A custom built octagonal sunken planter box, with sides measuring 0.36 m and depth of 1.2 m was built to fit the turntable in the downwind section of the wind tunnel. The planter box was filled with soil leaving 0.1 m depth unfilled. The tree sapling with root plate was placed in the middle of the planter box and the root plate was carefully buried in soil by filling the remaining soil box without causing any significant strain to the root plate and the attached instrumentation as shown in the Figure 3-1.
Trees growing in different soils have different advantages and limitations. Clay retains the moisture well, provides anchorage but unlocking the nutrients is a problem. Silty soil has
better drainage but has a risk of compaction. Sandy soils have free drainage, poor fertility and low anchorage. The loams (combination of clay, silt and sand) is the best soil for trees to grow. In this study, two soil media with contrasting states and properties were chosen to gain insight into the effect that soil has on tree response to wind loading to examine the change in root anchorage mechanics.
Sand tests were conducted with Barco #32 silica sand (Barco, 2015) in a dry state, to test the root response to wind loading in a purely frictional soil. The silica sand specifications are as shown in the Figure 3-6. The particle size distribution curve (ASTM D6913) is shown in Figure 3-6. The grading characteristics obtained from the particle size distribution curve are D10=0.3 mm; D30=0.395 mm and D60=0.52 mm. The uniformity coefficient, 𝑐𝑢 =
𝐷60
𝐷10 =1.73 and the coefficient of gradation, 𝑐𝑔 = (𝐷30)2
𝐷60×𝐷10 =1.0, indicate a well-graded
uniform sand. The maximum and minimum void ratio of the silica sand estimated by Varshoi [2012] in accordance with ASTM 4254 were 0.63 and 0.47 respectively. The maximum and minimum dry density of the silica sand based on void ratio estimates and specific gravity are 17.7 kN/m3 and 15.9 kN/m3. Peak friction angle (𝜙
𝑝′) and dilation angle (φ) of the silica sand were estimated by Deljoui [2012]. Peak friction angle (𝜙𝑝′) in accordance with ASTM D3080 varied from 42° to 47° at very low normal pressures and dilation angle (φ). The dilation angle (φ) estimated based on Bolton [1986] varied from 14° to 20°. For the wind tunnel tests, preparation of the dry silica sand was achieved with air pluviation through a # 10 sieve from constant height (0.63 m) to achieve uniform density in the planter box. The top 0.2 m was filled in three layer and compacted by hand using 0.08×0.08×0.39 m wooden block. Density of the top 0.2 m deep silica sand in the sand box was measured as 16.4 kN/m3 giving a relative density Dr=0.28.
The clay soil was prepared using Bentonite (sodium form). For every 25 % of Bentonite, 75 % of water was added and blended in a laboratory mixer; care was taken to ensure that the clay was blended effectively. The percentage of Bentonite or water in the clay mix is calculated as (weight of Bentonite or weight of water/total weight of the mix) ×100. The clay was sealed after preparing the mix to ensure consistent soil states. The moisture content, w (mass of water/dry soil particles) of the clay samples collected during the wind
tunnel testing period varied from 265 % to 250 %. Atterberg limits are widely used by geotechnical engineers to define the plastic properties of clay. Two important characteristics: liquid limit (moisture content at which soil just begins to flow) and plastic limit (lowest moisture content at which soil can rolled into 3 mm threads by hand without crumbling) of clay can be estimated based on B.S. 1377:1967. The Bentonite used was highly plastic in nature. The liquid limit (wL) and plastic limit (wP) of typical sodium
Bentonites were found by Feng [2000] to be 280 % and 37 % and by Zentar et al. [2009] to be 305 % and 52 %. In this study an average of the standard error (SE) values were taken as 45 % and 290 %. The liquidity index, IL [(w-wP/wL-wP)×100] at 255 % of water content
was 85.7 % and the plasticity index, IP (wL-wP) was 245 %. To estimate the undrained shear
strength of clay, Wood (1990) proposed an equation relating liquidity index and remoulded undrained shear strength as shown in Equation [3-1] below.
