A Case Study of Combined Drainage Consolidation-Preloading Methods for a Highway Subgrade on Peat Soils

A highway project of up to 100 km/h is currently being constructed between Colombo and Katunayake International Airport across a Sri Lankan muskeg area. At this site, peat deposit was initially 0.8∼15.3 m thick and was underlain by sand, clay, or gneiss. The ground improvement methods adopted in the project were combined drainage consolidation-preloading methods, pipe pile foundation, and geogrids. This paper provides a detailed insight into the implementation of combined drainage consolidation-preloading methods used in the project, including sand pile, gravel pile, and plastic drainage plate as the prefabricated vertical drains. Periodical field-level observations were taken during the ten years, including the construction and postconstruction periods. The results show that peat soils’ consolidation coefficient has been increased several times to tens of times due to ground improvement. After removing the temporary surcharge, the highway embankments did not heave and was followed by long-term settlements totaling 1.3∼7.4 cm over the following seven years of observations. Analysis of the settlement records shows that combined drainage consolidation-preloading methods have helped accelerate drainage consolidation and reduce postconstruction settlement.


Introduction
Peat soils are distributed in 59 countries and regions globally, accounting for 5% and 8% of the earth's surface area [1]. Peat soils, which have considerably high organic content, high water content, high compressibility, and low shear strength, are considered one of the worst foundation materials.
When peat deposits are relatively shallow (less than 5 m), excavation and replacement by granular materials are commonly performed. However, special foundation treatment is usually required when the deposits are deeper or of a large lateral extent. Combined drainage consolidation-preloading methods are widely accepted in soil engineering practice [11][12][13][14].
is technique involves removing pore water from the soil, leading to soil skeleton [15,16].
However, the application of this method in the amorphous peat foundation has not been reported. Given this, this paper reports the application of combined drainage consolidation-preloading methods in the amorphous peat soil foundation. Based on the analysis of the changes in settlement monitoring values and consolidation parameters, the foundation treatment scheme's applicability of combined drainage consolidation-preloading methods (sand pile, gravel pile, and prefabricated vertical drains) in amorphous peat soil foundation is evaluated.

Description of the Site
Sri Lanka is an island country in the Indian Ocean with a tropical monsoon climate, which is located between latitude 5°55′ to 9°50′ north and longitude 79°42′ to 81°53′ east. e highway connecting Colombo, the capital of Sri Lanka, and Katunayake International Airport is adjacent to the Indian Ocean, which is called the CKE principle line. e length of the mainline is 25.8 kilometers, and the length of ramps and branch lines is 4.8 kilometers; the site location is presented in Figure 1. e project began in August 2009 and was completed in September 2013. It has been open to traffic for seven years. e aerial view of the expressway during construction is provided in Figure 2(a), and the aerial view two years after completion is provided in Figure 2 e project is a bidirectional four-lane highway, and the design speed is 100 km/h. According to the technical specification, the maximum residual differential settlement had not to be more than 0.3% (100 km/h) and 0.6% (80 km/ h) change in grade over longitudinally. e postconstruction settlement does not exceed 180 mm within two years. e highway traverses through a muskeg area, and the soils encountered on the project site are peat soils, organic soils, and silt clay; the length of highway subgrade through peat soil is 13.7 km, and the thickness of the peat varies between 0.8 and 15.3 m. e peat soils contain incompletely decayed plant fragments and fine woody fibers, formed through accumulation and decomposition of natural vegetation.
e physical properties of peat soils, which were determined on samples taken utilizing a 7.6 cm piston sampler, are given in Table 1. Before construction, the water table was 0-0.5 m below the ground surface.

Design Considerations
For roads with strict postconstruction settlement control standards, preloading treatment is more effective for peat soil. It can reduce the settlements to acceptable values; furthermore, it has the advantage of resulting in an appreciable increase of its shear strength, which makes the preloading technique extremely interesting for different engineering applications.
Considering the cost and the technical feasibility of the different alternatives, the combined drainage consolidationpreloading methods for the highway subgrade on peat soils as the foundation improvement method are adopted and drainage consolidation methods include sand pile, gravel pile, and prefabricated vertical drains. After seven years of operation, the ground improvement work is proved to be successful, and the expected residual settlements are below the contract's allowable limit. In this paper, three typical stations of K2 + 600, K4 + 900, and K6 + 500 are presented. Effectiveness of the sand piles, gravel piles, and plastic drainage plate in the consolidation of peat soils are reported and discussed.
In the three sections of K2 + 600, K4 + 900, and K6 + 500, the thickest part of peat soil is 13.8 m and the thinnest part is 2.7 m. e physical and mechanical properties of soil are given in Table 2. e typical cone penetration test (CPT) plot at the test site on these sections is provided in Figures 3 According to the ground conditions, various drainage systems including sand pile, gravel pile, and plastic drainage plate were adopted. Details of design parameters are given in Table 3. e typical section of the combined drainage consolidation-preloading methods is given in Figures 4(a)-4(c). e height of the embankment of section K4 + 900 is 6.5 m, the height of the surcharge is 7.1 m, and the preloading time is 491 days, as shown in Figure 5

