Er. Vivek Abhyankar, SGAWings Civil Engineering Consultants, Mumbai
Dr. N. Subramanian, Consulting Structural Engineer, Gaithersburg, MD, USA
The five elements - Earth/Soils, Water, Air, Sun, and Space form our environmental system and keep the world alive through various daily and seasonal actions. Animals and trees form an integral part of our environmental system and are a god-given gift to humanity. Unlike animals, trees or plants can’t move but are the most dynamic and self-dependent systems (i.e. plants can extract energy from the environment and make food for themselves as well as animals and humans). Civil engineers only learn about concrete, steel, advanced materials, earthquakes, drone technology, etc., but may not have adequate knowledge of where to plant the trees to have maximum benefits but with minimum damage during cyclones.
Unplanned/uncontrolled growth of trees may often lead to increased problems during cyclonic winds and vigorous rains. Falling of trees on cars, roofs, sheds, or the public can cause severe damage every year. In the last year (i.e., in 2020) alone, India witnessed two devastating cyclones, namely Amphan (hit on the east coast) and Nisarg (hit on the west coast on 4th June’2020); in the year 2021 yet another cyclone ‘Taute’ hit the west coast and Gujarat on 10th May’2021 and caused tremendous damage to property and lives. During these cyclones, there were two types of damages: (i) direct damages due to high-speed winds, (ii) indirect damages due to falling of trees on structures/cars/humans. The first kind of damage can be controlled to a large extent through well-engineered structures but the second one is often neglected. During the recent cyclone, we observed great damage in and around Mumbai city due to many fallen trees on various structures/properties/objects and even on humans, as can be seen in Figs.1 and 2. The damage caused to mango farming in West-coast during the 4th June’2020 Nisarg cyclone is more than Rs. 6000 crores (https://timesofindia.indiatimes.com/). Still, the mechanism of tree stability and falling is not considered by civil engineers/architects. It is a myth that the ‘study of trees’ is to be done only by landscape architects/horticulturists/environmentalists. The traditional Civil engineering syllabus does not include any subject or a topic or tree stability mechanism (SM) and cautions/care to be taken. The present paper provides to lay a foundation for this subject.
Types of Trees
There are more than a thousand types of trees (small to big). But only the popular and taller than 8m trees, widely found in India are considered here. These trees include the Coconut tree, Banyan, Peepal tree, Jamun, Almond, Cashew, Mangos, Jackfruits, Nilgiri (eucalyptus), Chinar, Saag (teakwood), Rubbe, which are widely found alongside roads and buildings.
Benefits of Trees
As mentioned earlier, trees are an integral part of the earth’s ecological system. Most trees uptake carbon dioxide (CO2) and produce oxygen during the daytime, using the energy of the Sun, by a process called photosynthesis. While at night, the plants uptake oxygen and release carbon dioxide, which is called respiration. The Banyan Tree, which is the national tree of India, emits large amounts of oxygen during the day; other Indian trees which produce most oxygen are Neem, Peepal, Arjun, and Ashoka Tree(www.nelda.org.in). Peepal trees can uptake some amount of CO2 during the night also, due to a type of photosynthesis called Crassulacean Acid Metabolism. In addition, trees produce useful fruits and flowers for the benefit of mankind. Trees also give us shadow and help to maintain sustainable temperature on the earth. Most importantly, timber derived from several trees is used as a ‘renewable’ construction material (Subramanian,2019). Unfortunately, the ratio of ‘number of large trees cut every year’ for civil engineering projects (mega townships/roads/metros, etc.) vs. the ‘number of new plantation’ is not a balanced one. Many of the authorities impose action like - heavy penalty or even one-year imprisonment if someone cuts a tree. But rarely such things are implemented and unfortunately, the common public does not realize the importance of preserving old trees. Unfortunately, such tendencies are going to risk our future generations.
The Classification and Components of a Tree
To understand the mechanism of stability as well as falling of trees, the first one should know the classification of trees. In general, the vegetation/trees/plants may be classified as (1) Creepers, (2) Shrubs, (3) Crops, (4) Small plants, and (5) Large Trees.
Roots support and grip the soil and transfer the loads applied to the branches and the stem. The roots of some plants may tend to penetrate deep down the soil, whereas a few tend to grow laterally. This pattern of the roots and their growth depends on the type of trees, age of the tree, height and weight of that tree, type of surrounding environment (availability of water, type of soil/rock, manure, the slope of the ground, rains, wind, sunlight, the shadow from adjoining trees/buildings, salinity of water/wind, environmental pollution, and maintenance - pruning of leaves, branches. Fig. 3 shows a typical sketch of a tree showing these parts.
