Excavating Rocks - Rip-able or Un-rip-able

Road engineers often have to face the challenge on deciding whether the roads they designed at hilly/ mountainous areas, can be excavated cheaply so as to achieve safe and gentle road gradient for the road users. Of course, the engineers can design to the safest gradient theoretically with the latest high capacity excavating equipments and blasting, but the cost can be exorbitant which renders the project not feasible. This is particular true for under developed countries where budgets are limited. Modern living requires shortest distance and safest road gradient between the two points. In the mountain regions, mountains have to be excavated and valleys filled up or bridges built to satisfy such needs. But deep excavation often encounters hard rocks and rock excavation is expensive. It generally costs about US$2-4 to excavate and filling of soils, but will cost more than US$10-20 to excavate rocks, depending on the remoteness of the project sites. A balance has to be made between safety, comforts, distance and costs. The knowledge of the machineries available locally and rip-ability of local rocks become important so that the designers can decide which lowest and most economical excavated levels the local builders can achieve.

                                                                                       

        

These photos show the 20 ton capacity excavator being used to rip the rock. Obviously, this machinery is not suitable for ripping as capacity is low and the energy splits between the many teeth of the dipper. But as the rock is Shale, a soft rock, he may be able to rip slowly. But much energy is lost through friction and heat. Perhaps, this Contractor feels freer and economical to use his own available machine then to rent a D8 bulldozer with the ripper from others.

On the local scene, D8L with the ripper, having 335 horse power and weighing 25 tons, is the standard machinery specified to decide whether the rock is to be paid “rock rate”. If it is able to be ripped by this machine, then it is considered “earth or non-rock”. If not, it is paid “rock rate”. But in view of the high difference in cost, the Contractor obviously wants to claim rock rate while the client wishes to pay only non-rock rate. Thus Contractor may use an under-powered D8L to prove his point or even ask the drivers to cheat on the use of gears or operations so as to show the rock is not rip-able. Sometimes, road projects were abandoned because of this controversy, especially if the engineers provide low quantities of un-rip-able rock in the Contract. Of course, the design engineers can re-design the gradients to minimise rock excavation, but the safety of the roads may be compromised beside delay of projects. I remembered one road with a road gradient of 25% in some stretch to a hill resort, became accident prone stretch within one month of the opening. The cost and safety of the road is closely related.

But how to determine whether the rock is rip-able or not? Engineers nowadays easily acquire road design software to come out with beautiful lines, graphs, figures to even two decimals, cross-sections and quantities. But to estimate the non-rip-able rock from this standard software is beyond its capabilities. The engineers have to decide which level is the cut-off point as even 10m away may be different.

The knowledge of geology is the first pre-requisite. It is fortunate that the earlier geologists had come out with a Geological Map of Sarawak, mapping all the different features, geological history and types of rocks of Sarawak at different regions. These geologists of Sarawak Geological Department were great enthusiasts, scientists and explorers, who despite the inaccessible and difficult tropical terrains, ventured to the ground, studied diligently under the hot sun for years during the last century and came out with the geological reports and Maps, which had benefited the geologists/geotechnical engineers today, including me. I saluted them. The fourth edition dated 1986 could be purchased from the Sarawak Geological Department and I often referred to this Map.

Soil investigation is the next important step. Although geological map is a helpful preliminary investigation, detailed soil investigation will confirm the actual nature of the rocks beneath the proposed road. After all, earlier Geologists used generally visual observations of exposed outcrops, features, plus limited borehole drilling to come out the geological map. Some local variations or intrusions of rocks may happen during the tumultuous age of mountain buildings. Types of rocks will straight away determine whether the rocks are rip-able or not. Igneous rocks, such as Granite, Andesite, Gabbro, Basalt, Microtonalite, etc are generally difficult to rip, unless they are thinly laminated. Sedimentary rocks such as Shale, Siltstone, Mudstone, etc are generally easier to rip, but not the Limestone which is strongly bonded by chemical and can reach a strength of exceeding 30N/sq.mm unconfined strengths. As of metamorphosed rocks, Quartzite, Schist’s, Gneisses and Slates can be difficult to rip depending on the lamination, mica content and degree of weathering. Drilling boreholes can extract the rock cores beneath the ground for which the Geologists and the Geotechnical Engineers can study the rock samples. Together with Total Recovery Ratio (TCR) and Rock Recovery Designation (RQD), some feelings on the rock rip-ability can be made but not absolute. Drilling procedures, such as vibrations and water entries may produce totally shattered samples, which appear easily rip-able.

Seismic survey is sometimes deployed for larger road projects. Caterpillar (1983) and Smith(1986) had co-related Rip-able Charts with their research on seismic survey, and are helpful in determining the rip-ability of rocks. Seismic velocity travels 0.3km/s in loose unsaturated soils, but reaches 6km/s in hard rocks. Generally, velocity of 2.0km/s is rip-able while >2.5km/s is unlikely. In between, it is the grey area.

The most popular available excavating machineries used for road construction in Sarawak are Caterpillar D6 bull dozers and 20 tons excavators, mainly at lowlands and small hill areas. But D8 dozers are only owned by some big construction companies. D8L is probably the most common type which is specified in the local road specification. D8 Dozer had evolved from 132 horse powers (hp) in 1932 to 335hp D8L in 1982 and 310 hp D8T in 2004. Power is the most important factor to decide whether the rock is rip-able.

Dozer had since developed into even more powerful machineries, D9, D10 and even D11. These machineries are used to excavate non-rip-able rocks. Although blasting and hydraulically mounted breakers/chisels are another commonly deployed methods used to excavate hard rocks, but blasting requires a lot of safety procedures and relevant approvals which quite often cause delays. Chiselling is too slow for large quantities of rocks. Blasting is often prohibited near dam sites and other sensitive Equipments installations. Therefore, D9 and D10 dozers become the only options to excavate un-rip-able rocks in many areas. These machineries were often used by the large and rich timber corporations in the interior mountainous areas to build roads to transport their logs. With the dwindling of the forests, these timber companies had switched to construct highways which could deploy such machineries. In 1955, D9 had 286hp but by 2004, D9T had reached 410hp. But the most powerful D9 was 1980 D9L with 460hp.  Komatsu D275A probably posed a strong competitor to D9. In 1987, Caterpillar introduced 700hp D10 and 520hp D10N, weighing 82-86 tons. Komatsu came out with D455A with 620hp and weighed 76 tons. By 1986, Caterpillar produced 770hp D11N weighing about 102 tons with ripper, and by 2008 developed 850hp, 120 tons D11T. Komatsu responded with 1150hp D575A weighing 168tons. Such huge machines would be damaging to move on the local roads. I had seen D9 and D10 in the timber concessions, but I am not sure whether there is any D11 bull dozer in Sarawak.

Availability of large excavation machineries does not mean cheaper rock excavation cost. These machineries are expensive to procure, and probably not only the manufacturers monopolise the market, the owners also monopolise the local construction market. When budgets are limited or the country is poor, Engineers have to propose a design of steeper gradients with less excavation, slower speeds and longer routes. Road users have to be more careful, drive slowly and ensure that their cars are maintained road-worthy.

When you are poor, you cannot expect too much.