With the growing level of awareness and increasing demand for environmental protection, timber pile revetments with significant ecological effects have experienced increasingly wide application in China. The current timber pile revetment system has two problems: continuous dense piles have high construction costs and require large timber consumptions, while the discrete pile-bamboo fence systems have poor retaining effects. Considering these problems, an original revetment structure form is proposed: a discrete pile-mulberry straw arch structure. To investigate the mechanical properties of the mulberry straw, some mechanical tests are conducted on mulberry straw and its arch structure. It is found that the average compressive strength of mulberry straw is 2.30 MPa, the bending strength is 20.08 MPa and the shear strength is 2.99 MPa; considering the structural characteristics of arches, mulberry straw arches have a high bending strength, which affords immense potential for their application in revetment structures. Furthermore, by doing field study of the discrete timber pile-mulberry straw arch structure for bank revetment, it is found that the arch height of the mulberry straw arch changes 1.4–1.6 cm after 489 d observation. The soil settlement before the straw arch is 0.13–0.18 cm and the settlement of the pile is 0.66–0.72 mm. Compared with bamboo fence structure, the cost of using mulberry straw arch structure as revetment material can save up to 27.07%. Therefore, the discrete timber pile-mulberry straw arch has a rational and stable structure that provides effective support for protecting the riverbank and the toe of gentle slopes and effectively prevents the collapse of bank slopes in the rear. The new revetment structure proposed in this paper has significant ecological and economic effects, is easy to construct and feasible for standard production, and consequently has great application potential.
Timber pile revetments have gained prevalence in China due to their ecological effect [
Straw is a renewable and eco-friendly botanical resource [
There has been ample application of straw in slope engineering in the abovementioned literature, which provides a theoretical foundation. A majority of the literature has focused on rice and wheat straw, which have a high yield but easily perish and are not suitable for toe protection below the low water level. Moreover, mulberry straw is much more resistant to corrosion than rice and wheat straw [
To date, the application of straw in toe protection below the low water level has rarely been reported. The proposed discrete timber pile-mulberry straw arch structure has superior mechanical properties, significant economic and ecological advantages, and consequently great utilization potential and broad prospects for application. To further explore the feasibility and suitability of the proposed structure in the practical application of revetment engineering, a series of laboratory experiments and applied research is conducted at the project site.
Mulberry straw was taken from the Yancheng Dongtai Mulberry Field (32° 40′ 46″ N, 120° 29′ 23″ E), Dongtai City, Jiangsu Province, China. The area of this field is 160,000 mu. The mulberry branches are trimmed every year before germination in spring and after defoliation in the fall, which produces a large amount of mulberry straw waste. Straw is mostly incinerated, which causes air pollution. Here, this study aims to broaden its application prospects to ameliorate the problem of air pollution due to incineration.
The discarded mulberry straw from the Dongtai Mulberry Field was clipped. To ensure the repeatability of the tests, branches with diameters of 10–12 mm were chosen as experimental subjects. The basic mechanical properties of the mulberry straw and its arch structure were tested as follows: Compressive strength perpendicular to the grain
Mulberry straw of 18 mm length and 12 mm diameter was selected for the compressive strength test. To avoid contingency, 5 parallel samples were included. Compression tests were conducted in the direction perpendicular to the axial direction of individual mulberry straw to obtain the compressive strength according to the Chinese criterion for compression tests of timber (GB/T1939–2009).
The compressive strength of the samples was measured in a universal testing machine, which is under touch screen control and capable of performing mechanics performance testing and analytical investigation on various materials. Based on the specifications, the maximum load must be reached within 1–2 min. After the preliminary experiments, the press machine operating speed was set to 2 mm/min. Bending strength
Mulberry straw 150 mm in length and 12 mm in diameter was tested for bending strength. Similarly, 5 parallel samples are tested to avoid contingency. The 3-point bending test is used to obtain the bending strength according to the Chinese criterion (GB/T1936.1–2009). The universal testing machine described above was used. In this research study, the samples were placed on the two bases of the machine and imposed the load at an even rate on the diametric plane of the sample in the middle of the base. The loading rate was set to 5 mm/min. Shear strength
Mulberry straw 18 mm in length and 12 mm in diameter was tested for shear strength. The methods discussed by Li [ Mulberry straw arch and its mechanical properties
The mulberry straw arch was produced using standardized molds, with different molds for different sizes of arches. The mold to produce the arches in the laboratory tests is similar to those used for the engineering field, except in a smaller size. Details about the mold and production method are elaborated in
The sizes of the mulberry straw arch for the laboratory tests were as follows: the span of the arch was 135 mm; 4 sets of rise span ratios were used: 0.2, 0.3, 0.4, and 0.5; 4 sets of densities were used: 0.353, 0.396, 0.421, and 0.476 g/cm3, which were obtained through preliminary experiments. Beware of and avoid the case where the straw is too sparse in the arch in practical operation.