Another important factor is the slope of the ground and the slope of the tree itself. The trees which grow on the slopes are usually smaller in size and have firm, long roots gripping the soil below. Fig.4 shows trees on sloping ground and having inclined tree trunks. Such trees experience a large bending moment at the base. They are susceptible to falling and add weight during landslides.
The stability of any tree depends on the relationship/combination of four factors: (i) Strength of the roots, (ii) Height, width, and weight of the tree, (iii) Slope of ground and tree, and (iv) External forces.
The age of a tree can range from 10 years to 500 years (as per its type and other external factors explained earlier); In the United States, the tree with the longest lifespan is the bristle cone pine, which grows in the mountains of Nevada and southern California, which can live up to 5,000 years! (http://scienceline.ucsb.edu/). For an accurate assessment of the problem, an engineer must estimate the age of a tree. The approximate age of any tree can be assessed by any of the following rational methods: (i) Local inquiry with people in adjoining areas, (ii) By counting the number of circular rings formed inside the stem (this method is suitable in cut trees or partially exposed stem), (iii) By measuring the circumference of a tree and correlate with height and by using the formulae given below, (iv) Average annual growth rate. A few examples given below will be useful to explain the above methods clearly.
METHOD 1 (www.michigan.gov)
- Let us assume that the perimeter of a particular tree is measured as P = 2743 mm
- Hence, average Diameter D = P/ π = 873 mm and average Radius = 436.5 mm.
- Now subtract about ’12.5 mm’ from the Radius to account for bark. Thus R = 424 mm.
- If the rings are exposed on the stem then count the total in radius.
- Let us assume, number of rings as = 25. Hence average ring width,w = 424 / 25 = 17mm.
- Estimated age of that tree = R / w = 424 / 17 = 25 years (i.e. approximately equal to number of rings)
- Let us assume, the perimeter of a particular tree is measured to be P = 2743 mm
- If the average growth rate of the tree = 76 mm per year (this method is suitable for fast-growing trees)
- Then Age of the Tree = 2743/76 = 36 years
Now let’s understand the concept of ‘root-plate’. As the trees grow the roots spread radially in search of water, nutrients, minerals as well as to maintain the overall stability of the tree. The diameter and the depth of roots can be measured as shown in the figure below. This is called a ‘root-plate’.
Loads Acting on Trees
Let us now consider the forces acting on the tree and how to quantify them. Trees larger than 8m in height (i.e. about a two-story building structure) are found to be causing more damage than smaller height trees. Only trees taller than 8m are considered here.
Typical loads on the tree are:
- Self-weight of the tree inclusive of root-plate (unit weight of wood for various types of trees can be found from IS 339:1963 or IS 875 Part-I:1987, or IS 883:2016).
- Additional moments at the base due to the inclination of the stem.
- Lateral forces due to –
- Wind, base shaking (earthquake)
- Vehicular impact.
- Lateral soil load/Landslides
- Lateral snow loads/avalanches (in northern hilly areas of India)
During cyclones, if the forces exceed the resistance offered by the root-plate or the stem then either the stem shears off or the tree collapses by toppling of root-plate (RP) zone. The diameter of the RP and the distance from the nearest structure dictate the damage potential of trees. Often during hot sunny days or rains, people tend to park their vehicles under a large tree to protect vehicles from heat or rain but may get affected if that tree falls due to whatever reason. The falling of trees in cyclones does not give any forewarning for the public to act; hence there is an urgent need to costruct public parking structures in city areas. Apart from this, there are a couple of measures that should be taken while planting the trees or securing them against collapse.
Principles of Planting Trees
Late Architect Jerard D’Cunha, during 1996/97 showed how trees should be planted around the Prince of Wales museum, Mumbai (see Fig.7). According to him, tall trees should be planted adjacent to the compound walls (which purify the intake air from dust particles), followed by a 5m wide grass bed (which will cool down the clean air) and then small shrubs (which elevate the airflow), followed by a walkway and again small shrubs. Such arrangement was made to keep the intake air inside the museum clean, cool, and fairly dry (which is a primary requirement for any museum building for the longevity of articles stored inside).
