Keywords

1 Introduction

Rapid urbanisation already leads to significant impacts on surrounding environment [5, 7, 12, 19] and dramatically reduces the storage capacity of floodwater which is particularly important as the monsoon season sets in. Environment-friendly construction materials may resolve the raised problem despite many new technologies available for environment protection [1,2,3,4, 6, 8,9,10,11, 13, 15,16,17,18]. In Northwest China, residential houses have always been built using the post-harvest waste (PHW) such as straw bale for which its major constituents generally are loess, wheat straw, and corn cob. Most the residential houses have been used under extreme climate over decades, as shown in Fig. 1. This particular phenomenon indicates the superior performance of the loess-PHW mixture.

Fig. 1.
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Residential house built by the loess-PHW mixture: house wall (left-hand side) and enlargement of house wall (right-hand side)

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The objectives of this study are: (1) to carry out a series laboratory experiments to investigate the mechanical properties of the loess-PHW mixture, (2) to compare the failure mode, strength, and load-carrying capacity of the loess-PHW mixture to the loess, and (3) to evaluate the permeance of the loess-PHW mixture.

2 Methodology

This study conducted a series of laboratory experiments including UCS and strain- and stress-controlled DS tests. The UCS tests aimed to investigate the failure mode of the loess-PHW mixture specimens, while the strain- and stress-controlled DS tests targeted to assess the strength and load-carrying capacity of the specimens, respectively. The results were compared with the loess specimens.

3 Laboratory Experiments

3.1 Loess-PHW Mixture Specimen Preparation

Prior to the laboratory experiments, a large amount of wheat straw retrieved from the Loess Plateau located in Northwest China was boiled with distilled water and due to this reason, the wheat straw got softened, as shown in Fig. 2. The wheat straw was then mixed with the Q4 loess for preparing the loess-PHW mixture specimens.

Fig. 2.
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Wheat straw additive before (right-hand side) and after (left-hand side) the treatment

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In accordance with MIT soil classification system, grading curves for the Q4 loess can be presented, as shown in Fig. 3. The loess-PHW mixture specimens were prepared at the moisture contents ω of 14%, 18%, and 22%, respectively. For the loess-PHW mixture specimens with the same moisture content, various additives of PHW (0.3%, 0.45%, 0.6% and 0.75%) were introduced respectively.

Fig. 3.
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Grading curves for retrieved Q4 loess

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3.2 UCS Test

In the UCS tests, the loading ram displaced at a rate of 0.45 mm/min. The UCS tests were carried out on the loess-PHW mixture specimens at ω = 14, 18, and 22%, respectively, with the additives of PHW varying from 0.3 to 0.75%. The UCS test results are shown in Fig. 4.

Fig. 4.
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Axial stress-strain relationships of the loess-PHW mixture specimens at ω = 14%, 18%, and 22%, respectively

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3.3 Strain-Controlled DS Test

The strain-controlled DS tests were implemented on the loess-PHW mixture specimens of 62 mm in diameter and 20 mm in height. The DS apparatus made in Nanjing, China was to investigate their shear stress-displacement relationships, as shown in Figs. 5, 6 and 7. The loess-PHW mixture specimens of same moisture content were sheared at the normal pressures of 100 kPa, 200 kPa, and 300 kPa, respectively. While the shearing rate of 0.8 mm/min was adopted.

Fig. 5.
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Shear stress-displacement relationships of the loess-PHW mixture specimens at ω = 14% against various normal pressures of 100 kPa, 200 kPa, and 300 kPa, respectively

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Fig. 6.
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Shear stress-displacement relationships of the loess-PHW mixture specimens at ω = 18% against various normal pressures of 100 kPa, 200 kPa, and 300 kPa, respectively

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Fig. 7.
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Shear stress-displacement relationships of the loess-PHW mixture specimens at ω = 22% against various normal pressures of 100 kPa, 200 kPa, and 300 kPa, respectively

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3.4 Stress-Controlled DS Test

Since the strain rate applied significantly affects the particle dislocation interactions, the stress-controlled DS tests were conducted twice; the first test was on the specimen of 305 mm (length) by 305 mm (width) by 200 mm (height) without PHW additive, and the second test was on the specimen of same size with 0.75% PHW additive. In this study, the large-scale DS apparatus, Geocomp ShearTrac III (Fig. 8), was utilised enabling their shear displacement-time relationships against the shear stresses varying from 25 to 125 kPa to be investigated, as shown in Figs. 9 and 10.

