Experimental Study on Engineering Properties of Large-Thickness Loess in Dongzhiyuan Area

In this paper, the engineering properties of collapsible loess with large thickness in Dongzhiyuan Area is analyzed in detail through consolidation test, triaxial shear test, maximum dry density test, static penetration test, standard penetration test and site wave velocity test. The results show that: (1) the engineering properties of Malan loess in the upper part of collapsible loess in Dongzhiyuan Area are not so satisfying. With the increase of the depth, the void ratio of the loess becomes increasingly smaller while the cohesive force and internal friction angle increase gradually. The bearing capacity of the foundation increases, indicating that the engineering properties of the foundation soil become better. (2) At the same time, the static lateral resistance and point resistance of the foundation soil become increasingly larger, which is very beneficial to the friction piles in the large-thickness collapsible loess area. CLC number: TU443 Document code: A


Introduction
The central and western region of China is covered by the world's thickest and most extensive loess plateau, accounting for about 17% of the land in China [1] . The engineering properties of loess vary from region to region. Dongzhiyuan Area is located in Xifeng District, Qingyang City, Gansu Province. This area belongs to Zone II in the loess engineering geology division (The Eastern Gansu-Northern Shaanxi-Northern Shanxi District) [2] , is the largest loess tableland in China. According to a large number of engineering survey data, the thickness of loess in this area is up to about 200m.
All the above mentioned researches mainly analyzed loess foundation treatment and the bearing capability of piles from the perspective of the collapsibility of loess. However, there are few researches focusing on the systematic study on engineering properties of loess, especially the compressibility, density and shear strength of Malan loess and Lishi loess. Therefore, there is no complete theory till now. In light of this situation, this paper, taking the site of a media center project in Xifeng District of Qingyang City as the test site, analyzes the engineering properties of large-thickness collapsible loess in Dongzhiyuan Area through a series of tests such as consolidation test, triaxial shear test, maximum dry density test, static penetration test, standard penetration test and site wave velocity test, expecting to provide references for the construction in large-thickness collapsible loess areas. ③-20 buried soil (Q 2 eol ): reddish-brown, with a buried depth of 77.60m and a top elevation of 1307.58m.

Indoor test
(1) Take the original samples every 1m in the borehole with an automobile drill, seal the samples and send them to the laboratory for following use. The on-site sampling is shown as in Fig. 1. Fig.1 On-site sampling (2) Consolidation test, triaxial shear test, maximum dry density test, water content test, and loess collapsibility test was carried out with the samples.

On-site test
(1) Standard penetration test. After drilling to a predetermined depth, an in-situ standard penetration test is performed in the hole.  (2) Site wave velocity test. Four holes are selected in the site, and the wave velocity test is performed on the ground soil of the site with single hole method. In the test, a CJ-2000A suction-type three-component detector produced by Xi'an Electronic Instrument Factory was used. And its natural frequency is 28Hz. The collecting instrument FDP204s integrated motion tester used is produced by Institute of Rock and Soil Mechanics in Wuhan of the Chinese Academy of Sciences. The selected frequency bandpass of the instrument is 0.1-250Hz, and it has automatic gain and 1K sampling length.
(3) Static penetration test: select 10 points in the site to conduct static penetration test, as shown in Fig. 4.

Consolidation test
Consolidation test was then conducted on soil samples to obtain the e~p curve of loess, as shown in  Under the same pressure, except for certain soil layers, the void ratio of Lishi loess at the lower layer is much smaller than that of the upper Malan loess.
Therefore, as the increase of the depth of loess foundation, the void ratio of the foundation soil gradually decreases, the compressibility of the foundation soil, accordingly decreases and the

Triaxial shear test
Triaxial shear test was carried out on loess soil samples collected at different depths. The cohesive force and internal friction angles of the soil samples were measured to obtain the distribution map of the shear strength and the depth of the soil samples, shown in Fig. 6. Fig. 6 reveals that in the Malan loess section, the cohesive force of the soil is between 25 and 30 kPa, and the internal friction angle is between 28 and 29 degrees. In the section of Lishi loess, the internal friction of the soil increases with the depth. The angle gradually increases, which lies between 27° and 30°. Consequently, as the depth increases, the shear strength indexes of the foundation soil increases, and the shear strength increases as well.