Plantation of trees can be classified into three broad categories (i) plantation in and around buildings, (ii) plantation in and around infrastructure (roads/bridges/railways) and (iii) special/shore protection measures.
Large trees should be planted at a sufficient distance away from the nearest building. It is better to plant small trees 4.5 m away, and large trees at least 6.0 m away from a building (https://extension.missouri.edu/). Usually, if the expected height of the trees is known, then the minimum distance between the C/L of the tree stem and the face of the building should be kept equal to 0.6 times the height of a tree. Another thumb rule, which is a bit conservative is to plant the tree at a distance away from he building equal to the mature height of the tree (Wray,1995). In the case of trees with inclined stems, the distance has to be measured from the tip of the trees (remember that inclined trees are more susceptible to falling than upright trees). Figure 8 schematically shows the ideal distance of a tree from a house proportionate to the height of the tree. This is not just to avoid the falling of a tree on the structure but also considering the changes in wind pattern around the trees, bird flights, falling of leaves, etc.
Table 1 shows typical observed heights and ages of popular large-height Indian trees.
|Table 1 Typical age and height of popular Indian trees|
|Name of Tree||Maximum age (Years)||Maximum height (m)|
|Nilgiri (eucalyptus)||100 years||9-55|
|Saag (teak)||200 yrs||46|
However, it may often be noted that trees are planted very close to houses. In such cases, after full growth, the trees exert large pressure resulting in the upheaval of walls of the structures or compound walls (i.e. RP fouls with structure walls); in addition, such structures and trees are susceptible to damage during the heavy gust or cyclonic winds. Figures 10 and 11 show such closely planted trees and associated damage to the nearest structure/compound wall. Although people in private housing societies know the rules regarding car parking or laying drainage lines, they are unaware of the technical norms and practices towards planting trees near buildings! Also, many local municipal corporations may not consider educating the people about these aspects. The common public, in case they do not know all the technical details, should consult their builder/landscape architect/civil engineer before starting plantation around the new building. Remember that good gardening practices (watering plants till the first five years of growth, providing manure, pruning the branches, and overall maintenance) are extremely helpful to control the growth of trees and enjoy maximum benefits for a longer period!
While planting trees in road/rail/transportation projects certain principles of Roadside arboriculture need to be followed. The position of trees depends on the ‘overall width of the carriageway (inclusive of medians and shoulders)’ and classification of Highways (such as Expressways, Highways, local roads, and rural roads) as each category is designed for a specific speed. Speed of travel is always a factor in such landscape design. Some general guidelines are presented below (also see IRC IRC-SP 21:2009)
- For roads on flatter terrain, the planting of trees should be done so that trees can maintain shadow, coolness yet cleanliness. Plants parallel to both the outer edges and one row along the centerline of the median are commonly adopted (See Fig.12).
- If the road is high embankment type (i.e., roads formed at a higher elevation than adjoining land, on a compacted soil fill) then it is advisable to do the plantation of large trees at the base of the embankment slope for the longevity of the slopes (however the slopes to be protected using light grass/turfing, and mesh). The network of longitudinal and transverse drains and culverts etc. should be suitably positioned with the trees.
- In the case of hilly roads, the trees on ‘hill side’ and on ‘valley side’ need to be carefully assessed for the hassle-free operation of the road throughout the year (bigger trees may be retained on the valley side whereas shrubs and smaller plants up to 6 m may be retained on hillside-See Fig.13). The type of strata (rock/soil) and direction of rock slope need to be assessed for suitable slope stability analysis. If required, a suitable ‘drainage path’ for water needs to be provided between the trees especially on the hillside. Small trees hold the soil and even small falling rocks up to 30 cm (but not the landslide); in case of strata at any hill spot is susceptible for landslide, then rock nets/soil nails, etc. may be provided (depending on the degree of inclination of rock).
- In India typical duration of roadwork is two years with five years of maintenance period; this duration is sufficient to grow many trees up to 5 m in height. But most road contractors neglect the aspects of roadside arboriculture. Hence, long barren lands are often seen along main Indian roads!
- Till the plants are small they need to be preserved from cattle by using wire cages around the trees. After they grow, it is also important to preserve them from local woodcutters.
In many well-maintained old roads, ‘Arching of trees’ on both sides (usually this happens after about 30 years) present a pleasant drive with a cool shadow. But, often leaves or small branches (and entire trees during cyclones) may fall on the road and hence require frequent cleaning/maintenance of top surface and drains along the road. The adjoining picture shows the arching of trees in one of the roads. In any case, the location of trees on junctions should be carefully done so as to not affect the ‘sight distance’ (SD), in order to prevent head-on collision of vehicles.
