Study on Bottom Damp-Proof Method of Cave Dwelling

/e cave dwelling, as one of the most typical and traditional characteristic local housing styles in the loess area of Northwest China, has been adopted widely in many areas since ancient times./e construction of the cave dwellings supports environmental protection, makes use of innovative construction technology, and preserves historical cultural heritage. /e cave dwellings have practical significance and play an important role in the current society. Although there are many natural advantages to use cave dwellings, they also posemany problems, such as highmoisture, inadequate lighting, poor ventilation, insufficient strength, and so on. /e most serious problem is related to moisture, which affects the comfort, security, and safety of the residents; it needs to be addressed. /e authors propose to resolve this problem by developing a method of building a damp-proof course in the bottom of these cave dwellings. Meanwhile, specified experiments were carried out to validate the feasibility of this method./e results show that the method is reasonable and practical, and it can be implemented easily and conveniently in the future. /is research has significant practical value; the results can improve people’s living environment and increase comfort and safety of cave dwelling.


Introduction
e loess plateau of Northwest China is dry and cold in winter and hot in summer with scarce rainfall.For this reason, the region provides a good natural condition for the construction of cave dwellings, which has become an ancient and unique residence pattern on the loess plateau.e cave dwelling can be dated back to the cave-living period, which originated in ancient times.e archaeological cave sites in the Yangshao cultural period, which have been excavated in the Shaanxi Province, China, demonstrated that such cave dwelling appeared about 7000-5000 years ago.Now, there are still about 30 million people living in the caves of the loess area, which shows that cave dwelling is still widely used [1].
e cave dwelling, as a hot research topic in recent years, has been studied by many scholars.
ey have achieved a series of important research results.Yang, a professor of Xi'an University of Architecture and Technology, developed a thermal design of a zero energy consumption cavedwelling solar house by studying the characteristics of a traditional cave dwelling and considering the abundant solar radiation resources in the loess plateau of northern Shaanxi Province [2].In particular, Liu, an academician of the Chinese Academy of Engineering, has studied in depth on the sustainable developments of the cave dwelling.His results have been unanimously recognized and highly valued among China and the world experts [3].However, the research of cave dwelling is carried out relatively late in China. is research needs to be broadened and further investigated.
e cave dwelling is the most distinctive residence style in the loess region.Regarding the whole environment and the monomer cave dwelling, the architectural morphology of the cave dwelling are conformed to the nature and symbiotic coexistence with the ecological environment [4].e cave dwelling shows very strong vitality because there are so many advantages on it.e building material of the cave dwelling is natural eco-friendly.
e living condition is warm in winter and cool in summer.Its orientation and interior layout are reasonable and regular.e energy utilization is also very ingenious and efficient [5].As a coin has two sides, the traditional cave dwelling also has some problems, such as high moisture, inadequate lighting, poor ventilation, insufficient strength, and so on.Among these shortages, high moisture is the most serious problem, which can affect the comfort and security of the residency.In some situation, high moisture even threatens the safety of people's living.So, this problem must be solved [6].It is necessary to nd an e ective method of cave-dwelling damp-proof measures by drawing on modern building technology.
e research can help people to improve their living environment; increase their living comfort; and ensure their living, health, and safety.It has important application value and practical signi cance.