Water content and dry density test
The compaction test was conducted on disturbed loess samples to determine the change rule of water content and dry density, as shown in Fig. 7. (a) Correlation between water content and dry density of Malan loess (b) Correlation between water content and dry density of Lishi Loess Fig.7 Correlation between water content and dry density Fig.7(a) shows that the optimum water content of Malan Loess is 17.6% and the maximum dry density is 1.74g/cm 3 . It can be seen from Fig. 7(b) that the optimum water content of Lishi Loess is 18.9%, and the maximum dry density is 1.64g/cm 3 . Therefore, the loess at different depths has little difference in the maximum dry density and the optimum water content from top to bottom, and the properties of the foundation soil almost remain unchanged.  Figure 8 demonstrates the relationship between the collapsibility coefficient of loess and the depths under the pressure of 200 kPa. It is clear that the self-weight collapsibility coefficient of Malan loess is smaller than that Lishi loess, and the collapsibility coefficient decreases with the increase of depth. So it is calculated that the loess of the site is self-weight collapsible, the collapsibility level of which is III. The loess type is of great collapsibility, and the lower limit of which is about 25.0 m. Figure 9 is the curve relationship between the collapsibility coefficient and pressure of Malan loess and Lishi loess at different depths. Fig. 9(a) and (b) indicate that as the pressure increases, the collapsibility of the Malan loess and Lishi loess first increases and then decreases, and at 200 kPa, the collapsibility of loess is the largest.

Correlation between loess collapsibility coefficient and pressure
(a) Malan Loess (b) Lishi Loess Fig. 9 Curve relationship between collapsibility coefficient and pressure Under the same pressure, with the increase of depth, the collapsibility of loess gradually decreases. Comparing (a) and (b) in Figure 9, the collapsibility of Malan loess is greater than that of Lishi loess.

Water content test
The water content of the soil samples of different depths was measured, and the relationship between the water content and the depth of the soil samples have been obtained and listed in Fig.10.  Fig. 10 Correlation between water content and depth Fig. 10 demonstrates that the water content of the foundation soil is between 14% and 28% in the range of 70 m. The water content of Malan loess within 10m increases first and decreases later while that of Lishi loess within 10m~25m increases with depth. However, from 25m to 50m, it decreases with depth. Beyond 50m, it suddenly increases again, which is mainly because the soil is under groundwater beyond 50m, and the foundation soil is saturated  Table 1 is the reference values of the foundation soil bearing capacity indexes obtained in the standard penetration test according to project experience. It can be seen from Table 1 that with the increase of the depth, both the bearing capacity value and compression modulus of the foundation soil gradually increase. In general, the bearing capacity of Lishi loess is better than that of Malan loess.

Static penetration test results
Ten holes were selected from the site for static penetration test. And the average values of the static point resistance and lateral resistance of each layer were calculated. And accordingly, the change curve that static point resistance and lateral resistance vary with depth has been obtained and listed in Fig.11 and 12.  Fig. 11 and Fig. 12 reveal that the static point resistance of the Malan loess within 10m is about 0.6 MPa, and the lateral resistance is about 10~19 MPa. The static point resistance of Lishi loess when it is 10m deeper is about 1.8 to 7.8 MPa. The lateral resistance of it is about 20~228 MPa. As can be seen from the two figures, both the point resistance and lateral resistance of the foundation soil gradually increase with the increase of the depth. Therefore, as the depth of the foundation soil increases, the bearing capacity of the foundation soil becomes better and better, which is very advantageous for the friction piles in the large-thickness collapsible loess area.

Conclusion
(1) The engineering properties of Malan loess in the upper part of the large-thickness collapsible loess in Dongzhiyuan Area are not satisfying. With the increase of depth, the void ratio of Lishi loess gets smaller and smaller, the cohesion and internal friction angle gradually increase at the same time. The foundation bearing is gradually improved, and the engineering properties of the foundation soil are getting better and better.
(2) As the depth increases, the static point resistance and lateral resistance of the foundation soil become larger and larger. It is very advantageous for the friction pile in the large-thickness collapsible loess area.
(3) The large-thickness collapsible loess in Dongzhiyuan Area is a self-weight collapsible loess site, with a collapsible level of III, a serious degree of collapsibility, and a lower limit of which is about 25.0 m.