Similar measures are to be followed in the case of the railway/high-speed/elevated corridor or metro line project. Ample trees should be planted in such projects but at a suitable location so as not to affect the operations, when any tree falls during cyclones.
Restoration/Relocation of Fallen Trees
After any cyclone, the fallen trees need to be first removed from the operational roads/structures on a priority basis to reinstate the safe operations at the earliest, but the practice of cutting the falling trees into small pieces is not advisable. Instead, authorities should attempt to replant fallen trees at another suitable location. Types of equipment like small cranes / JCB / bulldozers are required to lift and shift the trees to another location (See Fig. 14). Often an electric saw is deployed/needed if the fallen tree is old and blocking a road or a structure. In the case of trees older than ten years, it is always desirable to safely lift the fallen tree (without toppling the crane during lifting) and replant it at a safe location, meeting the statutory norms of safe distance as explained already.
Structural Assessment of Trees - Pulling Test
Once it is found that any particular tree is not located at a suitable distance and needs to be relocated, it can be done as discussed above. But, if any tree is decided to be retained at the current location, then its structural adequacy should be assessed carefully. Pulling tests is one of the means of doing it. First, all the important physical dimensions of the tree, root-plate, age, etc., should be determined. Tree pulling at present is the most accepted method for evaluating the safety and stability of tree-root systems. It involves applying a known load on the trunk via a cable attached to the tree and measuring its response using Inclinometers attached to the buttress and Elastometers attached to the trunk (www.trees.org.uk/). The typical Load vs. displacement curve is plotted as shown in Fig.15.
After the structural assessment of desired large trees is made they can be protected by any one of the following methods (see Fig.16).
- Use of Metallic barricades
- Strengthening of the stem by wrapping the mesh around it.
- Providing external support to inclined stem/branches.
- Tying a leaning tree with a strap to a nearest tree/column.
- Steel fencing around the tree.
- Concrete/block wall protection around the tree stem.
- Tying the tree firmly with metallic chains.
Summary and Conclusions
Trees form an integral part of our ecological system and are a god-given gift to humanity. Civil engineers may not have adequate knowledge of where to plant the trees to have maximum benefits and at the same time with minimum damage during cyclones. Recent cyclones have caused tremendous damage to property and lives. The stability of any tree depends on the relationship/combination of four factors: (i) Strength of the roots, (ii) Height, width, and weight of the tree, (iii) Slope of ground and tree, and (iv) the external forces. Trees larger than 8m in height are found to be causing more damage than smaller height trees. Large trees should be planted at least 6 m away from the nearest building. The minimum longitudinal distance between two trees should be 0.25H to 0.4H subject to a minimum of 3m for ease of maintenance. Trees planted close to buildings will exert large pressure resulting in the upheaval of walls of the structures or compound walls (this effect will be enhanced in the case of expansive soils). While planting trees in road/rail/transportation projects certain principles of Roadside arboriculture need to be followed. Trees taller than 8 m are not advisable on medians. Pulling tests is one of the means of relocating trees. Several methods are available for protecting large trees.
The authors wish to acknowledge that the images used in this paper have been extracted from various sources on the Internet.
- IS 399:1963 Classification of Commercial Timbers and Their Zonal Distribution, Bureau of Indian Standards, New Delhi
- IS 875 : Part 1:1987 Code of practice for design loads (Other Than Earthquake) for buildings and structures: Part 1 dead loads - Unit weights of building materials and stored materials (Second Revision), Bureau of Indian Standards, New Delhi
- IRC-SP 21:2009 Guidelines on Landscaping and Tree Plantation, Indian Roads Congress, New Delhi, Nov., 90 pp.
- IS 883:2016 Design of Structural Timber in Buildings - Code of Practice (Fifth Revision), Bureau of Indian Standards, New Delhi
- NYPR (2016) Street Tree Planting Standards for New York City, City of New York Parks and Recreation, 31 pp.
- Subramanian, N. (2019) Building Materials, Testing and Sustainability, Oxford University Press, New Delhi, 788 pp.
- Wray, W.K. (ed.,) (1995) So Your Home is Built on Expansive Soils- A Discussion on How Expansive Soils Affect Buildings, American Society of Civil Engineers, New York, 59 pp.