About Landslides, Debris Flows, Flowslides and Such Unpleasant Happenings

TYPES OF LANDSLIDES

Depending on where we are and which expert we are talking to, the exact definition of landslide, debris flow, mud flow, earth slides, and earth flow may vary. As far as the people affected by these unpleasant happenings are concerned, they bear similar consequences regardless of what name one gives it. For their understanding, a simple explanation is provided.

Debris flow is a generic term used by engineers to describe the rapid movement of rocks, soil, water, and vegetation downhill. A debris flow could be a mudslide or a landslide, depending on the amount of water present. Flows contain many different-sized particles from sand grains to boulders, but the biggest rocks travel at the front of the flow.

Landslide is similar but may be drier. Landslide may be in the form of a block slip, which happens when the hillside is cut to make a road or for development in hilly terrains. Rotational slip is a term used to describe slumps. A slump is the slippage of a whole block of earth at once. The soil moves in "one piece".

Several forces work together to create slumps. Usually, resistance to downhill movement is created by roots holding soil together or by soil particles sticking together. Heavy rain, combined with stresses caused by wind on vegetation, work to loosen soil and pull the soil downward. Again, saturated soil can get loosened enough to move downhill, taking the vegetation and loose rocks with it.

Not all landslides occur rapidly even when triggered. Some, called creep, take a long time to develop and may slowly move over period of months or years. Although slow, almost all such happenings create signs which can be seen if one is on the lookout for them. The entire town of Kundasang, Sabah is moving ever so slowly, but moving nonetheless. This is an accepted fact by the residents there.

CAUSING LANDSLIDES AND SLIPS

A number of factors causes landslides or slope failures. In addition to these factors, one must always remember that slope failure problems are triggered by any combination of water saturation and flow, weak-heavy earth materials, and steep slopes.

WATER is the most common trigger of slope failure.

Any operation that allows ground to be saturated by water or entering of water has to be avoided.

High and steep slopes

Landslides are more likely to occur on steep slopes than gentler slopes. Wash gullies, which carry eroded soil downslope and are subject to rapid erosion, form more often on steep slopes. Debris flows are also more likely to take place on steep slopes. High slopes are more susceptible to damage due to water ingress and loss of soil cover, therefore needing much greater vigilance.

OTHER FATAL LANDSLIDES

1961 Ringlet, Cameron Highlands, Pahang - 16 fatalities

1973 Kampung Kacang Putih, Ipoh Perak - 42 fatalities

1995 Lebuhraya KL-Karak - 20 fatalities

1996 Keningau, Sabah - 302 fatalities

1996 Gunung Tempurung, Perak - 1 fatalities

1999 Kg. Gelam, Sandakan Sabah - 17 fatalities

2002 Simunjan, Sarawak - 16 fatalities

2002 Taman Hillview, Kuala Lumpur - 8 fatalities

2006 Taman Bukit Zooview, Hulu Klang - 4 fatalities

Dumping debris down the slope

The prevalent bad practice of simply discarding construction debris over the slope or using this method to build up a slope with cut soil, as we have seen, creates loose fill. Following rain, such loose fill gets saturated and can lead to floeslide or remain unstable and in time can permanently damage properties below. Some call this method as Tip Fill.

Position of structures on the slope

Where structures are built or some form of weight is put along the edge of a slope, weakening of slope and subsequent failure can occur.

Undercut at the toe of the slope

This removes supporting material at the toe of the slope and can aggravate landsliding and erosion. This situation is common in developments and road construction and can cause what is referred to as block slip.

GEOLOGY

GEOLOGY AND SLOPES

This section explains the relevance of geology to slope stability.

Geology is important because soil composition and bedrock also determine where landslides may occur. Soils are formed due to weathering (breaking down of rock into soil). The end results of weathering vary depending on the origin of the rock and its geological history. Soils with greater clay content stick together better because the clay particles absorb water slowly. Soils with more organic material, such as top soil under trees, act like sponges to soak up excess

water. When this top spongy layer is removed by logging, clearing, burning, or compacted by heavy machinery, excess water flows over the surface and erodes the top layer of the soil.