A universal testing machine was used to perform the 3-point bending test on the mulberry straw arch. To avoid horizontal displacement of the arch springing at the time of load application, the mulberry straw arch was fixed on the prop with superglue. The loading rate was 5 mm/min. When the mulberry straw arch manifested evident yield deformation, loading was stopped, and the experiment completed.
The field application of the discrete timber pile is mulberry straw arch structure in the Lianshen Lane Channel Regulation Project, China. Lianshen Lane is an inland canal along the coastland in northern Jiangsu Province, China, as shown in
Due to the scour of ship waves, the discrete timber pile-bamboo fence structure ecological revetment along the right bank of the 23 K+019–23 K+630 section north of the Gangwan Bridge in Fu’an is severely damaged, which affects the stability of the levees on the rear side, as shown in
Material
Mulberry straw with diameters of 10–12 mm was collected from the Dongtai Mulberry Field (32° 40′ 46″ N, 120° 29′ 23″ E), Dongtai City, Jiangsu Province, China. A 1:500 carbendazim solution was used to soak and sterilize the straws by coverage for 5 d, after which the straw was naturally dried and stored in a well-ventilated, sunproofed, and raintight environment. Mold for the arch
Take the mold at the engineering site as an example. The mold was made with bar iron and angle steel, 800 mm in length, and composed of the top and bottom pieces, as shown in Production of the arch
In this study, the treated mulberry straw was evenly placed on the bottom piece of the mold, the top piece was used to compact the straw and the density was set. The density for the engineering site was set at 0.476 g/cm3 based on laboratory tests. Strapping was used to tie the mulberry straw arch, after which it was removed from the mold, as illustrated in Preservative treatment
The mulberry straw arch and timber pile to be used at the engineering site requires preservative treatment. According to the preservative treatment for timber, the mulberry straw arch and timber pile were soaked in a blend solution (ammoniacal copper quaternary:water = 1:8) for 6 h; then, the mulberry straw arch was placed upright to let dry naturally.
Construction of the timber pile
According to the line type requirements of the design diagram, set the work platform to 0.1 m elevation with a digging machine. Form an earth cofferdam in front of the wall to ensure dry construction. Perform discrete piling, and the top mark of the pile should be at 1.3 m elevation, as shown in Installation of the mulberry straw arch
Fasten 6 × 10 × 80 cm self-locking battens with iron nails on the earth-facing side of the pile to prevent horizontal displacement of the arch, as shown in Layout of the geotextile bag and earthwork backfilling
Place the geotextile bag behind the installed mulberry straw arch. The geotextile bag is composed of 380 g/m2 machine-woven composite cloth. Fill the geotextile bag with earthwork. Fill the top layer of the geotextile bag with 20 cm of topsoil and planting soil. Backfill with earthwork from the side slope and neaten the geotextile bag, as shown in
Observe the monitoring factors, including the rise and span of the mulberry straw arch, settlement of the pile and the soil mass settlement behind the arch, as shown in
The stress-strain curves of the compression characteristics of the mulberry straw are presented in
The bending test is repeated five times for self-calibration. The load-displacement curves of the bending characteristics of mulberry straw are shown in
The load-displacement curves of the shear characteristics of mulberry straw are described by Li [
Similarly, the load-displacement curves, deformable models, and deformation diagrams of the three-point bending test is reported by Li [
The results of the 489 d field inspection are shown in
The soil settlement before the mulberry straw arch has significant impact on the deformation of straw arch.
The relationship of the cumulative settlement of the timber pile with time is presented in
Many scholars propose to use different plant material as revetment structure, such as timber piles, bamboo and Melaleua fences [
With the purpose to verify the advantage of mulberry straw arch structure from the aspect of the cost saving, the unit price of the mulberry straw arch structure and bamboo fence structure are estimated and presented in
Content | Unit price RMB | |
---|---|---|
Mulberry straw arch structure/m | Bamboo fence structure/m | |
Mulberry straw material |
1.2 | —— |
Bamboo material | —— | 10.4 |
Making mulberry straw arch | 16.3 | —— |
Making bamboo fence | —— | 15.6 |
Installation | 5.4 | 5.4 |
Total | 22.9 | 31.4 |
In order to verify the effective protective effect, the comparison pictures for bank revetment using discrete pile-mulberry straw arch structure and timber pile structure are shown in
Mulberry straw has good mechanical characteristics. The mulberry straw arch takes full advantage of the superior structural properties of arches to improve the retaining effect. Additionally, the mulberry straw arch is dense, compact and far more durable than bamboo fences, so it provides a good foundation as a bank protection structure.
The spaced timber mulberry-straw arch model is a feasible new revetment structure with good stability. It demonstrates fine support and good adaptability in the protection of the toe in gentle slopes. With water plants such as reeds, the proposed original model can efficiently prevent rear bank slopes from collapsing.
The discrete timer pile-mulberry straw arch structure has significant economic and ecological effects. It is also easy to construct and capable of standardized production. Therefore, the proposed new revetment structure has great potential and broad aspects in engineering applications.