Analysis of Settlement
e height of the embankment of section K6 + 500 is 3 m, the height of the surcharge is 1.5 m, and the preloading time is 399 days, as shown in Figure 5(c).
During the construction period, the total settlement of the subgrade soil of the three stations varies greatly, which is related to the height of the embankment and the height of surcharge, the thickness of peat soil, the method of consolidation and drainage, and so on. In station K4 + 900, the height of embankment is the highest and the roadbed settlement is as high as 1760 mm.

Consolidation Parameter
Analysis. e coefficient of consolidation C v is an important index to reflect the consolidation rate of foundation soil. e Asaoka method [17] and improved Asaoka method [18,19] are used to calculate the consolidation coefficient C v of subgrade in different project sections. e S n -S n-1 relationship of the K2 + 600 section, K4 + 900 section, and K6 + 500 section is presented in Figures 6(a)-6(c). e effects of both smear and wellresistance are considered in the improved Asaoka method, such that where C v is the coefficient of consolidation, H is the maximum drainage distance, and Δt is the time interval for         Advances in Civil Engineering settlement plot according to Asaoka [17]. e parameter β r can be calculated as where G is the factor expressing the effect of well-resistance, , k s is the horizontal permeability of the smear zone, k h is the coefficient of horizontal permeability; J is the factor expressing the effect of smear, J � ln(r s /r w )((k h /k s ) − 1), F a � (ln((n/s) + (k h /k s ))lns− (3/4))(n 2 /n 2 − 1) +(s 2 /n 2 − 1)(1 − (k h /k s ))(s 2 /4n 2 ) + (k h / k s )(1/n 2 − 1)(1 − (1/4n 2 )), n is the drain spacing ratio, s � (r s /r w ), r s is the diameter of the smear zone, and r w is the equivalent diameter of the drain. Depending on the Asaoka method and the consolidation theory of saturated soil with vertical drainage, an inverse analysis formula for the coefficient of consolidation is deduced: where k 1 � e − ((π 2 C v /4H 2 )+β r )Δt , k 0 � (1 − k 1 )S, and k 1 � β, where β can be obtained by the Asaoka method.
e results of an inverse analysis of the consolidation coefficient after the drainage consolidation treatment are summarized in Table 4. It can be seen that the coefficient of consolidation (C v ) of natural peat soils from laboratory tests    Advances in Civil Engineering is only 2.6-3.8 m 2 /year, which shows that the consolidation coefficient of foundation soil has increased several times to tens of times after using the drainage consolidation method. From the back-calculation results, the consolidation effect of sand pile and gravel pile drainage body is better than that of plastic drainage plate.

Postconstruction Settlement.
e long-time settlement of the subgrade by time after removing the surcharge is drawn in Figure 7.
It can be seen from Figure 7; after unloading, there have not been prominent rebound stages of the subgrade, which were different from the rebound phenomenon observed by Samson in the peat soil preloading project near Saint-Laurent River [20]. It experienced a period of settling stabilization, which was about 150, 350, and 800 days to the stations of K2 + 600, K4 + 900, and K6 + 500, respectively. After the stable settlement period, the foundation had experienced the settlement increase period, but the total settlement amount was not large; the settlement rate was 9.7 mm/year, 10.9 mm/year, and 1.8 mm/year, respectively. However, in section K4 + 900, the settlement was close to the warning value, worthy of great attention.

Conclusion
is paper presents a case study on peat soil ground improvement using combined drainage consolidation-preloading methods in Sri Lanka. e drainage consolidation method (sand pile, gravel pile, and plastic drainage plate) combined with an overloading preloading scheme in amorphous peat foundation is evaluated based on the field's statistical analysis data. e major conclusions drawn are summarized as follows: (1) e consolidation coefficient of foundation soil has increased several times to tens of times after using the drainage consolidation method. From the backcalculation results, the consolidation effect of sand pile and gravel pile drainage body is better than that of the plastic drainage plate. (2) After removing the temporary surcharge, the highway embankments did not heave and was followed by long-term settlements totaling 1.3∼7.4 cm over the following seven years of observations. (3) Combined drainage consolidation-preloading methods have been beneficial in accelerating drainage consolidation and reducing postconstruction settlement.

Data Availability
e data used to support the findings of this study are included within the article.

Conflicts of Interest
e authors declare that they have no Conflicts of Interest.