Research and Analysis
In order to further understand the living conditions of current cave dwellers, the authors carried out a survey in Yan'an city, Shaanxi Province.is questionnaire survey is mainly based on traditional cave-dwelling villages.So some typical old villages were chosen and most of the residents here live in cave dwelling for a long time.e participants' feelings of cave dwelling come from their real experience, and the reliability of the survey is high.In the questionnaire survey, 267 questionnaires were issued, and 243 were returned.Of the 243 questionnaires returned, 229 were valid.In order to ensure the e ectiveness of the results, the authors selected 243 questionnaires and eliminated all questionnaires which were incorrectly completed, for example, those with unanswered questions or those with the same answer to every question.According to the statistics, 91.0% of the questionnaires were returned and 85.8% were valid.is questionnaire is a comprehensive survey of the comfort of cave dwelling, which covers a wide range.In order to ensure the rationality and representativeness of the questionnaire, a lot of eld investigation and data consultation were carried out.e main aim of the questionnaire is to understand the actual feelings of the villagers on the living environment and to determine the practical signi cance of this study.e questions are simple and direct, and the result is clear at a glance.
e two questions related to this research are shown in Figures 1 and 2.
For the question "What is the main disadvantage of cave dwelling?", the results are shown in Figure 1.
For the question "Can you describe the air humidity in cave dwelling?", the results are shown in Figure 2.
Figure 1 shows clearly that more than 40% of people expressed frustration that the caves "easily get damp" due to the lack of damp-proof measures.Figure 2 shows that 10% people choose very damp and 57% people believed that their current residential cave was a little damp or even highly humid.e living condition is uncomfortable.Inside caves, the high air humidity will not only cause damage to indoor furniture, but will also a ect physical health, resulting in diseases like rheumatism, arthritis, and so on.e caves are not only uncomfortable, but they also have some safety issues [7].
us, for cave dwelling, the problem of high moisture has become one of the most undesirable issues.New measures and methods are required to decrease moisture, to improve damp-proof, and to reduce seepage of the cave dwellings.
Loess is the major building material for cave dwellings.Its particles are very small, containing calcium carbonate, and are rich in K, Cl, N, P, and other mineral elements.It easily deliquesces when meeting with the water.Because of the above reasons, water leakage, large deformation, and severe humid will occur when the cave dwelling su er from heavy and long-term rainfall withstand [8].e dry loess has better compression performance, while its tensile strength is usually very poor and easily causes brittle failure.When soaked by water, the shear strength of loess drops rapidly. 2 Advances in Civil Engineering is feature can threaten residents' safety.e loess on sides, top, and bottom of the cave dwelling are easily prone to get damp.is phenomenon happening on the top and lateral walls of the cave are related mainly with the water of rainfall and irrigation.For low precipitation in the loess plateau, it is helpful to reduce the damp on the top and lateral walls of the cave dwelling by maintaining a good rainwater drainage system and reducing water consumption by human activity.However, due to the in uence of groundwater and capillarity of loess, when water supply is adequate, it is easy to get damp.Especially, the phenomenon of getting damp is more obvious in the corners and bottom-side wall borders in cave dwelling.It tends to form white crystals due to the precipitation of salt from water.us, the cave dwelling dampproof technology should be focused on the bottom of caves.
e bottom of the cave dwelling is the foundation of the whole cave.For modern architecture, the damp-proof course can be directly set up in the foundation construction.In contrast, based on special material and construction methods, unlike modern architecture, it is unable for cave dwelling to set up damp-proof course directly after excavating foundation.erefore, traditional cave dwelling are seldom carried out e ective damp-proof treatment to its bottom.is also leads to serious dampness.In order to solve this problem, the authors proposed the idea of "reverse construction method" in the construction of cave dwelling.People should build the roof and wall rstly, and then construct damp-proof course on the bottom of the cave dwelling.

Processing Method
e authors proposed to use lime-soil as a damp-proof course at the bottom of cave dwelling.To enhance the performance of damp-proof course, moisture-proof linoleum is used in combination with lime-soil, as shown in Figure 3. Speci c practices are listed as follows.
First, dig 30-50 cm down from the surface of the cave dwelling to set the lime-soil cushion.e volume ratio of lime to loess can be about 1 : 9, 2 : 8, or 3 : 7. e cushion should be layered and compacted.
e thickness of each layer is about 30 cm before compaction, and the thickness is about 15 cm after compaction.After the nal compaction, the indoor oor is higher than the outdoor oor to prevent ooding.e outdoor cushion extends outward no less than 100 cm from the cave and no less than 50 cm on both sides of the cave dwelling.A layer of damp-proof linoleum plastic lm is set at the place where the side wall and the lime-soil cushion are connected.is approach can e ectively impede the ow channels of underground water, slow down the migration rate of soil moisture, and reduce moisture accumulation.e damp-proof course comprises lime-soil and linoleum, which helps to decrease dampness in the cave dwelling.e sketch drawing is shown in Figure 4.

Discussion on the Feasibility
In order to validate the feasibility of the proposed method of adding damp-proof cushion, the lime-soil permeability test and a cave penetration simulation test were designed and carried out.e basic physical parameters of the soil are shown in Table 1.

Lime-Soil Permeability Test.
In order to investigate the e ect of lime content of the lime-soil on its permeability, permeability coe cients with di erent lime contents were tested abiding by the variable head permeability test. is method could provide the experiment reference for improving damp-proof ability of cave dwellings.
e test equipment used is the TST-55 type variable water head permeability tester, as shown in Figure 5. e variable head method refers to the SD128-012-84 (hydroelectric earth test standard).
e method of preparation of samples applies JTJ051-93 (highway geotechnical test regulation).
e instrument and the schematic diagram of the variable water head permeability test are shown in Figures 5 and 6. e key points of the variable head permeability test are as follows.
Take four samples with the special cutting ring for the permeability coe cient test.e dry weight content of the lime in these four samples is 0%, 10%, 15%, and 20%, respectively.Weigh 3∼4 kg representative dry soil sample which is accurate to 1 gram, and measure the dry moisture  2)), make the water pass through the sample, and begin to measure the water head when the water over ows from the water outlet pipe (6).en record the initial water head H 1 .After the time t, the nal water head H 2 is recorded.e permeability coe cient (K t ) is given in the following equation: where F is the cross-sectional area of sample, L is the thickness of sample, a is the cross-sectional area of the piezometric tube, t is the time from the beginning to the end of the recording process, H 1 is the initial water head, and H 2 is water head after time t.Finally, the change curve of the permeability coe cient over the di erent lime contents is shown in Figure 7.
Figure 7 shows that the permeability coe cient of lime-soil decreased rapidly as the lime content increased.e lime-soil permeability coe cients were signi cantly smaller than that of the untreated sample.Furthermore, this showed that liming could e ectively decrease the permeability of loess and reduce the degree of the damp.e trend of the curve is relatively gentle when the lime content is greater than 10%, and this mass ratio is equivalent to the 2 : 8 volume ratio of lime-soil, so the volume ratio of 2 : 8 in lime-soil is a better choice.