𝑐𝑢 = 𝑐𝑙𝑅𝑀𝑊(1−𝐼𝐿) 𝑘𝑃𝑎 [3-1]
From Equation [3-1], cL=1.7 kPa (meaning cu=1.7 kPa at the liquid limit) and knowing the
mean value of Rwm (mineralogy function) for highly plastic clays is 21.3 [Vardarega and
Heigh 2014]. The remoulded undrained shear strength estimated from Equation [3-1] is cu=2.6 kPa. Skempton [1954] and Mitchell [1976] proposed Equations [3-2] and [3-3]
respectively, relating plasticity index to undrained shear strength cu, effective vertical
stress (𝜎𝑣′) and friction angle (ϕ'), which can be used to estimate shear strength of clay under various conditions. For this clay the following values were estimated:
𝑐𝑢 𝜎𝑣′
= 0 11 + 0 37𝐼𝑝 [3-2]
sin 𝜙′ = 0 3 − 0 1 ln 𝐼𝑝 = 0 26 [3-3]
From Equation [3-3], with a plasticity index (Ip) of 2.45, the frictional angle was ϕ’=15°.
Equation [3-2] does not give accurate results for soil with plasticity Index > 100 %, in such cases Equation [3-1] and [3-3] can be used.
3.4
Test procedure
The wind tunnel tests were conducted with two tree saplings (T1 and T2). T1 was tested in the sand and on a force balance with six degrees of freedom [Figures 3-1 and 3-3] and T2 was tested in both sand and clay. The tree saplings were tested with an incremental wind speed loading as shown in Figure 3-7. For the T1 testing, the complete data sampling was done with a 200 Hz sampling rate. Whereas T2 was tested with a 300 Hz sampling rate, to identify any possible electrical noise in the data. The Nyquist frequency is half of the sampling rate. As long as the desired natural frequency estimate is below the Nyquist frequency, the estimate should be reasonable.
The wind velocity profile was set to an “open country” profile. The power law profile for an open country terrain with drag coefficient of 0.00-0.005 (K for open grass land) needs to be taken as suggested by Davenport [1964] for further analysis. For the purpose of the analysis undertaken in this chapter, the wind speed was increased incrementally and recorded by the pitot tubes and the hot wire anemometer near the center of gravity of the tree structure.
After the installation of the strain gauges on the stem and roots, each strain gauge was calibrated. This was conducted using static pull tests; applied bending moment was equated to the Voltage response of the strain gauge. The slope of the volt response line to the applied bending moment was taken as the calibration constant for further calculations. The laser transducers were calibrated using a white strip of paper, which was placed in front of the laser transducer and at each 0.01 m increment, the response in Volts was recorded. The difference in the Voltage response after each increment was taken. The average of Voltage response increment at each 0.01 m displacement increment gave the calibration constant for the laser transducers. Accelerometers, hot wire anemometer and pitot static tubes were used for the testing. The calibrated response of tree saplings T1 & T2 to the applied wind field were measured with increasing incremental wind speeds.
The peak shear strength of the clay mix in the soil box was found using a Pilcon hand vane tester. The shear vane tests were conducted before and after the wind loading tests. The 33
mm diameter vane was used to measure the peak shear strength and the measured shear strength was around 4 kPa, which was the average of 4 tests conducted during the wind tunnel testing. Giving a low sensitivity of approximately 1.5 for the clay.
After the wind tunnel tests, the tree saplings were pruned, measured and weighed. The tree stems were cut in to three equal parts and stored in a refrigerator. Each root and stump were measured, weighed and stored in a lab refrigerator at 5°C. Three-point bending tests were conducted on the stem and root samples in accordance with ASTM D198-09 (Standard Test Methods of Static Tests of Lumber in Structural Sizes). The static bending tests were carried out using the MTS universal testing machine at University of Western Ontario, in the Structures lab. For the three-point bending tests, the distance between the two supports was kept 15 times the mid-point diameter for the stem sample and 60 mm or 100 mm for the roots. Resolution ratio of the test force and the displacement were 0.001 kN and 0.01 mm. A pushing probe of 20 mm radius was attached to the load cell and a lowering rate of 20 mm/min was used and half of the lowering rate was used for the root testing.