Fig. 8.
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Large-scale direct shear apparatus (Geocomp ShearTrac III)

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Fig. 9.
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Shear displacement-time relationships against the shear stresses of 25 kPa, 50 kPa, and 75 kPa for the loess specimen

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Fig. 10.
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Shear displacement-time relationships against the shear stresses of 25 kPa, 50 kPa, 75 kPa, 100 kPa, and 125 kPa for the loess-PHW mixture specimen

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4 Analysis and Discussion

4.1 UCS Test

It can be seen from Fig. 4a that the maximum axial stress for the loess-PHW mixture specimens at ω = 14% was reduced from 392 to 365 kPa while the PHW additive was raised from 0 to 0.75%. However, the corresponding axial strain was increased from 1.1 to 2%. In Fig. 4b, the maximum axial stress for the loess-PHW mixture specimen at ω = 18% showed a descending tendency with the increasing PHW additive; the maximum axial stress decreased from 226 to 204 kPa as the PHW additive was increased from 0 to 0.75%. The associated axial strain was however increased from 16 to 28%. Similar axial stress and strain variations presented in the previous two figures can also be found in Fig. 4c. In Fig. 4c, the maximum axial stress for the loess-PHW mixture specimen at ω = 22% presented a descending tendency with the increasing PHW additive; the maximum axial stress declined from 175 kPa for 0% PHW additive to 165 kPa for 0.75% PHW additive. But the corresponding axial strain was raised from 27 to 32%.

The greater the percentage PHW additive, the smaller the maximum axial stress, and the larger the associated axial strain. The phenomenon is most likely due to the fact that the PHW additive worked with the “honeycomb” structure of loess formed in its strata deposition process to develop a new structure with superior ability of withstanding larger plastic strains before failure. The failure mode was transferred from “shear failure” for the specimens with a small amount of the PHW additive to “ductile failure” for the specimens with a large amount of the PHW additive (see Fig. 11).

Fig. 11.
figure 11

Failure mode of specimen subjected to axial loading: (a) loess specimen (left-hand side) and house wall built by loess (right-hand side) and (b) loess-PHW mixture specimen (left-hand side) and house wall built by loess-PHW mixture (right-hand side)

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4.2 Strain-Controlled DS Test

The PHW additives of 0.3%, 0.45%, 0.6%, and 0.75% and the normal pressures of 100, 200, and 300 kPa were first applied to the specimens at ω = 14%. The cohesion c was reduced from 72.3 kPa for 0.3% PHW additive to 52.5 kPa for 0.75% PHW additive. While the friction angle ϕ was increased from 48.4 to 61.3° (Table 1). The same amounts of PHW additive and normal pressures were applied to the specimens at 18% and 22%, respectively. Similarly, the c values were in an inverse relation to the amount of the PHW additive, while the ϕ values were in a direct relation to the amount of the PHW additive; their shear strength parameters measured at c = 59.5 − 52.5 kPa, ϕ = 41.5 − 49.5° and c = 66.2 − 55.2 kPa, ϕ = 34.1 − 46.5°, respectively (Table 1). The greater the amount of the PHW additive, the smaller the mixture’s cohesion, and the larger the mixture’s friction angle. This phenomenon indicates that as subjected to a large amount of the PHW additive, the loess-PHW mixture specimens behaved as a “sand-like” material rather than a “clay-like” material. Since the PHW additive largely promoted the effect of particles inter-locking, particles dislocation or rearrangement became more difficult. Additionally, greater normal pressures, e.g., 200 kPa or 300 kPa, also made particles dislocation or rearrangement more difficult. This was reflected through the strain hardening behaviour after the specimens underwent the shear displacement of 1.7 mm (Figs. 5, 6 and 7).

Table 1. Interpretation of Mohr-Coulomb parameters based upon linear-fit results of Figs. 5, 6 and 7.
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4.3 Stress-Controlled DS Test

In the stress-controlled direct shear tests, the loess specimen was sheared by 25 kPa in the first stage and by 50 kPa and 75 kPa, respectively, in the second and final stages. The maximum shear displacement reached 0.28 mm in the first stage, with the small strain rate of 0.065 mm/min. The maximum shear displacement rose to 11.2 mm in the second stage, with the moderate strain rate of 0.153 mm/min. The maximum shear displacement increased sharply to 50 mm in the final stage, with the largest strain rate of 19.425 mm/min. Similar direct shear test was again conducted on the loess-PHW mixture specimen. The specimen was incrementally sheared up to the maximum shearing stress of 125 kPa, with an incremental shearing stress being equal to 25 kPa. The shearing stress for the loess-PHW mixture specimen corresponded to 75 kPa as subjected to the shear displacement of 12.23 mm (Fig. 10). The shearing stress for the loess specimen measured at 50 kPa as subjected to similar shear displacements (i.e., 11.15 mm) (Fig. 9). The results have shown a better ability of withstanding greater shearing stresses of the loess-PHW mixture specimen than the loess specimen as subjected to similar shear displacements. Similar phenomena have also been observed from the experimental results of the bamboo composite columns despite a time-consuming and complex fibrillation process of the bamboo [14].

5 Conclusions

The loess-PHW mixture specimens failed in ductile (barrel-shaped) mode as subjected to axial loads, implying that the specimens possessed an ability of withstanding larger plastic strains before failure. The loess-PHW mixture specimens also demonstrated a better strength and load-carrying capacity than the loess specimens.