When the underlying bedrock is porous, such as sandstone or some volcanic-formed rocks, the soil can drain faster. This prevents the soil from getting saturated and reduces the chances of a slip. Non-porous or impermeable bedrock (the rock mass underlying the soils) such as granite, keeps water in the soil above it. This increases the chances of saturation and eventually a slip of the soil above.

Joints in the rocks refer to fractures in the rock mass. Jointed bedrock (the fractured rock mass underlying soils) is subject to increased landslide risk depending on which way the joints are located, on how the fractures have formed, and how intense the fracturing is. Joints, parallel to the topsoil, can allow the soil above to slide easily, thus causing slip. The water lubricates the layers, creating a slip plane, and causes them to slide against one another like two pieces of paper. Vertical cracks or joints in bedrock, however, can actually help stabilize the slope by allowing water to enter into rocks and thus reducing the chances of a slip. If tree roots can get down into the cracks in the rocks, the soil above is further stabilized.

The type of bedrock can also determine slide potential. Some rocks, such as shales, lose strength when saturated and allow the soil above to move. Sandstones and granites wear down into coarse porous soils that can be easily shifted around.

VEGETATION

The effect that vegetation cover has on slopes and on the slope's tendency to slide is complicated. Trees intercept rain, lessening the impact that individual raindrops have on soil. Plants and undergrowth absorb the force of the rain and take up water for their own growth. It is easy to see what will happen when soil is not covered. Rain hitting the ground loosens the soil and washes away the soil particles, causing erosion.

Vegetation provides cover against rain and prevents shallow surface slips from occurring. Their roots hold the soil down. Trees have larger and deeper root systems than grasses or shrubs, and therefore are more effective in holding down the soil. When trees are cut, their roots die and quickly begin to decay. As the roots rot, they lose their ability to hold the soil. The roots eventually decay to the point where they become part of the soil. It must be noted that vegetation helps prevent shallow slip failures, which are typical of collapses around buildings and developments. Grasses, shrubs, and new trees begin to grow on the site where trees were cut. As the new trees grow, their roots gain strength in retaining soil. For a few years, the grasses and shrubs are the most important factor in holding down soil and preventing ugly erosions and shallow slip failures.

However, strong winds blowing on treetops can weaken the hold of the roots on the soil. The tree trunk acts like a crowbar: the wind pushes down on the treetop, which behaves like a 'handle' and pries up the soil with the roots. The resulting loss in slope stability can create a slump. Bare soil erodes rapidly. Removal of vegetation has been shown to increase runoff. If water is moving on top of the soil, it has a greater ability to move loose soil, rocks, and plant material.

PREVIOUS LAND USE

The influence of previous land use on slopes is also something that has to be taken into account in design.

In this category, we also include cases of bad construction practices. The problem with such practices is that all the mess goes unrecorded and is hidden under cosmetic work such as neat earth cover, grassing and trees. When cosmetic covers are put into place, all the visible traces of bad practices are hidden until there is a failure.

Remedial works on such slopes are very difficult. This is because it is impossible to assess how bad the existing grounds are. Even if the failed slope is properly re-designed and re-constructed, it will still be influenced by its surrounding areas which were badly constructed.

Since nobody keeps records of how badly the previous work was carried out, the engineer doing remedial works has to assume that the works were carried out as per specifications. This assumption forms the primary basis for almost all failure assessment and remedial designs. Since such assumptions are flawed, so are remedial designs and remedial works. An all-round NO WIN situation.

Previous land use is probably one of the most easily determined factors in predicting a landslide. Stacking, storing, or dumping along slopes with heavy material such as construction rubbish and dirt from cuttings on the slopes can cause failure. Slopes cleared for houses in the forest interface can create problems too. Such an area is cleared of trees and grass and then leveled to create flat building platforms at different levels. These are left exposed with little or no plastic covers and thus open to rapid erosion.

Some trees and shrubs may be planted after the houses are finished, but they cannot replace the root cohesion of the removed vegetation. Add to this, a badly compacted fill and the stage is set for rapid erosion.

Moreover, because of our bad earthworks, fills can easily be saturated and fail during rain in the form of small slides or slumps, which are easily seen at any development with their telltale erosion gullies.

Erosion damages such as gullies are filled up with loose fill before the project is handed over to the enduser, who now ends up facing difficult maintenance problem which may no longer be routine.