Tree risk case 3

Tree risk cases 3 – A mature Bombax ceiba with soil collapse incident

Background

The concerned tree is a mature tree of species Bombax ceiba with DBH of 580mm, Height of 23.6 m and crown spread of 13 m.  The tree is located at a planter within a construction site.  The tree is in fair form and the trunk is leaning slightly and with the trunk center growing towards southeast for about 1m at the level of about 4m and then self-corrected.  This abnormality of the trunk growth is due to its adaptation to the past environment and had happened for decades of year and it is common for the tree to compete for sufficient sunlight.   No significant defect was found except presence of some drooping, crooked twigs and some old wounds or scars of the detached lower limbs. The foliage density and size is in normal, and overall vigor and health is good at the time of inspection.

Incident

Shortly before the inspection, a sunk hole of size approximate 2m x 1.5m x 1m (depth) appear at its root zone when the adjacent construction site having its drilling operation (Figure 6.2). The hole was then backfilled with soil (Figure 6.3) and My Team was asked to assess the health and structural condition of the tree.

Findings

The background of the case described the general health and structural conduction for the aboveground part.  Regarding of the condition of trunk base and root system, it is observed that strong lateral roots were growing parallel to the planter and the boundary walls, which suggested that the extent of the tree root is mostly confined within the planter.  No observable crack in the tree roots nor the soil within about 2m from trunk base indicated the main root system is not affected severely except the location of soil collapsed. 

To confirm there are no further movement of tree trunk, four marks were made at the tree trunk and angles of tilting were recorded daily for 1 months (Figure 6.4) and the reading shows the tree tilting angle is stable during the measuring time.

Mitigation measures

Though soil was backfilled immediately, the incident had caused moderate damage to the root system and may induce stability problem to the tree structure.  Therefore, the failure potent of the tree has been increase from low to medium, regarding its large size and frequent landuse, its overall hazard rating become 2 + 3 + 3 =8.  To mitigate the increased risk, a set of temporary steel wire guying system was installed to provide additional support to the tree trunk (Figure 6.5) and the resulting hazard rating should be 1+ 3 + 3 =7.

The system had successfully protected the tree from failure as the adjacent tree of similar size has it trunk broken during the previous Super Typhoon Mangkhut in September 2018 with the peak speed of over 100 km/hr.