the calcareous sandstone, marl and clay and represent a mesa comprising of the Upper Miocene to Lower Pliocene Hufuf Formation, in front of the escarpment that marks the eastern edge of the As Summan or Shedgum Plateau.

Jabal Al-Qarah is known for its tall, linear cave passages and narrow canyons. The box-work of linear passages is better developed here than any other cave locations in the Eastern Province. Also, unlike most of the caves reported from the As Summan Plateau, formed by dissolution of limestone by ground water, the Jabal Al-Qarah caves represent an above ground (street level) cave system that appears to have been developed due to sub-aerial weathering and enlargement of the well-defined joint and fracture system in the Hufuf Formation. Due to sub-aerial development, the caves in Jabal Al-Qarah do not show many cave features typical of other caves in eastern Saudi Arabia.

Jabal Al-Qarah is a flat-topped hill with a maximum elevation of about 225 m above mean sea level. The eastern edge of the Jabal, close to the cave entrance, is interpreted to be bounded by several north-south trending high-angle normal faults with throws up to 10 m. The mushroom-like pillars of the Hufuf Formation observed close to the cave entrance appears to be one of these down fault blocks.

The cave system has approximately 28 linear passageways totaling about 1.5 Km, in a rectangular area roughly 132 m x 216 m. And unlike many of the limestone caves in the As Summan Plateau, where the cave floor, walls and ceilings are characterized by the presence of various features including stalactites, stalagmites, etc, the interior of the Jabal Al-Qarah caves is either clean or covered only by a thin veneer of wind-blown dust and guano.

Flash back – some 13 years ago when my present boss had interviewed me in Karachi-Pakistan for hiring me as his senior structural engineer for a big government project in the city of Hufuf he had specifically poked me then about two geotechnical complexities expected to be encountered at the project site. The issues per se related to the surface and sub-surface geology of the project site. One of them was about the expansive soil or swelling clay and the other about the sinkholes or the limestone solution channels (the cavities) found in the underlying bedrocks.

I must admit here, while I had some experiences of design of building foundations over expansive or swelling clay, the issue of sinkhole at that time was simply Greek to me. It is true, as a structural engineer I was aware of the geotechnical complexities that are often faced while undertaking structural design of building foundations – which are obviously founded on soils. And, technically speaking, Soil-Structure Interaction is a complex issue. Nevertheless, it is no wonder that the issue of sinkhole had aroused in me some curiosity.

It is important to mention here that those were the days when Internet accessibility was simply unknown in Pakistan and it was hard time for me finding any technical materials on the subject issue. Needless to underscore, the craving in me for learning about the sinkhole kept revving up. Fast forward – after arriving in the Kingdom, the professional wheels got into motion immediately and I was stationed in Riyadh assigned with taking care of the structural design issues related with the project per se in Hufuf.

The first thing that I did was plan a preliminary geotechnical investigation which was conducted at the project site. The geotechnical report revealed that the project site indeed had two geotechnical complexities of a serious nature. Two out of nine zones of the project sites had swelling clay or expansive soil problem. In simple words, it meant that the swelling soil (expansive clay) in the event of getting wet would swell & expand and consequently exert upward (heaving) pressures underneath the building foundations. All of this meant that the ordinary building foundations would simply not work. In the rest of the zones, yet another geotechnical complexity was encountered.

This was about the presence of sinkholes and the limestone solution channels (or cavities) in the bedrock underneath. This also warranted that some new kind of building foundation system be designed and built so as to circumvent this unique problem.

Fast forward further – I took up the challenge upfront and came up with special design system for foundations to circumvent & address these two unique geotechnical complexities. To address the expansive clay issue a special type of foundation system called Stiffened Slab Waffle Foundation was designed and built. In fact on Sunday the 7th of February 1999, a history was made when the concrete for the first ever Stiffened Slab Waffle Foundations was poured. Saudi Arabia thus became only the third country in the World, after USA and Australia, to have used this new foundation system to address the expansive soils problem.

For countering the sinkhole or the cavities issue yet another special type of foundation system called Semi-Rigid Raft Foundation was designed and built. Both of these foundations have performed well since they were built.

I take pride in the fact that later on I went on to produce two research papers on these two subject issues that were accepted for presentation in the 5th International Conference on Case Histories in Geotechnical Engineering held in New York during 13 thru 17 April 2004. 
 
Images courtesy: Cavinguk.co.uk