As promised, I still have several stories for you and this is another one. As you may remember, we’ve visited China for attending the WODCON in Shanghai1 and afterwards travelled to Beijing for sightseeing. A must see destination in Beijing is the Forbidden City. The epicentre of ancient Chinese power, the seat of the emperor. Once the exclusive domain of the supreme ruler, now a tourist attraction for the general public. The Forbidden City was mainly build in the Yongle era of the Ming dynasty2 between 1407 and 1420. It comprises numerous courtyards and halls and temples. All the buildings are surrounded by thick walls and a moat.
This moat is an impressive 6 meters deep and 52 meters wide. That is a big moat. But remember it is long: 3.5km around3. So, it is an impressive moat. Now consider this moat is dug in the fifteenth century. It has been dug by hand! Imagine, thousands of labourers digging, carrying and removing the soil from the moat. That is quite an operation.
To put this in perspective. The moat has a volume of 6x52x3,532m=1,101,984m³. Yes, that is over a million cubic meters. Even for a modern dredging project it is a serious volume. And digging a hole at one place is the first step. Where do you dispose it? At a dredging project, there is a reclamation area. As this was dry land, there was no reclamation area. So, what do you do with such a volume? If you pile it up, you can store a volume of V=1/3 pi r² h in a cone. Assume a slope of one third of the height to the radius, the height of the pile can be calculated and will be around 49 meter. And that is exactly what the ancient engineers did: they created the hill of Jingshan Park4. With its five peaks, it is not exactly a cone, but the estimated height was quite close!
The engineers had probably carefully planned how they constructed this hill and planned the delivery of the material accordingly. Nowadays, with the much higher production rates and shorter project delivery times, it is highly inadvisable to build a reclamation area with this height. There are several reasons why not to do it like that. First, it would take time to drain the pore water away from the core of the hill. Loading more on top quickly would make it very instable. Sometimes with disastrous results5. Another is when you create high banks, it will be easier for shear planes to form and collapse the structure that way. Lastly, a lower reclamation area will also have a larger surface area and more choice to select multiple locations to evenly distribute the material in volume and composition. A well designed reclamation area requires good knowledge of the deposited material and a skillful team that operates the equipment to manage the deposition.
Based on the exposed rocks sometimes seen on the sides of the Jingshang Park hill, the core is probably consisting of bigger rocks as a kind of backbone. But not every rock found in the moat ended up in the hill throughout the area. Several decorative rocks can be found that have a typical size that could just be handled by manual labour. Just another tribute to the perseverance of those classic engineers.
- WODCON XXII, EADA
- Yongle Emperor, Wikipedia
- Forbidden City, Wikipedia
- Jingshan Park, Wikipedia
- Aberfan disaster, Wikipedia