For six or seven years, local universities have been expanding their campuses to accommodate the increasing number of undergraduate students due to the curriculum changing from three to four years.
Some universities chose to construct new buildings on their existing campuses.
But for the University of Hong Kong, this was not possible - its existing campus, located in a built-up area, was already crowded. So the university came up with an innovative solution: it got approval to relocate three service reservoirs into underground caverns, and used the space freed up from the relocation to construct three new buildings.
The three buildings, housing the arts, social sciences and law faculties, make up HKU's new Centennial Campus.
And much to the surprise of many, the campus actually looks nice and green, complete with an array of environmentally friendly facilities.
But the point to note here is that HKU managed to create space to solve its land shortage problem by moving three service reservoirs into rock caverns excavated in the adjacent hillside.
Chief Executive Leung Chun-ying, in his recent policy address, referred to the use of underground space in relation to land supply for housing.
Specifically, if public works facilities such as the Shatin, Sai Kung and Sham Tseng sewage treatment works and the Diamond Hill service reservoirs could be relocated into rock caverns, the freed-up space could then be used for housing or other purposes.
Leung also mentioned the possibility of linking up underground spaces of structures in urban areas.
For the past few decades, rock caverns have been used in Scandinavian countries, France, the US and other countries for purposes including underground storage of fuel oil and gas, warehousing, shopping, sports and community facilities.
Train stations in many European and North American cities also often feature large underground shopping facilities that are well connected to office buildings. These places are warmer than the surface in winter and cooler in summer, providing a more comfortable environment for commuters and shoppers.
Most of the rock caverns were excavated from granitic rocks, which are generally stable and relatively impermeable to groundwater flow, except for the occasional fracture or fault zone.
These caverns generally require minimal artificial support (such as rock bolts and concrete linings) and minimal drainage provisions.
Rock caverns as wide as 30m in span have been constructed in granitic rocks in many countries, with proven construction technologies and satisfactory performance over many decades. The main bedrock types in Hong Kong consist of igneous rocks such as granite, granodionite, porphyry and rhyolite.
There is a decomposed zone near the surface due to tropical weathering. But below this zone, the igneous rocks are ideal for hosting large and stable rock caverns.
Technically speaking, it is feasible to relocate the aforementioned sewage treatment works and service reservoirs into rock caverns, thereby releasing the sites for housing development.
In the longer term, there are other urban facilities that could also be relocated underground. These include bus depots, warehouses, swimming pools, and waste and refuse treatment and disposal facilities.
In other countries, such facilities have been successfully accommodated in underground caverns.
If this could also be done in Hong Kong, it might help to free up more precious land for housing in our densely populated urban environment.
Professor Lee Chack-fan is the director of HKU's school of professional and continuing education