Cave-Dwelling Model Penetration Test.
In order to further investigate the practical application of the lime-soil damp-proof course at the bottom of the cave, the cave model penetration test was designed and carried out.Main test steps are as follows.
Samples were prepared in a rectangular mold of 40 cmlength, 20 cm-width, and 20 cm-height in accordance with the actual size of the cave (3.6 m-cave width, 1.8 m-side wall height, 1.8 m-arch height, and 6 m-depth).Considering the actual conditions of sampling, transportation, sample preparation, and test site, the proportion of this paper is 1 : 30.e loess of the model is taken from Yan'an, which is the same as the eldinvestigated cave dwelling.Two small size caves were dug into the two sides of the model, one has lime-soil cushion and the other does not.e wetting condition based on the largest rainfall of this area in history was considered.e most unfavorable situation has been taken into account.e bottom of the model was uniformly wetted with water at an interval of eight hours.200 ml water was added each time to ensure the bottom was evenly soaked.e model was sealed with a black plastic bag.
en, the soil moisture content was measured every 24 hours.After each measurement was completed, the model was continued to be added with water, sealed, and stood for a while.It is shown in Figure 8.A 2 : 8 volume ratio lime-soil cushion was set on a half of the bottom side.At the same time, there is no lime-soil cushion on the other half of the model.At the beginning of the experiment, the initial water content measured in the interior of the cave was the same.
Two models were tested.After ve days of continuous measurement, the relationship between water content and time was obtained, as is shown in Figure 9.
Figure 9 shows that after being soaked for ve days, the change in trend of water content is relatively smooth.What is more, the moisture contents measured of the treated half of the bottom were lower than that of the untreated half.
Because of the dimensional e ects, there is a certain di erence in the test dates between the scale model test and real cave dwelling.However, the comparison relationship between di erent soil permeability is similar because the scale ratio is same.e scale model test mainly investigates whether there are obvious di erences between the permeability of lime-soil and loess in cave dwelling and whether in the prototype or in the model, the trend that the lime-soil damp-proof layer can e ectively reduce the water penetration in cave dwelling is similar.e research concerned more about the trend than precision of the test result.It can be seen that the trend of the two curves has a clear  Advances in Civil Engineering distinction, which shows using lime-soil as damp-proof course in cave dwellings is feasible.

Conclusions
Aiming at reducing dampness of cave dwellings and in view of the actual situation of the loess region, the authors proposed the new processing method by which the dampproof course was set at the bottom of the cave, as it comprised the lime-soil cushion and the linoleum.Its feasibility was discussed and studied by the lime-soil permeability test and cave model penetration test.e results of this study are as follows: (i) e field investigation and questionnaire survey showed that humidity has become one of the main problems of cave dwelling environment and must be solved.(ii) e permeability coefficient of lime-soil decreased rapidly as the lime content increased and was significantly lower than that of the untreated soil.It proved that lime-soil could effectively decrease the permeability of loess, thus reducing the degree of dampness.Considering the test result, the volume ratio of 2 : 8 in lime-soil is a better choice.(iii) e results of cave model penetration test showed that the change of water content of the treated cave was relatively insignificant.And the measured moisture contents were lower than those of the untreated cave. is showed that lime-soil could effectively be damp-proof.
In addition, the estimates show that the retrofit cost of a common cave dwelling is about 1000 RMB or 170 USD by using the lime-soil layer bottom damp-proof method.e increased cost accounts for only 1.0%-1.6% of the total cost.In comparison with the current desiccant dehumidification methods, the cost of using lime-soil layer is the same as the cost of using two years desiccant.However, there is no subsequent cost to use lime-soil, but the desiccant must be used for a long time.Obviously, using lime-soil layer as damp-proof method for cave dwelling is very convenient and economical.
In conclusion, the method is reasonable and practical, which can be implemented easily and conveniently in the future.
is work also has practical values.Lime-soil cushions will improve living quality in the loess region and consequently benefit the preservation and development of cave architectures.

Figure 1 :Figure 2 :
Figure 1: e statistical survey data of the main disadvantages of cave dwelling.

Figure 3 :Figure 4 :
Figure 3: Damp-proof course at the bottom of the cave dwelling.

Figure 5 :
Figure 5: e instrument of the variable water head permeability test.

Figure 8 :
Figure 8: Front elevation (a), back elevation (b), and a longitudinal cross section (c) of the cave dwelling model (unit of dimensions: mm).

Table 1 :
Properties of the soil samples.