Failed Road Embankment and the Probable Causes

This almost 20m high new paved road embankment at a hilly terrain in Bintulu Division failed completely after heavy rain. Why?

The road embankment is crossing a slanting, narrow and deep valley, with hill less than 5m away.

There are five main factors which affect the road embankments:

(a)   the design, the slope provided, the foundation     and the earth materials used for the filled            embankment,

(b)   the method of construction of the filled                embankment,

(c)   the rain and effective drainage,

(d)   the traffic

(e)    the breakage of water supply pipe

(f)    the maintenance.

From the photograph, it can be seen that it was a complete collapse of the filled embankment, some soil slurry was seen flowing from the right side down the slope. There seemed to be a large depression on the right hand side of the road which was part of valley, and probably formed a pond after heavy rain. No concrete drain was seen and earth drain was     presumed. No much vegetation was seen on the right hand side indicating new construction. Turfing or vegetation had not covered the bare surface     completely. Water pipe was not seen on the             embankment.

The embankment was stable for a while after construction when there was no much rain. It was the rainfall and poor drainage which resulted water infiltrated into the fill embankment and weakened the soil.  It was also the accumulated pond water which continuously supplying water into the embankment soil until the soils were saturated with a lot of free water.

Just soak a compacted soil sample in the water, you will see the immense change of moisture content, CBR and hence the strength. If you vibrate it, the whole sample would liquefy and becomes slurry. Heavy lorries and machineries cause vibration.

As there was no water pipe, it was not the water leakage that infiltrated into the embankment. In fact many filled embankments failed because the water pipe broke. This often occurred at the junction of the excavated road and filled embankment which settled due to poor foundation, compaction and erosion.

Thus, from the sudden failure of the whole embankment, it looks more like slope stability/foundation sliding failure, which means average strength of the embankment materials has gone below the design requirements. Water seepage changed the properties of the earth-fill materials.

To prevent such problem, design-wise, always provide efficient drainage for this kind of high fill embankment.  Any depression shall be filled and levelled at the road side, must indicate on the Plan. No water pond shall be allowed to form. If the soil materials are very permeable and erodible, concrete drains have to be provided to prevent water seepage into the filled embankment. It is an advantage to have a few layers of coarse grained materials in the embankment to allow water to drain through to the other side efficiently, but shall not in a way allowing erosion of these layers. The bottom layers and toe of the slope are preferably large and small crusher stones mixed with coarse sand, to allow for such drainage.

On the construction, the valley bottom in the hilly area is often weathered to reach rocks, but sometimes, there are some shallow colluvials soils, which can be easily excavated and replaced with rocky materials. Higher valley sides need to excavate 0.3-0.5 m soil and replaced with compacted soil to ensure proper anchoring and no weak contact layer. This step will form firm foundation and prevent sliding along the slanting valley. 

The choice of fill materials from the borrow pits and degree of compaction will affect the performance of the embankment. Always investigate and test the materials before use. For road project, there is always a soil laboratory specified for doing such tests. If the materials are too silty, compaction is difficult, especially during raining. Clay also cannot be compacted during raining. Just look at the Compaction Curve of the Compaction Test  and see how sensitive is the moisture content on the field density. It is difficult to achieve 90-95% optimum density due to high water content. In fact, no work shall be done during raining, unless you use gravelly or sandy materials. Gravelly materials are hard to find generally though. The excavation of borrow pits often begin at he surface which is generally firm to hard residual soils followed by highly weathered rocks. The surface soils are often used for the foundation layers of the embankments while the rocky soils are used on the top. Planning shall be in such a way that the rocky materials are used for the bottom layers. 

Sarawak is in the tropical zone, hence rainfall is a common occurrence throughout the year. The dry season may be during May-August, but can construction wait until that period? 

Another problem with the local Contractors is they like to dump-fill into the valley to build up the embankment without compaction, sometimes not even make foundation clearing and preparation. This is the easiest and fastest way to build up high embankment, especially when the Supervisors are not around. Just bore down the embankment and do SPT tests, you can detect which layers are compacted and which are not. Well compacted soil has SPTs more than 5 (depending on the materials used, compacted sands have SPT more than 10),  those less than 3 were not likely compacted although the seepage of water would also lower the SPTs. Built-up embankments do not fail easily if there is no much water seepage or erosion, but uneven settlements on the road pavement will occur.

Maintenance includes inspection, clearing, repair of the drains and turfing of any bare ground, which inhibit water to infiltrate into the filled embankment. The Authority often has insufficient fund allocated for inspection and maintenance.